ML20027D185

Discusses 811216 Event Re Fuel Assembly Degradation While in Spent Fuel Storage Pool.Root Cause of Event Not Yet Determined

ML20027D185
Person / Time
Site:	Prairie Island
Issue date:	10/21/1982
From:	Ellen Brown NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:	Seyfrit K NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
References
TASK-AE, TASK-E242 AEOD-E242, NUDOCS 8211030016
Download: ML20027D185 (5)

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UNITED STATES 8

NUCLEAR REGULATORY COMMISSION N

WASHINGTON, D. C. 20555

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007 2 1 1982 AEOD/E242 MEMORANDUM FOR: Karl V. Seyfrit Chief Reactor Operations Analysis Branch Office for Analysis and Evaluation of Operational Data e

FROM:

Earl J. Brown, L6ad Engineer Engineering Systems Reactor Operations Analysis Branch

SUBJECT:

FUEL ASSEMBLY DEGRADATION WHILE IN THE SPENT FUEL STORAGE POOL

References:

1)

LER 81-031, Revision 0 and Revision 1 dated December 30, 1981 and May 12, 1982

2) Telecon with Prairie Island Resident Inspector

• 3)

Telecon between NRC staff (E. J. Brown and M. Tokar) and Westinghouse

, Safety Concerns The purpose of this memorandum is to inform you of an event which may be indicative of a mode of fuel assembly degradation not previously considered.

The primary safety concerns are: 1) the mode of degradation appears to be new and occurs while fuel is in the spent fuel pool. 2) the degradation may not be readily detectable, 3) the degradation resulted in separation of the top nozzle from the remainder of the fuel assembly while moving the asserrbly, and 4) if the mode of degradation is generic, there is the potential for dropped fuel assemblies with resultant damage to the spent fuel and possible fission product release.

Recommendations are given in th'e"Section' t'itliid Discussion and Possible Action.

LER Information Licensee event reports81-031 Revision 0, and 81-031. Revision 1, for the

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-. Prair.ie.. Island Nuclear Generating-Plant.are ' enc.losed.'forf.your inforwation.

Revision 0 of the LER describes the event and. Revision 1 provides a brief l

synopsis of an investigation conducted by the fuel vendor (Westinghouse).

l Conclusion 1 identifies the failure as stress corrosion.. cracking of the 30455 top nozzle.

In addition, conclusion 3 states that the stress corrosion A

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a, cracking is believed to have developed during pool storage. This mode of degradation is time dependent and even though the remaining 239 spent fuel assemblies were transferred without incident, there is the potential for subsequent damage during fuel transfer in the future if the problem is generic.

Furthermore, the conclusions in LER 81-031 Revision 1, are rather open ended in the sense that although stress corrosion cracking was identified as the failure mode, it was stated that pool chemistry d' ring the period u

reasonably met the vendor specification limits.

Therefore, the situation appeared to be unresolved because the failure mode was identified, but the belief seems to be that it should not have occurred because the pool chemistry specifications were met, 4

Additional Investigations I discussed the event with the resident inspector at Prairie Is' land after Revision 0 of the LER was received. He indicated there was little to add until results of a metallurgical examination were completed and the event had not resulted in physical damage or rupture of fuel rodi.

I sent a copy of the LER to Mike Tokar in the Core Performance Branch for information.

Since Revision 1 raised the issue of stress corrosion cracking occurring in the spent fuel pool that had not been anticipated, I again discussed the event with the resident inspector but he indicated there was no additional information available to him.

Subsequent discussions with Mike Tokar about.

the information in Revision 1 identified our mutual concerns and he arranged a conference telephone call with Westinghouse on September 30, 1982.

The information provided to us is sunmarized below.

1.

Description and Evaluation of Failed Top Nozzle On December 16, 1981, a top nozzle separated from a fuel assembly that was being transferred to the new high density fuel storage racks at-Prairie Island. The failed portions were examined at Battelle Laboratories. The failure occurred at a mechanical ball joint between stainless steel and zircaloy.

The failure was at all sixteen joints in the area of maximum curvature and was caused by stress corrosion cracking of the stainless steel. The cracks were intergranular and several cracks were present but did not propagate.

It was stated that the cracks were similar to those observed in TMI piping and exhibited oxidation on the surface.

