ML16011A519

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
References
FOIA/PA-2016-0203
Download: ML16011A519 (18)

Text

COMMUNICATOR THE PUBLICATION OF THE PROFESSIONAL REACTOR OPERA TOR SOCIETY FALL 2011 In th i s iss u e: NRC Fukushima Task Force Analysis of the 2003 Callaway Shutdown ----------

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

COMMUNICATOR PROFESSIONAL REACTOR OPERATOR SOCIETY PO Box 484 Byron , IL 61010 phone (8 15) 234-8 1 40 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> FAX and a n swe rin g BOARD OF DIRECTOR S President Mitch Taggart -Sequoyah Vice Presid e nt Casey Pfeiffer -Seq u oyah Treas ur er Mark Ra s mu sse n -Byron Sec retary Joe Eva n s -Perry Region I President vacant Region II President Brian Snyder -Oconee Region III President Jim Kelly -P erry Region IV Presid e nt Sheryl Breault -Fort Cal h oun Foreign and Domestic Affairs Pre s ident Bob Meyer -Yogt l e S TAFF Communicator Editor Mary Cay R asmussen Office Ma n ager Mark Rasmu sse n Web master vacant ON TH E COVER: TVA's Bellefonte Nuclear Power Plant i s scheduled to be online between 2018 and 2020. FALL2011 CONTENTS Bell efonte R eco mm e ndation s from the NRC Fukushima Ta sk Force Casey Pfeiff e r NFP A Standard 805 -Fire Prot ec tion Bri a n Snyder Analysis of the 2003 Callaway Shutdown Lawrence C ri sc ion e E thanol -The Answer to Our E n e r gy Needs? Mark R asm u ssen REGULAR FEATURES Members' Pag e Officer R e port s Joe SRO New Members PAGE 2 3 8 11 27 42 46 52 53 A D VERTISE IN THE COMMUNICATOR AD SIZE & PLACEMENT Back Cove r -F ull Page Back Cover -Half Page Inside Cove r -Fu ll P age Inside Cover Half P age Internal -Fu ll Page Int erna l -Half Page Int erna l -Quarter P age SINGLE ISSUE $1000. $ 600. $ 750. $ 400. $ 500. $ 300. $ 200. FULL YEAR (4 ISS UES) $2500. $1500. $1 875. $1000. $1250. $ 750. $ 500. M e mb e r s hip in th e Prof ess i o n a l Re ac t o r Operator Society i s open to a n y individual or co mpan y that s h a r es our goa l of prom o tin g safe reactor* o perati o n. M e mb e r s hip du es a r e $35 per yea r , which includes a s ubscription to th e Communicator.

The Co mmuni ca t or is published quarterly , a nd i s di s tribut e d to m e mb ers b y US m ai l. Additional co pies a r e avai labl e for $I 0 per copy. Inquiri es regarding distribution, d e li ve r y , o r c h a n ge of a ddr ess s hould be dir ec t e d to <thePROSoffi ce@nucpro s.co m>. The Co mmuni ca tor i s copyrighted b y the Profe ss ional Reactor Operator Society and may not be r e produced in who le o r in part without ex pr ess p erm i ss i on. Opinions ex pr esse d by a uthor s are th e ir own a nd d o not necessari l y refle c t th e po s itions of th e Pro fessional R eac tor Operator Society , the Comm uni ca t o r Ed it o r , or th e organization with which the a uth or i s affi liat e d. Publi ca tion of v i ewpo ints or d escriptions of materials , products, or serv i ces s h o uld n ot b e construed as e nd o r se ment s b y the Profe ss ional Reactor Operator Society or it s 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 Calla w a y Plant on O c tob e r 21 , 2003 , w hil e att e mpting to s tabili ze r e a c tor po we r durin g a for ced d e-rat e, X e non-135 buildup c au se d av e rag e r e a c tor c oolant t ep e ratur e to low e r at a 25 minut e p e riod , r es ultin g in an automati c i s olation of th e l e tdo w n sys t e m on lo w pr ess uri ze r wat e r l eve l and op e ration of th e r e a c tor b e lo w th e Minimum T e mp e ratur e for Criti c al Op e ration. Aft e r manuall y trippin g th e turbin e-g e n e rator to s i s t in t e mp e ratur e r ec ov ery, th e r e a c tor pa ss i ve ly s hut down du e to a s harp 4°F ri se in a veage c oolant t e mp e ratur e. For th e n e xt 110 ut es th e op e rator s p e rform ed s e condary and t e rtiary plant s hutdown a c tiviti es w hil e r e l y ing on an informal es timation that X e non-135 l eve l s we re s uffi c i e nt to pr e v e nt th e r e a c tor from v e rt e ntl y r es tartin g. Th e pa ss i ve r e actor sdown was not do c um e nt e d until it wa s un ce r e d 40 month s lat e r, and it wa s not s har e d with th e In s titut e of N ucl e ar Pow e r Op e ration s lo w in g th e r e qu es t w hi ch a cc ompani e d SOER 0 7 -1. Th e in c id e nt hi g hli g ht s th e pitfall s a ss o cat e d with att e mpting to maintain a c omm e r c ial pr ess uri ze d w at er r e a c tor c riti c al during MODE 2-D esce ndin g and d e mon s trat es how c on ce pt s t es t e d o n the N R C G e n e ri c Fundam e ntal s Exam appl y to a c tual r e a c tor op e ration. Th e in c id e nt al s o hi g hli g hts s om e non-c ons e rvativ e r e a c tivity mana ge m e nt pra c ti ces w hi c h mu st b e avoid e d b y Pr o f ess ion a l R eac tor Op e rat o r s. 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 l earned." Detail s of the incident were first pub-1 ical ly released by the Union of Concerned tists (UCS) in a 20 l 0 issue brief1 tit l ed 2003 S em e nt e d Shutdown at Callawa y, and then, in 20 l l , the US Nuclear Regulatory Commission (NRC) partially covered the incident as part of tion Notice 2011-02 , Op e rator P e rforman ce i ssu es Involving R e a c tivity Manag e m e nt at Nucl e ar Pow e r Plant s. 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-Fis s ion Heat Rate has on Temperature-Reactivity feedback. The challenges facing the operator during low power operation due to human factoring of control board instruments. (Co ntinu e d on n ex t pa ge) 1 h tt p://ww w.uc s u sa.o r g/nu c l ea r _powe r/nucl ea r _p owe r_ ri sk/sa fet y/2 00 3-segm ented-s hutd ow n-at-c a l !a w ay.html 2 http://pb a dupw s.nr c.gov/d ocs/ML I 01 8/ML I 01 8 10 282.pdf 11 1 2 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) It em 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 wi ll result in a negative in sertion of activity , thereby limitin g 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 limit ed window of their fuel cyc l e. October 21 , 2003 was l ate in fuel cycle 13 for Ca ll away Plant a nd 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 i s inherently stab l e. That is , the reactor " wa nt s" 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) w hil e the steam demand (i.e. turbine-generator loading) is held constant , reactor power wi ll decrease s li g htl y. Becau s e 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 in s erted, which further dampens the negative reactivity insertion. FALL20ll Temperature w ill continue to lower as long as there is a negative power mismatch. Eventually , eno u g h positive r eactivity w ill 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 an d the magnitude of the negative power mismatch wi ll lo wer , dampening the temperature drop. Once reactor power rises above steam demand , there will be a positive power mismatch which w ill now cause temperature to rise. The rising temperature will insert negative reactivity , causing reactor power to lo wer. These passive feedback processes will continue until , eventua ll y , reactor power again matches steam demand and there is no power mismatch to drive temperature.

At this point, temperature wi ll be l ower than it was prior to the negative reactivity insertion.

ca ll y , the change in temperature is: L'.l T = L'.lp/MTC). The reactor will passively respond to a positive insertion of reactivity in a simi l ar 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 coo l ant temperature (T avg) to the power match induced by the change in reactivity. 2. The passive counter insertion of reactivity due to the temperature response , which continue s 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 eve ntu ally results in the same steady state power at a new temperature. This generic fundamental is demonstrated later in this article by 3 Si n ce Ca ll away Pl a nt is a PWR , th e r eactiv i ty coeffic i e nt du e t o vo id s i s no t di sc u sse d in this a rticl e. 4 P o w e r mi s m a t ch i s t h e di ffe r e n ce b etwee n steam d e m a nd a nd r eac tor po wer.

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). (Co ntin u e d o n n ex t pa ge) 5 power d efe ct i s th e t e rm for th e re a ctivit y in s erted from a change in re a ctor power lev e l. 6 MOD E 1 r e fer s t o the s tate of op e ratin g the re a ctor a t power (5% to 100% rated re a ctor power). 7 MOD E 2 refer s to th e tran s iti o n s t a te b e tw ee n th e r e actor being s olidl y in the pow e r ran g e (i.e. b e yond th e point a t which th e NFHR e x e rts a ny s ub s tanti a l influ e nce) a nd th e r ea ctor b e ing s hutdown (i.e. d e finiti ve ly s ubcritical as indicat e d by ca lcul a tin g K e lT t o b e l ess than 0.9 9). T h e rea c tor e nter s MOD E 2-De sce ndin g when r ea ctor pow e r low e r s b e low 5% rated pow e r. 1 3 14 COMMUN I CATO R FALL 20 11 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% -0 1: 00 02*00 03:00 ' "" K o4: oo ""I' -.. ..........

