ML20082K591

RO 50-20/1991-1:on 910813,fission Product Gas Levels Rose from Normal Operating Levels to Max of 15% of Mpc.Caused by Small Blister on One of Element MIT-17 Fuel Plates. Corrective Action Consisted of Removing MIT-17 from Core

ML20082K591
Person / Time
Site:	MIT Nuclear Research Reactor
Issue date:	08/22/1991
From:	Bernard J, Kwok K MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
50-20-1991-1, NUDOCS 9108300133
Download: ML20082K591 (5)

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• cCb NUCLEAR REACTOR LABORATORY iO' AN INT ERDEPARit/E Nl AL CENT E R OF nMSSt.OHUSETlS INSTITUlE Of TLCHNOLOGY h 'I Oh.HARUNG 13J Attaany SPt+t C3 rudge. fAass fIL J A M RNAHD JR Director Teo ebi Na 317 W 7300 D"e c. tor of He e ty Op ' Ao"s Twi tb 501473 t/;T-CAM Te? Na 10m253 4202/4211 August 22,1991 U. S. Nuclear Regulatory Commission Washington, D.C. 20555 Attn: Document Corwol Desk

Subject:

Reportable Occurrence 50-20/1991-1, Detection of an incipient Fuel Element Failure Gentlemen:

Massachusetts Institute of Technology hereby submits this ten-day report of an occurrence at the MIT Research Reactor in accordance with paragraphs 7.13.2(d) and 1.15.4 of the Technical Sp-cifications. An initial report was made by telephone to Region I (Mr. Thomas F. Dragoun) on 13 August 1991, The format and content of this repon 're based on Regulatory Guide 1.16, Revision 1.

1. Renort No.: 50-20/1991 1 2a. Renon Date: 22 August 1991 2b. Date of Occuntnce: 12 August 1991

3. Facility: MIT Nuclear Reactor Laboratory 138 Albany Street Cambridge,MA 02139

4. Identification of Occurrence:

Slightly elevated levels of fission product gases in the air purge that is maintained across the surface of the primary coolant were observed following startup of the reactor to full power at 1722 on 5 August 1991. A sample of the purge gas was then drawn and analyzed. The preliminary analysis indicated that fission product gas levels were approximately 5-10% higher than normal and it was noted that these levels were decreasing 9108300133 910822 V

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Reportable Occurrence 50-20SI-l Page 2 toward the nonnal background level which was about 5% of MPC. It was decided to keep the reactor operating in order to generate enough activity so as to be detectable by the fuel element " sipping" process that is used to identify incipient fuel element failures. The reactor was shutdown at 1804 on 9 August 1991 with there having been no further indications of increased air purge activity. All fuel elements were sipped on 12 August 1991. This consists of drawing a quan of coolant through each element and then counting that liquid for activity. Element MIT-17 was identified as having an abnomially high rate of outgassing in that the activity in the water sample drawn through that element dunng the sipping procedure was a factor of two above the average. Subsequcnt visual inspection identified this element as having an occlusion. A refueling was then initiated and element MIT-17 was removed from the core. Reactor operation was resumed later on 12 August 1991 in accordance with the normal operating schedule and fission product gas release levels were observed to be normal.

5. Conditions Prior to Occurrence:

Element MIT-17 was first inserted in the reactor's B-Ring on 29 March 1982. It remained in core continuously until 9 August 1982 when it was removed. It was inserted in the B Ring for one operating week in September 1987. Regular use resumed on 29 August 1988 when it was agam placed in the B-Ring and kept there until 9 July 1990.

Element MIT-17 was next placed in the C Ring on 9 May 1991, it remained there until removed as part of the corrective hetion for this occurrence.

6. Description of Occurrence:

The fission product gas levels obtained in accordance with standard sampling procedt.res rose from normal operating levels to a maximum of 15% of MPC. This level was observed for less than a minute. It occurred about twenty minutes after the reactor startup on 5 August 1991. The level then rapidly decreased to about 10% of MPC and then over an hour to the current background level. All increases were well below pennitted concentrations (even without averaging over time).