" 'The frictu'r'e surface examination revealed concentrations cf iron,

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chromium, aluminum, silicon, nickel, and copper.

There were 'also impurities of chlorides, fluorides, and sulfur. They do not believe the stress corrosion cracking occurred in the reactor because the hydrog'en overpressure should result in low free oxygen.

However, the spent fus1 pool has highly oxygenated water atJlok t'empdrature-and could be a l+"-

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cause of cracking in the presence of high stresses and sensitized I

stainless steel.

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2.. Design and Fabrication The ball joint.is formed by a mechanical expansion to form bulges with plastica 11y deformed material with subsequent high residual stresses at these locations. The basic design and forming process has remained the same for 8 to 10 years.

There have been some changes (considered minor) with respect to the total nianber of bulges, but it does not appear related to this problem.

In short, there do not appear to be any changes in design or fabrication (the

'same tool and radii have been in use for several years) that could be a root cause of the cracking.

3.

Other Westinghouse Investigations Westinghouse indicated they have substan'tia1' experience ith approx-i imately'5000 fuel assemblies 'with various burnup levels and that a review of some of these was conducted without this type of probit:m.

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being identified. They performed post irradiation' examinations at.

Prairie Island and TV examinations at Trojan Kewaunee, Point Beach (all l

three have the same design as Prairie Island), and Zion. Additionally, i

a review of material lots was conducted and it was determined that assemblies with the same material as that used at Prairie Island were shipped to Point Beach and Kewaunee.

The TV examinations at all plants i

exce)t Prairie Island resulted in no evidence of stress corrosion l

crac cs in the top nozzle.

However ~a total of 27 assemblies at Prairie Island were examined and 12 showed evidence of corrosion. This did not result in additional nozzle failures when the assemblies were moved.

The nozzle that did fail had stress corrosion. cracks over approximately 90% of the rupture l

surface with ductile shear over' the remaining 10% surface.

l 4.

Westinghouse Conclusion It was stated that the only common point appears to be top nozzle I

corrosion in the spent fuel pool at Prairie Island.

However, the pool chemistry appears to have been maintained in accordance with specif-ications.

As a result, the current status and/or understanding is as follows:

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l a)

The spent fuel pool at Prairie Island is the only common point.

l b)

Since-no failures were observed at other plants, they 'have

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exhausted the search for immediate information and do not have a near term plan for additional examinations.

I c)

At Prairie Island, 440 of 500 fuel assemblies have been relocated to the new hi.gh density storage racks and.should. remain in place foY several years. -

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d) Westinghouse may consider providing general handling guide-lines such as elevation, path, and procedures.

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Westinghouse plans to monitor other plants for evidence of e) similar problems.

Discussion and Possible Action The infonnation provided h the LER and that obtained from the telephone conference substantiates at least four safety concerns which are:

1) the mode of degradation appears to be new to fuel and occurs in the the degradation may not be readily detectable, 3) the spent fuel pool. 2)d in separation of the top nozzle from the remainder degradation resulte of the fuel assembly while moving the assembly (the assembly dropped while and 4) if the degradation mode is generic, there 6s potential in the pool) fuel assemblies with potential for damage to spent fuel or' fuel for dropped within the reactor if an assembly was being returned for additional use.

Either situation could result in release of fission proddets.

i Although the concerns are real (three of the four have occurred), the investigations to date do not appear to identify a clear path toward immediate action that would provide a solution.

Since the corrosion has only been found at Prairie Island and initial investiga' tion at a few other plants has not yet identified additional incidents, the generic implic1tions do not yet appear to be widespread but they are potential concerns because of the relatively large incident rate (12 of 27) at Therefore, it would seem that a reasonable approach.to Prairie Island.

reduce undue risk would be associated with 6) awareness that the event occurred, b) alertness with respect to monitoring fuel in the spent fuel pool, c) review of guidance concerning transfer of fuel elements, and d) review of spent fuel pool water chemistry specifications relative to l

t this corrosion problem.

l It would appear that the most important aspect should be dissemination i

• of information about the corrosion event even though the root cause has not yet been established. My recommendations are as follows:

Notify the cognizant branches in NRR of this event for information 1) purposes without specific AEOD recommendations for immediate action but request that they consider the four items listed below for possible action.

a) 'N6tify licensees that may have plans for imminent transfer of l

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Westinghouse fuel in the spinit fuel pool.