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i -...._ ! "-. ' 'l i-f -, , , i j /i -.,. '\ / .. ' 20% % : 10° 0% i 10 0 p s 0°F i s t e .: 56 /55r F //554" F 551°F 560°F 557°F 554°F 551°F 1 00 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 Coo lant Temperature (T avg), Prim a r y Ca lorim e tri c pow e r (LiT) and Co ntrol B a nk 'D' rod h e i g ht s durin g th e October 21 , 2003 down pow er a nd pa ss i ve reactor s hutdown. Note th e seve re t e mperatur e transient which b ega n at 09:36. Turbine fir s t s t age s t e am pr ess ure data indi ca te s that th e operators s topp e d low e rin g ge n erato r lo a din g at 09:36 with reactor power a t 9%. Ov e r th e n ex t thr ee minut es, n ega ti ve reactivity du e to Xenon-135 ca u sed p owe r to co ntinu e to lo wer a noth e r I%. The p ower mismatch b e tw een the s t eam d e m a nd ed b y th e turbine throttle set p oi nt a nd the power b e in g produc ed b y fission ca u sed T avg to imm e di a t e l y b eg in to low e r , thereby in sert in g po sitive reactivity which co unt e r e d th e n ega tiv e rea c ti v it y b e ing in ser t ed b y the co ntinual buildup of Xenon-135. Around 09:39 the positive r eact ivit y bein g in se rt ed b y th e low e rin g t e mperatur e mat c h e d th e n ega ti ve r ea cti v it y b e in g in se rt ed b y Xenon -135 ca u sing r eactor p ower (as indicated b y core Li T) to s tabili ze a t a ppro x im a t e l y 8%. With a I% p owe r mi smatc h se nt , over the n ext twenty minut es T avg co ntinu ed t o stea dil y l owe r a nd thereby co unt erac t the continual buildup of xenon. Shortly afte r I 0:00 the c r ew again b ega n to low er turbine-generator lo ad in g in res pon se to th e Shift Manager's d ec i sio n to take th e turbine off-line following the l e tdown i so lation. The ren ewe d low e rin g of ge nerator loadin g caused s t ea m d em a nd to low er b e l ow fission pow e r a nd th e r e by a llo wed T avg to temporarily recover s li g htl y. Durin g this time period (I 0:03 to I 0:09), t h e n egative reactiv it y b e in g in se rt ed b y Xenon-135 was no w b ei n g counteracted b y the po s iti ve ity being in se rt ed by t h e l oad decrease (t h e pl a nt h ad a n egative p ower coefficie n t of reactivity).

Ge n erator l oad in g was aga in sta bili ze d aro und I 0:09 ca u s in g T avg to r es um e fa llin g, which i s the ex p ecte d passive respo n se of the r eactor plant to Xenon-135 buildup. The o p erators failed to gras p th e r eac tor d y n a mic s b e hind the transient and ass um e d th e I 0°F drop in T avg was b ei n g caused by malfunctioning steam line a nd turbine drain va l ves (w hi c h had co in c id e ntall y been placed in service at abo ut the same time the temperature transient b ega n).

FALL201 l By the time the reactor at Callaway Plant pa ssively 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 ments 8) 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 C OMMUNICATOR 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 s ince they were 2 Y2 hours ahead of schedule it is likely they intended to hold power at while 8 Th e re a r e thre e se t s o f nucl e ar in s trument s (th e power rang e, intermediat e ran ge a nd s our ce ran ge). Th e nu c l e ar in s trum e nt s m e a s ur e fi ss i o n rat e b y d e t e cting s tr ay n e utron s produ ced b y fi ss i o n. 9 T h e re a r e t wo se t s of ca lorim et ri c in s trum e nts a t Ca llaw ay Plant: L'.T in s trum e nt s (prim a r y ca lorim e tri c ca l c ul a t e d fro m th e t e mp e ratur e ri se acro ss the c ore) a nd th e rmal output c omput e r points (calcul a t e d from a se condary ca lorim e tri c). 15 16 COMMUNICATOR further troubleshooting occurred on the failed verter. Also around 09:36 the operators cycled the Group B turbine drains. One of th e switches for the drain s 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 b ee n occurring for the past 8 Yi 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 s teps 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 2 Y2 hours of the downpower , when the rate of xenon buildup was s till 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 pas s ively inserted from the downpower.

However , since Xenon-135 was still building up , negative reactivity was st ill being passively serte d. The crew did not have a detailed ity Management Plan 10 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 (Tav g) 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 , T avg lowered slightly.

Due to the -MTC , the lowering T avg 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 ofT avg* 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 T avg had lowered 9°F and the letdown system automatically isolated on low pressurizer water level. Also by 10: 00 , the crew recognized that T avg 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 turbin e be taken off-line.

Manual Turbine Trip and MODE 2 Entry After the letdown isolation, th e operators began lowering turbine-generator loading in preparation for removing the turbine from service. This caused a positive power mismatch which tempo-'0 T hose with access to proprietary documents from the World Association of N u clear Operators shou ld see the d a tions contained in WA 0 SOER 2007-1, R eac tivi ty Management, for expectations regarding Reactivity Management Pl a ns. 11 In the 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> s ince relieving th e watch at 06:3 0 the crew had n eeded to insert control rod s 30 steps in order to keep Tavg lowering a t the programmed rate. No active insertions of positive reactivity had been required to overcome X e non-135.

FALL 2 011 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 .... !Tre f (linear scale , 5°F/d i vis i on) ! ;:::::: ..:.-.:.:.-..:.*.:.:. I i--r--,.,., O'I 0 0 0 .-i .-i .-i ------C OMMUNICATOR 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 I n !Tavg (linear scale , 5°F/divisionl l Ff' --/ I I I I ------n o lo a d Ta v g (55 1'F) --------' ..... 5 50°F MT CO (55 1° F). L#------------l V 5 60°F 5 5 5°F 5 S0°F 10% 5% 10% AT 5% ... ftl " 1. E-04 u ... ::s 0. -..... I LH (linear scale , 5% rated ' --!NFHR*------; 0% ' ' "' ' ........_. ' ' 1.E-0 5 "' E c ftl " " a: E .a A verage Coolan t Temperature (Tavg) rose steeply -*---P OA H l.E-0 6 1. E--07 " 2 E ........ ftl fo llowing t he turbine trip (TT). The quick negative "' ftl ... .c *-c u c r e ac t iv i t y insertion which accompani ed th e -4°F spik e i n -""' l.E--0 8 Tavgcaused the reactor t o become s ubs t an t ially '-E .2 ... -1. E--09 ! subcr i tical and shut down. The POAH and a nom i nal I *<6' ** c -1/3 dpm SUR were reached around 10: 23. 1.E-11 % -1.E-1 0 l.E-11 Figure 2: Plot of Average Coolant Temperature (T a v g), Primary Calorimetric power (.1T) and Intermediate clear Instrument current s (IRNI) on October 21 , 2003. The sharp rise in T a vg 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 pa s sively 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 s ourc e range. See Figure 3 for plant evolutions occurring during this time frame. rarily caused T av g to s top lowering (the minimum T a v g 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 T av g 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 R e a c tor Pow e r c on s tant b y tran s f e rring load to the c ond e n se r s t e am dump s whil e r e du ci n g bin e Load. This w ill pr eve nt inadv e rt e nt e nt ry i nto Mod e 2 w h e n th e Turbin e i s tripp e d." Within 30 seconds of tripping the main turbine , reactor power lowered below 5% and the tors declared MODE 2. Rap id Ri se in T avg a nd P ass i ve S hutdo w n With the condenser steam dumps set to modulate at 1092 psig, upon tripping the turbine there was no steam demand until T avg rose to 557°F (corresponding to a steam pressure of 1092 psig). With the reactor initially around 6% power and with no steam demand , T a vg 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 decade s per minute (dpm); by 10: 18 SUR was -0.16 dpm -a chang e of 1600%. (C ontinu e d on n ex t pa ge) 1 7 18 COMMUN I CATOR FALL 2011 As the reactor neared the Non-Fission Heat Rate ( 1. 75% rated reactor power for thi s shutdown), temperature-reactivity feedback was lo s t (see ur e 4 on page 20); that is , lowering reactor power would no longer feed back po si ti ve reacti v ity via low e rin g temperatur

e. Thus , without a m a nual in se rtion of po s iti ve reactivity , power would tinue to lower into the so urce range. to lower to Y2 it s initial value, fission pow er (as indic a ted by IRNI currents) low e red to 1/6 its tial va lue. Thi s is further indication the fission reaction had s hut down and the Non-Fission Heat Rate was raising/maintaining reactor coolant tperatur e. Response to the Passive Shutdown A t 10: 13 , the instruments had indicated 5.17% and th e Intermediate Ran ge Nuclear In s trument s (IRN l s) had indicated l .52 E-5 ion chamber amps (ica). By I 0: 18 , T instruments indic a ted 2.4% and the IRNi s indicated 2.43E-6 ica. So in the time it took total power (as indicated by core While the reactor was passively s hutting down , the operators were performing the off-normal cedure for " Lo ss of L e tdown" (w hich had been entered at 10: 00). At 10: 1 8, a 75 gpm l e tdown orifice was plac e d in service and th e crew ex it e d the off-norma l procedure.

