IR 05000397/1992042
| ML17289B058 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 12/01/1992 |
| From: | Johnson P NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | |
| Shared Package | |
| ML17289B057 | List: |
| References | |
| 50-397-92-42-EC, NUDOCS 9212280296 | |
| Download: ML17289B058 (24) | |
Text
ENCLOSURE 2 U.S.
NUCLEAR REGULATORY COMMISSION
REGION V
Report No:
Docket No:
License No:
Licensee:
Facility Name:
Meeting at:
Date of Meeting:
Prepared by:
50-397/92-42 50-397 NPF-21 Washington Public Power Supply System P. 0.
Box 968 Richland, WA 99352 Washington Nuclear Project No.
(WNP-2)
Region V Office, Walnut Creek, California November 10, 1992 K. E. Johnston, Project Inspector Approved by:
P.
H.
hnson, Chief Reacto Projects Section
Date Signed Enforcement Conference on November
1992 Re ort No. 50-397 92-42 An Enforcement Conference was held on November 10, 1992, with the Washington Public Power Supply System (Supply System) to discuss issues related to the August 15, 1992 reactor power oscillation event.
A copy of information provided by the licensee at the conference is included as Enclosure 3.
92i2280296 92i20i PDR ADOCK 05000397
PDR,
ETAILS 1.
eet'n artici ants uclear Re ulator Commission J.
B.
K. E.
S. A.
F.
R.
J.
G.
L. F.
p.
H.
J.
W.
L. E.
W. P.
K. E.
Hartin, Regional Administrator Perkins, Director, Division of Reactor Safety and Projects Richards, Deputy Director, Division of Reactor Safety and Projects Huey, Enforcement Officer, RV Luehman, Enforcement Officer, Office of Enforcement (via Telephone)
Hiller, Chief, Reactor Safety Branch Johnson, Chief, Reactor Projects Section
Clifford, Project Hanager, NRR Phillips, Chief, Core Performance Section, Reactor Systems Branch, NRR Ang, Acting Senior Resident Inspector Johnston, Project Inspector Washin ton ublic Power Su
S ste A. L. Oxsen, Acting Hanaging Director J.
V. Parrish, Assistant Hanaging Director, Operations J.
C. Gearhart, Director, guality Assurance J.
W. Baker, WNP-2 Plant Hanager C. H. Powers, Director, Engineering G.
C.
Sor ensen, Regulatory Program Hanager H. J. Davidson, Staff Attorney Aha<<d D. L. Williams, Nuclear Engineer, Bonneville Power Authority 2.
Back round On August 15, 1992, Washington Nuclear Project-2 (WNP-2) experienced reactor power oscillations of approximately 25K power peak-to-peak.
With reactor power at 33.5X and operators decreasing core flow in anticipation of shifting the "A" recirculation pump to fast speed, operators observed power oscillations and initiated a manual scram.
The NRC dispatched an Augmented Inspection Team (AIT) to the site to review the cause of the event and evaluate its significance (Inspection Report 50-397/92-30).
The AIT concluded its inspection on August 29, 1992.
A special inspection was conducted from October 5 through October 21, 1992 to review issues identified by the AIT (Inspection Report 50-397/92-37).
The special inspection identified four apparent violations involving multiple barriers that failed to preclude the event.
In addition, it identified one related violation regarding the failure to adjust the Average Power Range Monitor flow-biased scram setpoints and one deviation regarding the Supply System's failure to fulfillits commitment to install and use the stability monitor required by the Technical
Specifications.
An enforcement conference was held with the licensee on November 10, 1992 to discuss the apparent violations and deviation.
fo cement Conference The enforcement conference convened at 8:30 a.m.
Hr. Martin opened the meeting by stating its purpose:
to come to a mutual understanding of the causes of the August 15, 1992 reactor power oscillation event, to understand the Supply System's point of view on the issues presented by the AIT and the special inspection, and to discuss the significance of the event.
Hr. Oxsen stated that the Supply System had taken this event very seriously and was moving quickly to understand the causes of the event and take rapid corrective actions.
Mr. Oxsen noted that the Supply System had a difference of opinion on two of the violations discussed in the special inspection report.
However, he emphasized that this difference of opinion did not detract from how serious the Supply System considered the overall issues to be.
Hr. Johnson summarized the apparent violations related to the reactor power oscillation event, the apparent violation concerning the flow biased scram setpoint, and the apparent deviation concerning installation of the stability monitor.
Mr. Perkins stated that the NRC considered this to be a significant safety event.
He stated that if the core power oscillations had been out-of-phase, the event may not have been readily identified and mitigated.
He observed that the root of the problems identified by the inspections could be attributed to a lack of sufficient management oversight.
Additionally, the Supply System failed to adequately assess industry guidance and did not perform adequate analyses of core stability.
Mr. Parrish summarized the Supply System's position on each of the apparent violations and the apparent deviation.
Hr. Parrish indicated that the Supply System did not feel that the failure of the Nuclear Safety Assurance Group (NSAG) to review the Boiling Water Reactor Owners'roup (BWROG) guidance on core stability was a Technical Specification violation as indicated in Inspection Report 92-37.
In addition, the Supply System did not feel that they had violated
CFR 50, Appendix A, General Design Criterion
(GDC 12).