Individual project managers could probably accomplish this through the resident Although we do not have evidence of this phenomenpn inspectors.

with fuel supplied by other vendors, it may be worthwhile to notify all PWRJicensees.-..

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Review the need for and feasibility of nionitoring fuel in the b) spent fuel pool to detect stress corrosion c, racking.

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Review the adequacy of current guidance and/or need and feasibility of developing additional guidance for transfer (elevation, path, etc.) of fuel assemblies in the spent fuel j

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Review whether spent fuel pool water chemistry specifications are adequate.

2) Consider this event for inclusion in the next issue of Power Reactor Events.

3) Consider the event for inclusion in the NEA Incident Reporting System (IRS) (foreign reactors).

These recommendations are predicated on information that the ron cause l

has not yet been determined.

Hence, recommendation 1 will alert NRR staff and pennit them to take action as needed. Some of the concerns-identified can probably be considered in the reviews for spent fuel pool modifications involving high density storage. Also, recommendations 2 and 3 will provide wide dissemination of the event to operating plants and others in a longer time frame.

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I Earl J. Brown, Lead Engineer i

Engineering Systems

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g PRAIRIE TSLAND NUCLEAR GINERATING PLANT Docket No. 50-282 Licens e No. DPR-42 l

50-3'06 DPR-60 Spent Tuel' Assembly D-34 Top Nczzle Event s

The Licensee. Event Report for this occurrence is attached.

This event is reported in.. compliance with Technical Specification i

6.7.3.1.1, due to the potentially generic nature of the event.

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[,yith both units operatint at cover, stsent fuel was beint noved in the spent fuel l

o A s' ent fuel asse=bly top nozzle came apart from the rest of 'the i

g istorate pools.

o E I asse=ble causine thie assemble to tio toward the edee of the pool.

The lower end ofI I'5'TT'1 I m am -Mv is ner.r restine on the too of the fuel rack and the top is restine in I h l a gate opening in the vall. No detectable release of radioactivity occurred.

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CAuss DEscatrTION AND CONNECTIVE ACTIONS 27 lThe cause is not known at this time.

Tuel handling operations have been suspended 1 3 o i until the fuel assenbiv can be noved.

Cause of the failure is beine investiested.1

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Docket No. 50-282 50-306 1.I1 81-031/01T-0 Detailed Descriptien of Event Spent fuel was being transferred from pool #2 to pool #1 in preparation for the re-racking of pool #2. Two hundred and two (202) out of 441 spent fuel assenblies had been transferred to pool #1 including 11, Unit I, Re*gion D asse=blies. When fuel handling in the spent fuel pools was resused the morning of Decenbar 16, the next step in the fuel transfer log etiled for noving spent fuel assembly D-34 with no insert (i.e. no thimble plug,'RCCA or 3PRA) from location S-J47 in pool #2 to location S.IO6 in pool #1. The

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operator latched assembly D-34, removed ii: from locaion S-J47 and moved it to a position above location S.206. No diffidulties were encountered during these operations. At approximately 0730 the operator began lowering D-34 when the bottom nozzle, came to rest on the top of the spent fuel rack indi-cating the asse=bly needed to be re-indexed on location.S.IO6.

As the operstor attempted to raise the asse=bly in order to re-index, the

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top nozzle (which remained latched to the spent fuel handling tool) separated from the remainder of asse=bly D-34.

The asse=bly, ninus the top nozzle, then tipped towards the spent fuel pool vall and came to rest at approximitely a "

30 degree angle from vertical in both the'I and Y planes. The botton nozzle is supported on two faces by the top o".the spent fuel rack at location s'IO6 and the top of the assembly is supported by the weir gate opening in the vall which exists between the transfer canal and pool fl. The assembly is vedged between the veir gate and fuel rack and is considered stabilized. There was no' detectable release of radioactivity and visual examinations do not reveal any damaged fuel pins.

Ca'use of Occurrence and Corrective Action A cause has not been determined at this tine. Metalurgical examination of the top nozzle by the fuel vendor Oill aid in deter =ining an appa' rent cause.

