ML20024D287
| ML20024D287 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 06/15/1983 |
| From: | Ellen Brown NRC |
| To: | |
| Shared Package | |
| ML20024D284 | List: |
| References | |
| TASK-AE, TASK-E313 AEOD-E313, TAC-49353, NUDOCS 8308030485 | |
| Download: ML20024D287 (5) | |
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l AE0D ENGINEERENG EVALUATION REPORTO UNITS: Prairie Island 1 and 2 EE REPORT No.: AEOD/E313 DOCKET Nos: 50-282 and 50-306 DATE: June 15, 1983 i
LICENSEES: Northern States Power EVALUATOR / CONTACT: E. J. Brown NSSS/AE: Westinghouse / Fluor 4
Power Services Company
SUBJECT:
POTENTIAL CONTAMINATION OF THE SPENT FUEL P0OL AND PRIMARY REACTOR SYSTEM EVENT DATES: December 16,1981 (LER 81-031, Revision 0 and Revision 1)
SUMMARY
A previous engineering evaluation report, E242, identified potential safety issues concerning stress corrosion cracking of the top nozzle of fuel assem-1 blies in the spent fuel pool at Prairie Island. The program office, NRR,
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started an investigation of the event in response to E242. This report provides an assessment of the information obtained as part of the program office investigation.
Based on a review' of the information, it appears that the failure mechanism was stress corrosion. However, the cause of cracking does not appear to be related to the few chloride excursions which have occurred in the spent fuel pool as originally believed by the licensee. Hence the cause has not been definitively established. However, evaluation of pipe cracking of the line from the boric acid storage tank to the safety injection system (identified subsequent to the nozzle cracking) revealed that sulfate contami-nation was the probable cause. This type of contamination was potentially available to the fuel assembly nozzle because there are pathways to both the spent fuel pool and the primary reactor system from the boric acid storage
. tank piping. The potential source of contamination was postulated to be d
either a contaminated batch of boric acid or resin intrusion due to the recycling system.
Therefore, the primary concerns raised by this evaluation are (1) identification of potential contaminants and (2) possible pathways for the contaminants to reach both the spent fuel pool and primary reactor system. A very important aspect of the events is that they appear symptomatic of a potential generic issue that could involve contamination of the primary reactor system with possible degradation of primary system components. The issue has the potential for affecting all PWR plants. With this background, it-would seem prudent to include monitoring for sulfur contaminants as part of the water chemistry programs for the primary reactor system and spent fuel pool..
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- This document supports ongoing AE00 and NRC activities and does not represent the position or requirements of the responsible NRC program. office.
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. DISCUSSION This engineering evaluation covers potential generic implications that evolv-ed from an assessment of NRC staff investigations in response to AE0D/E242
( Re f. 1). As a result of Reference 1, NRR initiated an evaluation of the event and obtained information from the Licensee (Ref. 2 and 3).
The subject of interest was degradation of fuel assemblies while in the spent fuel storage pool at Prairie Island. Subsequent to meeting with the licensee, NRC staff from NRR, IE, and AE0D met to review and discuss individual assessments based on the information we had reviewed.
Reference 4 is an informal note to P. Wu from E. J. Brown that contains a perception of the inportant staff concerns that were discussed at the staff meeting.
The evidence seems to support a conclusion that the failure mechanism was stress corrosion.
However, the evidence submitted with respect to limited chloride excursions in the spent fuel pool does not substantiate those events as the cause of the fuel assembly cracking.
Therefore, the primary safety concern is that the cause of the corrosion cracking has not been identified.
However, it seems logical to conclude, that the fuel assemblies were exposed to a contaminant of some kind. The technical issues related to possible contami-nants are:
(1) What is the contaminant and what is the source of the contaminant?
(2) What is the pathway for the contaminant to reach the fuel? This question should address possible pathways to the primary system and/or to the spent fuel pool.
In addition, if the source is through a pathway to the primary system, is degradation occurring in the primary system or is the damage occurring only after fuel is transferred to the spent fuel pool?
Some of these questions were raised in the NRC staff meeting indicated in Reference 4.
Reference 5 responds to the concern about possible degra-dation that might result in failure to insert control rods. This could be a possible common mode failure, but it appears that the fuel vendor and licensee have reviewed this aspect even though the information was not submitted to NRC for review.
An event reported in LER 82-029 (Docket 282) concerns cracks found in piping in the concentrated boric acid line that connects the boric acid storage tanks to the safety injection system. The event indicates a possible source of contaminants to the fuel assemblies. The investigation of that event was reported in Reference 6.
The primary contaminants of sufficient concentration to cause cracking of the 304 stainless steel pipes l
which had high residual stresses were sulfates.
Small amounts of chlorides and fluorides were also found. The possible sources of these contaminants were believed to be a contaminated batch of boric acid or resin intrusion.
due to the recycling system.
FINDINGS l
l The determination of sulfates as the possible source of cracking of the pipes from the boric acid storage tank to the safety injection lines suggests a possible source of contamination for the fuel assembly top nozzles. A review e
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. of flow diagrams from the FSAR (Ref. 7) indicates possible flow paths to the reactor primary system from the boric acid storage tanks.
In addition, the pump used for recirculating stagnant lines of the concentrated boric acid system were also used for makeup boric acid to the spent fuel pool water.