Hr. Hartin asked if the Supply System had determined who in the organization was responsible for the reactor core design.
Hr. Powers stated that as the Director of Engineering, he was responsible, and that Engineering had failed to identify constraints for rod control patterns during startup, providing too much flexibilityto the Shift Nuclear Engineer.
Mr. Oxsen added that there had been no independent party reviewing core design.
Hr. Powers stated that the generic response to this event should include the new technology available in assessing core stability.
In addition, many BWR operators applied approximately the same control to startup that WNP-2 had.
Hr. Powers stated that the Supply System would be providing these observations to the BWROG.-
-3-Hr. Hartin asked who had performed the core stability analysis and what was the Supply System's assessment of its quality.
Mr. Powers stated that Siemens Power Corporation had performed the analysis as prescribed by the Supply System using a licensed code and looking at the requisite stability areas.
Hr. Phillips noted that Siemens may have used the code inaccurately.
Siemens had modeled its 9X9 fuel assemblies as an 8X8 geometry since the code was not designed to assess reactor cores with mixed fuel geometries such as the WNP-2 Cycle 8 core.
Hr. Powers indicated that the work had been performed over two years ago and that the Supply System had performed limited confirmatory analysis.
Hr. Powers stated that despite the problems with the analysis of core stability, calculations indicate that the aggressive rod pattern used during the startup consumed 75X to 80X of the margin to core instability, and that is where the Supply System has initially focused its efforts.
Mr. Johnson noted that there appears to have been little communication between Station Technical and Design Engineering regarding control of control rod movement.
Hr. Powers stated that although this is true for core instability, there appears to have been adequate communications regarding other core control factors.
Mr. Johnson asked if the issue of instability had been raised.
Mr. Powers stated that Siemens had reviewed the issue but concluded that the new core would not be less stable.
Mr.
Powers indicated that the Supply System should have pursued this issue.
Hr. Baker stated that a questioning attitude had not been developed and that the Supply System had become too comfortable with the stability limits provided in the Technical Specifications.
Mr. Ang indicated that during his review he had observed that a questioning attitude had been demonstrated on several of the aspects of changing to a 9X9 fuel assembly geometry; however, instability was not one of them.
There was further discussion of the two apparent violations with which the licensee had taken issue.
The Supply System argued they had complied with GDC 12.
They further stated that the actions taken by the operator during the event and the licensee's determination that an out-of-phase core oscillation event was detectable support the argument that operators would have taken appropriate action to scram if the core oscillations had been out-of-phase.
The Supply System concluded that since operators demonstrated that they could detect and mitigate a core oscillation event, they had complied with GDC 12.
With respect to the failure of the NSAG to perform a review of the March 18, 1992 BWROG letter, the Supply System felt the character of the letter was not appropriate for the NSAG's review.
The Supply System considered the letter to be industry subcommittee-type information and felt that if the NSAG reviewed all such documents they would be inundated with infor-mation that may not be sufficiently developed for its consideration.
In addition, the licensee considered responsibility for review of industry information to belong to the entire organization, not just the NSAG.
Therefore, the Supply System considered that a violation focusing solely on the failure of the NSAG to perform a review was not appropriate.
Mr.
Parrish indicated that the Supply System had initiated a comprehensive review of how industry advisories are reviewe Hr. Johnson responded that although the NSAG apparently did not see the March 1992 BWROG letter, it is the group to which the Technical Specifi-cations assign responsibility for the review of industry information, including "industry advisories."
He noted that the NSAG charter did not specifically direct them to review correspondence from owners'roups, and that this should be reviewed.
Hr. Johnson also noted that other cognizant licensee groups should have recognized the BWROG letter as falling within the NSAG's responsibility and forwarded it for their review.
Mr. Richards asked Mr. Gearhart if he had assessed the scope of the guality Assurance involvement prior to the event.
Hr. Gearhart stated that gA had not been involved in the core physics review.
He committed that gA would become involved in future core physics reviews and would contract outside help if necessary.
In addition, gA appears not to have performed a review of the BWROG letter discussing instability.
He committed to assess the focus of gA on industry information.
Hr. Martin, recognizing the abundance of industry issues and information routinely provided to licensees, asked if the Supply System's process for identifying appropriate priority of review was well defined.
Mr. Oxsen stated that the problem was a lack of Supply System sensitivity to industry information.
In response to the August 15 event, he has emphasized to Supply System employees reviewing industry information that the foremost question should be "why can't this happen here7" Hr.
Parrish stated that within the Supply System these issues should not be pursued from a parochial point of view.
When an issue is received by one of the Supply System's organizations, they should consider who else may need to take action.
Hr. Powers indicated that they were reevaluating how information is reviewed and disseminated.
Hr. Perkins concluded the meeting by thanking the Supply System for their participation and by stating that the information presented would be considered during the review of the enforcement issues.
The meeting adjourned at 10:30 ENCLOSURE
Washington Public Power Supply System Enforcement Conference Power Oscillation Event of August 16, 1992 Enforcement Conference Date:
November 10, 1992 Region V, Walnut Creek, CA
MANAGEMENTSTATEMENT RELATED T THE WNP-2 RE P WER ILLATIONEVENT The Supply System has reviewed the Augmented Inspection Team (AIT) report and the Special Inspection Report related to the August 15, 1992 core power oscillation event at WNP-2. In addition, we have had the benefit of a number of discussions and meetings with members of the AIT, the resident inspector and regional management regarding this event.