Tne history.of fuel asser51y h34 is as follo s: D-34 underwent three cycles of irradiation and accumulated a discharge bu:nup of 29,424.KJD/M. The itssetbly resided in the Unit 1 core at locations J-02 during cycle 2, E-04 during cycle 3, and I-04 during cycle 4.

In all three cycles, it cohtained a.

thi ble plugging device.

In April 1978 it was discharged from the core to spent fuel pool location S-J47 uhere it retained until Dece:bar 16, 1981. A review of fuel handling records did not uncover any difficulties in the

.: _handli.ng of this ass,er.517.,

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Tne failure of fuel asse=bly D-34's top nozzle occurred at the first bulge joint bet een the sixteen stainless steel sleeves and the zircalloy-RCCA guide tubes.

During fuel handling the weight ef a fuel.asse=bly is supported V

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.Dece=ber. 30, 1981 Attach = ant (page 2 of 2) 2 NORTP.IRN STATES POWER COMPANT

. PRAIRIE.ISI.AND NUCLEAR GENERATING Pl. ANT Do:ket No. 50-282 50-306 LER 81-031/01T-0 fro = the spent fuel handling tool which is latched cnto the fuel asse=bly rep

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nozzle. The load is than transmitted do.n through the asse= Sly via the

- 16 RCCA guide tubes which are connected to the botton nozzle.

The top nozzle is attached to the guide tubes as.follows: A stainlass steel sleeve, approxi-mately 8 inches long, is velded to the top norria at each of the 16 guide tuba locations. The zircalloy guide tubes arq than inserted into these sleeves and coupled by a fornfit where the zirealloy tube is bulged out into the sleeva'.'.

at 3 alevations.

From visual exahnations of the fuel asser.hly 34 and its top nezzle, with the aid of a undervater TV csmera, it appears that the 16 stainless steel sleeves failed at the same elevation corresponding to the first.bulga joint. The zircalloy guide tubes appear to,have remained intact.

Fual handling operations in the spent fuel pools have been suspended. Pro-cadures and tooling are being developed to upright the assembly and place it into a spent fuel storage rack. Further corrective action may be taken when the apparent cause of the occurrence is determined.

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IL 60137 PRAIRIE ISLAND NUCLEAR GINir. RATING PLANI Docket No. 50-282 License No. DPR-42

. Uedate Renort - Seent Fuel Asse:bly D-34 Top Nozzle Event The Updated License Event Report for this occurrence is attached.

This event was previously report'ed to you on December 30, 1981.

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50-282

'50-306 LER 81-031/01X-1 Cause of Occurrence and Corrective Action Metallurgical hot cell examinations have shown that failure occurred at Ehe.

first bulge joint on the stainless steel sleeves and 1.

The fractures s@Trad..).ittle ductility (N 10%oftfesurface).

2., The grain boundaries showed axtensive separation and pitting.

3.

Corrosion products on the surface and particles on the fractura surface are high in Si, A1 Cu and C1.

4.

Extensive grain boundary separation has occur ed over the bulged araa.

5.

Cracks appear to originate on the outer surface.

6.

The surface' features show rust colored flow patterns in tha cracked area.

7.

Chamical analysis of.tha sleeve indicates nor=al constituents for 304 stainless steel.

3ased on the above observations, the fuel vendor has drawn the following conclusions:

1.

The fracture is classified as stress corrosica cracking.

2.

Since stress c6frssion ' cracking takes ti=e to.degelop it must have been present prior to the Dece=ber 16,.1981 handling.

3.

Stress corrosion cracking is not likely to have develope'd during. operation (i.a...yhile the assenbly was resident in the reactor. core) and therefore it is believed to have develeped during pool storage.

4.

pool che=istry during the storage period reasenably =et the vendor supplied specification limits.

5 b'seque'nt 'to the occurreness fue1 Mandling in the's' pent ' fuel' pools was

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suspended en Dace:ber' 16, 1981.

Recovery and stcpage of the asse:bly was.

a:::=plished en January 19 and 20, 1982.

The asse:bly was than placed in

• a ne:=21 spent fuel storage location.. Tuel hssdling.in the pools was l

resu ed ef. February 19, 1c82.

The recaining 219 spent fuel assemblies vere t snsf erred *.-itheut incident.

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