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{{#Wiki_filter:COMMUNICATOR THE PUBLICATION OF THE PROFESSIONAL
{{#Wiki_filter:COMMUNICATOR THE PUBLICATION OF THE PROFESSIONAL REACTOR OPERA TOR SOCIETY FALL 2011 In this issue: NRC Fukushima Task Force Analysis of the 2003 Callaway Shutdown
----------
-------------------
-\\t --30 YEARS OF OPERA TORS SPEAKING FOR OPERA TORS -----------
COMMUNICATOR PROFESSIONAL REACTOR OPERATOR SOCIETY PO Box 484 Byron, IL 61010 phone (815) 234-8140 24 hour FAX and answering BOARD OF DIRECTORS President Mitch Taggart -Sequoyah Vice President Casey Pfeiffer
-Sequoyah Treasurer Mark Rasmussen -Byron Secretary Joe Evans -Perry Region I President vacant Region II President Brian Snyder -Oconee Region III President
* a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
* a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
* an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
* an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
* a signal to the Boron Dilution and Mitigation System (BDMS
* a signal to the Boron Dilution and Mitigation System (BDMS) which causes an automatic swap over of charging pump suctions from the Volume Control Tank to the Refueling Water Storage Tank (RWST) in the event that source range counts increase by 70% in a rolling 10 minute period (since the RWST is borated to -2500 ppm, this BDMS circuit provides tection against inadvertent reactivity additions caused by xenon-135 decay, inadvertent tions, and inadvertent cooldowns)
. Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi zed, this is so a reactor start up can be perfonned.
15 During a reactor startup, administrative controls 16 are in place which mitigate the loss of safety margin from blocking the automatic safety circuits driven by the SRNis. The designers of Callaway Plant never intended for the plant to be operated in
* a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
* a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
* an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
* an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
* a signal to the Boron Dilution and Mitigation System (BDMS) which causes an automatic swap over of charging pump suctions from the Volume Control Tank to the Refueling Water Storage Tank (RWST) in the event that source range counts increase by 70% in a rolling 10 minute period (since the RWST is borated to -2500 ppm, this BDMS circuit provides tection against inadvertent reactivity additions caused by xenon-135 decay, inadvertent tions, and inadvertent cooldowns). Although the Technical Specifications for Callaway Plant permit operation in the source range with the SRNis de-energized, this is so a reactor start up can be perfonned. 15 During a reactor startup, administrative controls 16 are in place which mitigate the loss of safety margin from blocking the automatic safety circuits driven by the SRNis. The designers of Callaway Plant never intended for the plant to be operated in the source range with the control rods at their critical rod heights and with none of the SRNI driven automatic protections in place. Although the NRC is technically correct in stating that this condition did not violate the plant's licensing requirements, there is more to ensuring reactor safety than forcing a verbatim interpretation of the Technical Specifications; not all conditions can be exactly defined by the Technical Specifications and a competent professional reactor operator should be able to discern when the plant is in a condition in which the designer never intended. (Continued on next page) 14Note that from I 0:23 to 11 :25 all indications other than the IRNis were steadily indicating the plant was low in the power range: the PRNis were reading -I% rated power, the t-.T instruments were reading 1.75% power and the secondary calorimric computer points were reading 62 MWth. In order to realize they were in the source range the operators would have either had to note the !RNI readings or question why they had not needed to add positive reactivity to account for xenon buildup. With regard to noting the IRNI readings, because of their units (ion chamber amps) and their scaling (logarithmic) the operators do not normally use these instruments while at power. With regard to questioning why they had not needed to dilute or pull rods to make up for xenon, understanding the reactor dynamics of Iodine/Xenon was a weakness of this crew as demonstrated by their response to the 9°F temperature drop which occurred from 09:36 to I 0:00. 15It is impossible to do a successful reactor startup without blocking the SRNI flux trip and BDMS; therefore, once the IRNI signal reaches I E-10 ica during a reactor startup, the operators are permitted to de-energized the SRNis (which by then have had all their protective functions blocked). 16For example: a Reactor Engineer present in the control room, an Estimated Critical Position has been calculated, the crew is intently performing a procedure which warns them to "expect criticality at any time," etc. 25 26 COMMUNICATOR CONCLUSIONS There is much to be learned from the October 21, 2003 passive shutdown at Callaway Plant. The incident highlights a number of issues: the ner by which NRC licensed operators might fail to appreciate the magnitude of the effect xenon is having on core reactivity while that effect is being masked by power defect, the need for specific and thoughtful procedural guidance for stabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback poses to the operator as the NFHR is approached, the manner in which an erator focusing on calorimetric instruments while at low power might fail to recognize fission power lowering below the Point of Adding Heat, and the importance of documenting incidents in the rective action process. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "generic mentals" of reactor dynamics. No analysis of the incident has been done by INPO and the NRC's analysis in Information Notice 2011-02 is not very thorough; those interested in a deeper analysis of the details of the incident are encouraged to view the references at the end of this article. DISCLAIMER The views expressed in this article are those of the author and in no way reflect the position of the US Nuclear Regulatory Commission or the sional Reactor Operator Society. To participate in an online analysis of this dent, send an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro ups. com (anonymous participation is accommodated). FALL 2011 Anyone wishing additional information on this incident is encouraged to contact me at: cione@hotmail.com (573) 230-3959 REFERENCES l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator Performance Issues Involving Reactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at laway, November 2010. 3. Non-Concurrence on NRC Information Notice 2011-02, Operator Performance Issues ing Reactivity Management At Nuclear Power Plants (ADAMS #MLl 10420293). 4. September 17, 2010 letter from L. Criscione to William Borchardt (ADAMS #ML 102640674). 5. April 27, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #MLl 0120040 l ). 6. April 30, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #ML101230100). 7. G2010059/EDATS: OED0-2010-0775 -tion Closure Letter to Lawrence S. Criscione Related to Requested Action Under 1 OCFR 2.206 Regarding October 21, 2003 Event at Callaway Plant, Unit 1 (TAC No. ME4721), ADAMS #MLl 10140104, January 19, 2011. Note from the Author In my opinion, this is an important piece of Operating Experience which is only available through PROS. The event was never submitted to INPO and, although the NRC included it in an Information Notice, most of the significant Lessons Learned from the incident were not addressed. More than anything, the event is an example of: ( 1) licensed reactor tors being "set up for failure" by impractical operating practices and expectations and (2) the importance of honestly reporting events and accurately analyzing them so that future reactor operators (both at the plant and throughout the industry) do not fall victim to the same poor practices and knowledge gaps.}}
* a signal to the Boron Dilution and Mitigation System (BDMS) which causes an automatic swap over of charging pump suctions from the Volume Control Tank to the Refueling Water Storage Tank (RWST) in the event that source range counts increase by 70% in a rolling 10 minute period (since the RWST is borated to -2500 ppm, this BDMS circuit provides tection against inadvertent reactivity additions caused by xenon-135 decay, inadvertent tions, and inadvertent cooldowns)
. Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi zed, this is so a reactor start up can be perfonned.
15 During a reactor startup, administrative controls 16 are in place which mitigate the loss of safety margin from blocking the automatic safety circuits driven by the SRNis. The designers of Callaway Plant never intended for the plant to be operated in the source range with the control rods at their critical rod heights and with none of the SRNI driven automatic protection s in place. Although the NRC is technically correct in stating that this condition did not violate the plant's licensing requirement s, there is more to ensuring reactor safety than forcing a verbatim interpretation of the Technical Specifications
; not all conditions can be exactly defined by the Technical Specifications and a competent professional reactor operator should be able to discern when the plant is in a condition in which the designer never intended.  
(Continued on next page) 14Note that from I 0:23 to 11 :25 all indications other than the IRNis were steadily indicating the plant was low in the power range: the PRNis were reading -I% rated power, the t-.T instruments were reading 1.75% power and the secondary calorimric computer points were reading 62 MWth. In order to realize they were in the source range the operators would have either had to note the !RNI readings or question why they had not needed to add positive reactivity to account for xenon buildup. With regard to noting the IRNI readings, because of their units (ion chamber amps) and their scaling (logarithmic) the operators do not normally use these instruments while at power. With regard to questioning why they had not needed to dilute or pull rods to make up for xenon, understanding the reactor dynamics of Iodine/Xenon was a weakness of this crew as demonstrat ed by their response to the 9°F temperature drop which occurred from 09:36 to I 0:00. 15It is impossible to do a successful reactor startup without blocking the SRNI flux trip and BDMS; therefore, once the IRNI signal reaches I E-10 ica during a reactor startup, the operators are permitted to de-energized the SRNis (which by then have had all their protective functions blocked). 16For example: a Reactor Engineer present in the control room, an Estimated Critical Position has been calculated, the crew is intently performin g a procedure which warns them to "expect criticality at any time," etc. 25 26 COMMUNICATOR CONCLUSIONS There is much to be learned from the October 21, 2003 passive shutdown at Callaway Plant. The incident highlights a number of issues: the ner by which NRC licensed operators might fail to appreciate the magnitude of the effect xenon is having on core reactivity while that effect is being masked by power defect, the need for specific and thoughtful procedural guidance for stabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback poses to the operator as the NFHR is approached, the manner in which an erator focusing on calorimetric instruments while at low power might fail to recognize fission power lowering below the Point of Adding Heat, and the importance of documenting incidents in the rective action process. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "generic mentals" of reactor dynamics. No analysis of the incident has been done by INPO and the NRC's analysis in Information Notice 2011-02 is not very thorough; those interested in a deeper analysis of the details of the incident are encouraged to view the reference s at the end of this article.
DISCLAIMER The views expressed in this article are those of the author and in no way reflect the position of the US Nuclear Regulatory Commission or the sional Reactor Operator Society.
To participate in an online analysis of this dent, send an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro ups. com (anonymous participation is accommodated).
FALL 2011 Anyone wishing additional information on this incident is encouraged to contact me at: cione@hotmail.com (573) 230-3959 REFERENCES  
: l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator Performance Issues Involving Reactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at laway, November 2010. 3. Non-Concurrence on NRC Information Notice 2011-02, Operator Performance Issues ing Reactivity Management At Nuclear Power Plants (ADAMS #MLl 10420293)
. 4. September 17, 2010 letter from L. Criscione to William Borchardt (ADAMS #ML 102640674).  
: 5. April 27, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #MLl 0120040 l ). 6. April 30, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #ML101230100).  
: 7. G2010059/EDATS: OED0-2010-0775  
-tion Closure Letter to Lawrence S. Criscione Related to Requested Action Under 1 OCFR 2.206 Regarding October 21, 2003 Event at Callaway Plant, Unit 1 (TAC No. ME4721),
ADAMS #MLl 10140104, January 19, 2011. Note from the Author In my opinion, this is an important piece of Operating Experience which is only available through PROS. The event was never submitted to INPO and, although the NRC included it in an Information Notice, most of the significant Lessons Learned from the incident were not addressed.
More than anything, the event is an example of: ( 1) licensed reactor tors being "set up for failure" by impractical operating practices and expectations and (2) the importance of honestly reporting events and accurately analyzing them so that future reactor operators (both at the plant and throughout the industry) do not fall victim to the same poor practices and knowledge gaps.}}

Revision as of 15:26, 30 June 2018

Article from the Fall 2011 Communicator of the Professional Reactor Operator Society on the October 21, 2003 Inadvertent Passive Reactor Shutdown at Callaway Plant
ML16011A519
Person / Time
Site: Callaway 
Issue date: 11/01/2011
From:
NRC/RES/DRA/OEGIB
To:
Criscione L S
References
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Text

COMMUNICATOR THE PUBLICATION OF THE PROFESSIONAL REACTOR OPERA TOR SOCIETY FALL 2011 In this issue: NRC Fukushima Task Force Analysis of the 2003 Callaway Shutdown

-\\t --30 YEARS OF OPERA TORS SPEAKING FOR OPERA TORS -----------

COMMUNICATOR PROFESSIONAL REACTOR OPERATOR SOCIETY PO Box 484 Byron, IL 61010 phone (815) 234-8140 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> FAX and answering BOARD OF DIRECTORS President Mitch Taggart -Sequoyah Vice President Casey Pfeiffer

-Sequoyah Treasurer Mark Rasmussen -Byron Secretary Joe Evans -Perry Region I President vacant Region II President Brian Snyder -Oconee Region III President Jim Kelly -Perry Region IV President Sheryl Breault -Fort Calhoun Foreign and Domestic Affairs President Bob Meyer -Yogtle STAFF Communicator Editor Mary Cay Rasmussen Office Manager Mark Rasmussen Web master vacant ON THE COVER: TVA's Bellefonte Nuclear Power Plant is scheduled to be online between 2018 and 2020. FALL2011 CONTENTS Bellefonte Recommendations from the NRC Fukushima Task Force Casey Pfeiffer NFP A Standard 805 -Fire Protection Brian Snyder Analysis of the 2003 Callaway Shutdown Lawrence Criscione Ethanol -The Answer to Our Energy Needs? Mark Rasmussen REGULAR FEATURES Members' Page Officer Reports Joe SRO New Members PAGE 2 3 8 11 27 42 46 52 53 ADVERTISE IN THE COMMUNICATOR AD SIZE & PLACEMENT Back Cover -Full Page Back Cover -Half Page Inside Cover -Full Page Inside Cover Half Page Internal

-Full Page Internal -Half Page Internal -Quarter Page SINGLE ISSUE $1000. $ 600. $ 750. $ 400. $ 500. $ 300. $ 200. FULL YEAR (4 ISSUES) $2500. $1500. $1875. $1000. $1250. $ 750. $ 500. Membership in the Professional Reactor Operator Society is open to any individual or company that shares our goal of promoting safe reactor*

operation. Membership dues are $35 per year, which includes a subscription to the Communicator.

The Communicator is published quarterly

, and is distributed to members by US mail. Additional copies are available for $I 0 per copy. Inquiries regarding distribution, delivery, or change of address should be directed to <thePROSoffi ce@nucpros.com>. The Communicator is copyrighted by the Professional Reactor Operator Society and may not be reproduced in whole or in part without express permission. Opinions expressed by authors are their own and do not necessari ly reflect the positions of the Professional Reactor Operator Society, the Communicator Editor, or the organization with which the author is affiliated. Publication of viewpoints or descriptions of materials

, products, or services should not be construed as endorsements by the Professional Reactor Operator Society or its officers.

FALL 2011 COMMUNICATOR Analysis of the October 21, 2003 Passive Reactor Shutdown at Callaway Plant Lawrence S. Criscione PE Larry Criscione works in the Nuclear Regulatory Commission's Office of Research (RES) where he analyzes equipment and human performance data in support of nuclear regulatory research.

The views expressed in this cle are his own and in no way reflect the position of the US NRC. Larry worked at Callaway Plant from 2002 through 2007 where he was a licensed Senior Reactor Operator and a Shift Technical Advisor.