Because of the significance with which we have viewed the event, prompt actions were taken by management and staff at the Supply System to investigate and correct the causes of the event.
Prompt corrective actions were taken for the technical factors which contributed to the event.
Additionally, disciplinary actions were taken with those individuals (including senior managers) who were in positions where we would have expected their level of knowledge, responsibility and expertise to have caused them to take actions before the event to prevent its occurrence.
Also, we recognized broader failings in our management practices as well as our processes which allowed this event to occur. Therefore, in addition to the corrective actions related to this particular event, we are evaluating other areas of WNP-2 operations to determine where else the lessons learned from this event may apply.
As an example, one of the failures contributing to this event was the way in which BWROG correspondence was handled.
We have corrected this deficiency and are reviewing how information from other sources, such as technical committees and
"user groups" is currently processed.
We recognize that no system is failure-proof and that we must instill in our personnel a management expectation that information which they become aware of must be evaluated for potential impact in areas other than their own.
During the course of our discussions with the NRC staff, we have realized that there are some differences of opinion related to some of the apparent violations.
These differences involve whether certain weaknesses should be also considered a violation of a WNP-2 technical specification, an NRC rule, or a regulation.
These differences are minor, in that they do not cloud the fundamental position expressed by Supply System management in our October 30, 1992 response to the AIT report.
Namely, that the Supply System recognizes the significance of the August 15 core power oscillation event and that the event should not have occurred at WNP-2.
This document provides the Supply System's summary of apparent violations, its position regarding those violations, root cause analyses, safety and regulatory significance assessment, and corrective action R F
L E ARDIN APPARENT VI LATI N u in fA n Vil i VI I
nad ua e Pr edu The NRC inspection report states that the Supply System failed to provide adequate procedures for developing control rod patterns, in that PPM 9.3.9 and PPM 9.3.12 provided inadequate direction (i.e., qualitative or quantitative acceptance criteria for power distribution parameters)
to prevent power oscillations.
I temPoii n
The Supply System agrees with the NRC findings.
The subject procedures did not provide adequate qualitative and/or quantitative direction that would ensure that operation of the plant would remain within design basis assumptions.
It is noteworthy, however, that the addition ofquantitative guidance willnot by itself, prevent future recurrence ofa power oscillation event.
The quantitative direction must be considered in conjunction with qualitative judgements to determine appropriate operating parameters.
As further discussed in Section D, the Supply System Nuclear Engineering department will enhance its core design review plan by incorporating a
requirement to identify and understand the core design criteria or safety analysis assumptions regarding stability, and willprovide applicable limitations, criteria, and recommendations to the plant for incorporation into appropriate plant procedures.
Among other things, procedures have been modified to:
(a) limit acceptable bases for deviating from the approved rod withdrawal sequence above 20% power, (b) provide restraints on power distribution, and (c) require peer review ofStation Nuclear Engineer (SNE) changes to prescribed control rod order sheets.
Also, in the interim, outside expertise has been brought in to assist in core design methodology evaluations.
Ad'u m nt f APRM Flow-ias d Tri pints "T-Fact r" Technical Specification (TS) 3.2.2 requires that Average Power Range Monitor (APRM) flow-biased simulated thermal power-upscale scram trip setpoint and flow biased neutron flux-upscale control rod block trip setpoint be established (using a defined methodology) when thermal power is greater than or equal to 25% of rated thermal power.
Page
Surveillance Requirement (SR) 4.2.2 requires a calculation of the above factors (a) at least once per 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, (b) within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after completion of a thermal power increase of at least 15% of rated thermal power, and (c) initiaHy, and at least once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> when the reactor is operating with maximum fraction of limiting power density greater than or equal to fraction rated thermal power.
According to the NRC, the Supply System did not adjust these setpoints consistent with TS 3.2.2 and SR 4.2.2 requirements.
I emP i't appears that subsequent to the power oscillation event, the Supply System and NRC Staff were on parallel tracks when evaluating whether the Supply System had incorrectly interpreted this technical specification and associated surveillance requirements.
The Supply System agrees that it incorrectly interpreted when it was appropriate to perform the "T-factor," APRM flow-biased simulated thermal power-upscale scram trip setpoint, and flow-biased neutron flux-upscale control rod block trip setpoint calculations, in that it allowed 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after achieving 25% power before "T-factor" calculations were performed.
Therefore, a
violation of TS 3.2.2 occurred.
As further discussed in Section D, the Supply System has corrected the misinterpretation ofthe technical specification and corrected associated procedures to prevent recurrence.
ne
i ri ri n
General Design Criterion 12 of Title 10, Part 50 of the Code of Federal Regulations states that, "tt]he reactor core and associated coolant, control, and protection systems shall be designed to assure that power oscillations which can result in conditions exceeding specified acceptable fuel design limits are not possible or can be reliably and readily detected and suppressed."
Since it is readily known that power oscillations are possible in boiling water reactors, licensees must ensure that the second half ofGDC 12 is satisfied.
That is, power oscillations must be reliably and readily detected and suppressed.