Therefore, it is evident that there are potential pathways for contaminants from the boric acid system to both the primary reactor system and the spent fuel pool at Prairie Island. Also, based on the information presented for the fuel assembly nozzle cracking, the licensee did not analyze liquid samples for sulfate and fluoride contamination.
Based on the available infonnation, it appears that the fuel assembly nozzle cracking could be related to contamination which somehow got into the spent fuel pool. Also, it seems that most of the safety concerns raised by staff concerning dropped fuel assemblies or possible inability to insert control rods if nozzle damage occurred in the reactor have at least been reviewed, even though the information was not presented to the NRC staff. The primary safety issues then appear to resolve to 1) adequate treatment of safety aspects raised about fuel assembly nozzle cracking and 2) potential generic implications related to the introduction of contaminants into the spent fuel pool and primary reactor system.
Based on the preceding discussion, the following findings are provided:
(1) The actual cause of stress corrosion cracking of the fuel assembly nozzles at Prairie Island has not been definitely established.
Available evidence suggests that contaminants such as sulfates could have been a possible cause.
(2) The licensee appears to have addressed most of the NRC staff questions concerning specific saftey issues. Although imediate concerns may have been resolved, the apparent relationship between the fuel assembly degradation and contamination suggests that the issue about the NRC position on long term storage of fuel assemblies raised in Reference 3 (D. G. Eisenhut to R. H. Vollmer and E. Jordan), has not been completely resolved. Specifically, should contamination induced degradation of fuel assemblies in long term storage be con-sidered, and if so, how should it be monitored and controlled?
(3) Since sulfates were identified with cracking of pipes from the boric acid storage tank to the safety injection system, it is evident that such contamination could have been introduced into the primary reactor system and the spent fuel storage pool at the Prairie Island site.
(4) The suspected source of contaminants at the Prairie Island site are common to all PWRs. Therefore, the event appears symptomatic of a potential generic issue that could involve.possible degradation of primary system components. A similar issue related to contamination of j
reactor coolant water is discussed in Reference's 8, 9, and 10.
l CONCLUSIONS Based on the review of this event and related information, it appears that con-tamination of plant systems could be a precursor to unanticipated degradation
.. of fuel, during long term storage, ar.d possibly could result in degradation of primary reactor system components. Sir.ce the possible sources of contamination were postulated to be a contaminated batch of boric acid or resin intrusion from the recycling system, the issue is potentially generic to PWRs.
The important aspects appear to be that:
(1) this type of contamination was not anticipated, (2) the monitoring programs for the spent fuel pool and the primary system generally analyze for chlorides but do not analyze for sulfur or fluoride contaminants, and (3) an unsuspecting licensee could inadvertantly place a plant in a condition that may result in significant degradation of safety-related equipment. With this background, it appears that both identifi-cation of potential contaminants (sources and species) and methods to monitor water chemistry for potentially serious concentration of specific species, such as sulfates, should be reviewed for applicability to operating PWR plants.
To further support the concern about sulfur contamination, steam generator tube cracking at TMI-1 indicates both the extent and type of damage that can result from sulfur species incursions into the primary system even though the source of contamination and pathway were dif:ferent than at Prairie Island. There has also been some evidence of corrodants in equipment connected to the pressurizer.
Although there is insufficient information to draw firm conclusions, there may be some basis for concern relative to the effect of corrodants on operation of the PORV and safety valves. Previous events involving corrodant attack en stainless steel piping of PWRs was addressed by an NRC Bulletin and Circula:-
(Refs. 11 and 12).
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REFERENCES 1.
NRC, Engineering Evaluation AE0D/E242, " Fuel Assembly Degradation While in the Spent Fuel Pool," October 21, 1982.
2.
Letter to Director, NRR from Northern States Power Company, " Transmittal i
of Presentation of Material Related to Fuel Assembly Failure for Prairie Island Units 1 and 2," March 22,1983.
3.
NRC, Memorandum for R. H. Yo11mer and E. Jordan from D. G. Eisenhut, " Fuel Assembly Degradation While in the Spent Fuel Storage Pool," March 31, 1983.
4.
NRC, Informal note to P. Wu from E. J. Brown, " Comments on Prairie Island Fuel Assembly Degradation While in the Spent Fuel Pool" (Meeting of NRC Staff on 4/20/83), dated April 21, 1983.
5.
NRC, Memorandum for P. Wu from M. Tokar, " Prairie Island Spent Fuel Pool Fuel Assembly Degradation," May 16, 1983.
6.
NRC, Meeting Summary of " Boric Acid Line Crack Indications," April 18, 1983 for Prair.ie Island Units 1 and 2.
7.
NRC, FSAR for Prairie Island Units 1 and 2.
8.
LER 50-321/82-028, E. I. Hatch, Unit 1.
9.
LER 50-324/81-027, Brunswick, Unit 2.
- 10. LER 50-333/81-074, James A. FitzPatrick.
- 11. NRC, IE Bulletin 79-17, " Pipe Cracks in Stagnant Borated Water Systems at PWR Plants", July 26, 1979 and Revision 1, October 29, 1979.
- 12. NRC, IE Circular 76-06, " Stress Corrosion Cracks in Stagnant, Low Pressure Stainless Piping Containing Boric Acid Solution at PWRs", November 26, 1976.
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