ML19345B994
| ML19345B994 | |
| Person / Time | |
|---|---|
| Site: | MIT Nuclear Research Reactor |
| Issue date: | 11/26/1980 |
| From: | Lisa Clark MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE |
| To: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| RO-50-020-79-4A, RO-50-20-79-4A, NUDOCS 8012030686 | |
| Download: ML19345B994 (5) | |
Text
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h .a s NUCLEAR REACTOR LABORATORY ,' ,
I MASSACHUSETTS INSTITUTE OF TECHNOLOGY 'DY o K. MARLING 138 Albany Street Cambrid;;e, Mass. 02139 L cL ARK JR.
f- oersctor (61 h 253-4202 o'c'e' c' ***cto' 00"*oa$
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l 11/26/80 Mr. Boyce H. Grier, Director U. S. Nuclear Regulatory Commission Region #1 l
! 631 Park Avenue l King of Prussia, PA 19496 l Re: Reportable Occurrence 50-20-/79-4A, License R-37
! Apparent Fuel Element Cladding Failure, Further Information l
Gentlemen:
Massachusetts Institute of Technology hereby submits further information concerning the above occurrence, a written report of which was initially sent to USNRC, Region #1, on July 2, 1979.
As dsscribed in the original report, the MIT Research Reactor was shut down on June 22, 1979 to investigate an increase in the concentration of fission product gases routinely measured at a low level in the core purge effluent from the top of the core tank. On June 25 and 26, 17 of the 24
}
l elements were removed from the core, replaced with other partially-used
! elements,.and the reactor was restarted on June 27 without incident.
l As stated in the report, plans were made for identification of the faulty element, including reuse of the discharged fuel.other than the faulty element.
l Faulty Element - Identification and History.
Subsequent to issuing the report, the faulty element was identified as #4M10 by using " sipping" techniques on July 3, 1979. A one-litet sample of water pumped from that element was found to contain the fission products,.de-133andI-131atconcentrationsseveraltimesthatinsamples from other elements that had also been in the core at the time of the 1 V I failure. These other elements, after consultation with a subcommittee of I I
the MIT Reactor Safeguards Committee, were returned to the core on July 23, 1979, and again the reactor was restarted with no recurrence of the previous problem, providing. confirmation that #4M10 was indeed the element responsible gp12030IM ~h
.' p:g3 2 of 4 e
'for the sacreased' fission product concentrations.
The fuel history records show that element #4M10 had been continuously in the core since the MITR-II was first loaded in August 1975. The reactor was operated primarily at low power (a few' kilowatts) for startup testing during the remainder of calendar 1975 and at half power (2.5 MW or less) during most of 1976. Since early December 1976, operation has-been princi-pally at 4.9 MW on a routine schedule averaging 80-90 hours / week.
Total energy produced by the core was 60,880 MWH during the period.
Initial and final U-235 contents for element #M410 were 445.20 and 300.46 grams, respectively, for an average burnup of 32.5%. The peak fission density 21 21 was 1.32X10 f/ce, or 73.5% of the 1.8X10 f/cc limit allowed by Technical Specification 3.11-2c. In order to allow for uncertainties in the factors used for calculating peak fission density, fuel is removed before the fission density exceeds our present self-imposed limit of 85% of the Technical 21 Specification limit, i.e.1.53X10 f/cc. Consequently, element #4M10 was at approxicst 2.y 86.5% of its useful life at the time of removal from the core.
Cause of Failure Subsequent to its identification as the faulty element, #4M10 was examined under water in the core tank. The exterior appeared normal, but light transmitted through the coolant channels revealed the presence of a blister on the center fuel plate (plate #8). The blister could be seen on both sides of the fuel plate, extending about half way into the channels (roughly r.040I.010" on each' side). It covered about 3/4" along the width of the plasm. Longitudinally its extent could not be determined; if circular, it would be about the size of a five cent piece. It appeared to be located an inch or two from the end of the plate at the numbered end of the element.
This means that,at the time of failure, it was'in or near the elevated flux that occurs at the bottom of the core (and'also in the coolest water).
Quality assurance records related to the procurement of element #4M10 were reviewed, including water gap traces, plate fin heights, plate and.
clement dimensions, blister inspection reports, radiographs and ultrasonic
^
test scans. Inspections of the last two types during fabrication were done i
on'a sampling basis; no radiograph and no scans were done for the blistered plate, but no defects were revealed for other plates tested by these methods '
in the same element.