Larry has worked at Clinton Power Station (2000-02) and FirstEnergy (2008-09).

In the US Navy he was qualified as Engineering Officer of the Watch at DlG prototype (1994) and aboard the fleet ballistic missile submarine USS GEORGIA (1995-98).

He holds a branch license in nuclear engineering from the State of Iowa. He is a 1993 graduate of the University of Missouri-Rolla.

Abstract:

At Callaway Plant on October 21, 2003, while attempting to stabilize reactor power during a forced de-rate, Xenon-135 buildup caused average reactor coolant teperature to lower at a 25 minute period, resulting in an automatic isolation of the letdown system on low pressurizer water level and operation of the reactor below the Minimum Temperature for Critical Operation. After manually tripping the turbine-generator to sist in temperature recovery, the reactor passively shut down due to a sharp 4°F rise in aveage coolant temperature. For the next 110 utes the operators performed secondary and tertiary plant shutdown activities while relying on an informal estimation that Xenon-135 levels were sufficient to prevent the reactor from vertently restarting. The passive reactor sdown was not documented until it was uncered 40 months later, and it was not shared with the Institute of Nuclear Power Operations lowing the request which accompanied SOER 07 -1. The incident highlights the pitfalls assocated with attempting to maintain a commercial pressurized water reactor critical during MODE 2-Descending and demonstrates how concepts tested on the NRC Generic Fundamentals Exam apply to actual reactor operation. The incident also highlights some non-conservative reactivity management practices which must be avoided by Professional Reactor Operators. This article describes the events leading up to and immediately following a passive reactor shutdown which occurred at Ameren Corporation

's nuclear plant in Callaway County, MO on October 21, 2003. An assessment of the NRC's response to the incident is included along with key "lessons to be learned."

Details of the incident were first pub-1 ical ly released by the Union of Concerned tists (UCS) in a 20 l 0 issue brief1 titled 2003 Semented Shutdown at Callaway, and then, in 20 l l, the US Nuclear Regulatory Commission (NRC) partially covered the incident as part of tion Notice 2011-02, Operator Performance issues Involving Reactivity Management at Nuclear Power Plants. 2 Also discussed in the article are: The manner by which the effect of Xenon-135 buildup can be masked by other passive tivity insertions during a plant transient.

The effect operation near the Non-Fission Heat Rate has on Temperature-Reactivity feedback. The challenges facing the operator during low power operation due to human factoring of control board instruments

. (Continued on next page) 1http://www.ucsusa.org/nuclear _power/nuclear _power_ risk/safety/2003-segmented-shutdown-at-cal !away.html 2http://pbadupws.nrc.gov/docs/ML I 018/ML I 01810282.pdf 11 12 COMMUNICATOR REACTOR DYNAMICS REFRESHER Passive Response to Reactivity Changes Commercial Pressurized Water Reactors (PWRs) in the United States are designed to passively spond to changes in reactivity.

They do this through two primary methods:3 1. A negative power coefficient of reactivity

2. A negative Moderator Temperature ficient of reactivity

(-MTC) Item I is a required safety feature of all US Commercial designs:

a negative power coefficie nt of reactivity ensures that an uncontrolled rise in tor power will result in a negative insertion of activity, thereby limiting the power rise. Item 2 is normally present throughout the fuel cycle at most PWRs; however, some plants do mit a slight +MTC during a limited window of their fuel cycle. October 21, 2003 was late in fuel cycle 13 for Callaway Plant and a -MTC was sent so discussions in this article assume a -MTC. The combined result of items l and 2 is that, on a US commercia l PWR, power is inherently stable. That is, the reactor "wants" to stay at a steady power and resists power increases and decreases.

Response to a reactivity insertion with steady state steam demand: When negative reactivity (L'.lp) is inserted (e.g. insertion of control rods, dition of boron, buildup of Xenon-135) while the steam demand (i.e. turbine-generator loading) is held constant, reactor power will decrease slightly. Because of the negative power coefficient of tivity, positive reactivity is passively inserted as power lowers, dampening the negative reactivity insertion

. With steam demand unchanged

, the new lower power will cause a negative power match to develop.4 This negative power mismatch will cause temperature to lower. Due to the -MTC, as temperature lowers positive reactivity is passively inserted, which further dampens the negative reactivity insertion

. FALL20ll Temperature will continue to lower as long as there is a negative power mismatch. Eventually

, enough positive reactivity will be inserted by the temperature drop to result in a net increase in activity.

This point is called the point of power "turning."

At this point, reactor power will start to rise and the magnitude of the negative power mismatch will lower, dampening the temperature drop. Once reactor power rises above steam demand, there will be a positive power mismatch which will now cause temperature to rise. The rising temperature will insert negative reactivity

, causing reactor power to lower. These passive feedback processes will continue until, eventually, reactor power again matches steam demand and there is no power mismatch to drive temperature.

At this point, temperature will be lower than it was prior to the negative reactivity insertion

.

cally, the change in temperature is: L'.l T = L'.lp/MTC). The reactor will passively respond to a positive insertion of reactivity in a similar manner, ing in the reactor operating at a higher temperature than prior to the reactivity insertion

. The response of the reactor described in the graphs above is called "Temperature-Reactivity feedback.

" Temperature-Reactivity feedback consists of two things: 1. The passive response of the average reactor coolant temperature (Tavg) to the power match induced by the change in reactivity

. 2. The passive counter insertion of reactivity due to the temperature response, which continues until power turns and re-approache s steam demand. So, without any operator action, US commercial PWRs passively respond to reactivity changes in a manner that eventually results in the same steady state power at a new temperature

. This generic fundamental is demonstrated later in this article by 3Since Callaway Plant is a PWR, the reactivity coefficient due to voids is not discussed in this article. 4Power mismatch is the difference between steam demand and reactor power.

FALL201 I the way the reactor at Callaway Plant responded to Xenon-135 buildup when the turbine-generator loading was kept constant from 09:36 to l 0:03 (see Figure 1 on next page). Passive response to a change in steam demand (for a PWR): When the steam demanded by the turbine is lowered, a negative power mismatch will result, causing temperature to rise. The rising temperature will insert negative reactivity, causing reactor power to lower. The lowering reactor power will result in a lowering of the power match, dampening the temperature rise. As long as there is a positive power mismatch, temperature will continue to rise. The negative reactivity sertion from rising temperature will continue until reactor power falls below steam demand resulting in a negative power mismatch which thereby causes temperature to lower. The lowering perature will insert positive reactivity

, causing power to tum and approach steam demand. tor power will eventually become steady at the new steam demand level. Due to the negative power coefficient of reactivity

, the lower power level will have resulted in a passive positive t1v1ty insertion.

Temperature will passively spond to this positive reactivity insertion by steadying out at a higher level and thus inducing a negative reactivity insertion which cancels out the power defect. 5 A pressurized water reactor will respond similarly to an increase in steam demand. The response of the reactor described in the graphs above is characterized as "reactor power follows steam demand." Without any operator action, US PWRs passively respond to steam demand changes in a manner that eventually results in reactor power matching steam demand at a new temperature

. This generic fundamental is demonstrated later in this article by the way the reactor at Callaway Plant responded to COMMUNICATOR the lowering of turbine-generator loading between 10:03 and 10:10 (see Figure 1). The Effect of Decay Heat Following the initial criticality of the fuel cycle, some level of decay heat is always present.

The amount of decay heat present is determined by the reactor's power history. At 100% rated power, decay heat typically accounts for 7% of the power being generated in the core. During a down power, decay heat accounts for a slightly larger percentage of reactor power than at steady state power. This is because the longer lived fission product daughters which were produced at 100% power are exerting a disproportional influence on the decay heat spectrum than they normally would at a steady state power level. This influence is not easily noticed in MODE 1. 6 However, as reactor power nears MODE 2,7 the effects of decay heat become substantial.

The Non-Fission Heat Rate: The Non-Fission Heat Rate (NFHR) is the power produced by the reactor plant from sources other than fission. though there are other contributors to the NFHR besides decay heat (e.g. friction heat from the actor Coolant Pumps), this article is primarily cerned with the effect of decay heat. The NFHR is about 7% of rated power when the reactor is operating at 100% power. The contribution of short-lived fission product daughters to the NFHR is roughly proportional to the fission rate so it lowers proportionally to reactor power. However, the change in the population of long-lived fission product daughters lags the change in fission rate as the reactor is down powered. As the fission rate falls to zero, there is still a substantial amount of heat being generated by the long lived fission product daughters.

This NFHR varies with power history, but, following a I 0%/hour shutdown of the reactor, the half-life spectrum of the remaining daughters is long enough that the NFHR is tively constant when measured in hours (i.e. it lowers by just a few percent every hour). (Continued on next page) 5power defect is the term for the reactivity inserted from a change in reactor power level. 6MODE 1 refers to the state of operating the reactor at power (5% to 100% rated reactor power). 7MODE 2 refers to the transition state between the reactor being solidly in the power range (i.e. beyond the point at which the NFHR exerts any substantial influence) and the reactor being shutdown (i.e. definitively subcritical as indicated by calculating KelT to be less than 0.99). The reactor enters MODE 2-Descending when reactor power lowers below 5% rated power. 13 14 COMMUNICATOR FALL 2011 Average Reactor Coolant Temperature (Tavg), Control Band 'D' Rod Heights and Reactor Power (LlT) during the October 21, 2003 Passive Reactor Shutdown at Callaway Plant 587°F 584°F 581°F 578°F 575°F 572°F 569°F 220 steps 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% -01:00 02*00 03:00 ' "" K o4:oo ""I' -.. ..........

... ' ..... ""i... '"""'-20% 09:30 09:40 09:50 10:00 10:10 10:20 10:30 0 i 10% 120 i steps ' l .........

.,,.,,:> i-... -....... ... , " .,,_ IL -.. *-.. , .2'-....

I'... *-......

I\ --....,_ i \ " ....._ IL--I 1 "' i ' !'.... .........

.......__,i'o...

i -...._ ! "-. ' 'l i-f -, , , i j /i -.,. '\ / .. ' 20% % : 10° 0% i 10 0 ps 0°F i ste .: 56 /55r F //554"F 551°F 560°F 557°F 554°F 551°F 100 80 60 40 10% 0% 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 --Tavg (scale: 3°F/division

, maximum:

586.7°F, minimum:

549.9°F) -CTRL ROD BANK D (scale: 20 steps/div, max: 216 steps, min: 0 steps) --lff power (scale: 10% rated reactor power/div, max: 100.9%, min: 1.6%) Figure I: Plot of Average Coolant Temperature (Tavg), Primary Calorimetric power (LiT) and Control Bank 'D' rod heights during the October 21, 2003 down power and passive reactor shutdown. Note the severe temperature transient which began at 09:36. Turbine first stage steam pressure data indicates that the operators stopped lowering generator loading at 09:36 with reactor power at 9%. Over the next three minutes, negative reactivity due to Xenon-135 caused power to continue to lower another I%. The power mismatch between the steam demanded by the turbine throttle setpoint and the power being produced by fission caused T avg to immediately begin to lower, thereby inserting positive reactivity which countered the negative reactivity being inserted by the continual buildup of Xenon-135

. Around 09:39 the positive reactivity being inserted by the lowering temperature matched the negative reactivity being inserted by Xenon -135 causing reactor power (as indicated by core Li T) to stabilize at approximately 8%. With a I% power mismatch sent, over the next twenty minutes T avg continued to steadily lower and thereby counteract the continual buildup of xenon. Shortly after I 0:00 the crew again began to lower turbine-generator loading in response to the Shift Manager's decision to take the turbine off-line following the letdown isolation. The renewed lowering of generator loading caused steam demand to lower below fission power and thereby allowed Tavg to temporarily recover slightly. During this time period (I 0:03 to I 0:09), the negative reactivity being inserted by Xenon-135 was now being counteracted by the positive ity being inserted by the load decrease (the plant had a negative power coefficie nt of reactivity).

Generator loading was again stabilized around I 0:09 causing T avg to resume falling, which is the expected passive response of the reactor plant to Xenon-135 buildup. The operators failed to grasp the reactor dynamics behind the transient and assumed the I 0°F drop in Tavg was being caused by malfunctioning steam line and turbine drain valves (which had coincidentally been placed in service at about the same time the temperature transient began).

FALL201 l By the time the reactor at Callaway Plant passively shut down on October 21, 2003, the NFHR was 1.75% of rated reactor power. About half of this was due to RCP pump heat. The Point of Adding Heat: The NFHR mines the reactor's Point of Adding Heat (POAH). The POAH is the amount of fission power needed to noticeably affect reactor power. During a tor startup, the POAH is the point at which raising reactor power (as measured by the nuclear ments8) will noticeably affect total power (as measured by the calorimetric Instruments 9). The POAH is significant during a reactor startup cause it is the point at which Reactivity feedback starts to occur: once reactor power ascends above the POAH, it becomes cult for the reactor operator to pull control rods to produce a set Start Up Rate (SUR) because as positive reactivity is actively inserted with the control rods the resultant reactor power increase causes temperature to rise and thereby feed back negative reactivity which lowers the SUR. Prior to reaching the POAH, the reactor operator uses the control rods to actively control reactivity

. yond the POAH, the control rods are used to tively control average coolant temperature via the passive response that temperature has to manual reactivity changes.

On a shutdown, the POAH cannot be recognized until the reactor is already below it. During a shutdown, the POAH is the point at which ing fission power (as indicated by the Intermediate Range Nuclear Instruments) has no effect on total power. This generic fundamental is demonstrated on Figure 4 by the way the /).. T trace steadies out at 1.75% while the IRNI trace continues to lower. EVENT NARRATIVE DESCRIPTION Cause of the Forced De-Rate At 07:21 on October 20, 2003 a safety-related verter (NN 11) failed, causing the unit to enter a 24 -hour Technical Specification (T/S 3.8.7.A) to COMMUNICATOR either repair the failed inverter or begin a plant shutdown. At 00:37 on October 21, 2003, after repair tempts by Electrical Maintenance

, the operators placed the inverter in service for a retest. The verter failed its retest and at 01 :00 the operators began down powering the reactor at 10%/hour in preparation for a reactor shutdown. By 07:21 reactor power was just below 40% with the inverter still unrepaired so the unit entered the 6-hour Technical Specification (T/S 3.8.7.B) to either repair the failed inverter or shut down the reactor.

Entry into Off-Normal Procedure for Loss of Safety-Related Instrument Power At 08:21 the inverter was again placed in service for a retest. The inverter failed its retest and the crew responded by performing the off-normal cedure for a "Loss of Safety Related Instrument Power." By 08:36 the control room operators had completed their actions, but the off-normal dure could not be closed until an equipment tor could become available to perform an ment check of some valves in the Auxiliary water system. This alignment check was not pleted until 11 :34, resulting in the off-normal cedure remaining open until 11 :37. Although this off-normal procedure administratively remaining open should not, in and of itself, have caused a problem, for unexplained reasons the operators claim they could not perform the step in the tor Shutdown procedure for inserting the control banks until this off-normal procedure had been exited (see discussion in the "Safety and PI&R Concerns" section). Xenon-135 induced Cooldown At 09:36 the unit was at 9% power and the tors discontinued down powering the generator.