By thi s point (10: 18), had they recogni ze d the reactor was s hut 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 10:3 0 10:45 11 :00 11 : 15 11 :30 11 : 45 12:00 12:15 Figure 3: Pl ot of Co nt ro l Bank rod h eig ht s and Int e rm e di ate R a n ge (IRNI) currents on October 2 1 , 2003. The reactor passively sh ut down short l y afte r the turbine was manually tripped at l 0: l 3 an d reached the so ur ce range about 26 minute s l ater. A n ominal -1 /3 dpm SUR developed as power fe ll below the PO AH. The s li ght drop in reactor power from I 0: 39 to 1 2:05 was ca u sed by a l owe rin g of subc riti ca l multiplication resulting from the co ntinued buildup of Xenon-135. The operato r s began in se rting the control b a nk s at 1 2:05 a nd completed at 1 2: 15.

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

After exiting the off-norm a l procedure for " Loss of Letdown" the Control Room Supervisor signed the Reactor Operator the ta s k 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 wa s prioritized over actively controlling core reactivity (i.e. over inserting the contro l bank s to en s ure the reactor remained shutdown). Thi s ta s k involves multiple manipulations of charging system components and took 30 minutes to comp l ete; in comparison , manuall y driving in the control banks takes 10 minute s. As reactor power was decaying through five a de s of power to reach the s ource range , licensed Reactor Operators were assigned to place Cooling Tower Blowdown in s ervice and to s ecure th e ond of three intake pumps (cooling Tower down had been s ecured a coup l e of hours earlier to support Chemistry surveillances and the intake pump was s ecured becau s e two pumps were no longer ne eded due to the forced de-rate causing evaporation rate to lower). These tasks were both logged compl e te at l 0:34. It i s unclear why these ta s ks wer e prioritized over in s erting the control bank s. Operation in the So urc e Rang e At 10: 39 , reactor pow e r entered the s ource range , a s evident on Fi g ure 3 (page 18) by the IRNI rents s tabilizing.

As at mo s t reactor plants , the C OMMUNICATOR Source Range Nuc l ear Instruments (SRNl s) at Callaway remain de-energized until bistable s on the IRNis validate reactor power is in the source range. Because the control rods were still at their l a s t critical rod heights , there wa s more subcritical multiplication than is normally pre s ent when these lRNI bistables are calibrated.

As a result , the SRNis did not energize upon initially entering the source range. It took 45 minute s of additional Xenon-135 buildup to lower subcritical cation to the point at which the first SRNI channel was able to automatically energi z e. At 11:01 a 1 icensed operator was as s igned to scure the second of three condensate pump s. It is unclear why , while in the s ource range with no SRNis energized and with the control rods s till at their last critical rod height s, the lic e nsed tors prioritized manipulation of the conden s ate s ystem over inserting the control banks. To s ome (e.g. thi s author) the crew's action s indicate that they were unaware the reactor had s ively shut down. That i s, the most reasonable explanation for the crew " prioriti z ing" ancillary tasks 12 over deliberate contro l of the nuclear fi ss ion reaction i s that for 67 minute s they failed to recognize the reactor had shut down.13 At 11 : 25 the channe l 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 sourc e range. At 11 : 3 8 the channel 1 SRNI energized. (Continu e d on n ex t pa ge) 1 2 Fo r exa mpl e: pl ac ing a n ext ra 4 5 gpm l e td ow n o rifi ce in se r v i ce, pla c ing C oolin g To w e r Sl o wdown in se r v ic e , sec urin g unn ecessa r y in ta ke a nd co nd e n sa t e pump s. A lth o u g h o pt i mi z in g wa t e r c h e mi s tr y of t h e prim a r y pl a nt a nd coo l i n g tower i s imp o rt a nt a nd a lth o u g h minimi z in g " h o u se" e l ec tri c l oa d s b y sec urin g l a r ge a nd n o l o n ger n ee d e d pump s i s imp o rt a nt , th ese t as ks a r e " a ncill a r y" w ith r ega rd to th e prim a ry fo c u s of t h e r eac t o r s hutdo w n pro ce dur e: in se rtin g th e co ntrol b a nk s t o d efi niti ve ly e n s ur e th e r ea ct or i s in a s hutd o wn c o nditi o n a nd w ill r e m ain in th a t s t a t e r ega rdl ess of p ass iv e (e.g. xe n on d ecay) o r un ex p ec t e d (e.g. in a d ve rt e nt diluti o n s or coo ld o wn s) c h a n ges in co r e r eac ti v it y. 13 It s h o uld b e n o t ed h e re t h a t the c r ew h as co n s i s t e ntl y asse rt e d th a t pri or t o m a nu a ll y trippin g th e turbin e th ey we r e awa r e th e r eac t o r would p ass i ve ly s hut d o wn o n ce s t e am d e m a nd w as r e mov e d. T hi s asse rti o n a mount s t o the c r ew d e lib era t e l y a ll ow ing t h e r eac t o r to p ass i ve ly s hut d ow n w hil e th ey p e r fo rm e d the a n c ill ary it e m s m e ntion e d in not e 1 2. T he a uth o r of thi s artic l e be li eves t h a t , if t ru e , t h is a m o unt s to in co mp ete n ce. T h at i s, i t is in co mp ete nt fo r a n N R C li ce n se d o p erato r to pri o riti ze a n c ill ary tas k s over d e lib era t e l y co ntr o llin g th e r eac t o r, a nd it i s in co mp e t e nt t o d e lib e rat e l y re ly on p ass i ve mur es to s hu t d ow n t h e reac t o r w h e n act i ve m ea n s (e.g. ro ds a nd b o ron) are ava il a bl e. S in ce th e US N R C h as r e fu sed t o quti o n t he o p era t ors' asse rti o n s , a t t hi s po int th e qu es ti o n re m a in s unr eso l ve d as to w h e th e r or n o t , pr ior t o th e SRN i s e n e r g ii n g , t he o p era t o r s were awa re t h e r eactor h ad p ass i ve ly s hu t d ow n. A lth o u g h t h e In s titut e of N u c l ea r Power Op erat i o n s (fNP O) i s awa r e of t h e di sc r e p a n c i es s urr o unding t h e O c t o b e r 2 I , 2 00 3 s hutd o wn , rNPO h as s imil a rl y d ec lin ed t o eva lu a t e t he c l a im s m a d e by the o p era t o r s; s i nce rNPO mu s t re ly o n A m e r en t o vo lun ta ril y r e p o rt th e in c id e nt , rNPO h as s t a t e d th at i t i s in no p os iti on t o co ndu c t its own assess m e n t. For t h ose in tereste d, t he c l a im s of the o p erato rs a re s umm a r ize d in e n c l os ur e 2 to N R C AD A M S d oc um e nt MLI 101 4 01 0 4 a nd a re a n a l yze d in d e t a il in ADAM S d oc um e n t ML102640674. 1 9 2 0 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: 1 5 10: 1 8 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 Fiss i on Heat Rate (NFHR) for this shutdown was 1. 75% o f ra t e d reactor power (about62MWth). The Po i nt o f A dd ing Heat (POAH) was reached around 10: 23. IRN I currents at the POAH correspond to a fission power o f about 2.4 MWth. 2.8E-06 IRNI 2.8E-07 2.8E-08 Figure 4: Logarithmic plot s of Total Power (as represented by 6T instrument reading s) and fission power (as resented b y Intermedi ate Ran ge Nuclear In s trument currents). Starting around 5% rated reactor power , as fission power lower s exponentially , total power asymptotically approaches the Non-Fission Heat Rate (NFHR). The match between fi ss ion power a nd total power ha s a stro n g impact on Temperature-Reactivity feedback causin g it to degrade upon entry into MODE 2-Descending a nd causing it to completely disappear at the Point of Addin g Heat (POAH). Althou g h temperature continue s to directly affect reactivity as the NFHR i s approached , ture-Reactivity i s lo s t becau se fa llin g fission power from a negative reactivity insertion do es not immediately fect temperature s ince non-fission heat sources "buffe r" temperature from dramatically lowerin g.

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-Fis s ion 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 s ustained 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 repair s in MODE 2-Descending.