The NRC inspection report states in part that, because (a) the WNP-2 Cycle 8 core could have experienced in-phase or out-of-phase power oscillations, and (b)
training that should have prepared responsible personnel to be able to readily and reliably detect these power oscillations was inadequate, and (c) procedures did not provide adequate direction, to reliably and readily detect power oscillations, GDC 12 was violated.
Page 2
The Supply System agrees that procedure and training deficiencies regarding the prevention of power oscillation events may not have adequately supported personnel who were responsible for reliably and readily detecting power oscillation events.
More quantitative direction should have been provided to personnel responsible for minimizing the likelihood of a power oscillation event.
These weaknesses could have adversely affected a consistent ability to predict unstable operation under other circumstances.
Supply System analyses to date indicate that the dominant core instability mode is core-wide oscillations when compared to regional oscillations. In other words, core-wide oscillations would occur before regional oscillations would be experienced.
The LAPUR calculations support these results.
Furthermore, the uniform distribution of9x9 assemblies, and the uniform rod pull of the single rod sequence (SRS) would indicate that a regional oscillation is unlikely. It is very likely that LPRM upscale/downscale alarms would occur under regional oscillations, providing "reliable and ready" detection.
Further, if regional oscillations had occurred, they would probably have been bounded by a limit cycle of approximately the same magnitude as the core wide oscillations that occurred.
The August 15, 1992, power oscillation occurred core-wide, was detected and suppressed by plant operators based upon uncertainty in plant response.
This supports the basic "detect and suppress" interim approach to the stability issue.
The Supply System believes that the above issues are a generic industry concern.
As such, quantitative additions to WNP-2 procedures may evolve as the industry refocuses on revising power oscillation guidance.
ucl ar afet A
n e r
Review The NRC inspection report states that the Nuclear Safety Assurance Group (NSAG) did not properly review and provide recommended corrective actions for the March 18, 1992, BWROG letter. This failure was categorized as an apparent violation of TS 6.2.3.1.
This technical specification provides that:
The NSAG shall function to examine unit operating h
',NRCi,izddd Licensee Event Reports, and other sources of unit design and operating experience information, including units of similar design, which may indicate areas for improving unit safety.
The NSAG shall make detailed recommendations for Page 3
revised procedures, equipment and modifications, maintenance activities, operations activities, or other means ofimproving unit safety to the Director of Licensing and Assurance.
The Supply System acknowledges that NSAG did not review the March 18, 1992, BWROG letter.
However, it does not appear that this lack of review constitutes a violation ofTS 6.2.3.1.
The March 18, 1992 letter was issued by the BWROG Stability Committee and ATWS/StabilityTask Force on the implementation ofthe Stability Interim Corrective Actions gCAs) contained in NRC Bulletin 88-07, Supplement l. While such information would be reviewed by personnel that may be most directly affected, it is not common practice to automatically submit subcommittee guidance to NSAG. In fact, ifthe Supply System were to modify its approach regarding industry subcommittee-type information, NSAG would be inundated with "opinions" that may not be sufficiently developed for its consideration.
It must be noted that the Supply System did not treat the BWROG letter as though it was of little significance.
The letter was reviewed by the WNP-2 Executive Committee member, the Stability Committee member, and the Reactivity Controls Committee member.
Also, it was reviewed at the supervisory level for its applicability to WNP-2 at this time. A management decision was made that the information was significant enough to be provided to STAs, operators, and shift nuclear engineers.
Atraining module also was created.
However, a decision was made that since the information was qualitative (similar to existing procedure guidance), plant procedures did not require modification. This was an incorrect decision.
To better ensure that significant industry information is properly evaluated and acted upon, the Supply System has taken several steps.
A Corporate Standard, including review criteria, willbe developed that requires personnel involved in industry organizations to formally evaluate information received and provide feedback to management regarding significant issues.
These formalized review criteria will better sensitize management personnel and lower level correspondence reviewers to look for industry guidance that on its face may appear to be minor, but actually has significant implications. Ifsuch information is believed to be significant or potentially significant, itwillbe sent to NSAG for review and resolution.
Also, procedure modifications are being developed that will require increased focus on owners'roup correspondence.
Finally, and perhaps most importantly, personnel at all levels have been sensitized to have a questioning attitude that extends beyond individual responsibilities whenever industry guideline documents are reviewed. The combination ofprocess changes, sensitization, and questioning attitude should result in improved judgement Page 4
whenever industry documents that could impact the plant are reviewed, notwithstanding whether the review is a technical specification requirement.
vi w f ix The NRC inspection report states that the Supply System 10 C.F.R. 50.59 evaluations for Cycle 7 and Cycle 8 reloads appear to be inadequate in that they did not sufficiently consider the changes in thermal hydraulic characteristics of the reload cores and did not appropriately evaluate their effect on, or possible changes to, the various conditions analyzed pursuant to 10 C.F.R. 50.59.
The Supply System used best available information and licensed computer codes when performing its analyses regarding Cycle 7 and 8 reload cores.
As noted in the NRC inspection report, the basis for 10 C.F.R. 50.59 evaluations was the Core Operating Limits Report (COLR). Reload Analysis Reports (RAR) for both cycles werc supporting reports for the COLRs.
The RARs evaluated core hydrodynamic stability and determined that the decay ratios for both reloads were within acccptablc i~mits. The decay ratios were determined using the COTRAN code.