It is not clear why this occurred, but since they were 2Y2 hours ahead of schedule it is likely they intended to hold power at while 8There are three sets of nuclear instruments (the power range, intermediat e range and source range). The nuclear instruments measure fission rate by detecting stray neutrons produced by fission. 9There are two sets of calorimetric instruments at Callaway Plant: L'.T instruments (primary calorimetric calculated from the temperature rise across the core) and thermal output computer points (calculated from a secondary calorimetric). 15 16 COMMUNICATOR further troubleshooting occurred on the failed verter. Also around 09:36 the operators cycled the Group B turbine drains. One of the switches for the drains was not indicating

properly, requiring the operators to locally observe the operation of the thirteen valves controlled by the malfunctioning switch. By 09:36, the 10%/hour downpower which had been occurring for the past 8Yi hours was causing a significant Xenon-135 transient.

The constant build up of xenon was inserting negative reactivity at a significant rate; however, prior to 09:36 it was having little effect on reactor plant parameters.

The build up of xenon went largely unnoticed cause, although significant, it was not great enough to overcome the large amounts of positive reactivity being inserted by the 10%/hour ing of reactor power and the 3°F/hour lowering of reactor coolant temperature

. In fact, prior to 09:36 the operators were occasionally having to actively insert negative reactivity because the positive activity being passively inserted from the down power/cool down was slightly greater than the negative reactivity being passively inserted by xenon. Through 09:36, 114 inward steps of rod movement and 220 gallons of boron were required to keep temperature lowering at the desired rate (the boron additions were done during the first 2Y2 hours of the downpower

, when the rate of xenon buildup was still low; see Figure 1 for the control rod movements)

. When the crew ceased lowering turbine-generator load at 09:36, positive reactivity was no longer being passively inserted from the downpower.

However, since Xenon-135 was still building up, negative reactivity was still being passively serted. The crew did not have a detailed ity Management Plan10 and, because of their periences during the past three hours, 11 failed to recognize that, with the downpower no longer oc-FALL2011 curnng, they needed to actively insert positive reactivity to keep average coolant temperature stable. Starting at 09:36, average reactor coolant ture (Tavg) began to lower at about 22°F/hr. With Xenon-135 continuing to insert negative ity, the reactor would occasionally become slightly subcritical causing power to lower below steam demand. With power less than steam mand, Tavg lowered slightly.

Due to the -MTC, the lowering Tavg inserted positive reactivity and caused the reactor to return to a critical state. In this manner, the reactor passively remained cal (i.e. passively overcame the negative reactivity being inserted by Xenon-135) by responding to the buildup of xenon with a lowering ofTavg* The crew mistakenly believed that malfunctioning turbine drains were causing the drop in T avg, so instead of aggressively inserting positive reactiity (e.g. by diluting boron or withdrawing rods), they coordinated with equipment operators in the turbine building to troubleshoot the turbine drains. The only positive reactivity actively inserted the entire day was a 360 gallon add of water to the Volume Control Tank which occurred between 09:47 and 10:00. Letdown Isolation By 10:00 Tavg had lowered 9°F and the letdown system automatically isolated on low pressurizer water level. Also by 10:00, the crew recognized that Tavg had fallen below 55 l °F, the Minimum Temperature for Critical Operations (MTCO) at Callaway Plant. To assist in recovering ture, the Shift Manager directed that the turbine be taken off-line.

Manual Turbine Trip and MODE 2 Entry After the letdown isolation, the operators began lowering turbine-generator loading in preparation for removing the turbine from service.

This caused a positive power mismatch which tempo-'0Those with access to proprietary documents from the World Association of Nuclear Operators should see the dations contained in WA 0 SOER 2007-1, Reactivity Management, for expectations regarding Reactivity Management Plans. 11 In the 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> since relieving the watch at 06:30 the crew had needed to insert control rods 30 steps in order to keep Tavg lowering at the programmed rate. No active insertions of positive reactivity had been required to overcome Xenon-135.

FALL 2011 0 N .,., 00 .-i '<I" " 'fl 'fl 'fl 0 O'I O'I O'I O'I O'I O'I .-i -__ J _____ J_ ____ J__ -__ J_ ___ .l .. --.L.. --560°F .. 555°F .... !Tref (linear scale, 5°F/division) ! ;:::::: ..:.-.:.:.-..:.*.:.:. I i--r--,.,., O'I 0 0 0 .-i .-i .-i ------COMMUNICATOR N .,., 00 .-i '<I" " 0 ,.,., O'I N ':"1 ':"1 ':"1 \"! \"! \"! <:"! <:"! <:"! <:"! 0 0 0 0 0 0 0 0 0 0 0 .-i .-i .-i .-i .-i .-i .-i .-i .-i .-i .-i In !Tavg (linear scale, 5°F/divisionl l Ff' --/ I I I I ------no load Tavg (551'F) --------' ..... 550°F MTCO (551° F). L#------------lV 560°F 555°F 5S0°F 10% 5% 10% AT 5% ... ftl " 1. E-04 u ... ::s 0. -..... ILH (linear scale, 5% rated ' --!NFHR*------; 0% ' ' "' ' ........_

. ' ' 1.E-05 "' E c ftl " " a: E .a Average Coolant Temperature (Tavg) rose steeply -*---POAH l.E-06 1. E--07 " 2 E ........

ftl following the turbine trip (TT). The quick negative "' ftl ... .c *-c u c reactivity insertion which accompani ed the -4°F spike in -""' l.E--08 Tavgcaused the reactor to become substantially '-E .2 ... -1. E--09 ! subcritical and shut down. The POAH and a nominal I *<6' ** c -1/3 dpm SUR were reached around 10:23. 1.E-11 % -1.E-10 l.E-11 Figure 2: Plot of Average Coolant Temperature (Tavg), Primary Calorimetric power (.1T) and Intermediate clear Instrument currents (IRNI) on October 21, 2003. The sharp rise in T avg was caused by the power mismatch resulting from manually tripping the turbine at 6% power and 550.4°F with the steam dumps set at 1092 psig (557°F). The negative reactivity inserted by this temperature rise caused the reactor to passively shut down. The leveling out of the .1 T trace at I 0:23 indicated the Point of Adding Heat. The leveling out of the IRNI traces at I 0:39 indicates entry into the source range. See Figure 3 for plant evolutions occurring during this time frame. rarily caused Tavg to stop lowering (the minimum Tavg occurring at 10:03 in Figures 1 and 2 sponds to the lowering of turbine load below tor power). Between 10:03 and 10:09 the negative reactivity being inserted by xenon was addressed with power defect instead of temperature defect. At l 0: 12:35 the operators manually tripped the turbine-generator with reactor power just under 6% and Tavg at 550.4°F. Prior to tripping the turbine, the operators had, per their procedure, set the condenser steam dumps to open at 1092 psig (which corresponds to 557°F, the "no-load" age coolant temperature at Callaway Plant). ever, because of the confusion resulting from the temperature transient and automatic letdown tion, the crew missed the procedure step to "Hold Reactor Power constant by transferring load to the condenser steam dumps while reducing bine Load. This will prevent inadvertent entry into Mode 2 when the Turbine is tripped." Within 30 seconds of tripping the main turbine, reactor power lowered below 5% and the tors declared MODE 2. Rapid Rise in T avg and Passive Shutdown With the condenser steam dumps set to modulate at 1092 psig, upon tripping the turbine there was no steam demand until Tavg rose to 557°F (corresponding to a steam pressure of 1092 psig). With the reactor initially around 6% power and with no steam demand, Tavg rose rapidly:

1°F within the first 20 seconds, 2.5°F in the first ute, 4°F in the first two minutes, and the full 6.6°F rise (corresponding to 557°F) within five minutes. The sharp insertion of negative reactivity resulting from this temperature rise caused the reactor to passively shut down, as indicated by the Start Up Rate (SUR) data. When the turbine was tripped at I 0: 12:35, SUR was -0.01 decades per minute (dpm); by 10:18 SUR was -0.16 dpm -a change of 1600%. (Continued on next page) 17 18 COMMUNICATOR FALL 2011 As the reactor neared the Non-Fission Heat Rate ( 1. 75% rated reactor power for this shutdown),

temperature-reactivity feedback was lost (see ure 4 on page 20); that is, lowering reactor power would no longer feed back positive reactivity via lowering temperatur

e. Thus, without a manual insertion of positive reactivity

, power would tinue to lower into the source range. to lower to Y2 its initial value, fission power (as indicated by IRNI currents) lowered to 1/6 its tial value. This is further indication the fission reaction had shut down and the Non-Fission Heat Rate was raising/maintaining reactor coolant tperature. Response to the Passive Shutdown At 10:13, the instruments had indicated 5.17% and the Intermediate Range Nuclear Instruments (IRNls) had indicated l .52E-5 ion chamber amps (ica). By I 0: 18, T instruments indicated 2.4% and the IRNis indicated 2.43E-6 ica. So in the time it took total power (as indicated by core While the reactor was passively shutting down, the operators were performing the off-normal cedure for "Loss of Letdown" (which had been entered at 10:00). At 10:18, a 75 gpm letdown orifice was placed in service and the crew exited the off-norma l procedure.

By this point (10: 18), had they recognized the reactor was shut down, it 1.E-03 -(/) E°1.E-04 <<I .... Q) 1.E-05 < c <<I a:: 1.E-10 Q) -<...l.. 0 10:00 10:15 10:30 10:45 11 :00 11 :15 11 :30 11 :45 12:00 12:15 Figure 3: Plot of Control Bank rod heights and Intermediate Range (IRNI) currents on October 21, 2003. The reactor passively shut down shortly after the turbine was manually tripped at l 0: l 3 and reached the source range about 26 minutes later. A nominal -1 /3 dpm SUR developed as power fell below the POAH. The slight drop in reactor power from I 0:39 to 12:05 was caused by a lowering of subcritical multiplication resulting from the continued buildup of Xenon-135

. The operators began inserting the control banks at 12:05 and completed at 12: 15.

FALL 2011 was already too late to prudently try to recover criticality.

After exiting the off-normal procedure for "Loss of Letdown" the Control Room Supervisor signed the Reactor Operator the task of raising letdown flow to 120 gpm by placing the 45 gpm orifice in service per the normal operating dure. It is unclear why this task was prioritized over actively controlling core reactivity (i.e. over inserting the control banks to ensure the reactor remained shutdown)

. This task involves multiple manipulations of charging system components and took 30 minutes to complete; in comparison

, manually driving in the control banks takes 10 minutes. As reactor power was decaying through five ades of power to reach the source range, licensed Reactor Operators were assigned to place Cooling Tower Blowdown in service and to secure the ond of three intake pumps (cooling Tower down had been secured a couple of hours earlier to support Chemistry surveillances and the intake pump was secured because two pumps were no longer needed due to the forced de-rate causing evaporation rate to lower). These tasks were both logged complete at l 0:34. It is unclear why these tasks were prioritized over inserting the control banks. Operation in the Source Range At 10:39, reactor power entered the source range, as evident on Figure 3 (page 18) by the IRNI rents stabilizing.

As at most reactor plants, the COMMUNICATOR Source Range Nuclear Instruments (SRNls) at Callaway remain de-energized until bistables on the IRNis validate reactor power is in the source range. Because the control rods were still at their last critical rod heights, there was more subcritical multiplication than is normally present when these lRNI bistables are calibrated.

As a result, the SRNis did not energize upon initially entering the source range. It took 45 minutes of additional Xenon-135 buildup to lower subcritical cation to the point at which the first SRNI channel was able to automatically energize. At 11:01 a 1 icensed operator was assigned to scure the second of three condensate pumps. It is unclear why, while in the source range with no SRNis energized and with the control rods still at their last critical rod heights, the licensed tors prioritized manipulation of the condensate system over inserting the control banks. To some (e.g. this author) the crew's actions indicate that they were unaware the reactor had sively shut down. That is, the most reasonable explanation for the crew "prioritizing" ancillary tasks 12 over deliberate control of the nuclear fission reaction is that for 67 minutes they failed to recognize the reactor had shut down.13 At 11 :25 the channel 2 SRNT energized.

Since a Main Control Board alarm annunciate s whenever a SRNI channel energizes

, it can be confidently assumed that at 11 :25 the crew was aware they were in the source range. At 11 :38 the channel 1 SRNI energized.

(Continued on next page) 12For example: placing an extra 45 gpm letdown orifice in service, placing Cooling Tower Slowdown in service, securing unnecessary intake and condensate pumps. Although optimizing water chemistry of the primary plant and cooling tower is important and although minimizing "house" electric loads by securing large and no longer needed pumps is important, these tasks are "ancillary" with regard to the primary focus of the reactor shutdown procedure: inserting the control banks to definitively ensure the reactor is in a shutdown condition and will remain in that state regardless of passive (e.g. xenon decay) or unexpected (e.g. inadvertent dilutions or cooldowns) changes in core reactivity. 13It should be noted here that the crew has consistently asserted that prior to manually tripping the turbine they were aware the reactor would passively shut down once steam demand was removed. This assertion amounts to the crew deliberately allowing the reactor to passively shut down while they performed the ancillary items mentioned in note 12. The author of this article believes that, if true, this amounts to incompetence. That is, it is incompetent for an NRC licensed operator to prioritize ancillary tasks over deliberately controlling the reactor, and it is incompetent to deliberately rely on passive mures to shut down the reactor when active means (e.g. rods and boron) are available. Since the US NRC has refused toqution the operators' assertions, at this point the question remains unresolved as to whether or not, prior to the SRNis energiing, the operators were aware the reactor had passively shut down. Although the Institute of Nuclear Power Operations (fNPO) is aware of the discrepancies surrounding the October 2 I, 2003 shutdown, rNPO has similarly declined to evaluate the claims made by the operators; since rNPO must rely on Ameren to voluntarily report the incident, rNPO has stated that it is in no position to conduct its own assessment. For those interested, the claims of the operators are summarized in enclosure 2 to NRC ADAMS document MLI 10140104 and are analyzed in detail in ADAMS document ML102640674. 19 20 COMMUNICATOR At 11 :40 a licensed operator placed the motor driven Start Up Feed pump (SIU FP) in service in preparation for securing the second of two turbine driven Main Feed pumps (MFPs). At 11 :42 a actor Operator initiated a Containment Purge. At 11 :51 the final MFP was secured.