By prudently conducting the turbine repairs in MODE 1 , Callaway Plant learned from its past mistakes and s et 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 i s 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 deca y heat and other fission heat sources , both primary calorimetric (e.g. L1 T instruments) and s econdary calorimetric instrumentation a re poor indicators of fis s ion power in MODE 2. Due to cold-leg shielding and C OMMUNICATOR 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 a s cension 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 pas s ive reactor shutdown , the organization wa s unaware that it had an event which it could analyze for " problems" needin g 2 1 22 COMMUN ICATOR " re so lution." The purpo se of writing a condition report i s 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 e dge weaknesses, unreali stic management tations, e tc.) and the se problem s can then be ly ze d 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 ur e." The general operating procedure for ducting the down power and reactor shutdown was poorly s tructured. The procedure assumed that in order to stop the down power the operators needed to do nothing more than dela y 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 procedur e 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. affec t the NFHR and deca y gammas h ave on total power met e rs) in MODE 2-De sce nding. Management expectat ions were unr ea li s tic; it was unreali s tic to ex p ect the crew, with procedural guidance written for a continuous (i.e. " non-seg mented") s hutdown , to be able to hold pow e r at 10% power during the seve re xenon tran s ient which i s 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 Octob e r 21 , 2003 pa ss ive reactor s hutdown was not document e d until it was dentally uncovered 40 month s after the fact, these gross procedur al d efic ienci es and unreali st ic agement expectations went uncorrected until 2007. On June 17 , 2005 a s imilar pa ss ive reactor sdown occurred durin g a forced de-rate for a failed power supply in a n E ngineered Safeguards ture (ESF) cabinet. Durin g this de-rate , the tor pas sive ly s hut down du e to a 2°F spike in T avg w hich occurred upon manuall y tripping the main t urbin e. The s hutdown occurred two minute s prior to the failed power s uppl y being s ucce ssfu lly retested and 54 minut es prior to the expiration of the s hutdown action of the Technical Specifica-FALL 20 11 tion. That is, since the broken equipment was scessfully repaired prior to the planned shutdown tim e, had the reactor not pa ss ively s hut down the crew could have immediately returned to power. In s tead , resultant delay s in returning to power lowing the inadvertent passive s hutdown cost the utility 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> of lo s t generation.

Like the 2003 pa ss ive s hutdown , the 2005 pa ss ive shutdown was not documented until it, too , was accidently covered in February 2007. Had the October 21, 2003 pa ss ive reactor s hutdown been evaluated b y the utility's Problem Identification

& Resolution process , it is likely the 2005 pas s ive reactor sdown would ne ve r have occurred.

Although the inadvertent pa ssive shutdown of a commercial PWR might see m like a commercial concern v ice a safety concern, failing to recogni ze it can readily jeopardize r eac tor 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 hour s 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 so urce range. Like the October 2003 pa ssive r eac tor s hutdown a t 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 lnstitut e of Nuclear Power Operation s and shared with the indu stry v ia a Significant Event tion. Sharing OpE with INPO Both the October 21 , 2003 and June 1 7 , 2005 ps ive reactor shutdowns were accidently unco ve red in February 2007 during a review of critical rameter data from pa s t s hutdowns to support a maJor revision to the Re actor Shutdown dure. The two s hutdown s were documented along with seve n other s hutdown s in Callaway Action Rquest 200701278, Anal y sis of Past R eactor downs -RF 15 Pr eparation 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 o c currences 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, "Th e in s p ec tor's r e view of th e op e rating pro ce dur e s did not find an y tim e line s s guidan ce on performing th e st e ps to ins e rt the c ontrol rods." It is unclear why the NRC inspector expected the Reactor Shutdown procedure to contain " tim e lin e ss guidan ce on p e rforming the s t e p s to ins e rt th e 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-2 3 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 th e st e ps to insert the co ntrol 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 licen se d operator that the crew failed to perform flawle ss ly. 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 thi s. It is not a procedure violation to fail to recogni ze a passive reactor s hutdown , it is a human performance error and no mor e. 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 u se one's training and experience to place the plant in an FALL 2011 analyzed condition (e.g. if the plant has passively s hut 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 s hutting 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 "wa nt" 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. (Con tinu e d on n e xt pa ge)

FALL 2011 Operation without SRNis The reactor entered the so urce range a t 10:3 9; ye t , no Source Range Nuclear Instrument (SRNI) gized for another 45 minutes ( 11 :25). Each SRNI at Callaway Plant i s powered through a contact on it s channel's as s ociated IRNI. Thi s contact automatically closes at 5E-l l ica. cause of the subcritica l multiplication afforded by the control bank s still being at their last critical rod height s, both channels of IRNi s were reading greater than 5 E-I I ic a when the reactor first tered the s ource range. It took 45 minutes of tional Xenon-135 buildup for the channel 2 IRNI to lower below 5E-l l ica and 59 minute s for channel I. The SRNis can al so b e manually energized once the PRNI signa l ha s lowered b e low 10% rated reactor power. Had they , prior to I l :25 , ni ze d they were in the so urce range, the operators could have manu a lly energized either or both SRNi s. The fact that they did not do this is one of many indications to thi s author that , prior to the SRNis automatically e nergizing at l 1: 25 , the erators were unaware they were in the so urce ran ge 14* The SRNls add s ignificant defen se in depth during operation in the so urce range by providing:

• s tart up rate indication

• an audible count rate which quickly a l erts the operator to ri s ing reactivity

• a met e r indication better s uit e d for monitoring power in the s ource range than the more broadly ranged IRNI meter s; C OMMUNI CATO R

• an a utomatic high flux reactor trip which i s se t a bout 5 decade s earlier than the IRNI high flux trip; and

• a signal to the Boron Dilution and Mitigation Sy s tem (BDMS) which causes an automatic swap over of charging pump s uction s from the Vo lum e Control T a nk to the Refueling Water Storage Tank (RWST) in the event that source range counts increa se by 70% in a rolling 10 minute period (since the RWST i s borated to -2500 ppm , thi s BDMS circuit provide s tection against inadvertent reactivity additions ca u se d by xenon-135 decay , inadvertent tion s, and inadvertent cooldowns). Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi ze d , this i s so a r eac tor s tart up can be perfonned.

15 During a reactor s tartup, a dministrative contro l s 16 are in place which mitigate the lo ss of safety margin from blocking the automatic safety circuit s driven by the SRNi s. The de s igners of Callaway Plant never intended for the plant to be operated in th e source range with the control rod s at their critical rod heights and with none of the SRNI driven automatic protection s in place. Although th e NRC is technically correct in stat in g that thi s condition did not v iolate the plant's licen s in g requirement s, there i s more to ensuring reactor safe ty than forcing a verbatim interpretation of the Technical Specifications

not all conditions can be exactly defined by the Technical Specifications and a competent profes s ional reactor operator s hould be ab l e to di sce rn when the plant i s in a condition in which the designer never intended. (Continued o n n ex t pag e) 14 Note that from I 0

23 to 11 :25 a ll indi cat i ons other than the IRNi s were stea dil y indi cat in g the plant was l ow in th e power range: the PRNis were r ead in g -I% rated power , the t-.T in stru m ents were reading 1.75% power and the seco nd ary ca l o rimric co mput e r po i nt s were reading 62 MWth. In or d er to realize they were in the so ur ce range the operators wo uld h ave e i t h er h a d to n o t e the !RNI r ea din gs or qu es ti o n why the y h ad not n ee d e d to add po s iti ve reactivity to acco unt fo r xenon buildup. With regard to noting t h e I RNI r ea din gs, because of their units (i on c h a mb er a mp s) a nd their sca lin g (logarithmic) the o p erato r s do not normally u se these instruments w hil e at power. With regard to questioning w h y they had n ot n eeded to dilute or pull rods to make up for xeno n , understanding t h e reactor dynamics of I odi n e/Xe n o n was a weakness of this crew as demonstrat e d by t h eir response to t h e 9°F temper a ture drop wh i c h occurred from 09:36 to I 0:00. 1 5 I t is impossible t o do a s u ccessfu l reactor s tartup without blocking the SRN I flux trip and BDMS; t h erefore, o n ce t h e IRNI sig n a l reaches I E-10 i ca during a r eactor s t artu p , the o p erators are p e rmitt e d to de-e n ergize d the SRNis (w hi c h by then have h ad all their protective functions bl ocked). 16 For examp l e: a R eactor E n gi n eer pres e n t in t h e contro l room, an Est im ated Cr i tica l P ositio n h as been calc ul ated , the crew is intent l y performin g a procedure which warns them to " expect criticality at a n y 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 s tabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback po ses 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 proce ss. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "ge neric mental s" 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 detail s 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 sio nal Reactor Operator Society. To participate in an online analysis of this dent, se nd an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro up s. com (anonymous participation is accommodated).

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

l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator P erformance I ssues Involvin g R eactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at la way , 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 #ML102640674).