The Supply System agrees that apparently COTRAN may not be adequate for evaluating dccai ratios. and thus, led to a non-conservative reload analysis report.
The August 1S.
1992.
power oscillations confirm this conclusion.
This conclusion is also confirmed by additional analyses using other computer codes, that werc pcrformcd after the power oscillation event.
Evaluations using the latest statc~f-thc.art technology indicates that power distribution stability margins decrease during startup.
More conservative analytical methods likelywould have informed fuel design personnel that stability margins were being challenged during startups.
This would result in the establishment of more conservative operating limits. However, this information is only available by using unlicensed computer codes such as LAPUR.
In addition, calculations using LAPUR and STAIF indicated margins to instability for oscillations were small. Ifsuch calculations had been performed before the event, the Supply System would have been aware that in-phase or out-of-phase instability could have occurred. While this additional information certainly would have been of benefit during the evaluation of 9x9 fuel, since these codes are not approved, the Supply System could not have relied on these results to make initial design adequacy decisions.
However, the Supply System acknowledges that codes such as LAPUR or STAIF could have been used to add more conservatism (beyond regulatory requirements) to operating parameter guidelines.
Page 5
v n r n n I A
ni r n
vi in The NRC inspection report notes that the Supply System did not adequately install and use ANNAin that a 0.3 Hz filter rendered ANNAineffective for technical specification purposes.
This failure apparently constitutes a deviation from a commitment made as part of TS Amendment No. 71, dated June 23, 1989.
The Supply System agrees that ANNA,as installed with the 0.3 Hz filter, would not have provided adequate early warning that the core was entering instability regions.
The instrument was installed and tested at WNP-2 in March 1989, and modified and tested in February 1990.
However, instead of a 0.3 Hz filter, a 5 Hz filter would have been more appropriate.
This was a design oversight and willbe corrected as discussed in Section D.
oot Cause Discu i n This event was analyzed by Plant staff with representatives.
from General Electric, Institute for Nuclear Power Operation gNPO) and Siemens Power Corporation (reload fuel vendor).
The primary root cause for this oscillation event has been determined to be unanticipated interaction ofoperating conditions and components.
The Supply System staff and vendor personnel failed to identify the extent of the tendency for this core design to become unstable under certain operating practices.
In addition to the primary root cause, there were several contributing root causes.
The firstroot cause involves Plant/System Operation since the effects ofchanging operating parameters were not fullyevaluated in that:
aO The SNE selected a start up rod pattern with characteristics of aggressive critical power ratios and high radial peaking. This was standard operating practice to minimize stress on the Reactor Recirculation pumps and the fuel.
Maximizing the amount ofcontrol rod movement during low speed recirculation pump operation minimizes the amount of rod movement required after the shift to high speed.
With the control rods set to support power increases principally by increasing flow, the number of high power changes associated with rod movement is significantly reduced.
This results in less local fuel stress at higher power levels.
In addition, recirculation flowand power could then be increased quickly minimizing the time spent at high speed with the flow control valves at the minimum position when the pump is subject to increased vibration.
Page 6
b.
Past startups and operating regimes during cycle 8 had not disclosed problems with similar patterns.
C.
The plant stability monitor (ANNA)was not employed to provide early warning of the potential changes in core instability.
This was, in part, due to past experience and a confirmation that the existing core exclusion region versus actual and planned plant conditions for this start up were acceptable.
Thus, the SNE failed to consider the need to be conservative in his rod pattern selection and use of monitoring tools for conditions that could promote and predict core stability.
The second root cause was design related.
Specifically, there was inadequate independent review of design changes in that:
a0 The Supply System design review process for the mixed core consisting of9x9 and 8x8 fuel assemblies failed to discern the impact the differences in hydraulic resistance of the fuel assemblies would have on the core's susceptibility to instabilities outside existing instability regions defined by Technical Specification 3.2.7 and current BWROG guidance.
Design review included assurances of conformance to license requirements, but did not discern that core stability licensing analyses did not consider the effects of high peaking on core stability at operating conditions which existed at conditions other than the licensing basis.
Thus, the design review program responsible for setting limits on the plant failed to constrain the rod pattern and peaking conditions to assure core stability.
The third root cause was analysis deficiencies in that:
a.
The fuel designer did not perform sensitivity analyses for core instability at reactor conditions other than those required to perform reload licensing analyses.
b.
Current licensing methodologies do not require these sensitivity analysis on start up power distributions.
C.
Computer models to analyze cores to this level of sophistication are in development and not licensed for use.
Page 7
d.
The designer believed the hydraulic differences between 8x8 and advanced 9x9 would be offset by the void coefficient and would not contribute to the likelihood of instability under nominal start up conditions.
Thus, the fuel designers failed to identify that this core was less stable and did not provide recommendations for compensation through conservative operating conditions.
Acontributing causal factor was management methods. AManagement Oversight and Risk Tree (hlORQ analysis has been completed for this event.
This analysis found significant weaknesses in the barriers management has established to prevent thc cvcnt and in the management controls in place for the design and operation of thc core.
Management acknowledges it should have responded to prior industry information and critically questioned the design oversight and operating philosophy to minimize the potential for core power oscillations, Specific findings resulted from this review included the following:
aO Managcmcnt's response to the Implementation Guidance for Stability Interim Corrective Actions issued on March 18, 1992, was weak.