It is unclear why these tasks were prioritized over serting the control banks. From 12:05 to 12: 15 the Reactor Operator inserted the control banks. Control bank insertion was not completed until over two hours after the 4°F perature spike which caused the passive reactor shutdown. HUMAN PERFORMANCE ASPECTS Xenon-135 Cooldown The temperature transient which significantly tributed to the confusion that resulted in the pas-10:12 10 10:15 10:18 FALL201 l sive reactor shutdown was a result of the operators failing to account for Xenon-135 when they stopped the turbine downpower at 09:36. though operators might well understand the ics of Xenon-135

, applying this knowledge while conducting a busy forced de-rate and while being distracted by equipment malfunctions is much more difficult than applying this knowledge while taking a Generic Fundamentals Exam. Two ble solutions to aid the operators in adequately assessing xenon are to have readily available erating Experience (OpE) listed on specific pre-job brief forms and to require Reactor Engineering to prepare detailed Reactivity agement Plans for forced de-rates.

Challenges of MODE 2-Descending Due to the degradation of Temperature-Reactivity feedback which occurs in MODE 2-Descending (see Figure 4, below), if there is a need to remain 10:21 10:24 10:27 2.8E-05 l\T instrument channel 1 (%rated power) 1 lH 0.1 0.01 Non Fission Heat Rate (NFHR) for this shutdown was 1. 75% of rated reactor power (about62MWth)

. The Point of Adding Heat (POAH) was reached around 10:23. IRNI currents at the POAH correspond to a fission power of about 2.4 MWth. 2.8E-06 IRNI 2.8E-07 2.8E-08 Figure 4: Logarithmic plots of Total Power (as represented by 6T instrument readings) and fission power (as resented by Intermedi ate Range Nuclear Instrument currents)

. Starting around 5% rated reactor power, as fission power lowers exponentially

, total power asymptotically approaches the Non-Fission Heat Rate (NFHR). The match between fission power and total power has a strong impact on Temperature-Reactivity feedback causing it to degrade upon entry into MODE 2-Descending and causing it to completely disappear at the Point of Adding Heat (POAH). Although temperature continues to directly affect reactivity as the NFHR is approached

, ture-Reactivity is lost because falling fission power from a negative reactivity insertion does not immediately fect temperature since non-fission heat sources "buffer" temperature from dramatically lowering.

FALL 2011 critical at low powers then the reactor should main m low MODE l (i.e. greater than 5% power). Because of the Temperature-Reactivity feedback afforded in MODE 1, operators can rely on perature to passively respond to react1v1ty changes. Near the Non-Fission Heat Rate (i.e. in MODE 2) the operator must directly respond to reactivity changes (e.g. xenon buildup) with active reactivity manipulations (e.g. rods or boron/ water). Whereas it is not very difficult to maintain temperature through the active insertion of tivity, it can be extremely difficult to actively spond to reactivity changes directly (while at the same time ensuring the reactor neither exceeds 5% power nor drops below the POAH). ln 2007 Callaway Plant's procedures were changed to minimize sustained operations in MODE 2-Descending.

On April 13 and 14, 2009 Callaway Plant successfully performed turbine control valve repairs while maintaining the reactor critical in low MODE 1. Based on their past perience with low power operations

, it is unlikely that they would have successfully remained cal during the turbine control valve repairs had they attempted these repairs in MODE 2-Descending.

By prudently conducting the turbine repairs in MODE 1, Callaway Plant learned from its past mistakes and set its operators up for cess. Recognizing the Passive Shutdown Although the operators claim otherwise, it appears that for 67 minutes (from 10: 18 to 11 :25) they failed to realize the reactor was shutdown. Whether or not the operators were aware of the passive shutdown as it was occurring, it is still worth exploring some of the "human factors" falls associated with attempting to maintain MODE 2-Descending

. There are no adequate instruments for indicating fission power when attempting to maintain MODE 2-Descending.

Due to decay heat and other fission heat sources, both primary calorimetric (e.g. L1 T instruments) and secondary calorimetric instrumentation are poor indicators of fission power in MODE 2. Due to cold-leg shielding and COMMUNICATOR decay gammas, Power Range Nuclear Instruments (PRNis) do not accurately reflect fission power and wi II continue to read -1 % rated reactor power even after the reactor has entered the source range. The only accurate indications of fission power in MODE 2-Descending are the IRNls; however, these instruments are human factored for ing reactor startups and not for maintaining MODE 2. Because of the significant range of these instruments (i.e. 10 decades of power) they have substantial calibration errors. These errors have little effect on the operator as long as the operator is using these instruments to detect CHANGES in fission power and not as an lute measure of fission power. For this reason, these instruments are intentionally scaled in ion chamber amps instead of percent rated power. That is, the calibration errors prevent these ments from accurately indicating absolute power levels so they were intentionally "human factored" to use units which are not easily converted into percent rated power or into MWth, thus ing the operator from using them while attempting to maintain discrete power bands. Attempting to use the IRNis to maintain a power band from the POAH to 5% is unwise. Furthermore, although recognizing when the Point of Adding Heat has been attained during a power ascension is straight forward, during a downpower it is impossible to recognize the POAH until fission power is stantially below it. See References 2 and 6 for more details on cal Lessons Learned.

SAFETY AND PI&R CONCERNS No PI&R effort in 2003 For unknown reasons the passive reactor down was not documented in the plant's tive action program in 2003. The failure of the crew to document the passive reactor shutdown resulted in the organization failing to perform quate Problem Identification

& Resolution (PI&R). That is, without a condition report menting either the xenon induced letdown tion or the inadvertent passive reactor shutdown, the organization was unaware that it had an event which it could analyze for "problems" needing 21 22 COMMUNICATOR "resolution." The purpose of writing a condition report is not to "tum yourself in for making rors," it is to provide the organization a record of the known (or perceived) facts so that these facts can be analyzed for potential "problems" (e.g. adequate procedural guidance, operator edge weaknesses, unrealistic management tations, etc.) and these problems can then be lyzed for "resolution s" (e.g. improved guidance).

There are some (e.g. this author) who believe that on October 21, 2003 the crew was "set up for ure." The general operating procedure for ducting the down power and reactor shutdown was poorly structured

. The procedure assumed that in order to stop the down power the operators needed to do nothing more than delay continuing in the procedure

. The procedure made no tion that the actions the operators needed to take for "holding" power during a xenon transient were different than the actions needed for "reducing" power. The procedure did not take into account the limitation s of the operator's control equipment (i.e. the degradation of Temperature-Reactivity feedback) and monitorin g equipment (i.e. affect the NFHR and decay gammas have on total power meters) in MODE 2-Descending. Management expectations were unrealistic; it was unrealistic to expect the crew, with procedural guidance written for a continuous (i.e. "non-segmented") shutdown, to be able to hold power at 10% power during the severe xenon transient which is induced from an aggressive 9 hour1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> down power at 10%/hour. ever, since the October 21, 2003 passive reactor shutdown was not documented until it was dentally uncovered 40 months after the fact, these gross procedural deficiencies and unrealistic agement expectations went uncorrected until 2007. On June 17, 2005 a similar passive reactor sdown occurred during a forced de-rate for a failed power supply in an Engineered Safeguards ture (ESF) cabinet.

During this de-rate, the tor passively shut down due to a 2°F spike in T avg which occurred upon manually tripping the main turbine. The shutdown occurred two minutes prior to the failed power supply being successfully retested and 54 minutes prior to the expiration of the shutdown action of the Technical Specifica-FALL 2011 tion. That is, since the broken equipment was scessfully repaired prior to the planned shutdown time, had the reactor not passively shut down the crew could have immediately returned to power. Instead, resultant delays in returning to power lowing the inadvertent passive shutdown cost the utility 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> of lost generation.

Like the 2003 passive shutdown, the 2005 passive shutdown was not documented until it, too, was accidently covered in February 2007. Had the October 21, 2003 passive reactor shutdown been evaluated by the utility's Problem Identification

& Resolution process, it is likely the 2005 passive reactor sdown would never have occurred.

Although the inadvertent passive shutdown of a commercial PWR might seem like a commercial concern vice a safety concern, failing to recognize it can readily jeopardize reactor safety. In ary 2005, the operators of a reactor in Virginia were attempting to maintain the reactor in MODE 2-Descending while repairs were being conducted on the secondary plant. The reactor passively shut down and the operators failed to notice it. Two hours later, the reactor inadvertently restarted lowing a manual positive reactivity addition which was conducted by operators who had failed to rognize the reactor had entered the source range. Like the October 2003 passive reactor shutdown at Callaway Plant, the operators failed to ment the event. Unlike the Callaway incident, when the incident in Virginia was brought to the attention of plant management

, an investigation was performed and the results were reported to the lnstitute of Nuclear Power Operation s and shared with the industry via a Significant Event tion. Sharing OpE with INPO Both the October 21, 2003 and June 1 7, 2005 psive reactor shutdowns were accidently uncovered in February 2007 during a review of critical rameter data from past shutdowns to support a maJor revision to the Reactor Shutdown dure. The two shutdowns were documented along with seven other shutdowns in Callaway Action Rquest 200701278, Analysis of Past Reactor downs -RF 15 Preparation Concerns. In their FALL 2011 August l 0, 2007 cover letter distributing WANO SOER 07-01, Reactivity Management, INPO quested that their member utilities "provide mation on similar occurrences and solutions at their plants."

For unexplained

reasons, Ameren determined that neither the October 2003 nor the June 2005 passive reactor shutdowns were worthy of sharing with the industry.

Since no INPO SEN concerning the October 2003 passive shutdown has been released since the NRC's issuance of IN 2011-02, it appears that INPO agrees with Ameren's decision that a passive reactor shutdown resulting in a two hour delay in inserting control banks does not meet the threshold for a Significant Event Notification.

In the absence of a detailed INPO document on the incident, interested nuclear professionals should review the issue brief leased by the Union of Concerned Scientists (see Reference 2). Informally Relying on Xenon-135 One of the more troubling aspects of the tors' claim that they were consciously aware the reactor had passively shut down is that this claim amounts to informally relying on Xenon-135 to prevent the reactor from inadvertently restarting.

Several times during the downpower

, the tors performed a "Xenon Prediction."

A Xenon Prediction estimates Xenon-13 5 levels based on projected power history, and it is used as a tool to assist the operators in maintaining the reactor critical.

A Xenon Prediction is very different from a Shutdown Margin Calculation.

Although there are times when a Shutdown Margin tion will rely on Xenon-135 for Shutdown Margin (SDM), when this is done it is based on actual power history. Another major difference between the two calculations is their uses: a SDM tion is used to ensure the reactor will not tently return to criticality during postulated tive reactivity additions (e.g. inadvertent dilutions

, inadvertent cool downs, etc.) whereas a Xenon Prediction assumes no failures and is used to mate the amount of negative reactivity which must be overcome to maintain the reactor critical.

Since a SDM calculation was not completed on October 21, 2003 until forty minutes after the trol banks had been inserted, the crew, for the 106 minutes they claim they knew the reactor was COMMUNICATOR shutdown

( l 0: 18 to 12:04) yet were still retaining the rods at their last critical rod heights, bly relied on an informal estimate that Xenon-135 levels were large enough to prevent an inadvertent restart.

Following the Shift Technical Advisor's calculation of Shutdown Margin (at 12:49), the crew added over 3600 gallons of boron in order to meet the required SDM. Since Xenon-135 is a radioactive isotope with a half-life of 9.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, a reactor requiring Xenon-135 to maintain it subcritical will eventually turn to power. Although the physics of dine-135 and Xenon-135 are well understood

, formally relying on estimations when formal culations are available is contrary to the principles of conservative reactor operation

. If there is a commercial reason to rely on Xenon-135 to tain Shutdown Margin, a formal SDM calculation should be performed and reviewed PRIOR to ing on Xenon-135 to maintain the reactor shut down. Operating Beyond Procedure Guidance In 2007 the US NRC investigated the October 21, 2003 passive reactor shutdown.

Although they issued non-cited violations (NCVs) for the tors failing to make a log entry documenting eration below the MTCO and for the operators failing to document the passive shutdown with a condition report, the NRC found no problems garding the two hour delay in the insertion of the control rods. Concerning this delay, the NRC stated, "The inspector's review of the operating procedures did not find any timeliness guidance on performing the steps to insert the control rods." It is unclear why the NRC inspector expected the Reactor Shutdown procedure to contain "timeliness guidance on performing the steps to insert the control rods." Like the nonnal (i.e. non -faulted) reactor shutdown procedures at all US commercial reactors, Callaway Plant's Reactor Shutdown procedure contained no provisions for intentionally allowing the reactor to passively shut down. Per the procedure

, the only way to shut down the reactor was to manually insert the trol banks. Since the procedure inherently sumes it is followed, and since the procedure re-23 24 COMMUNICATOR quires the control banks be manually inserted to effect the shutdown, then it would be nonsensical for the procedure to contain "timeliness guidance on performing the steps to insert the control rods." That is, since the reactor is shut down by manually inserting the control banks, it would not make sense for the procedure to dictate a time frame for inserting the control banks ING a passive reactor shutdown.

Nonetheless, the NRC has thus far maintained its 2007 position that no violations occurred other than the two NCYs concerning the lack of a log entry and condition report. In Information Notice 2011-02 the NRC specifically avoided addressing whether or not they believed the operators were aware of the passive reactor shutdown prior to the first SRNI channel energizing.

[Note: The author of this article was a reviewer for IN 2011-02 and is the owner of the initial block which has a "NoConcur" in it on the routing page. For those ested, the Non-Concurrence Form, which includes the NRC's response, can be found in the NRC's public ADAMS library as MLl 10420293.] Note that it is the opinion of this author it is not a procedure violation to unknowingly allow the actor to passively shut down. Operating a large commercial PWR at low power during an sive xenon transient is not an easy task; combined with the challenges already mentioned above (e.g. loss of Temperature-Reactivity

feedback, physical limitations of calorimetric indications of "fission" power near the NFHR, poor procedural guidance, lack of a detailed Reactivity Management Plan, equipment malfunction s) it should not be ing to any NRC licensed operator that the crew failed to perform flawlessly. Although most erators would like to think that it would never take them 67 minutes to recognize the reactor had shut down, most do recognize that, given the wrong set of circumstances

, any operator is capable of ing a mistake such as this. It is not a procedure violation to fail to recognize a passive reactor shutdown, it is a human performance error and no more. And it is not a procedure violation to, due to a human performance error, find oneself in cumstances not expected by the procedure

. When this occurs, the proper response is to use one's training and experience to place the plant in an FALL 2011 analyzed condition (e.g. if the plant has passively shut down, then manually insert the control banks). Note that failing to recognize a passive shutdown as it is occurring is very different from recognizing the reactor is passively shutting down and then intentionally prioritizing other actions above the deliberate control of reactivity

. Whether or not the NRC chooses to address it, intentionally allowing a large commercial reactor plant to passively shut down constitutes a mental misunderstanding of the principles of servative reactor plant operations.