7. Description of Anoarent Cause of Occurrence:

The apparent cause of this occurrence is a small blister on one of element MIT-17's fuel plates. This can only be seen when the element is backlighted with Cerenkov radiation. As shown in the attached Figure One,it is located near the center of either the 6th or 71h fuel plates. It is approximately one inch below the top of the element. (Note: ,

There is some possibility that it is a foreign inclusion, not a blister, llowever, this is unlikely given its location near the top of the element and the upward flow of coolant through the core.)

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Reportable Occurrence 50-2(W1-1 .

Page 3

8. Analvsis of Occum nce:

Quality assurance records on the manufacture of element h!IT-17 do not indicate any deviations from h1ITR element specifications. Element htIT-17 had been in-core while 42197.27 hiWH of energy were produced. It has 403.59 grams of its original 506 gram loading of U-235 remaimng. The point of peak burnup on hilt-17 was 49.35% of the allowed fission density (1.8x101 2 fissions /cc). Primary coolant chemistry is carefully monitored. 'the three parameters measured (pil, chloride, conductivity) have generally been as specified. The few deviations that have occurred were too brief and of too low a magnitude to have caused this occurrence.

9. Corrective Action:

The immediate corrective action consisted of removing element hilt 17 from the core. Fission aroduct gas levels were normal prior to the occurrence and have remained normal. The c ad failure was detected and addressed during its incipient stage.

10. Fr.ilum Data:

A cladding failure occurred to a "4ht" series element in June 1979. Refer to ROR

1. 50-20n9-4 dated 2 July 1979 and #50-20n9-4A dated 26 November 1980. Excess outgassing occurred in an " hilt" series element in September 1983, in July 1985, in February 1986, in October 1986, and in hiay 1988. Refer to ROR #5D-20/83 2,

1. 50-20/85-2,50-20/86-1,50-20/86-2, and 50-20S81. The current failure is not the most severe of those noted. ("h11T" series fuel was made by Atomics International. The "4ht" series was made by Gulf Atomic. Currently, fuel is being procured from Babcock and Wilcox.)

Operating experience with fuel from all ncnufac'arers has been good, except as noted below. Of 43 elements made by Gulf Atomic,42 elements (630 plates) have been permanently discharged from the core after peak burnup approached the license limit. *lhe other 4h1 series element failed in 1979 as reported earlier, at an average bumup of 32.5%.

' Average burnups on the Gulf Atomic fuel are in the range 42-44%. Of the 39 elements made by Al and subsequently used in the hilTR core,26 elements (390 plates) have achieved the maximum permissible burnup and have peak fission densities of 1.58x1021 fissions /cc and average burnups of 37.4%. The thirteen other Al series elements (195 plates) now in use or awaiting reuse have peak fission densities in the range 0.057 to 1.52x101 2 f ssior,s/cc. As previously reported, five other Al series elements were previously discharged due to excess outgassing.

Reportable Occurrence 50-20N1-1 Page 4 Th failure of this sixth Al element means that six plates from a total of 585 plates have now exhibited excessive outgassing. Because of the sensitivity of MITR Il monitoring equipment, radiation releases have been insignificant.

Fuel for the MIT Research Reactor is currently being manufsetured by the Babcock and Wilcox (B&W) Company. Twelve B&W clements (180 plates) are curn:ntly in-core with the most utilized of those elements currently having achieved a peak fission density of 2

0.96x101 f ssions/cc. There have been no failures of B&W fuel to date.

Sincerely, M

Kwan S. Kwok, Ph.D.

[L Superintendent MIT Research Reactor

<k Ci b4 ohn A. Bernard, Ph.D.

Director of Reactor Operations MIT Research Reactor JAB /KSK:gw

Attachment:

Figure One cc: MITRSC USNRC - Region 1 - Chief, Reactor Pmjects Section No. 3A USNRC - Region I- Reactor Engineer, Reactor Projects Section No. 3A USNRC - Senior Resident Inspector, Pilgrim Nuclear Station USNRC - Project Manger, Standaniization and Non-Power Reactor Project Directorate USNRC - Region I- Chief, Effluents Radiation Protection Branch

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Reponable Occurrence S20/1991 1 Attachment j i

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Figure One

Horizontal Cross Section of MITR Fuel Element Showing Blister Location  !

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