Training v as provided to STAs, SNEs and Plant Operators but procedures were not updated.
b.
Managcmcnt Policy allowed too much flexibilityfor the SNE/STA to dctcrminc thc core flux profile.
C.
management methods used to review the reload design did not ask pcnctntmg questions in the area of core stability.
d.
hfanagcmcnt decisions and reasoning for reanalysis and acceptance of a lower feed water interlock value were not well communicated.
Conscqucntly, procedures were inconsistently amended and SNE operating stratcgics werc not appropriately influenced to take advantages of the lower fccduatcr value.
The root cause for the deviation associated with the "T-factor" for the APRM setpoints was inadequate procedures.
The technical specification requirements were not adequately written into the procedure requiring the adjustment prior to 25 percent power.
A contributing root cause was management methods which failed to recognize and take corrective action for the non-compliant condition created by Technical Specification Surveillance 7.4.2.1.
Page 8
6.
The root cause for the problem with the filtered input to ANNA was a design configuration and analysis deficiency.
The interface between the existing plant hardware and the ANNA hardware received a less than adequate review and analysis.
A contributing cause was an inadequate review and test of the design change to assure operability after installation.
fet i
ifi n e
I i
ifi n
The P wer cilia'
Ev n P r Ina i /Evl n
n rv 'm As a slight modification of the position taken in the LER submitted to the NRC on November 2, 1992, the Supply System concludes that this event had minimal actual safety significance.
However, the potential existed for safety significance since another event, with more significant reactivity changes, could have adversely impacted fuel integrity.
Notwithstanding this safety significance conclusion, the Supply System recognizes that the power oscillation event had great regulatory significance.
A water chemistry analysis performed shortly after the event did not indicate of fuel failure.
Similarly, analysis of the primary coolant did not provide any indication of iodine spiking, which would have been indicative of fuel failure following depressurization.
A comparison of the coolant chemistry with previous shutdowns did not demonstrate any other abnormalities.
During the initialassessment followingthe event, a bounding POWERPLEX analysis was performed to ensure minimum critical power ratio limits had not been exceeded.
The available APRM data indicated the maximum neutron fluxmagnitude was approximately
+ 9 percent.
As a bounding case this steady state analysis assumed power was increased by 9 percent with no increase in flow. The results showed a delta CPR of 0.5.
With an initial CPR of 1.946, the minimum CPR would be below the Operating Limit Minimum CPR of 1.795 but well above the Safety Limit Minimum CPR of 1.07 including uncertainties in data and assumptions.
A transient analysis also was performed based on the peak to peak oscillations noted on the Local Power Range Monitors (LPRMs). The analysis was performed with the Supply System VIPRE Transient code and was independently performed by Siemens Power Corporation using the XCOBRA-T code.
A conservative hot channel analysis using a 30 percent peak-to-peak input resulted in a delta CPR of approximately 0.20 from both codes.
Based on the above, the Supply System concludes that the power oscillations did not result in any fuel failures or Safety LimitMinimum Critical Power Ratio (SLMCPR)
limits being exceeded.
Page 9
The Supply System concludes that the failure to perform the "T" factor adjustment prior to exceeding 25 percent power had minimal, ifany, safety significance.
The purpose of this adjustment at non-rated (lowpower) conditions is to provide added protection against a highly peaked power distribution by temporarily adjusting the APRM sensitivity to a more conservative value.
Had this been performed at the 25 percent power point it would have resulted in a 15 percent decrease in the margin-to-trip setpoint.
Such a decrease could have provided earlier indication ofpower oscillation, but would not have by itself, prevented the event.
~NN M
The Supply System concludes that 0.3 Hertz filters associated with the ANNAinput had minimal safety significance. IfANNA had been used with these filters it would have provided results that were non-conservative.
The ANNA monitoring system was only one of the systems being relied upon for the "detect and suppress" strategy associated with core oscillations when operating in Region "C" on the power to flow map.
The degraded condition ofthis equipment because of filtered input was potentially significant since it could have allowed the plant to go into the region of instability without accurate decay ratio information ifthe system had been used.
rrec iv A i
Immedi rr v A'
At 0415 hours0.0048 days <br />0.115 hours <br />6.861772e-4 weeks <br />1.579075e-4 months <br /> on August 15, 1992, preparation began to place the reactor in cold shutdown and the unusual event was terminated at 0430 hours0.00498 days <br />0.119 hours <br />7.109788e-4 weeks <br />1.63615e-4 months <br />.
Later on August 15, a root cause team was appointed and an analysis plan was formulated to investigate the cause of the event, assist the NRC Augmented Inspection Team (AIT) and recommend corrective actions.
The Supply System root cause team was supplemented by experts from Siemens Nuclear Power, General Electric, and INFO.
h-Trm rr tiv i n The following short-term corrective actions have been completed.
In order to maintain assembly power to flowratio as low as possible, as well as maintaining radial peaking as low as possible, procedures were revised to require Critical Power Ratio (CPR) greater than 2.2 between 25 percent power and 50 percent core flow.