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 R egar ding 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 th i s iss u e: NRC Fukushima Task Force Analysis of the 2003 Callaway Shutdown ----------

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

COMMUNICATOR PROFESSIONAL REACTOR OPERATOR SOCIETY PO Box 484 Byron , IL 61010 phone (8 15) 234-8 1 40 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> FAX and a n swe rin g BOARD OF DIRECTOR S President Mitch Taggart -Sequoyah Vice Presid e nt Casey Pfeiffer -Seq u oyah Treas ur er Mark Ra s mu sse n -Byron Sec retary Joe Eva n s -Perry Region I President vacant Region II President Brian Snyder -Oconee Region III President Jim Kelly -P erry Region IV Presid e nt Sheryl Breault -Fort Cal h oun Foreign and Domestic Affairs Pre s ident Bob Meyer -Yogt l e S TAFF Communicator Editor Mary Cay R asmussen Office Ma n ager Mark Rasmu sse n Web master vacant ON TH E COVER: TVA's Bellefonte Nuclear Power Plant i s scheduled to be online between 2018 and 2020. FALL2011 CONTENTS Bell efonte R eco mm e ndation s from the NRC Fukushima Ta sk Force Casey Pfeiff e r NFP A Standard 805 -Fire Prot ec tion Bri a n Snyder Analysis of the 2003 Callaway Shutdown Lawrence C ri sc ion e E thanol -The Answer to Our E n e r gy Needs? Mark R asm u ssen REGULAR FEATURES Members' Pag e Officer R e port s Joe SRO New Members PAGE 2 3 8 11 27 42 46 52 53 A D VERTISE IN THE COMMUNICATOR AD SIZE & PLACEMENT Back Cove r -F ull Page Back Cover -Half Page Inside Cove r -Fu ll P age Inside Cover Half P age Internal -Fu ll Page Int erna l -Half Page Int erna l -Quarter P age SINGLE ISSUE $1000. $ 600. $ 750. $ 400. $ 500. $ 300. $ 200. FULL YEAR (4 ISS UES) $2500. $1500. $1 875. $1000. $1250. $ 750. $ 500. M e mb e r s hip in th e Prof ess i o n a l Re ac t o r Operator Society i s open to a n y individual or co mpan y that s h a r es our goa l of prom o tin g safe reactor* o perati o n. M e mb e r s hip du es a r e $35 per yea r , which includes a s ubscription to th e Communicator.

The Co mmuni ca t or is published quarterly , a nd i s di s tribut e d to m e mb ers b y US m ai l. Additional co pies a r e avai labl e for $I 0 per copy. Inquiri es regarding distribution, d e li ve r y , o r c h a n ge of a ddr ess s hould be dir ec t e d to <thePROSoffi ce@nucpro s.co m>. The Co mmuni ca tor i s copyrighted b y the Profe ss ional Reactor Operator Society and may not be r e produced in who le o r in part without ex pr ess p erm i ss i on. Opinions ex pr esse d by a uthor s are th e ir own a nd d o not necessari l y refle c t th e po s itions of th e Pro fessional R eac tor Operator Society , the Comm uni ca t o r Ed it o r , or th e organization with which the a uth or i s affi liat e d. Publi ca tion of v i ewpo ints or d escriptions of materials , products, or serv i ces s h o uld n ot b e construed as e nd o r se ment s b y the Profe ss ional Reactor Operator Society or it s 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 Calla w a y Plant on O c tob e r 21 , 2003 , w hil e att e mpting to s tabili ze r e a c tor po we r durin g a for ced d e-rat e, X e non-135 buildup c au se d av e rag e r e a c tor c oolant t ep e ratur e to low e r at a 25 minut e p e riod , r es ultin g in an automati c i s olation of th e l e tdo w n sys t e m on lo w pr ess uri ze r wat e r l eve l and op e ration of th e r e a c tor b e lo w th e Minimum T e mp e ratur e for Criti c al Op e ration. Aft e r manuall y trippin g th e turbin e-g e n e rator to s i s t in t e mp e ratur e r ec ov ery, th e r e a c tor pa ss i ve ly s hut down du e to a s harp 4°F ri se in a veage c oolant t e mp e ratur e. For th e n e xt 110 ut es th e op e rator s p e rform ed s e condary and t e rtiary plant s hutdown a c tiviti es w hil e r e l y ing on an informal es timation that X e non-135 l eve l s we re s uffi c i e nt to pr e v e nt th e r e a c tor from v e rt e ntl y r es tartin g. Th e pa ss i ve r e actor sdown was not do c um e nt e d until it wa s un ce r e d 40 month s lat e r, and it wa s not s har e d with th e In s titut e of N ucl e ar Pow e r Op e ration s lo w in g th e r e qu es t w hi ch a cc ompani e d SOER 0 7 -1. Th e in c id e nt hi g hli g ht s th e pitfall s a ss o cat e d with att e mpting to maintain a c omm e r c ial pr ess uri ze d w at er r e a c tor c riti c al during MODE 2-D esce ndin g and d e mon s trat es how c on ce pt s t es t e d o n the N R C G e n e ri c Fundam e ntal s Exam appl y to a c tual r e a c tor op e ration. Th e in c id e nt al s o hi g hli g hts s om e non-c ons e rvativ e r e a c tivity mana ge m e nt pra c ti ces w hi c h mu st b e avoid e d b y Pr o f ess ion a l R eac tor Op e rat o r s. 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 l earned." Detail s of the incident were first pub-1 ical ly released by the Union of Concerned tists (UCS) in a 20 l 0 issue brief1 tit l ed 2003 S em e nt e d Shutdown at Callawa y, and then, in 20 l l , the US Nuclear Regulatory Commission (NRC) partially covered the incident as part of tion Notice 2011-02 , Op e rator P e rforman ce i ssu es Involving R e a c tivity Manag e m e nt at Nucl e ar Pow e r Plant s. 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-Fis s ion Heat Rate has on Temperature-Reactivity feedback. The challenges facing the operator during low power operation due to human factoring of control board instruments. (Co ntinu e d on n ex t pa ge) 1 h tt p://ww w.uc s u sa.o r g/nu c l ea r _powe r/nucl ea r _p owe r_ ri sk/sa fet y/2 00 3-segm ented-s hutd ow n-at-c a l !a w ay.html 2 http://pb a dupw s.nr c.gov/d ocs/ML I 01 8/ML I 01 8 10 282.pdf 11 1 2 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) It em 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 wi ll result in a negative in sertion of activity , thereby limitin g 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 limit ed window of their fuel cyc l e. October 21 , 2003 was l ate in fuel cycle 13 for Ca ll away Plant a nd 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 i s inherently stab l e. That is , the reactor " wa nt s" 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) w hil e the steam demand (i.e. turbine-generator loading) is held constant , reactor power wi ll decrease s li g htl y. Becau s e 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 in s erted, which further dampens the negative reactivity insertion. FALL20ll Temperature w ill continue to lower as long as there is a negative power mismatch. Eventually , eno u g h positive r eactivity w ill 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 an d the magnitude of the negative power mismatch wi ll lo wer , dampening the temperature drop. Once reactor power rises above steam demand , there will be a positive power mismatch which w ill now cause temperature to rise. The rising temperature will insert negative reactivity , causing reactor power to lo wer. These passive feedback processes will continue until , eventua ll y , reactor power again matches steam demand and there is no power mismatch to drive temperature.

At this point, temperature wi ll be l ower than it was prior to the negative reactivity insertion.

ca ll y , the change in temperature is: L'.l T = L'.lp/MTC). The reactor will passively respond to a positive insertion of reactivity in a simi l ar 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 coo l ant temperature (T avg) to the power match induced by the change in reactivity. 2. The passive counter insertion of reactivity due to the temperature response , which continue s 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 eve ntu ally results in the same steady state power at a new temperature. This generic fundamental is demonstrated later in this article by 3 Si n ce Ca ll away Pl a nt is a PWR , th e r eactiv i ty coeffic i e nt du e t o vo id s i s no t di sc u sse d in this a rticl e. 4 P o w e r mi s m a t ch i s t h e di ffe r e n ce b etwee n steam d e m a nd a nd r eac tor po wer.

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). (Co ntin u e d o n n ex t pa ge) 5 power d efe ct i s th e t e rm for th e re a ctivit y in s erted from a change in re a ctor power lev e l. 6 MOD E 1 r e fer s t o the s tate of op e ratin g the re a ctor a t power (5% to 100% rated re a ctor power). 7 MOD E 2 refer s to th e tran s iti o n s t a te b e tw ee n th e r e actor being s olidl y in the pow e r ran g e (i.e. b e yond th e point a t which th e NFHR e x e rts a ny s ub s tanti a l influ e nce) a nd th e r ea ctor b e ing s hutdown (i.e. d e finiti ve ly s ubcritical as indicat e d by ca lcul a tin g K e lT t o b e l ess than 0.9 9). T h e rea c tor e nter s MOD E 2-De sce ndin g when r ea ctor pow e r low e r s b e low 5% rated pow e r. 1 3 14 COMMUN I CATO R FALL 20 11 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% -0 1: 00 02*00 03:00 ' "" K o4: oo ""I' -.. ..........