As discussed above, US commercial PWRs "want" to be critical and "want" to match steam demand. The inherent passive response of the reactor as xenon decays is to eventually return to criticality and to match steam demand. As soon as it is noted that the actor has passively shut down, and as long as tive means to control the nuclear fission reaction are available, they should be used to ensure the reactor is taken to, and remains in, a shutdown condition

. Loss of Safety Related Instrument Power During the investigation of the October 21, 2003 passive reactor shutdown, the Shift Manager cated that the biggest delay in inserting the control banks was the fact that the crew was still ing the off-normal procedure for the "Loss of Safety-Related Instrument Power" which had been entered at 08:21 but was not exited until 11:37. Since all the control room actions were completed by 08:36 (an hour before the temperature transient which led to the passive reactor shutdown),

it is unclear exactly how this off normal procedure delayed the insertion of the control banks during the hour following the turbine trip. Nonetheless

, for unstated reasons the NRC has decided to take the operators at their word and not question how the performance of this procedure inhibited the insertion of the control banks yet did not inhibit the operators from placing the 45 gpm letdown orifice in service, placing Cooling Tower down in service, lowering intake flow, or lating the feed and condensate systems. Those interested in this topic should consult References 4, 5 and/or 6, cited on page 26. (Continued on next page)

FALL 2011 Operation without SRNis The reactor entered the source range at 10:39; yet, no Source Range Nuclear Instrument (SRNI) gized for another 45 minutes ( 11 :25). Each SRNI at Callaway Plant is powered through a contact on its channel's associated IRNI. This contact automatically closes at 5E-l l ica. cause of the subcritica l multiplication afforded by the control banks still being at their last critical rod heights, both channels of IRNis were reading greater than 5E-I I ica when the reactor first tered the source range. It took 45 minutes of tional Xenon-135 buildup for the channel 2 IRNI to lower below 5E-l l ica and 59 minutes for channel I. The SRNis can also be manually energized once the PRNI signal has lowered below 10% rated reactor power. Had they, prior to I l :25, nized they were in the source range, the operators could have manually energized either or both SRNis. The fact that they did not do this is one of many indications to this author that, prior to the SRNis automatically energizing at l 1:25, the erators were unaware they were in the source range14* The SRNls add significant defense in depth during operation in the source range by providing:

  • start up rate indication
  • an audible count rate which quickly alerts the operator to rising reactivity
  • a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
  • an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
  • a signal to the Boron Dilution and Mitigation System (BDMS) which causes an automatic swap over of charging pump suctions from the Volume Control Tank to the Refueling Water Storage Tank (RWST) in the event that source range counts increase by 70% in a rolling 10 minute period (since the RWST is borated to -2500 ppm, this BDMS circuit provides tection against inadvertent reactivity additions caused by xenon-135 decay, inadvertent tions, and inadvertent cooldowns)

. Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi zed, this is so a reactor start up can be perfonned.

15 During a reactor startup, administrative controls 16 are in place which mitigate the loss of safety margin from blocking the automatic safety circuits driven by the SRNis. The designers of Callaway Plant never intended for the plant to be operated in the source range with the control rods at their critical rod heights and with none of the SRNI driven automatic protection s in place. Although the NRC is technically correct in stating that this condition did not violate the plant's licensing requirement s, there is more to ensuring reactor safety than forcing a verbatim interpretation of the Technical Specifications

not all conditions can be exactly defined by the Technical Specifications and a competent professional reactor operator should be able to discern when the plant is in a condition in which the designer never intended.

(Continued on next page) 14Note that from I 0:23 to 11 :25 all indications other than the IRNis were steadily indicating the plant was low in the power range: the PRNis were reading -I% rated power, the t-.T instruments were reading 1.75% power and the secondary calorimric computer points were reading 62 MWth. In order to realize they were in the source range the operators would have either had to note the !RNI readings or question why they had not needed to add positive reactivity to account for xenon buildup. With regard to noting the IRNI readings, because of their units (ion chamber amps) and their scaling (logarithmic) the operators do not normally use these instruments while at power. With regard to questioning why they had not needed to dilute or pull rods to make up for xenon, understanding the reactor dynamics of Iodine/Xenon was a weakness of this crew as demonstrat ed by their response to the 9°F temperature drop which occurred from 09:36 to I 0:00. 15It is impossible to do a successful reactor startup without blocking the SRNI flux trip and BDMS; therefore, once the IRNI signal reaches I E-10 ica during a reactor startup, the operators are permitted to de-energized the SRNis (which by then have had all their protective functions blocked). 16For example: a Reactor Engineer present in the control room, an Estimated Critical Position has been calculated, the crew is intently performin g a procedure which warns them to "expect criticality at any time," etc. 25 26 COMMUNICATOR CONCLUSIONS There is much to be learned from the October 21, 2003 passive shutdown at Callaway Plant. The incident highlights a number of issues: the ner by which NRC licensed operators might fail to appreciate the magnitude of the effect xenon is having on core reactivity while that effect is being masked by power defect, the need for specific and thoughtful procedural guidance for stabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback poses to the operator as the NFHR is approached, the manner in which an erator focusing on calorimetric instruments while at low power might fail to recognize fission power lowering below the Point of Adding Heat, and the importance of documenting incidents in the rective action process. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "generic mentals" of reactor dynamics. No analysis of the incident has been done by INPO and the NRC's analysis in Information Notice 2011-02 is not very thorough; those interested in a deeper analysis of the details of the incident are encouraged to view the reference s at the end of this article.

DISCLAIMER The views expressed in this article are those of the author and in no way reflect the position of the US Nuclear Regulatory Commission or the sional Reactor Operator Society.

To participate in an online analysis of this dent, send an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro ups. com (anonymous participation is accommodated).

FALL 2011 Anyone wishing additional information on this incident is encouraged to contact me at: cione@hotmail.com (573) 230-3959 REFERENCES

l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator Performance Issues Involving Reactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at laway, November 2010. 3. Non-Concurrence on NRC Information Notice 2011-02, Operator Performance Issues ing Reactivity Management At Nuclear Power Plants (ADAMS #MLl 10420293)

. 4. September 17, 2010 letter from L. Criscione to William Borchardt (ADAMS #ML 102640674).

5. April 27, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #MLl 0120040 l ). 6. April 30, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #ML101230100).
7. G2010059/EDATS: OED0-2010-0775

-tion Closure Letter to Lawrence S. Criscione Related to Requested Action Under 1 OCFR 2.206 Regarding October 21, 2003 Event at Callaway Plant, Unit 1 (TAC No. ME4721),

ADAMS #MLl 10140104, January 19, 2011. Note from the Author In my opinion, this is an important piece of Operating Experience which is only available through PROS. The event was never submitted to INPO and, although the NRC included it in an Information Notice, most of the significant Lessons Learned from the incident were not addressed.

More than anything, the event is an example of: ( 1) licensed reactor tors being "set up for failure" by impractical operating practices and expectations and (2) the importance of honestly reporting events and accurately analyzing them so that future reactor operators (both at the plant and throughout the industry) do not fall victim to the same poor practices and knowledge gaps.

COMMUNICATOR THE PUBLICATION OF THE PROFESSIONAL REACTOR OPERA TOR SOCIETY FALL 2011 In this issue: NRC Fukushima Task Force Analysis of the 2003 Callaway Shutdown

-\\t --30 YEARS OF OPERA TORS SPEAKING FOR OPERA TORS -----------

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FALL 2011 COMMUNICATOR Analysis of the October 21, 2003 Passive Reactor Shutdown at Callaway Plant Lawrence S. Criscione PE Larry Criscione works in the Nuclear Regulatory Commission's Office of Research (RES) where he analyzes equipment and human performance data in support of nuclear regulatory research.

The views expressed in this cle are his own and in no way reflect the position of the US NRC. Larry worked at Callaway Plant from 2002 through 2007 where he was a licensed Senior Reactor Operator and a Shift Technical Advisor.

Larry has worked at Clinton Power Station (2000-02) and FirstEnergy (2008-09).

In the US Navy he was qualified as Engineering Officer of the Watch at DlG prototype (1994) and aboard the fleet ballistic missile submarine USS GEORGIA (1995-98).

He holds a branch license in nuclear engineering from the State of Iowa. He is a 1993 graduate of the University of Missouri-Rolla.

Abstract:

At Callaway Plant on October 21, 2003, while attempting to stabilize reactor power during a forced de-rate, Xenon-135 buildup caused average reactor coolant teperature to lower at a 25 minute period, resulting in an automatic isolation of the letdown system on low pressurizer water level and operation of the reactor below the Minimum Temperature for Critical Operation. After manually tripping the turbine-generator to sist in temperature recovery, the reactor passively shut down due to a sharp 4°F rise in aveage coolant temperature. For the next 110 utes the operators performed secondary and tertiary plant shutdown activities while relying on an informal estimation that Xenon-135 levels were sufficient to prevent the reactor from vertently restarting. The passive reactor sdown was not documented until it was uncered 40 months later, and it was not shared with the Institute of Nuclear Power Operations lowing the request which accompanied SOER 07 -1. The incident highlights the pitfalls assocated with attempting to maintain a commercial pressurized water reactor critical during MODE 2-Descending and demonstrates how concepts tested on the NRC Generic Fundamentals Exam apply to actual reactor operation. The incident also highlights some non-conservative reactivity management practices which must be avoided by Professional Reactor Operators. This article describes the events leading up to and immediately following a passive reactor shutdown which occurred at Ameren Corporation

's nuclear plant in Callaway County, MO on October 21, 2003. An assessment of the NRC's response to the incident is included along with key "lessons to be learned."

Details of the incident were first pub-1 ical ly released by the Union of Concerned tists (UCS) in a 20 l 0 issue brief1 titled 2003 Semented Shutdown at Callaway, and then, in 20 l l, the US Nuclear Regulatory Commission (NRC) partially covered the incident as part of tion Notice 2011-02, Operator Performance issues Involving Reactivity Management at Nuclear Power Plants. 2 Also discussed in the article are: The manner by which the effect of Xenon-135 buildup can be masked by other passive tivity insertions during a plant transient.

The effect operation near the Non-Fission Heat Rate has on Temperature-Reactivity feedback. The challenges facing the operator during low power operation due to human factoring of control board instruments

. (Continued on next page) 1http://www.ucsusa.org/nuclear _power/nuclear _power_ risk/safety/2003-segmented-shutdown-at-cal !away.html 2http://pbadupws.nrc.gov/docs/ML I 018/ML I 01810282.pdf 11 12 COMMUNICATOR REACTOR DYNAMICS REFRESHER Passive Response to Reactivity Changes Commercial Pressurized Water Reactors (PWRs) in the United States are designed to passively spond to changes in reactivity.

They do this through two primary methods:3 1. A negative power coefficient of reactivity

2. A negative Moderator Temperature ficient of reactivity

(-MTC) Item I is a required safety feature of all US Commercial designs:

a negative power coefficie nt of reactivity ensures that an uncontrolled rise in tor power will result in a negative insertion of activity, thereby limiting the power rise. Item 2 is normally present throughout the fuel cycle at most PWRs; however, some plants do mit a slight +MTC during a limited window of their fuel cycle. October 21, 2003 was late in fuel cycle 13 for Callaway Plant and a -MTC was sent so discussions in this article assume a -MTC. The combined result of items l and 2 is that, on a US commercia l PWR, power is inherently stable. That is, the reactor "wants" to stay at a steady power and resists power increases and decreases.

Response to a reactivity insertion with steady state steam demand: When negative reactivity (L'.lp) is inserted (e.g. insertion of control rods, dition of boron, buildup of Xenon-135) while the steam demand (i.e. turbine-generator loading) is held constant, reactor power will decrease slightly. Because of the negative power coefficient of tivity, positive reactivity is passively inserted as power lowers, dampening the negative reactivity insertion

. With steam demand unchanged

, the new lower power will cause a negative power match to develop.4 This negative power mismatch will cause temperature to lower. Due to the -MTC, as temperature lowers positive reactivity is passively inserted, which further dampens the negative reactivity insertion

. FALL20ll Temperature will continue to lower as long as there is a negative power mismatch. Eventually

, enough positive reactivity will be inserted by the temperature drop to result in a net increase in activity.

This point is called the point of power "turning."

At this point, reactor power will start to rise and the magnitude of the negative power mismatch will lower, dampening the temperature drop. Once reactor power rises above steam demand, there will be a positive power mismatch which will now cause temperature to rise. The rising temperature will insert negative reactivity

, causing reactor power to lower. These passive feedback processes will continue until, eventually, reactor power again matches steam demand and there is no power mismatch to drive temperature.

At this point, temperature will be lower than it was prior to the negative reactivity insertion

.

cally, the change in temperature is: L'.l T = L'.lp/MTC). The reactor will passively respond to a positive insertion of reactivity in a similar manner, ing in the reactor operating at a higher temperature than prior to the reactivity insertion

. The response of the reactor described in the graphs above is called "Temperature-Reactivity feedback.

" Temperature-Reactivity feedback consists of two things: 1. The passive response of the average reactor coolant temperature (Tavg) to the power match induced by the change in reactivity

. 2. The passive counter insertion of reactivity due to the temperature response, which continues until power turns and re-approache s steam demand. So, without any operator action, US commercial PWRs passively respond to reactivity changes in a manner that eventually results in the same steady state power at a new temperature

. This generic fundamental is demonstrated later in this article by 3Since Callaway Plant is a PWR, the reactivity coefficient due to voids is not discussed in this article. 4Power mismatch is the difference between steam demand and reactor power.

FALL201 I the way the reactor at Callaway Plant responded to Xenon-135 buildup when the turbine-generator loading was kept constant from 09:36 to l 0:03 (see Figure 1 on next page). Passive response to a change in steam demand (for a PWR): When the steam demanded by the turbine is lowered, a negative power mismatch will result, causing temperature to rise. The rising temperature will insert negative reactivity, causing reactor power to lower. The lowering reactor power will result in a lowering of the power match, dampening the temperature rise. As long as there is a positive power mismatch, temperature will continue to rise. The negative reactivity sertion from rising temperature will continue until reactor power falls below steam demand resulting in a negative power mismatch which thereby causes temperature to lower. The lowering perature will insert positive reactivity

, causing power to tum and approach steam demand. tor power will eventually become steady at the new steam demand level. Due to the negative power coefficient of reactivity

, the lower power level will have resulted in a passive positive t1v1ty insertion.

Temperature will passively spond to this positive reactivity insertion by steadying out at a higher level and thus inducing a negative reactivity insertion which cancels out the power defect. 5 A pressurized water reactor will respond similarly to an increase in steam demand. The response of the reactor described in the graphs above is characterized as "reactor power follows steam demand." Without any operator action, US PWRs passively respond to steam demand changes in a manner that eventually results in reactor power matching steam demand at a new temperature

. This generic fundamental is demonstrated later in this article by the way the reactor at Callaway Plant responded to COMMUNICATOR the lowering of turbine-generator loading between 10:03 and 10:10 (see Figure 1). The Effect of Decay Heat Following the initial criticality of the fuel cycle, some level of decay heat is always present.