Core total peaking factor will be maintained less than 3.4 prior to pump shift. These are initial parametric values and willbe reevaluated for each cycle during the transition from an 8X8/9X9 to a uniform 9X9 core.
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Fifteen case studies were run by the reload fuel vendor (Siemens) to validate the stability of the Cycle 8 core.
This was accomplished utilizing one dimensional and three dimensional modeling codes.
The calculations were performed under a variety of conditions including but not limited to: August 15, 1992 restart conditions, August 2, 1992 restart conditions before and after FCV closure, restart conditions with worst case under corrective action restraints both now (500 MWD/M'HJ), at 1000 MVD/MTU,and at 1500 hGVD/MTU. The results of these stability analysis show decay ratios for this core to be less than 1.00. All cases showed decay ratios to be between 0.2 and 0.6 indicating all situations to be self dampening.
In an effort to minimize the inlet sub-cooling, which can contribute to power oscillations, a change was made to the Minimum Feedwater Temperature Curve in Plant Procedure, PPM 3.1.2, "Reactor Plant Cold Startup."
Procedures for monitoring power oscillations willrequire that the ANNAsystem be operable and in service from greater than 25% reactor power and less than 50% core flow.
In addition to general precautions recommended by the BWROG on stability, specific requirements were put in place to minimize plant testing and time spent below 50% core rated flow and above 25% core rated power.
This willrequire shifting recirculation pumps from 15Hz to 60Hz speed at power less than 33%.
An approved startup plan was written and approved that controlled and specified rod patterns for the Cycle 8 startup.
This plan utilized a rod pattern that was analyzed for stability prior to closing the first flowcontrol valve for recirculation pump shift to fast speed.
Revisions, except for minor rod position deviations due to instrument or equipment malfunctions, willrequire a new stability evaluation of the alternate rod pattern and Plant Operating Committee approval prior to recirculation pump upshift.
The Power-to-Flow Map was revised by designating an
"INCREASED AWARENESS" region. BWROG Implementation Guidance for Stability Interim Corrective Actions have been implemented in Plant procedures and reinforced by training sessions and exam testing.
Plant Shutdown and Abnormal Condition Procedures were modified to provide increased monitoring, precautions, and direction regarding potential core instabilities.
The Supply System will continue its involvement in the BWROG activities involving stability.
A Supply System Principal Engineer has been spending Page
approximately half-time participating in these activities.
This includes work as a primary representative on the Stability Committee, ATWS/Stability Task Force and the committee involved with the long term hardware proposal.
As discussed in Section A, feedback mechanisms will be improved to ensure increased sensitivity to potentially significant issues.
A memo was issued by the Plant Manager to all plant personnel informing them of the significance of this event.
The memo provided information on the seriousness of the event, a summary of the causes, and an outline of corrective actions.
10.
Plant Procedure PPM 7.4.2.1, Power Distribution Limits has been modified to require a "T-factor" adjustment prior to exceeding 25 percent core power.
11.
The six APRM and eighteen LPRM input signals to ANNAhave been modified to eliminate the 0.3 Hertz low pass filter. AllANNA signals now have a 5.0 Hertz low pass filter.
12.
A new Plant Procedure PPM 2.1.8, ANNA Stability Monitoring System, was written to describe the operation ofANNAoutside of Region C on the power to flow map.
13.
A peer review by the BWR Owners Group was performed of the current WhP-2 operating practices related to prevention, detection, and suppression of power oscillations.
The Supply System will continue to evaluate the appropriateness of the operating strategies implemented as short-term corrective actions.
The actions taken will be considered during the evaluations that will be made during the implementation of the long-term actions.
These results may identify changes needed in the operating strategies initiallyestablished.
n-Trm rr
'v
'n The following long-term design process corrective actions have been identified.
These long-term corrective actions address the design process and the reviews by Supply System personnel to validate fuel vendor's calculations to ensure unstable regions are avoided throughout core operating cycles. Additionally, verification of the fuel vendor s analysis to support design reviews for future cycles willbe performed.
An important aspect of the development of the long-term corrective actions and the assignment ofpriorityto each activity was in the determination ofthe implications on the design process and the operating strategies.
Although the WNP-2 Cycle 8 core met the Page 12
required reload design criteria, the power oscillation occurred outside of the currently identified region of instability. It has become apparent that a core reload design could satisfy all of the regulatory requirements and still allow undesirable operational situations.
The Supply System has concluded that, as a long-term corrective action, it will be necessary to supplement our review of the design process.
Because the root cause identified problems with operating strategies, the design process and the review of those designs must address those strategies.
Additionally, the Supply System recognizes the importance of continued involvement with the industry in the resolution of the core stability issues.
The followingis a summary of the changes to be evaluated in order to supplement the existing design process.
The scope of Supply System design reviews will be expanded to provide additional oversight ofvendor reload design and analysis.
This expanded review process will be implemented for the Cycle 9 reload design and will include increased scope and technical depth of the design.
A plan for the design review for Cycle 9 willbe developed by March 15, 1993 and the enhanced reviews will include:
a.
increased emphasis on the operating performance of the core in addition to meeting the licensing requirements; b. increased awareness of the impact of core and fuel design changes on plant operations.
c.
increased attention to core'tability and thermal hydraulic characteristics.
d. fuel vendor willbe required to perform additional stability analysis beyond the current licensing requirements.