... ' ..... ""i... '"""'-20% 09:30 09:40 09:5 0 10:00 10:10 10: 20 10:3 0 0 i 10% 120 i s t eps ' l .........

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

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i -...._ ! "-. ' 'l i-f -, , , i j /i -.,. '\ / .. ' 20% % : 10° 0% i 10 0 p s 0°F i s t e .: 56 /55r F //554" F 551°F 560°F 557°F 554°F 551°F 1 00 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 Coo lant Temperature (T avg), Prim a r y Ca lorim e tri c pow e r (LiT) and Co ntrol B a nk 'D' rod h e i g ht s durin g th e October 21 , 2003 down pow er a nd pa ss i ve reactor s hutdown. Note th e seve re t e mperatur e transient which b ega n at 09:36. Turbine fir s t s t age s t e am pr ess ure data indi ca te s that th e operators s topp e d low e rin g ge n erato r lo a din g at 09:36 with reactor power a t 9%. Ov e r th e n ex t thr ee minut es, n ega ti ve reactivity du e to Xenon-135 ca u sed p owe r to co ntinu e to lo wer a noth e r I%. The p ower mismatch b e tw een the s t eam d e m a nd ed b y th e turbine throttle set p oi nt a nd the power b e in g produc ed b y fission ca u sed T avg to imm e di a t e l y b eg in to low e r , thereby in sert in g po sitive reactivity which co unt e r e d th e n ega tiv e rea c ti v it y b e ing in ser t ed b y the co ntinual buildup of Xenon-135. Around 09:39 the positive r eact ivit y bein g in se rt ed b y th e low e rin g t e mperatur e mat c h e d th e n ega ti ve r ea cti v it y b e in g in se rt ed b y Xenon -135 ca u sing r eactor p ower (as indicated b y core Li T) to s tabili ze a t a ppro x im a t e l y 8%. With a I% p owe r mi smatc h se nt , over the n ext twenty minut es T avg co ntinu ed t o stea dil y l owe r a nd thereby co unt erac t the continual buildup of xenon. Shortly afte r I 0:00 the c r ew again b ega n to low er turbine-generator lo ad in g in res pon se to th e Shift Manager's d ec i sio n to take th e turbine off-line following the l e tdown i so lation. The ren ewe d low e rin g of ge nerator loadin g caused s t ea m d em a nd to low er b e l ow fission pow e r a nd th e r e by a llo wed T avg to temporarily recover s li g htl y. Durin g this time period (I 0:03 to I 0:09), t h e n egative reactiv it y b e in g in se rt ed b y Xenon-135 was no w b ei n g counteracted b y the po s iti ve ity being in se rt ed by t h e l oad decrease (t h e pl a nt h ad a n egative p ower coefficie n t of reactivity).

Ge n erator l oad in g was aga in sta bili ze d aro und I 0:09 ca u s in g T avg to r es um e fa llin g, which i s the ex p ecte d passive respo n se of the r eactor plant to Xenon-135 buildup. The o p erators failed to gras p th e r eac tor d y n a mic s b e hind the transient and ass um e d th e I 0°F drop in T avg was b ei n g caused by malfunctioning steam line a nd turbine drain va l ves (w hi c h had co in c id e ntall y been placed in service at abo ut the same time the temperature transient b ega n).

FALL201 l By the time the reactor at Callaway Plant pa ssively 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 ments 8) 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 C OMMUNICATOR 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 s ince they were 2 Y2 hours ahead of schedule it is likely they intended to hold power at while 8 Th e re a r e thre e se t s o f nucl e ar in s trument s (th e power rang e, intermediat e ran ge a nd s our ce ran ge). Th e nu c l e ar in s trum e nt s m e a s ur e fi ss i o n rat e b y d e t e cting s tr ay n e utron s produ ced b y fi ss i o n. 9 T h e re a r e t wo se t s of ca lorim et ri c in s trum e nts a t Ca llaw ay Plant: L'.T in s trum e nt s (prim a r y ca lorim e tri c ca l c ul a t e d fro m th e t e mp e ratur e ri se acro ss the c ore) a nd th e rmal output c omput e r points (calcul a t e d from a se condary ca lorim e tri c). 15 16 COMMUNICATOR further troubleshooting occurred on the failed verter. Also around 09:36 the operators cycled the Group B turbine drains. One of th e switches for the drain s 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 b ee n occurring for the past 8 Yi 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 s teps 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 2 Y2 hours of the downpower , when the rate of xenon buildup was s till 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 pas s ively inserted from the downpower.

However , since Xenon-135 was still building up , negative reactivity was st ill being passively serte d. The crew did not have a detailed ity Management Plan 10 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 (Tav g) 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 , T avg lowered slightly.

Due to the -MTC , the lowering T avg 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 ofT avg* 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 T avg had lowered 9°F and the letdown system automatically isolated on low pressurizer water level. Also by 10: 00 , the crew recognized that T avg 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 turbin e be taken off-line.

Manual Turbine Trip and MODE 2 Entry After the letdown isolation, th e operators began lowering turbine-generator loading in preparation for removing the turbine from service. This caused a positive power mismatch which tempo-'0 T hose with access to proprietary documents from the World Association of N u clear Operators shou ld see the d a tions contained in WA 0 SOER 2007-1, R eac tivi ty Management, for expectations regarding Reactivity Management Pl a ns. 11 In the 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> s ince relieving th e watch at 06:3 0 the crew had n eeded to insert control rod s 30 steps in order to keep Tavg lowering a t the programmed rate. No active insertions of positive reactivity had been required to overcome X e non-135.

FALL 2 011 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 .... !Tre f (linear scale , 5°F/d i vis i on) ! ;:::::: ..:.-.:.:.-..:.*.:.:. I i--r--,.,., O'I 0 0 0 .-i .-i .-i ------C OMMUNICATOR 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 I n !Tavg (linear scale , 5°F/divisionl l Ff' --/ I I I I ------n o lo a d Ta v g (55 1'F) --------' ..... 5 50°F MT CO (55 1° F). L#------------l V 5 60°F 5 5 5°F 5 S0°F 10% 5% 10% AT 5% ... ftl " 1. E-04 u ... ::s 0. -..... I LH (linear scale , 5% rated ' --!NFHR*------; 0% ' ' "' ' ........_. ' ' 1.E-0 5 "' E c ftl " " a: E .a A verage Coolan t Temperature (Tavg) rose steeply -*---P OA H l.E-0 6 1. E--07 " 2 E ........ ftl fo llowing t he turbine trip (TT). The quick negative "' ftl ... .c *-c u c r e ac t iv i t y insertion which accompani ed th e -4°F spik e i n -""' l.E--0 8 Tavgcaused the reactor t o become s ubs t an t ially '-E .2 ... -1. E--09 ! subcr i tical and shut down. The POAH and a nom i nal I *<6' ** c -1/3 dpm SUR were reached around 10: 23. 1.E-11 % -1.E-1 0 l.E-11 Figure 2: Plot of Average Coolant Temperature (T a v g), Primary Calorimetric power (.1T) and Intermediate clear Instrument current s (IRNI) on October 21 , 2003. The sharp rise in T a vg 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 pa s sively 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 s ourc e range. See Figure 3 for plant evolutions occurring during this time frame. rarily caused T av g to s top lowering (the minimum T a v g 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 T av g 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 R e a c tor Pow e r c on s tant b y tran s f e rring load to the c ond e n se r s t e am dump s whil e r e du ci n g bin e Load. This w ill pr eve nt inadv e rt e nt e nt ry i nto Mod e 2 w h e n th e Turbin e i s tripp e d." Within 30 seconds of tripping the main turbine , reactor power lowered below 5% and the tors declared MODE 2. Rap id Ri se in T avg a nd P ass i ve S hutdo w n With the condenser steam dumps set to modulate at 1092 psig, upon tripping the turbine there was no steam demand until T avg rose to 557°F (corresponding to a steam pressure of 1092 psig). With the reactor initially around 6% power and with no steam demand , T a vg 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 decade s per minute (dpm); by 10: 18 SUR was -0.16 dpm -a chang e of 1600%. (C ontinu e d on n ex t pa ge) 1 7 18 COMMUN I CATOR FALL 2011 As the reactor neared the Non-Fission Heat Rate ( 1. 75% rated reactor power for thi s shutdown), temperature-reactivity feedback was lo s t (see ur e 4 on page 20); that is , lowering reactor power would no longer feed back po si ti ve reacti v ity via low e rin g temperatur

e. Thus , without a m a nual in se rtion of po s iti ve reactivity , power would tinue to lower into the so urce range. to lower to Y2 it s initial value, fission pow er (as indic a ted by IRNI currents) low e red to 1/6 its tial va lue. Thi s is further indication the fission reaction had s hut down and the Non-Fission Heat Rate was raising/maintaining reactor coolant tperatur e. Response to the Passive Shutdown A t 10: 13 , the instruments had indicated 5.17% and th e Intermediate Ran ge Nuclear In s trument s (IRN l s) had indicated l .52 E-5 ion chamber amps (ica). By I 0: 18 , T instruments indic a ted 2.4% and the IRNi s indicated 2.43E-6 ica. So in the time it took total power (as indicated by core While the reactor was passively s hutting down , the operators were performing the off-normal cedure for " Lo ss of L e tdown" (w hich had been entered at 10: 00). At 10: 1 8, a 75 gpm l e tdown orifice was plac e d in service and th e crew ex it e d the off-norma l procedure.