The amount of decay heat present is determined by the reactor's power history. At 100% rated power, decay heat typically accounts for 7% of the power being generated in the core. During a down power, decay heat accounts for a slightly larger percentage of reactor power than at steady state power. This is because the longer lived fission product daughters which were produced at 100% power are exerting a disproportional influence on the decay heat spectrum than they normally would at a steady state power level. This influence is not easily noticed in MODE 1. 6 However, as reactor power nears MODE 2,7 the effects of decay heat become substantial.

The Non-Fission Heat Rate: The Non-Fission Heat Rate (NFHR) is the power produced by the reactor plant from sources other than fission. though there are other contributors to the NFHR besides decay heat (e.g. friction heat from the actor Coolant Pumps), this article is primarily cerned with the effect of decay heat. The NFHR is about 7% of rated power when the reactor is operating at 100% power. The contribution of short-lived fission product daughters to the NFHR is roughly proportional to the fission rate so it lowers proportionally to reactor power. However, the change in the population of long-lived fission product daughters lags the change in fission rate as the reactor is down powered. As the fission rate falls to zero, there is still a substantial amount of heat being generated by the long lived fission product daughters.

This NFHR varies with power history, but, following a I 0%/hour shutdown of the reactor, the half-life spectrum of the remaining daughters is long enough that the NFHR is tively constant when measured in hours (i.e. it lowers by just a few percent every hour). (Continued on next page) 5power defect is the term for the reactivity inserted from a change in reactor power level. 6MODE 1 refers to the state of operating the reactor at power (5% to 100% rated reactor power). 7MODE 2 refers to the transition state between the reactor being solidly in the power range (i.e. beyond the point at which the NFHR exerts any substantial influence) and the reactor being shutdown (i.e. definitively subcritical as indicated by calculating KelT to be less than 0.99). The reactor enters MODE 2-Descending when reactor power lowers below 5% rated power. 13 14 COMMUNICATOR FALL 2011 Average Reactor Coolant Temperature (Tavg), Control Band 'D' Rod Heights and Reactor Power (LlT) during the October 21, 2003 Passive Reactor Shutdown at Callaway Plant 587°F 584°F 581°F 578°F 575°F 572°F 569°F 220 steps 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% -01:00 02*00 03:00 ' "" K o4:oo ""I' -.. ..........

... ' ..... ""i... '"""'-20% 09:30 09:40 09:50 10:00 10:10 10:20 10:30 0 i 10% 120 i steps ' l .........

.,,.,,:> i-... -....... ... , " .,,_ IL -.. *-.. , .2'-....

I'... *-......

I\ --....,_ i \ " ....._ IL--I 1 "' i ' !'.... .........

.......__,i'o...

i -...._ ! "-. ' 'l i-f -, , , i j /i -.,. '\ / .. ' 20% % : 10° 0% i 10 0 ps 0°F i ste .: 56 /55r F //554"F 551°F 560°F 557°F 554°F 551°F 100 80 60 40 10% 0% 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 --Tavg (scale: 3°F/division

, maximum:

586.7°F, minimum:

549.9°F) -CTRL ROD BANK D (scale: 20 steps/div, max: 216 steps, min: 0 steps) --lff power (scale: 10% rated reactor power/div, max: 100.9%, min: 1.6%) Figure I: Plot of Average Coolant Temperature (Tavg), Primary Calorimetric power (LiT) and Control Bank 'D' rod heights during the October 21, 2003 down power and passive reactor shutdown. Note the severe temperature transient which began at 09:36. Turbine first stage steam pressure data indicates that the operators stopped lowering generator loading at 09:36 with reactor power at 9%. Over the next three minutes, negative reactivity due to Xenon-135 caused power to continue to lower another I%. The power mismatch between the steam demanded by the turbine throttle setpoint and the power being produced by fission caused T avg to immediately begin to lower, thereby inserting positive reactivity which countered the negative reactivity being inserted by the continual buildup of Xenon-135

. Around 09:39 the positive reactivity being inserted by the lowering temperature matched the negative reactivity being inserted by Xenon -135 causing reactor power (as indicated by core Li T) to stabilize at approximately 8%. With a I% power mismatch sent, over the next twenty minutes T avg continued to steadily lower and thereby counteract the continual buildup of xenon. Shortly after I 0:00 the crew again began to lower turbine-generator loading in response to the Shift Manager's decision to take the turbine off-line following the letdown isolation. The renewed lowering of generator loading caused steam demand to lower below fission power and thereby allowed Tavg to temporarily recover slightly. During this time period (I 0:03 to I 0:09), the negative reactivity being inserted by Xenon-135 was now being counteracted by the positive ity being inserted by the load decrease (the plant had a negative power coefficie nt of reactivity).

Generator loading was again stabilized around I 0:09 causing T avg to resume falling, which is the expected passive response of the reactor plant to Xenon-135 buildup. The operators failed to grasp the reactor dynamics behind the transient and assumed the I 0°F drop in Tavg was being caused by malfunctioning steam line and turbine drain valves (which had coincidentally been placed in service at about the same time the temperature transient began).

FALL201 l By the time the reactor at Callaway Plant passively shut down on October 21, 2003, the NFHR was 1.75% of rated reactor power. About half of this was due to RCP pump heat. The Point of Adding Heat: The NFHR mines the reactor's Point of Adding Heat (POAH). The POAH is the amount of fission power needed to noticeably affect reactor power. During a tor startup, the POAH is the point at which raising reactor power (as measured by the nuclear ments8) will noticeably affect total power (as measured by the calorimetric Instruments 9). The POAH is significant during a reactor startup cause it is the point at which Reactivity feedback starts to occur: once reactor power ascends above the POAH, it becomes cult for the reactor operator to pull control rods to produce a set Start Up Rate (SUR) because as positive reactivity is actively inserted with the control rods the resultant reactor power increase causes temperature to rise and thereby feed back negative reactivity which lowers the SUR. Prior to reaching the POAH, the reactor operator uses the control rods to actively control reactivity

. yond the POAH, the control rods are used to tively control average coolant temperature via the passive response that temperature has to manual reactivity changes.

On a shutdown, the POAH cannot be recognized until the reactor is already below it. During a shutdown, the POAH is the point at which ing fission power (as indicated by the Intermediate Range Nuclear Instruments) has no effect on total power. This generic fundamental is demonstrated on Figure 4 by the way the /).. T trace steadies out at 1.75% while the IRNI trace continues to lower. EVENT NARRATIVE DESCRIPTION Cause of the Forced De-Rate At 07:21 on October 20, 2003 a safety-related verter (NN 11) failed, causing the unit to enter a 24 -hour Technical Specification (T/S 3.8.7.A) to COMMUNICATOR either repair the failed inverter or begin a plant shutdown. At 00:37 on October 21, 2003, after repair tempts by Electrical Maintenance

, the operators placed the inverter in service for a retest. The verter failed its retest and at 01 :00 the operators began down powering the reactor at 10%/hour in preparation for a reactor shutdown. By 07:21 reactor power was just below 40% with the inverter still unrepaired so the unit entered the 6-hour Technical Specification (T/S 3.8.7.B) to either repair the failed inverter or shut down the reactor.

Entry into Off-Normal Procedure for Loss of Safety-Related Instrument Power At 08:21 the inverter was again placed in service for a retest. The inverter failed its retest and the crew responded by performing the off-normal cedure for a "Loss of Safety Related Instrument Power." By 08:36 the control room operators had completed their actions, but the off-normal dure could not be closed until an equipment tor could become available to perform an ment check of some valves in the Auxiliary water system. This alignment check was not pleted until 11 :34, resulting in the off-normal cedure remaining open until 11 :37. Although this off-normal procedure administratively remaining open should not, in and of itself, have caused a problem, for unexplained reasons the operators claim they could not perform the step in the tor Shutdown procedure for inserting the control banks until this off-normal procedure had been exited (see discussion in the "Safety and PI&R Concerns" section). Xenon-135 induced Cooldown At 09:36 the unit was at 9% power and the tors discontinued down powering the generator.

It is not clear why this occurred, but since they were 2Y2 hours ahead of schedule it is likely they intended to hold power at while 8There are three sets of nuclear instruments (the power range, intermediat e range and source range). The nuclear instruments measure fission rate by detecting stray neutrons produced by fission. 9There are two sets of calorimetric instruments at Callaway Plant: L'.T instruments (primary calorimetric calculated from the temperature rise across the core) and thermal output computer points (calculated from a secondary calorimetric). 15 16 COMMUNICATOR further troubleshooting occurred on the failed verter. Also around 09:36 the operators cycled the Group B turbine drains. One of the switches for the drains was not indicating

properly, requiring the operators to locally observe the operation of the thirteen valves controlled by the malfunctioning switch. By 09:36, the 10%/hour downpower which had been occurring for the past 8Yi hours was causing a significant Xenon-135 transient.

The constant build up of xenon was inserting negative reactivity at a significant rate; however, prior to 09:36 it was having little effect on reactor plant parameters.

The build up of xenon went largely unnoticed cause, although significant, it was not great enough to overcome the large amounts of positive reactivity being inserted by the 10%/hour ing of reactor power and the 3°F/hour lowering of reactor coolant temperature

. In fact, prior to 09:36 the operators were occasionally having to actively insert negative reactivity because the positive activity being passively inserted from the down power/cool down was slightly greater than the negative reactivity being passively inserted by xenon. Through 09:36, 114 inward steps of rod movement and 220 gallons of boron were required to keep temperature lowering at the desired rate (the boron additions were done during the first 2Y2 hours of the downpower

, when the rate of xenon buildup was still low; see Figure 1 for the control rod movements)

. When the crew ceased lowering turbine-generator load at 09:36, positive reactivity was no longer being passively inserted from the downpower.

However, since Xenon-135 was still building up, negative reactivity was still being passively serted. The crew did not have a detailed ity Management Plan10 and, because of their periences during the past three hours, 11 failed to recognize that, with the downpower no longer oc-FALL2011 curnng, they needed to actively insert positive reactivity to keep average coolant temperature stable. Starting at 09:36, average reactor coolant ture (Tavg) began to lower at about 22°F/hr. With Xenon-135 continuing to insert negative ity, the reactor would occasionally become slightly subcritical causing power to lower below steam demand. With power less than steam mand, Tavg lowered slightly.

Due to the -MTC, the lowering Tavg inserted positive reactivity and caused the reactor to return to a critical state. In this manner, the reactor passively remained cal (i.e. passively overcame the negative reactivity being inserted by Xenon-135) by responding to the buildup of xenon with a lowering ofTavg* The crew mistakenly believed that malfunctioning turbine drains were causing the drop in T avg, so instead of aggressively inserting positive reactiity (e.g. by diluting boron or withdrawing rods), they coordinated with equipment operators in the turbine building to troubleshoot the turbine drains. The only positive reactivity actively inserted the entire day was a 360 gallon add of water to the Volume Control Tank which occurred between 09:47 and 10:00. Letdown Isolation By 10:00 Tavg had lowered 9°F and the letdown system automatically isolated on low pressurizer water level. Also by 10:00, the crew recognized that Tavg had fallen below 55 l °F, the Minimum Temperature for Critical Operations (MTCO) at Callaway Plant. To assist in recovering ture, the Shift Manager directed that the turbine be taken off-line.

Manual Turbine Trip and MODE 2 Entry After the letdown isolation, the operators began lowering turbine-generator loading in preparation for removing the turbine from service.

This caused a positive power mismatch which tempo-'0Those with access to proprietary documents from the World Association of Nuclear Operators should see the dations contained in WA 0 SOER 2007-1, Reactivity Management, for expectations regarding Reactivity Management Plans. 11 In the 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> since relieving the watch at 06:30 the crew had needed to insert control rods 30 steps in order to keep Tavg lowering at the programmed rate. No active insertions of positive reactivity had been required to overcome Xenon-135.

FALL 2011 0 N .,., 00 .-i '<I" " 'fl 'fl 'fl 0 O'I O'I O'I O'I O'I O'I .-i -__ J _____ J_ ____ J__ -__ J_ ___ .l .. --.L.. --560°F .. 555°F .... !Tref (linear scale, 5°F/division) ! ;:::::: ..:.-.:.:.-..:.*.:.:. I i--r--,.,., O'I 0 0 0 .-i .-i .-i ------COMMUNICATOR N .,., 00 .-i '<I" " 0 ,.,., O'I N ':"1 ':"1 ':"1 \"! \"! \"! <:"! <:"! <:"! <:"! 0 0 0 0 0 0 0 0 0 0 0 .-i .-i .-i .-i .-i .-i .-i .-i .-i .-i .-i In !Tavg (linear scale, 5°F/divisionl l Ff' --/ I I I I ------no load Tavg (551'F) --------' ..... 550°F MTCO (551° F). L#------------lV 560°F 555°F 5S0°F 10% 5% 10% AT 5% ... ftl " 1. E-04 u ... ::s 0. -..... ILH (linear scale, 5% rated ' --!NFHR*------; 0% ' ' "' ' ........_

. ' ' 1.E-05 "' E c ftl " " a: E .a Average Coolant Temperature (Tavg) rose steeply -*---POAH l.E-06 1. E--07 " 2 E ........

ftl following the turbine trip (TT). The quick negative "' ftl ... .c *-c u c reactivity insertion which accompani ed the -4°F spike in -""' l.E--08 Tavgcaused the reactor to become substantially '-E .2 ... -1. E--09 ! subcritical and shut down. The POAH and a nominal I *<6' ** c -1/3 dpm SUR were reached around 10:23. 1.E-11 % -1.E-10 l.E-11 Figure 2: Plot of Average Coolant Temperature (Tavg), Primary Calorimetric power (.1T) and Intermediate clear Instrument currents (IRNI) on October 21, 2003. The sharp rise in T avg was caused by the power mismatch resulting from manually tripping the turbine at 6% power and 550.4°F with the steam dumps set at 1092 psig (557°F). The negative reactivity inserted by this temperature rise caused the reactor to passively shut down. The leveling out of the .1 T trace at I 0:23 indicated the Point of Adding Heat. The leveling out of the IRNI traces at I 0:39 indicates entry into the source range. See Figure 3 for plant evolutions occurring during this time frame. rarily caused Tavg to stop lowering (the minimum Tavg occurring at 10:03 in Figures 1 and 2 sponds to the lowering of turbine load below tor power). Between 10:03 and 10:09 the negative reactivity being inserted by xenon was addressed with power defect instead of temperature defect. At l 0: 12:35 the operators manually tripped the turbine-generator with reactor power just under 6% and Tavg at 550.4°F. Prior to tripping the turbine, the operators had, per their procedure, set the condenser steam dumps to open at 1092 psig (which corresponds to 557°F, the "no-load" age coolant temperature at Callaway Plant). ever, because of the confusion resulting from the temperature transient and automatic letdown tion, the crew missed the procedure step to "Hold Reactor Power constant by transferring load to the condenser steam dumps while reducing bine Load. This will prevent inadvertent entry into Mode 2 when the Turbine is tripped." Within 30 seconds of tripping the main turbine, reactor power lowered below 5% and the tors declared MODE 2. Rapid Rise in T avg and Passive Shutdown With the condenser steam dumps set to modulate at 1092 psig, upon tripping the turbine there was no steam demand until Tavg rose to 557°F (corresponding to a steam pressure of 1092 psig). With the reactor initially around 6% power and with no steam demand, Tavg rose rapidly:

1°F within the first 20 seconds, 2.5°F in the first ute, 4°F in the first two minutes, and the full 6.6°F rise (corresponding to 557°F) within five minutes. The sharp insertion of negative reactivity resulting from this temperature rise caused the reactor to passively shut down, as indicated by the Start Up Rate (SUR) data. When the turbine was tripped at I 0: 12:35, SUR was -0.01 decades per minute (dpm); by 10:18 SUR was -0.16 dpm -a change of 1600%. (Continued on next page) 17 18 COMMUNICATOR FALL 2011 As the reactor neared the Non-Fission Heat Rate ( 1. 75% rated reactor power for this shutdown),

temperature-reactivity feedback was lost (see ure 4 on page 20); that is, lowering reactor power would no longer feed back positive reactivity via lowering temperatur

e. Thus, without a manual insertion of positive reactivity

, power would tinue to lower into the source range. to lower to Y2 its initial value, fission power (as indicated by IRNI currents) lowered to 1/6 its tial value. This is further indication the fission reaction had shut down and the Non-Fission Heat Rate was raising/maintaining reactor coolant tperature. Response to the Passive Shutdown At 10:13, the instruments had indicated 5.17% and the Intermediate Range Nuclear Instruments (IRNls) had indicated l .52E-5 ion chamber amps (ica). By I 0: 18, T instruments indicated 2.4% and the IRNis indicated 2.43E-6 ica. So in the time it took total power (as indicated by core While the reactor was passively shutting down, the operators were performing the off-normal cedure for "Loss of Letdown" (which had been entered at 10:00). At 10:18, a 75 gpm letdown orifice was placed in service and the crew exited the off-norma l procedure.

By this point (10: 18), had they recognized the reactor was shut down, it 1.E-03 -(/) E°1.E-04 <<I .... Q) 1.E-05 < c <<I a:: 1.E-10 Q) -<...l.. 0 10:00 10:15 10:30 10:45 11 :00 11 :15 11 :30 11 :45 12:00 12:15 Figure 3: Plot of Control Bank rod heights and Intermediate Range (IRNI) currents on October 21, 2003. The reactor passively shut down shortly after the turbine was manually tripped at l 0: l 3 and reached the source range about 26 minutes later. A nominal -1 /3 dpm SUR developed as power fell below the POAH. The slight drop in reactor power from I 0:39 to 12:05 was caused by a lowering of subcritical multiplication resulting from the continued buildup of Xenon-135

. The operators began inserting the control banks at 12:05 and completed at 12: 15.

FALL 2011 was already too late to prudently try to recover criticality.

After exiting the off-normal procedure for "Loss of Letdown" the Control Room Supervisor signed the Reactor Operator the task of raising letdown flow to 120 gpm by placing the 45 gpm orifice in service per the normal operating dure. It is unclear why this task was prioritized over actively controlling core reactivity (i.e. over inserting the control banks to ensure the reactor remained shutdown)

. This task involves multiple manipulations of charging system components and took 30 minutes to complete; in comparison

, manually driving in the control banks takes 10 minutes. As reactor power was decaying through five ades of power to reach the source range, licensed Reactor Operators were assigned to place Cooling Tower Blowdown in service and to secure the ond of three intake pumps (cooling Tower down had been secured a couple of hours earlier to support Chemistry surveillances and the intake pump was secured because two pumps were no longer needed due to the forced de-rate causing evaporation rate to lower). These tasks were both logged complete at l 0:34. It is unclear why these tasks were prioritized over inserting the control banks. Operation in the Source Range At 10:39, reactor power entered the source range, as evident on Figure 3 (page 18) by the IRNI rents stabilizing.

As at most reactor plants, the COMMUNICATOR Source Range Nuclear Instruments (SRNls) at Callaway remain de-energized until bistables on the IRNis validate reactor power is in the source range. Because the control rods were still at their last critical rod heights, there was more subcritical multiplication than is normally present when these lRNI bistables are calibrated.

As a result, the SRNis did not energize upon initially entering the source range. It took 45 minutes of additional Xenon-135 buildup to lower subcritical cation to the point at which the first SRNI channel was able to automatically energize. At 11:01 a 1 icensed operator was assigned to scure the second of three condensate pumps. It is unclear why, while in the source range with no SRNis energized and with the control rods still at their last critical rod heights, the licensed tors prioritized manipulation of the condensate system over inserting the control banks. To some (e.g. this author) the crew's actions indicate that they were unaware the reactor had sively shut down. That is, the most reasonable explanation for the crew "prioritizing" ancillary tasks 12 over deliberate control of the nuclear fission reaction is that for 67 minutes they failed to recognize the reactor had shut down.13 At 11 :25 the channel 2 SRNT energized.

Since a Main Control Board alarm annunciate s whenever a SRNI channel energizes

, it can be confidently assumed that at 11 :25 the crew was aware they were in the source range. At 11 :38 the channel 1 SRNI energized.

(Continued on next page) 12For example: placing an extra 45 gpm letdown orifice in service, placing Cooling Tower Slowdown in service, securing unnecessary intake and condensate pumps. Although optimizing water chemistry of the primary plant and cooling tower is important and although minimizing "house" electric loads by securing large and no longer needed pumps is important, these tasks are "ancillary" with regard to the primary focus of the reactor shutdown procedure: inserting the control banks to definitively ensure the reactor is in a shutdown condition and will remain in that state regardless of passive (e.g. xenon decay) or unexpected (e.g. inadvertent dilutions or cooldowns) changes in core reactivity. 13It should be noted here that the crew has consistently asserted that prior to manually tripping the turbine they were aware the reactor would passively shut down once steam demand was removed. This assertion amounts to the crew deliberately allowing the reactor to passively shut down while they performed the ancillary items mentioned in note 12. The author of this article believes that, if true, this amounts to incompetence. That is, it is incompetent for an NRC licensed operator to prioritize ancillary tasks over deliberately controlling the reactor, and it is incompetent to deliberately rely on passive mures to shut down the reactor when active means (e.g. rods and boron) are available. Since the US NRC has refused toqution the operators' assertions, at this point the question remains unresolved as to whether or not, prior to the SRNis energiing, the operators were aware the reactor had passively shut down. Although the Institute of Nuclear Power Operations (fNPO) is aware of the discrepancies surrounding the October 2 I, 2003 shutdown, rNPO has similarly declined to evaluate the claims made by the operators; since rNPO must rely on Ameren to voluntarily report the incident, rNPO has stated that it is in no position to conduct its own assessment. For those interested, the claims of the operators are summarized in enclosure 2 to NRC ADAMS document MLI 10140104 and are analyzed in detail in ADAMS document ML102640674. 19 20 COMMUNICATOR At 11 :40 a licensed operator placed the motor driven Start Up Feed pump (SIU FP) in service in preparation for securing the second of two turbine driven Main Feed pumps (MFPs). At 11 :42 a actor Operator initiated a Containment Purge. At 11 :51 the final MFP was secured.

It is unclear why these tasks were prioritized over serting the control banks. From 12:05 to 12: 15 the Reactor Operator inserted the control banks. Control bank insertion was not completed until over two hours after the 4°F perature spike which caused the passive reactor shutdown. HUMAN PERFORMANCE ASPECTS Xenon-135 Cooldown The temperature transient which significantly tributed to the confusion that resulted in the pas-10:12 10 10:15 10:18 FALL201 l sive reactor shutdown was a result of the operators failing to account for Xenon-135 when they stopped the turbine downpower at 09:36. though operators might well understand the ics of Xenon-135

, applying this knowledge while conducting a busy forced de-rate and while being distracted by equipment malfunctions is much more difficult than applying this knowledge while taking a Generic Fundamentals Exam. Two ble solutions to aid the operators in adequately assessing xenon are to have readily available erating Experience (OpE) listed on specific pre-job brief forms and to require Reactor Engineering to prepare detailed Reactivity agement Plans for forced de-rates.

Challenges of MODE 2-Descending Due to the degradation of Temperature-Reactivity feedback which occurs in MODE 2-Descending (see Figure 4, below), if there is a need to remain 10:21 10:24 10:27 2.8E-05 l\T instrument channel 1 (%rated power) 1 lH 0.1 0.01 Non Fission Heat Rate (NFHR) for this shutdown was 1. 75% of rated reactor power (about62MWth)

. The Point of Adding Heat (POAH) was reached around 10:23. IRNI currents at the POAH correspond to a fission power of about 2.4 MWth. 2.8E-06 IRNI 2.8E-07 2.8E-08 Figure 4: Logarithmic plots of Total Power (as represented by 6T instrument readings) and fission power (as resented by Intermedi ate Range Nuclear Instrument currents)

. Starting around 5% rated reactor power, as fission power lowers exponentially

, total power asymptotically approaches the Non-Fission Heat Rate (NFHR). The match between fission power and total power has a strong impact on Temperature-Reactivity feedback causing it to degrade upon entry into MODE 2-Descending and causing it to completely disappear at the Point of Adding Heat (POAH). Although temperature continues to directly affect reactivity as the NFHR is approached

, ture-Reactivity is lost because falling fission power from a negative reactivity insertion does not immediately fect temperature since non-fission heat sources "buffer" temperature from dramatically lowering.

FALL 2011 critical at low powers then the reactor should main m low MODE l (i.e. greater than 5% power). Because of the Temperature-Reactivity feedback afforded in MODE 1, operators can rely on perature to passively respond to react1v1ty changes. Near the Non-Fission Heat Rate (i.e. in MODE 2) the operator must directly respond to reactivity changes (e.g. xenon buildup) with active reactivity manipulations (e.g. rods or boron/ water). Whereas it is not very difficult to maintain temperature through the active insertion of tivity, it can be extremely difficult to actively spond to reactivity changes directly (while at the same time ensuring the reactor neither exceeds 5% power nor drops below the POAH). ln 2007 Callaway Plant's procedures were changed to minimize sustained operations in MODE 2-Descending.

On April 13 and 14, 2009 Callaway Plant successfully performed turbine control valve repairs while maintaining the reactor critical in low MODE 1. Based on their past perience with low power operations

, it is unlikely that they would have successfully remained cal during the turbine control valve repairs had they attempted these repairs in MODE 2-Descending.

By prudently conducting the turbine repairs in MODE 1, Callaway Plant learned from its past mistakes and set its operators up for cess. Recognizing the Passive Shutdown Although the operators claim otherwise, it appears that for 67 minutes (from 10: 18 to 11 :25) they failed to realize the reactor was shutdown. Whether or not the operators were aware of the passive shutdown as it was occurring, it is still worth exploring some of the "human factors" falls associated with attempting to maintain MODE 2-Descending

. There are no adequate instruments for indicating fission power when attempting to maintain MODE 2-Descending.

Due to decay heat and other fission heat sources, both primary calorimetric (e.g. L1 T instruments) and secondary calorimetric instrumentation are poor indicators of fission power in MODE 2. Due to cold-leg shielding and COMMUNICATOR decay gammas, Power Range Nuclear Instruments (PRNis) do not accurately reflect fission power and wi II continue to read -1 % rated reactor power even after the reactor has entered the source range. The only accurate indications of fission power in MODE 2-Descending are the IRNls; however, these instruments are human factored for ing reactor startups and not for maintaining MODE 2. Because of the significant range of these instruments (i.e. 10 decades of power) they have substantial calibration errors. These errors have little effect on the operator as long as the operator is using these instruments to detect CHANGES in fission power and not as an lute measure of fission power. For this reason, these instruments are intentionally scaled in ion chamber amps instead of percent rated power. That is, the calibration errors prevent these ments from accurately indicating absolute power levels so they were intentionally "human factored" to use units which are not easily converted into percent rated power or into MWth, thus ing the operator from using them while attempting to maintain discrete power bands. Attempting to use the IRNis to maintain a power band from the POAH to 5% is unwise. Furthermore, although recognizing when the Point of Adding Heat has been attained during a power ascension is straight forward, during a downpower it is impossible to recognize the POAH until fission power is stantially below it. See References 2 and 6 for more details on cal Lessons Learned.

SAFETY AND PI&R CONCERNS No PI&R effort in 2003 For unknown reasons the passive reactor down was not documented in the plant's tive action program in 2003. The failure of the crew to document the passive reactor shutdown resulted in the organization failing to perform quate Problem Identification

& Resolution (PI&R). That is, without a condition report menting either the xenon induced letdown tion or the inadvertent passive reactor shutdown, the organization was unaware that it had an event which it could analyze for "problems" needing 21 22 COMMUNICATOR "resolution." The purpose of writing a condition report is not to "tum yourself in for making rors," it is to provide the organization a record of the known (or perceived) facts so that these facts can be analyzed for potential "problems" (e.g. adequate procedural guidance, operator edge weaknesses, unrealistic management tations, etc.) and these problems can then be lyzed for "resolution s" (e.g. improved guidance).

There are some (e.g. this author) who believe that on October 21, 2003 the crew was "set up for ure." The general operating procedure for ducting the down power and reactor shutdown was poorly structured

. The procedure assumed that in order to stop the down power the operators needed to do nothing more than delay continuing in the procedure

. The procedure made no tion that the actions the operators needed to take for "holding" power during a xenon transient were different than the actions needed for "reducing" power. The procedure did not take into account the limitation s of the operator's control equipment (i.e. the degradation of Temperature-Reactivity feedback) and monitorin g equipment (i.e. affect the NFHR and decay gammas have on total power meters) in MODE 2-Descending. Management expectations were unrealistic; it was unrealistic to expect the crew, with procedural guidance written for a continuous (i.e. "non-segmented") shutdown, to be able to hold power at 10% power during the severe xenon transient which is induced from an aggressive 9 hour1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> down power at 10%/hour. ever, since the October 21, 2003 passive reactor shutdown was not documented until it was dentally uncovered 40 months after the fact, these gross procedural deficiencies and unrealistic agement expectations went uncorrected until 2007. On June 17, 2005 a similar passive reactor sdown occurred during a forced de-rate for a failed power supply in an Engineered Safeguards ture (ESF) cabinet.

