ML19327B589
| ML19327B589 | |
| Person / Time | |
|---|---|
| Issue date: | 09/26/1989 |
| From: | Fieno D Office of Nuclear Reactor Regulation |
| To: | Thadani A Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8911020131 | |
| Download: ML19327B589 (3) | |
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NUCLEAR REGULATORY COMMISSION o
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1 CEP 2 61989
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MEMORANDUM FOR:
Ashok C. Thadani, Acting Chief i
Reactor Systems Branch j
Division of Systems Technology i
i
- THRU:
Robert C. Jones, Section Chief j
Section B.
4 Reactor Systems Branch Oivision of Systems Technology FROM:
Daniel Fieno, Senior Reactor Engineer (Nuclear)
Section B Reacto: Systems Branch Division of Systems Technology
SUBJECT:
MEETING WITH ABB ATOM INC. ON OSKARSHAMN 2 FUEL FAILURES 4
1 ThestaffmetwithrepresentativesofABBAtomInc.(ABB)onJuly20 1989 in Rockville, Maryland to discuss the fuel failurcs at Oskarshamn 2 in late 1987 and early 1988.
The meeting was held as an open meeting so that licensees and fuel vendors could be fully informed about the event and its causes. The t
enclosure presents a list of meeting attendees.
L Oskarthamn 2 experienced stepwise increases in offgas and primary coolant activities during the period from December 1987 through mid February 1988.
L During the refueling outage in August 1988, four failed fuel rods in separate.
u fresh SVEA fuel. assemblies were identified. Subsequent evaluations led to the conclusion that these rods failed because the fuel assemblies had (Nrated under dryout conditions for an extended period o' time (between 2 and 7 days) during norinal power operating conditions. Each of the failed fuel rods was a corner rod located closest to the control rod center. The failed fuel rods also exhibited secondary failures caused by hydriding.
3 ABB performed an extensive evaluation of the cause of the failed fuel rods and concluded that the dryout failures were caused by two principal effects that I
p fncreased the power in each of the failed fuel rods. To understand why the J
power in these fuel rods increased, a brief discussion of the fuel loading in
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an Oskarthamn 2 supercell will be given. A supercell is defined here as an array of four fuel assemblies having a cruciform shaped control rod located at
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the center of the array and in the. water gaps between the fuel assemblies.
Two of the four fuel assemblies were previously burned and located diagonally) across from each other. TheothertwoassemblieswerefreshSVEA(watercross
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fuel assemblies located in the cther diagonal locations of the supercell. The I
previously burned fuel assemblics were contained in channel boxes that were on
-their second fuel assembly lifetime. These channel boxes exhibited bowing l
that increased the amount of water in the gap, adjacent to the control rod, of 8911020131gg26 g
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~ Ashok C. ' Thadani i h
the bypass water region of the supercell.
In addition, the Oskarshamn 2 staff i
assumed that the previously burned fuel assemblies were moved closer to the center of.the gap, as they would be in a core with all SVEA fuel assemblies.
Thus, even without the channel bow, the gap was larger then assumed in plant
- process computer predictions of the critical power ratio (CPR).
Therefore, the two effects, channel bow and geometrical positioning, combined to produce l
a wider gap than assumed in operations. The increased water gap led to t
increased neutron moderation and higher powers in the corner rods of the fresh SVEA fuel. The ABB analysis showed that the corner rods had operated below i
the safety limit CPR. The lower powers of the burned fuel assemblies did not result in dryout for its corner rods.
L ABB con:1uded that the fuel failures were caused by dryout for the reasons l
stated above. ABB also concluded that its analytical methods were capable of l
predicting the dryout when the bypass water region gaps were properly taken into account.
Finally, ABB concluded that the failures were not caused by any features of.the SVEA watercross design and that other BWR fuel designs would have failed under similar circumstances, t
A copy of the two documents on this Oskarshamn 2 failure event may be obtained from the Reactor Systems Branch.
l t
DatiielFieno,SeniorReactorEngineer(Nuclear)
Section B Reactor Systems Branch Division of Systems Technology Fnclosure:
As stated i
cc:
M. W. Hodges L. Phillips P. Wen-i I
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ENCLOSURE
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Meeting with ABB on Oskarsharm 2 Fuel Failures g
ABB M. W. Hodges George Griffiths Daniel Tieno Nils 0. Jonsson Johann Lindna, Per Sollbe Harry Thornt cg GE Uanice Charnley Steve Congdon ANF S. M. Stoller Larry Nielsen Istvan Frankl Scott State KeithSheppard(consultant)
Westinghouse Toshiba Bill Harri-Tetsuro Takeshita -
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Hitachi Chuba E.P.C.
Hideaki Utsuno-Yoshihiro Noguchi l
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