By thi s point (10: 18), had they recogni ze d the reactor was s hut 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 10:3 0 10:45 11 :00 11 : 15 11 :30 11 : 45 12:00 12:15 Figure 3: Pl ot of Co nt ro l Bank rod h eig ht s and Int e rm e di ate R a n ge (IRNI) currents on October 2 1 , 2003. The reactor passively sh ut down short l y afte r the turbine was manually tripped at l 0: l 3 an d reached the so ur ce range about 26 minute s l ater. A n ominal -1 /3 dpm SUR developed as power fe ll below the PO AH. The s li ght drop in reactor power from I 0: 39 to 1 2:05 was ca u sed by a l owe rin g of subc riti ca l multiplication resulting from the co ntinued buildup of Xenon-135. The operato r s began in se rting the control b a nk s at 1 2:05 a nd completed at 1 2: 15.

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

After exiting the off-norm a l procedure for " Loss of Letdown" the Control Room Supervisor signed the Reactor Operator the ta s k 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 wa s prioritized over actively controlling core reactivity (i.e. over inserting the contro l bank s to en s ure the reactor remained shutdown). Thi s ta s k involves multiple manipulations of charging system components and took 30 minutes to comp l ete; in comparison , manuall y driving in the control banks takes 10 minute s. As reactor power was decaying through five a de s of power to reach the s ource range , licensed Reactor Operators were assigned to place Cooling Tower Blowdown in s ervice and to s ecure th e ond of three intake pumps (cooling Tower down had been s ecured a coup l e of hours earlier to support Chemistry surveillances and the intake pump was s ecured becau s e two pumps were no longer ne eded due to the forced de-rate causing evaporation rate to lower). These tasks were both logged compl e te at l 0:34. It i s unclear why these ta s ks wer e prioritized over in s erting the control bank s. Operation in the So urc e Rang e At 10: 39 , reactor pow e r entered the s ource range , a s evident on Fi g ure 3 (page 18) by the IRNI rents s tabilizing.

As at mo s t reactor plants , the C OMMUNICATOR Source Range Nuc l ear Instruments (SRNl s) at Callaway remain de-energized until bistable s on the IRNis validate reactor power is in the source range. Because the control rods were still at their l a s t critical rod heights , there wa s more subcritical multiplication than is normally pre s ent when these lRNI bistables are calibrated.

As a result , the SRNis did not energize upon initially entering the source range. It took 45 minute s of additional Xenon-135 buildup to lower subcritical cation to the point at which the first SRNI channel was able to automatically energi z e. At 11:01 a 1 icensed operator was as s igned to scure the second of three condensate pump s. It is unclear why , while in the s ource range with no SRNis energized and with the control rods s till at their last critical rod height s, the lic e nsed tors prioritized manipulation of the conden s ate s ystem over inserting the control banks. To s ome (e.g. thi s author) the crew's action s indicate that they were unaware the reactor had s ively shut down. That i s, the most reasonable explanation for the crew " prioriti z ing" ancillary tasks 12 over deliberate contro l of the nuclear fi ss ion reaction i s that for 67 minute s they failed to recognize the reactor had shut down.13 At 11 : 25 the channe l 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 sourc e range. At 11 : 3 8 the channel 1 SRNI energized. (Continu e d on n ex t pa ge) 1 2 Fo r exa mpl e: pl ac ing a n ext ra 4 5 gpm l e td ow n o rifi ce in se r v i ce, pla c ing C oolin g To w e r Sl o wdown in se r v ic e , sec urin g unn ecessa r y in ta ke a nd co nd e n sa t e pump s. A lth o u g h o pt i mi z in g wa t e r c h e mi s tr y of t h e prim a r y pl a nt a nd coo l i n g tower i s imp o rt a nt a nd a lth o u g h minimi z in g " h o u se" e l ec tri c l oa d s b y sec urin g l a r ge a nd n o l o n ger n ee d e d pump s i s imp o rt a nt , th ese t as ks a r e " a ncill a r y" w ith r ega rd to th e prim a ry fo c u s of t h e r eac t o r s hutdo w n pro ce dur e: in se rtin g th e co ntrol b a nk s t o d efi niti ve ly e n s ur e th e r ea ct or i s in a s hutd o wn c o nditi o n a nd w ill r e m ain in th a t s t a t e r ega rdl ess of p ass iv e (e.g. xe n on d ecay) o r un ex p ec t e d (e.g. in a d ve rt e nt diluti o n s or coo ld o wn s) c h a n ges in co r e r eac ti v it y. 13 It s h o uld b e n o t ed h e re t h a t the c r ew h as co n s i s t e ntl y asse rt e d th a t pri or t o m a nu a ll y trippin g th e turbin e th ey we r e awa r e th e r eac t o r would p ass i ve ly s hut d o wn o n ce s t e am d e m a nd w as r e mov e d. T hi s asse rti o n a mount s t o the c r ew d e lib era t e l y a ll ow ing t h e r eac t o r to p ass i ve ly s hut d ow n w hil e th ey p e r fo rm e d the a n c ill ary it e m s m e ntion e d in not e 1 2. T he a uth o r of thi s artic l e be li eves t h a t , if t ru e , t h is a m o unt s to in co mp ete n ce. T h at i s, i t is in co mp ete nt fo r a n N R C li ce n se d o p erato r to pri o riti ze a n c ill ary tas k s over d e lib era t e l y co ntr o llin g th e r eac t o r, a nd it i s in co mp e t e nt t o d e lib e rat e l y re ly on p ass i ve mur es to s hu t d ow n t h e reac t o r w h e n act i ve m ea n s (e.g. ro ds a nd b o ron) are ava il a bl e. S in ce th e US N R C h as r e fu sed t o quti o n t he o p era t ors' asse rti o n s , a t t hi s po int th e qu es ti o n re m a in s unr eso l ve d as to w h e th e r or n o t , pr ior t o th e SRN i s e n e r g ii n g , t he o p era t o r s were awa re t h e r eactor h ad p ass i ve ly s hu t d ow n. A lth o u g h t h e In s titut e of N u c l ea r Power Op erat i o n s (fNP O) i s awa r e of t h e di sc r e p a n c i es s urr o unding t h e O c t o b e r 2 I , 2 00 3 s hutd o wn , rNPO h as s imil a rl y d ec lin ed t o eva lu a t e t he c l a im s m a d e by the o p era t o r s; s i nce rNPO mu s t re ly o n A m e r en t o vo lun ta ril y r e p o rt th e in c id e nt , rNPO h as s t a t e d th at i t i s in no p os iti on t o co ndu c t its own assess m e n t. For t h ose in tereste d, t he c l a im s of the o p erato rs a re s umm a r ize d in e n c l os ur e 2 to N R C AD A M S d oc um e nt MLI 101 4 01 0 4 a nd a re a n a l yze d in d e t a il in ADAM S d oc um e n t ML102640674. 1 9 2 0 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: 1 5 10: 1 8 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 Fiss i on Heat Rate (NFHR) for this shutdown was 1. 75% o f ra t e d reactor power (about62MWth). The Po i nt o f A dd ing Heat (POAH) was reached around 10: 23. IRN I currents at the POAH correspond to a fission power o f about 2.4 MWth. 2.8E-06 IRNI 2.8E-07 2.8E-08 Figure 4: Logarithmic plot s of Total Power (as represented by 6T instrument reading s) and fission power (as resented b y Intermedi ate Ran ge Nuclear In s trument currents). Starting around 5% rated reactor power , as fission power lower s exponentially , total power asymptotically approaches the Non-Fission Heat Rate (NFHR). The match between fi ss ion power a nd total power ha s a stro n g impact on Temperature-Reactivity feedback causin g it to degrade upon entry into MODE 2-Descending a nd causing it to completely disappear at the Point of Addin g Heat (POAH). Althou g h temperature continue s to directly affect reactivity as the NFHR i s approached , ture-Reactivity i s lo s t becau se fa llin g fission power from a negative reactivity insertion do es not immediately fect temperature s ince non-fission heat sources "buffe r" temperature from dramatically lowerin g.

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-Fis s ion 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 s ustained 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 repair s in MODE 2-Descending.