During this de-rate, the tor passively shut down due to a 2°F spike in T avg which occurred upon manually tripping the main turbine. The shutdown occurred two minutes prior to the failed power supply being successfully retested and 54 minutes prior to the expiration of the shutdown action of the Technical Specifica-FALL 2011 tion. That is, since the broken equipment was scessfully repaired prior to the planned shutdown time, had the reactor not passively shut down the crew could have immediately returned to power. Instead, resultant delays in returning to power lowing the inadvertent passive shutdown cost the utility 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> of lost generation.

Like the 2003 passive shutdown, the 2005 passive shutdown was not documented until it, too, was accidently covered in February 2007. Had the October 21, 2003 passive reactor shutdown been evaluated by the utility's Problem Identification

& Resolution process, it is likely the 2005 passive reactor sdown would never have occurred.

Although the inadvertent passive shutdown of a commercial PWR might seem like a commercial concern vice a safety concern, failing to recognize it can readily jeopardize reactor safety. In ary 2005, the operators of a reactor in Virginia were attempting to maintain the reactor in MODE 2-Descending while repairs were being conducted on the secondary plant. The reactor passively shut down and the operators failed to notice it. Two hours later, the reactor inadvertently restarted lowing a manual positive reactivity addition which was conducted by operators who had failed to rognize the reactor had entered the source range. Like the October 2003 passive reactor shutdown at Callaway Plant, the operators failed to ment the event. Unlike the Callaway incident, when the incident in Virginia was brought to the attention of plant management

, an investigation was performed and the results were reported to the lnstitute of Nuclear Power Operation s and shared with the industry via a Significant Event tion. Sharing OpE with INPO Both the October 21, 2003 and June 1 7, 2005 psive reactor shutdowns were accidently uncovered in February 2007 during a review of critical rameter data from past shutdowns to support a maJor revision to the Reactor Shutdown dure. The two shutdowns were documented along with seven other shutdowns in Callaway Action Rquest 200701278, Analysis of Past Reactor downs -RF 15 Preparation Concerns. In their FALL 2011 August l 0, 2007 cover letter distributing WANO SOER 07-01, Reactivity Management, INPO quested that their member utilities "provide mation on similar occurrences and solutions at their plants."

For unexplained

reasons, Ameren determined that neither the October 2003 nor the June 2005 passive reactor shutdowns were worthy of sharing with the industry.

Since no INPO SEN concerning the October 2003 passive shutdown has been released since the NRC's issuance of IN 2011-02, it appears that INPO agrees with Ameren's decision that a passive reactor shutdown resulting in a two hour delay in inserting control banks does not meet the threshold for a Significant Event Notification.

In the absence of a detailed INPO document on the incident, interested nuclear professionals should review the issue brief leased by the Union of Concerned Scientists (see Reference 2). Informally Relying on Xenon-135 One of the more troubling aspects of the tors' claim that they were consciously aware the reactor had passively shut down is that this claim amounts to informally relying on Xenon-135 to prevent the reactor from inadvertently restarting.

Several times during the downpower

, the tors performed a "Xenon Prediction."

A Xenon Prediction estimates Xenon-13 5 levels based on projected power history, and it is used as a tool to assist the operators in maintaining the reactor critical.

A Xenon Prediction is very different from a Shutdown Margin Calculation.

Although there are times when a Shutdown Margin tion will rely on Xenon-135 for Shutdown Margin (SDM), when this is done it is based on actual power history. Another major difference between the two calculations is their uses: a SDM tion is used to ensure the reactor will not tently return to criticality during postulated tive reactivity additions (e.g. inadvertent dilutions

, inadvertent cool downs, etc.) whereas a Xenon Prediction assumes no failures and is used to mate the amount of negative reactivity which must be overcome to maintain the reactor critical.

Since a SDM calculation was not completed on October 21, 2003 until forty minutes after the trol banks had been inserted, the crew, for the 106 minutes they claim they knew the reactor was COMMUNICATOR shutdown

( l 0: 18 to 12:04) yet were still retaining the rods at their last critical rod heights, bly relied on an informal estimate that Xenon-135 levels were large enough to prevent an inadvertent restart.

Following the Shift Technical Advisor's calculation of Shutdown Margin (at 12:49), the crew added over 3600 gallons of boron in order to meet the required SDM. Since Xenon-135 is a radioactive isotope with a half-life of 9.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, a reactor requiring Xenon-135 to maintain it subcritical will eventually turn to power. Although the physics of dine-135 and Xenon-135 are well understood

, formally relying on estimations when formal culations are available is contrary to the principles of conservative reactor operation

. If there is a commercial reason to rely on Xenon-135 to tain Shutdown Margin, a formal SDM calculation should be performed and reviewed PRIOR to ing on Xenon-135 to maintain the reactor shut down. Operating Beyond Procedure Guidance In 2007 the US NRC investigated the October 21, 2003 passive reactor shutdown.

Although they issued non-cited violations (NCVs) for the tors failing to make a log entry documenting eration below the MTCO and for the operators failing to document the passive shutdown with a condition report, the NRC found no problems garding the two hour delay in the insertion of the control rods. Concerning this delay, the NRC stated, "The inspector's review of the operating procedures did not find any timeliness guidance on performing the steps to insert the control rods." It is unclear why the NRC inspector expected the Reactor Shutdown procedure to contain "timeliness guidance on performing the steps to insert the control rods." Like the nonnal (i.e. non -faulted) reactor shutdown procedures at all US commercial reactors, Callaway Plant's Reactor Shutdown procedure contained no provisions for intentionally allowing the reactor to passively shut down. Per the procedure

, the only way to shut down the reactor was to manually insert the trol banks. Since the procedure inherently sumes it is followed, and since the procedure re-23 24 COMMUNICATOR quires the control banks be manually inserted to effect the shutdown, then it would be nonsensical for the procedure to contain "timeliness guidance on performing the steps to insert the control rods." That is, since the reactor is shut down by manually inserting the control banks, it would not make sense for the procedure to dictate a time frame for inserting the control banks ING a passive reactor shutdown.

Nonetheless, the NRC has thus far maintained its 2007 position that no violations occurred other than the two NCYs concerning the lack of a log entry and condition report. In Information Notice 2011-02 the NRC specifically avoided addressing whether or not they believed the operators were aware of the passive reactor shutdown prior to the first SRNI channel energizing.

[Note: The author of this article was a reviewer for IN 2011-02 and is the owner of the initial block which has a "NoConcur" in it on the routing page. For those ested, the Non-Concurrence Form, which includes the NRC's response, can be found in the NRC's public ADAMS library as MLl 10420293.] Note that it is the opinion of this author it is not a procedure violation to unknowingly allow the actor to passively shut down. Operating a large commercial PWR at low power during an sive xenon transient is not an easy task; combined with the challenges already mentioned above (e.g. loss of Temperature-Reactivity

feedback, physical limitations of calorimetric indications of "fission" power near the NFHR, poor procedural guidance, lack of a detailed Reactivity Management Plan, equipment malfunction s) it should not be ing to any NRC licensed operator that the crew failed to perform flawlessly. Although most erators would like to think that it would never take them 67 minutes to recognize the reactor had shut down, most do recognize that, given the wrong set of circumstances

, any operator is capable of ing a mistake such as this. It is not a procedure violation to fail to recognize a passive reactor shutdown, it is a human performance error and no more. And it is not a procedure violation to, due to a human performance error, find oneself in cumstances not expected by the procedure

. When this occurs, the proper response is to use one's training and experience to place the plant in an FALL 2011 analyzed condition (e.g. if the plant has passively shut down, then manually insert the control banks). Note that failing to recognize a passive shutdown as it is occurring is very different from recognizing the reactor is passively shutting down and then intentionally prioritizing other actions above the deliberate control of reactivity

. Whether or not the NRC chooses to address it, intentionally allowing a large commercial reactor plant to passively shut down constitutes a mental misunderstanding of the principles of servative reactor plant operations.

As discussed above, US commercial PWRs "want" to be critical and "want" to match steam demand. The inherent passive response of the reactor as xenon decays is to eventually return to criticality and to match steam demand. As soon as it is noted that the actor has passively shut down, and as long as tive means to control the nuclear fission reaction are available, they should be used to ensure the reactor is taken to, and remains in, a shutdown condition

. Loss of Safety Related Instrument Power During the investigation of the October 21, 2003 passive reactor shutdown, the Shift Manager cated that the biggest delay in inserting the control banks was the fact that the crew was still ing the off-normal procedure for the "Loss of Safety-Related Instrument Power" which had been entered at 08:21 but was not exited until 11:37. Since all the control room actions were completed by 08:36 (an hour before the temperature transient which led to the passive reactor shutdown),

it is unclear exactly how this off normal procedure delayed the insertion of the control banks during the hour following the turbine trip. Nonetheless

, for unstated reasons the NRC has decided to take the operators at their word and not question how the performance of this procedure inhibited the insertion of the control banks yet did not inhibit the operators from placing the 45 gpm letdown orifice in service, placing Cooling Tower down in service, lowering intake flow, or lating the feed and condensate systems. Those interested in this topic should consult References 4, 5 and/or 6, cited on page 26. (Continued on next page)

FALL 2011 Operation without SRNis The reactor entered the source range at 10:39; yet, no Source Range Nuclear Instrument (SRNI) gized for another 45 minutes ( 11 :25). Each SRNI at Callaway Plant is powered through a contact on its channel's associated IRNI. This contact automatically closes at 5E-l l ica. cause of the subcritica l multiplication afforded by the control banks still being at their last critical rod heights, both channels of IRNis were reading greater than 5E-I I ica when the reactor first tered the source range. It took 45 minutes of tional Xenon-135 buildup for the channel 2 IRNI to lower below 5E-l l ica and 59 minutes for channel I. The SRNis can also be manually energized once the PRNI signal has lowered below 10% rated reactor power. Had they, prior to I l :25, nized they were in the source range, the operators could have manually energized either or both SRNis. The fact that they did not do this is one of many indications to this author that, prior to the SRNis automatically energizing at l 1:25, the erators were unaware they were in the source range14* The SRNls add significant defense in depth during operation in the source range by providing:

  • start up rate indication
  • an audible count rate which quickly alerts the operator to rising reactivity
  • a meter indication better suited for monitoring power in the source range than the more broadly ranged IRNI meters; COMMUNICATOR
  • an automatic high flux reactor trip which is set about 5 decades earlier than the IRNI high flux trip; and
  • a signal to the Boron Dilution and Mitigation System (BDMS) which causes an automatic swap over of charging pump suctions from the Volume Control Tank to the Refueling Water Storage Tank (RWST) in the event that source range counts increase by 70% in a rolling 10 minute period (since the RWST is borated to -2500 ppm, this BDMS circuit provides tection against inadvertent reactivity additions caused by xenon-135 decay, inadvertent tions, and inadvertent cooldowns)

. Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi zed, this is so a reactor start up can be perfonned.

15 During a reactor startup, administrative controls 16 are in place which mitigate the loss of safety margin from blocking the automatic safety circuits driven by the SRNis. The designers of Callaway Plant never intended for the plant to be operated in the source range with the control rods at their critical rod heights and with none of the SRNI driven automatic protection s in place. Although the NRC is technically correct in stating that this condition did not violate the plant's licensing requirement s, there is more to ensuring reactor safety than forcing a verbatim interpretation of the Technical Specifications

not all conditions can be exactly defined by the Technical Specifications and a competent professional reactor operator should be able to discern when the plant is in a condition in which the designer never intended.

(Continued on next page) 14Note that from I 0:23 to 11 :25 all indications other than the IRNis were steadily indicating the plant was low in the power range: the PRNis were reading -I% rated power, the t-.T instruments were reading 1.75% power and the secondary calorimric computer points were reading 62 MWth. In order to realize they were in the source range the operators would have either had to note the !RNI readings or question why they had not needed to add positive reactivity to account for xenon buildup. With regard to noting the IRNI readings, because of their units (ion chamber amps) and their scaling (logarithmic) the operators do not normally use these instruments while at power. With regard to questioning why they had not needed to dilute or pull rods to make up for xenon, understanding the reactor dynamics of Iodine/Xenon was a weakness of this crew as demonstrat ed by their response to the 9°F temperature drop which occurred from 09:36 to I 0:00. 15It is impossible to do a successful reactor startup without blocking the SRNI flux trip and BDMS; therefore, once the IRNI signal reaches I E-10 ica during a reactor startup, the operators are permitted to de-energized the SRNis (which by then have had all their protective functions blocked). 16For example: a Reactor Engineer present in the control room, an Estimated Critical Position has been calculated, the crew is intently performin g a procedure which warns them to "expect criticality at any time," etc. 25 26 COMMUNICATOR CONCLUSIONS There is much to be learned from the October 21, 2003 passive shutdown at Callaway Plant. The incident highlights a number of issues: the ner by which NRC licensed operators might fail to appreciate the magnitude of the effect xenon is having on core reactivity while that effect is being masked by power defect, the need for specific and thoughtful procedural guidance for stabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback poses to the operator as the NFHR is approached, the manner in which an erator focusing on calorimetric instruments while at low power might fail to recognize fission power lowering below the Point of Adding Heat, and the importance of documenting incidents in the rective action process. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "generic mentals" of reactor dynamics. No analysis of the incident has been done by INPO and the NRC's analysis in Information Notice 2011-02 is not very thorough; those interested in a deeper analysis of the details of the incident are encouraged to view the reference s at the end of this article.

DISCLAIMER The views expressed in this article are those of the author and in no way reflect the position of the US Nuclear Regulatory Commission or the sional Reactor Operator Society.

To participate in an online analysis of this dent, send an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro ups. com (anonymous participation is accommodated).

FALL 2011 Anyone wishing additional information on this incident is encouraged to contact me at: cione@hotmail.com (573) 230-3959 REFERENCES

l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator Performance Issues Involving Reactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at laway, November 2010. 3. Non-Concurrence on NRC Information Notice 2011-02, Operator Performance Issues ing Reactivity Management At Nuclear Power Plants (ADAMS #MLl 10420293)

. 4. September 17, 2010 letter from L. Criscione to William Borchardt (ADAMS #ML 102640674).

5. April 27, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #MLl 0120040 l ). 6. April 30, 2010 letter from Lawrence Criscione to William Borchardt (ADAMS #ML101230100).
7. G2010059/EDATS: OED0-2010-0775

-tion Closure Letter to Lawrence S. Criscione Related to Requested Action Under 1 OCFR 2.206 Regarding October 21, 2003 Event at Callaway Plant, Unit 1 (TAC No. ME4721),

ADAMS #MLl 10140104, January 19, 2011. Note from the Author In my opinion, this is an important piece of Operating Experience which is only available through PROS. The event was never submitted to INPO and, although the NRC included it in an Information Notice, most of the significant Lessons Learned from the incident were not addressed.

More than anything, the event is an example of: ( 1) licensed reactor tors being "set up for failure" by impractical operating practices and expectations and (2) the importance of honestly reporting events and accurately analyzing them so that future reactor operators (both at the plant and throughout the industry) do not fall victim to the same poor practices and knowledge gaps.

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