2.
The Supply System willevaluate the feasibility ofchanging the fuel design to one that is more stable than the current 9x9-9X design.
The long-term objective is the use of fuel designs which create known and manageable stability characteristics during plant operations and transients.
The goal is that the next fuel fabrication contract, for fuel to be delivered in 1995, willmeet the criteria necessary to satisfy this objective.
The potential for earlier changes in fuel design willbe evaluated, but to ensure that the impact of a new fuel design on the existing mixed core are fullyunderstood, the Supply System does not expect to be able to implement fuel design changes prior to the 1995 fuel delivery. This evaluation willbe completed by March 15, 1993.
3.
To support enhanced reload design reviews and implementation of operating strategies, the Supply System willpursue obtaining core stability analysis codes.
The Supply System will evaluate the existing codes and their availability in an Page 13
attempt to implement their use in support of Cycle 10 core design review. This evaluation willbe completed by January 1, 1994.
The Supply System willencourage the fuel vendor to accelerate the validation of the present stability code used for assessing selected rod patterns for the startup plan.
The stability of the existing core will be evaluated as part of the startup plan discussed above under short term corrective action to ensure that the corrective actions are valid during plant startups that may occur for the remainder of Cycle 8 operation.
The frequency specified for the surveillance requirement for power distribution limits and the determination of the "T-factor" for APRM set points is condition-based and leads to some confusion.
The Supply System willpursue a Technical Specification change to eliminate the confusion and to eliminate the requirement for calculating "T". A Technical Specification change request willbe submitted by January 31, 1994 following completion of the necessary analysis.
A short-term corrective action involved splitting signals for the LPRMs input into ANNA, the stability monitoring program.
This implementation approach associated with the modification decreased the flexibilityofANNA. The Supply System willevaluate the impact of this reduction in flexibility. This evaluation willbe completed by January 1, 1993.
A reliability improvement evaluation will be performed of the core stability monitor (ANNA). This investigation willinclude assessment of power supply, CPU redundancy, auto alarm features, and enhanced surveillance techniques to verify continued hardware and software operability.
This evaluation will be completed by January 1, 1993.
The Reactor Engineering Group within the Plant Technical Department provides on-shift direction to the operating crews during power operation and maneuvering.
The Fuels Engineering Group within Engineering, is the primary interface with the fuel vendor.
The Supply System willevaluate this division of responsibilities and the working relationship between the two groups in establishing strategies during start up and fullpower operation.
This evaluation willbe completed by March 1, 1993.
Replace Flow Control Valves (FCVs) and two speed pump operation with Adjustable Speed Drive (ASD) pumps.
This willeliminate the need to conduct operations under the restrictions on FCVs and 15 Hz speed pumps.
The current two speed recirculation pumps will be powered from adjustable speed power Page 14
supplies, allowing continuous low adjustments from 15 to 60 Hz.
With this modification, recirculation fiow control valves are not required.
Due to implementation restraints and concerns, this modification is now scheduled to be complete by June 30, 1994.
To regain fiow margin for cycle 9 the Supply System is aggressively pursuing jet pump cleaning for the next refueling outage.
11.
An evaluation willbe performed oflong term shutdown strategies to ensure the correct procedures are in place for all conditions.
This action willbe complete by January 1, 1993.
12.
For cycle 9, a revalidation of the startup plan will be performed to assure this approach provides adequate margins for stability.
This will be completed January 1, 1993.
13.
Further reviews willbe performed to identify actions to be taken to improve the effectiveness ofthe Supply System's participation in industry activities. This will be completed by February 1, 1993.
14.
Management issues associated with this event are being addressed and corrective actions are ongoing.
The Management Oversight and Risk Tree (MORT)
analysis has been completed.
Management has reviewed this report and initiated the following actions:
a. Action is being taken to strengthen programs and practices used to review and assimilate industry information. Specific changes to plant practices have been incorporated to ensure BWROG and NUMARC information is critically screened for specific and for general relevance to WNP-2. An examination willbe performed to identify the need to review other documents which may strengthen the influence of industry information on Supply System practices.
This wiH be completed by December 1, 1992.
b. Action willbe taken to strengthen our reactivity management program.
Our existing process willbe reviewed and contact willbe made with other utilities to emulate the best features of their programs.
This willinclude a review of the clarity of responsibilities between the Supply System and the contractor.
In addition, the Supply System recognizes that lessons learned in this event ware applicable to other areas of our operation.
Programs exist to ensure design constraints are integrated into operating practices.
An assessment will be conducted to ensure the objective of a strong link between design bases and operating constraints is met.
These actions will be completed by April 15, 1993.
Page
c. A corporate level review be made of the overall relationship between Engineering Services, the fuel vendor, operators, STAs, and the SNEs to assure that responsibilities and duties forall aspects of fuel design, operation, fuel design related independent review, quality assurance, limit setting and recommendations on operating modes is well defined, active, effective and understood by all concerned.
Included in this evaluation willbe a review of all barriers that are assumed to be in place (such as ANNA) to prevent reactivity related events are actually being used in a fashion that the barrier is effective in performing its intended function. This item is complimentary to item 14b.
This action willbe complete by April 15, 1993.
15.
Disciplinary action has been implemented for responsible individuals at all levels of management associated with this event.
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