. .. - . - .._ , . - - .- _ . --. _ . . . . ~. - . - - , ,
/
pegn 3 cf 4 l'
Records of primary coolant PR and~ conductivity.-were also reviewed.
1
. The . former 'normally. is 6.0-6.5, with occasional reas ugs as low as'5.8 or j as high as.6.8. In August of 1977, weekly values of 7.0 and 7.2 were
- . recorded, but are not considered significant.because they. occurred two years before the failure. . Conductivity has run in the range 1.0-2.0 micromhos/cmi.' occasionally ~a little lower or higher for brief periods.
] The MIT cladding-failure was discussed with others who hais t.ad experiences of a similar type, principally EG&G- Idaho, Inc. , and the University of Missouri Research Reactor. The former reported a number of I' failures attributed to various causes such as cladding porosity, thin cladding, foreign material in the cladding and corrosion in stagnant water. They have had extensive experience with plate-type elements having
- a uranium-aluminide core clad in aluminum and now routinely run such fuel I
to a lission density of 2.3X10 f/cc without problems except for the types j identified above.. It is.my understanding that they are now considering the establishment of a higher burn.sp limit.
The University of Missouri Research Reactor experienced a cladding failure that was revealed in a hot cell examination undertaken for another purpose. Aerojet Nuclesr Company performed a destructive hot' cel1~ examination of MURR fuel element #755-F3 and reported the results in " Post-Irradiation 7xamination of MURR Fuel Element", by R.R. Hobbins and W.C. Francis (August 17, 1973). A pin-hole failure of the cladding was found in a region that had experienced only 2.3X10Of/ce, and the cause was determined to be a small
, . iron particle embedded:in the cladding.-
Limited information has also been received that others have experienced blistered plates ma'de from materials furnished at about t.he same time by the same fabricator who made the MITE fud bctch containing element #4M10.-
' Performance of Other MITR Fuel Elemants Eltment #4M10 was one of 44 elements fabricated in a batch in 1973-74.
~
One was a'special element intended for low power reactor physics measure-ments only, but the.other 43 have all been used to varying degrees of burnup~at 5 MW. Twelve of these-were permanently removed from the core between May and October:1980 just before reaching the 85% fission density.
limit-(actually. 83.7-84.9%).' Average burnups were in the range 39.5-40.6%.-
, Three other elements have reached the same or hight.s. burnup than #4M10.
at the time it failed. .The' remaining 27 elements made by that fabricator J ail have some, but ' smaller. degrees ~of burnup. The first three of a new batch i 4
+, ,- , , ,- .>m
, - , . , ,, cw . = . , - , - - +
png) 4of4 I
of elements made by another fabricator were put in the core last January.
The design was the same, although the initial loading was 506 grams of U-235 instead of 445 grams. The burnup on t!ese is now about 9%, and there have been no problems.
Qnclusion Based principally on the performance of other fuel elements from the same fabrication batch and, to a slight deg:ee, on the performance of similar elements made by another fabricator, it can be concluded that the cladding failure was the result of a manufacturing deficiency. This is confirmed by the successful use' of thousands of other elements of similar design (except for the 0.010 inch high fins on the surfaces of the MITR plates) in several other reactors, many to higher burnups.
Consideration was given to the possibility of contracting for a destructive hot cell examination in an effort to pin point the precise cause of failure. This was not undertaken because of high estimates received for the conduct of such work, on the order of $50,000. It was believed that, at best, such an examination would simply reveal a defect of a type experienced in the past by other reactors and, at vorst, it might be in-conclusive. In addition, the time lapse necessary for cooling of the element prior to shipping, for obtaining a shipping license, and for conduct of the eynmination was estimated to be comparable to the time required to achieve the 85% burnup limit on a siginificant number of companion elements.
Element #4M10 remains in the fuel storage ring of the reactor core tank.
It is continously monitored here by the same core purge monitor that detected the 1979 cladding-failure. In one to two years it will be shipped either separately or together with other spent elements to a Department of Energy reprocessing plant.
~
Please let me know if there is any further information that I can provide concerning this matter.
Sincerely, m* e.-s .
Lincoln Clark, Jr.
LC/sbs cc: MIT Reactor Safeguards-Committee
.USNRC, Division of Licensing, att'n: H. Bernard
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