By prudently conducting the turbine repairs in MODE 1 , Callaway Plant learned from its past mistakes and s et 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 i s 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 deca y heat and other fission heat sources , both primary calorimetric (e.g. L1 T instruments) and s econdary calorimetric instrumentation a re poor indicators of fis s ion power in MODE 2. Due to cold-leg shielding and C OMMUNICATOR 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 a s cension 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 pas s ive reactor shutdown , the organization wa s unaware that it had an event which it could analyze for " problems" needin g 2 1 22 COMMUN ICATOR " re so lution." The purpo se of writing a condition report i s 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 e dge weaknesses, unreali stic management tations, e tc.) and the se problem s can then be ly ze d 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 ur e." The general operating procedure for ducting the down power and reactor shutdown was poorly s tructured. The procedure assumed that in order to stop the down power the operators needed to do nothing more than dela y 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 procedur e 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. affec t the NFHR and deca y gammas h ave on total power met e rs) in MODE 2-De sce nding. Management expectat ions were unr ea li s tic; it was unreali s tic to ex p ect the crew, with procedural guidance written for a continuous (i.e. " non-seg mented") s hutdown , to be able to hold pow e r at 10% power during the seve re xenon tran s ient which i s 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 Octob e r 21 , 2003 pa ss ive reactor s hutdown was not document e d until it was dentally uncovered 40 month s after the fact, these gross procedur al d efic ienci es and unreali st ic agement expectations went uncorrected until 2007. On June 17 , 2005 a s imilar pa ss ive reactor sdown occurred durin g a forced de-rate for a failed power supply in a n E ngineered Safeguards ture (ESF) cabinet. Durin g this de-rate , the tor pas sive ly s hut down du e to a 2°F spike in T avg w hich occurred upon manuall y tripping the main t urbin e. The s hutdown occurred two minute s prior to the failed power s uppl y being s ucce ssfu lly retested and 54 minut es prior to the expiration of the s hutdown action of the Technical Specifica-FALL 20 11 tion. That is, since the broken equipment was scessfully repaired prior to the planned shutdown tim e, had the reactor not pa ss ively s hut down the crew could have immediately returned to power. In s tead , resultant delay s in returning to power lowing the inadvertent passive s hutdown cost the utility 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br /> of lo s t generation.

Like the 2003 pa ss ive s hutdown , the 2005 pa ss ive shutdown was not documented until it, too , was accidently covered in February 2007. Had the October 21, 2003 pa ss ive reactor s hutdown been evaluated b y the utility's Problem Identification

& Resolution process , it is likely the 2005 pas s ive reactor sdown would ne ve r have occurred.

Although the inadvertent pa ssive shutdown of a commercial PWR might see m like a commercial concern v ice a safety concern, failing to recogni ze it can readily jeopardize r eac tor 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 hour s 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 so urce range. Like the October 2003 pa ssive r eac tor s hutdown a t 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 lnstitut e of Nuclear Power Operation s and shared with the indu stry v ia a Significant Event tion. Sharing OpE with INPO Both the October 21 , 2003 and June 1 7 , 2005 ps ive reactor shutdowns were accidently unco ve red in February 2007 during a review of critical rameter data from pa s t s hutdowns to support a maJor revision to the Re actor Shutdown dure. The two s hutdown s were documented along with seve n other s hutdown s in Callaway Action Rquest 200701278, Anal y sis of Past R eactor downs -RF 15 Pr eparation 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 o c currences 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, "Th e in s p ec tor's r e view of th e op e rating pro ce dur e s did not find an y tim e line s s guidan ce on performing th e st e ps to ins e rt the c ontrol rods." It is unclear why the NRC inspector expected the Reactor Shutdown procedure to contain " tim e lin e ss guidan ce on p e rforming the s t e p s to ins e rt th e 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-2 3 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 th e st e ps to insert the co ntrol 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 licen se d operator that the crew failed to perform flawle ss ly. 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 thi s. It is not a procedure violation to fail to recogni ze a passive reactor s hutdown , it is a human performance error and no mor e. 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 u se one's training and experience to place the plant in an FALL 2011 analyzed condition (e.g. if the plant has passively s hut 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 s hutting 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 "wa nt" 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. (Con tinu e d on n e xt pa ge)

FALL 2011 Operation without SRNis The reactor entered the so urce range a t 10:3 9; ye t , no Source Range Nuclear Instrument (SRNI) gized for another 45 minutes ( 11 :25). Each SRNI at Callaway Plant i s powered through a contact on it s channel's as s ociated IRNI. Thi s contact automatically closes at 5E-l l ica. cause of the subcritica l multiplication afforded by the control bank s still being at their last critical rod height s, both channels of IRNi s were reading greater than 5 E-I I ic a when the reactor first tered the s ource range. It took 45 minutes of tional Xenon-135 buildup for the channel 2 IRNI to lower below 5E-l l ica and 59 minute s for channel I. The SRNis can al so b e manually energized once the PRNI signa l ha s lowered b e low 10% rated reactor power. Had they , prior to I l :25 , ni ze d they were in the so urce range, the operators could have manu a lly energized either or both SRNi s. The fact that they did not do this is one of many indications to thi s author that , prior to the SRNis automatically e nergizing at l 1: 25 , the erators were unaware they were in the so urce ran ge 14* The SRNls add s ignificant defen se in depth during operation in the so urce range by providing:

• s tart up rate indication

• an audible count rate which quickly a l erts the operator to ri s ing reactivity

• a met e r indication better s uit e d for monitoring power in the s ource range than the more broadly ranged IRNI meter s; C OMMUNI CATO R

• an a utomatic high flux reactor trip which i s se t a bout 5 decade s earlier than the IRNI high flux trip; and

• a signal to the Boron Dilution and Mitigation Sy s tem (BDMS) which causes an automatic swap over of charging pump s uction s from the Vo lum e Control T a nk to the Refueling Water Storage Tank (RWST) in the event that source range counts increa se by 70% in a rolling 10 minute period (since the RWST i s borated to -2500 ppm , thi s BDMS circuit provide s tection against inadvertent reactivity additions ca u se d by xenon-135 decay , inadvertent tion s, and inadvertent cooldowns). Although the Technical Specification s for Callaway Plant permit operation in the source range with the SRNis de-energi ze d , this i s so a r eac tor s tart up can be perfonned.

15 During a reactor s tartup, a dministrative contro l s 16 are in place which mitigate the lo ss of safety margin from blocking the automatic safety circuit s driven by the SRNi s. The de s igners of Callaway Plant never intended for the plant to be operated in th e source range with the control rod s at their critical rod heights and with none of the SRNI driven automatic protection s in place. Although th e NRC is technically correct in stat in g that thi s condition did not v iolate the plant's licen s in g requirement s, there i s more to ensuring reactor safe ty than forcing a verbatim interpretation of the Technical Specifications

not all conditions can be exactly defined by the Technical Specifications and a competent profes s ional reactor operator s hould be ab l e to di sce rn when the plant i s in a condition in which the designer never intended. (Continued o n n ex t pag e) 14 Note that from I 0

23 to 11 :25 a ll indi cat i ons other than the IRNi s were stea dil y indi cat in g the plant was l ow in th e power range: the PRNis were r ead in g -I% rated power , the t-.T in stru m ents were reading 1.75% power and the seco nd ary ca l o rimric co mput e r po i nt s were reading 62 MWth. In or d er to realize they were in the so ur ce range the operators wo uld h ave e i t h er h a d to n o t e the !RNI r ea din gs or qu es ti o n why the y h ad not n ee d e d to add po s iti ve reactivity to acco unt fo r xenon buildup. With regard to noting t h e I RNI r ea din gs, because of their units (i on c h a mb er a mp s) a nd their sca lin g (logarithmic) the o p erato r s do not normally u se these instruments w hil e at power. With regard to questioning w h y they had n ot n eeded to dilute or pull rods to make up for xeno n , understanding t h e reactor dynamics of I odi n e/Xe n o n was a weakness of this crew as demonstrat e d by t h eir response to t h e 9°F temper a ture drop wh i c h occurred from 09:36 to I 0:00. 1 5 I t is impossible t o do a s u ccessfu l reactor s tartup without blocking the SRN I flux trip and BDMS; t h erefore, o n ce t h e IRNI sig n a l reaches I E-10 i ca during a r eactor s t artu p , the o p erators are p e rmitt e d to de-e n ergize d the SRNis (w hi c h by then have h ad all their protective functions bl ocked). 16 For examp l e: a R eactor E n gi n eer pres e n t in t h e contro l room, an Est im ated Cr i tica l P ositio n h as been calc ul ated , the crew is intent l y performin g a procedure which warns them to " expect criticality at a n y 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 s tabilizing the reactor at low power levels following a power, the challenge that loss of Reactivity feedback po ses 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 proce ss. For PWR trainers/ operators, data from the incident provide practical demonstrations of many of the "ge neric mental s" 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 detail s 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 sio nal Reactor Operator Society. To participate in an online analysis of this dent, se nd an email to: RCSOTP _16_Reactivity Con tro 1-s u bscri be@yah oogro up s. com (anonymous participation is accommodated).

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

l. US Nuclear Regulatory Commission, mation Notice 2011-02, Operator P erformance I ssues Involvin g R eactivity Management at Nuclear Power Plants, January 31, 2011. 2. Union of Concerned Scientists, Issue Brief 20101100, 2003 Segmented Shutdown at la way , 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 #ML102640674).

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 R egar ding 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.