%DONRleak062255; ]> Study of Incident Involving Fuel Leak in North Carolina State College Reactor Machine readable transcription Mann, Marvin M. Creation of machine-readable version: James M. Jackson Sanborn Creation of digital images: James M. Jackson Sanborn Conversion to TEI.2-conformant markup: James M. Jackson Sanborn ca. 15 kilobytes NCSU Libraries Raleigh, NC DONRleak062255

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copyright 2000, by North Carolina State University

 August 28, 1955

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Nuclear Reactor

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Library of Congress Subject Headings June 22, 1955 English LCSH manuscript 24-bit color: 400 dpi Study of Incident Involving Fuel Leak in North Carolina State College Reactor

Curtis A Nelson, Director, Division of Inspection

Marvin M. Mann, Assistant Director for compliance

STUDY OF INCIDENT INVOLVING FUEL LEAK IN <name type="corporate">NORTH CAROLINA <lb/> STATE COLLEGE</name> REACTOR

SYMBOL:INS:MMM

I spent the day with Dr. C. K. Beck in Raleigh on June 16, 1955. As you know, Herman Roth of Oak Ridge had reported to us that the core of the Raleigh reactor had developed a moderate leak; and that it was estimated at the time that about 100 cc's of fuel solution had made its way out of the core. After I had a short talk with Dr. Beck, he took me over the reactor installation. We then spent the rest of the day (with the staff) constructing as best we could the chronological history of the incident, which follows:

May 6, 1955: On this date, Dr. Beck himself operated the reactor for several hours at a power of approximately 6 kilowatts. This is the last date on which the reactor was operated at power.

May 9, 1955: It was noted that the pressure in the core had in- creased over the weekend. Normal pressure was said to be between 5 and 15 inches of water below atmos- pheric pressure. In this occasion there was no apparent reason for the pressure increase, so investi- gation was begun. The core pressure was returned to normal, that is, approximately 15 inches below atmos- pheric pressure and on subsequent readings during the day it was observed to rise slowly.

May 10, 1955: Pressure had returned to atmospheric by morning Samples were taken from the gas in the aluminum envelope that surrounds the stainless steel core and evidence of fission products was found. Subsequently, it was determined that the primary activity was Xe-133 and its magnitude was estimated at one microcurie per cc. It was presumed then that fission product gases were leaking from the gas recombination system, there being no evidence that liquid had leaked out of the core vessel. Note: There is a small wire suspended between the core and the aluminum envelope very near the bottom of the two containers. When liquid enters this space then the resistance to ground from this wire drops off sharply.

During the next few days confirmatory tests were made of the activity and its magnitude. A detailed procedure was formulated for disposal of the radio- active gas. During all this time the core was at atmospheric pressure.

May 20, 1955: Execution of the procedure just referred to was begun and consisted of the following: --- Gas from both the envelope and the core was bled slowly to the main exhaust stack for the reactor building. Normal flow to the stack is 20,000 cfm and the magnitude of the active gas flow was controlled in such a way as to maintain activity of the stack effluent at less than 10-9 microcuries per cc. This rate was based on the conservative assumption that all the activity was from iodine 131, although it had been shown earlier that the ratio of Xe-133 to iodine 131 was greater than 1000.

May 28, 1955: A slight increase in the gamma activity of the cooling water was noted. This is water that flows through coils of 1/4" stainless steel tubing in the core. This is a "once through" cooling system but there had been no actual flow for some days. The activity noted was that from a sample taken from a hold up tank, which is about 60 feet from the reactor. The next step was to insti- tute flow for a sufficient time to obtain a sample which had actually come from the reactor core. It was found that activity of this sample was about 25 times geater than that of the first. The cooling coils were then put under pressure of a approximately 55 pounds and the system was closed off. However, no appreciablc drop in pressure occurred over a period of several hours and it was concluded that although a leak apparently existed in the cooling coils, it must be rather small.

May 31, 1955: The presence of uranium in the cooling water samples taken late in the evening of May 28 was established by alpha counts.

June 2, 1955: It appeared on this date that the purge was essentially complete, that is, the activity of the stack effluent was now approximately that of normal background. However, purging continued and is still going on.

June 3, 1955: On this day, Dr. Beck decided to execute a criticality test as a check, via critical rod position, on both earlier and current measurements which indicated that the liquid level in the core had not changed. The critical rod position was found to be normal within limits of reasonable error.

June 6, 1955: It was found that the "wet wire monitor" mentioned in the May 10th note, indicated the presence of liquid outisde the core, that is, between the core and the envelope. The liquid level was checked and found to be low by about 4 millimeters, indicating leakage of approximately 250 cc's of fuel solution into the envelope. Samples of cooling water showed somewhat greater activity than on May 28th. It was then decided that the situation was serious enough to warrant transfer of the fuel solu- tion to storage containers which had been placed in a room below the reactor for this purpose. This was done on June 7.

Between June 7 and June 10, that portion of the concrete shield immediately above the core and plenum was removed and the shim rods and control rod drives and supports were taken out so as to yield access to the reactor core. Incidentally, the tips of the shim rods, which in shut down condition reside in the narrow space between core and envelope are covered with a thin deposit which is quite active.

On June 10, the cooling coils were drained and analysis of samples resulted in Wentification of a number of fission products. It would appear that, in truth, at least two leaks developed in this reactor system; one in the cooling system, and the other in the stainless steel core. On the basis of experience, one might feel inclined to blame the leaks on faulty welds, although it has been shown that a water solution of uranium sulphate can actually corrode through stainless steel plate at points of local surface imperfection.

My own view of this situation is that the safest, and indeed very probably the cheapest remedy is that of complete replacement of the core assembly. At the same time, I think that a unique opportunity exists to learn from this incident and I would therefore recommend that careful and thorough disassembly of this reactor core be made in some location, probably 0ak Ridge where adequate facilities exist, for handling and study of highly radioactive equipment.

The other problem of the moment is that of formulating a safe and adequate procedure for removal of the reactor core, recovery of what liquid has leaked into the envelope, and shipment to the point of disassembly and study, I suggested to Dr. Beck, that if he were interested in calling on the experience of Chalk River personnel in this regard that I will be glad to arrange contact for him.

Dr. Beck has already been in contact with Babcock and Wilcox and with North American Aviation in regard to construction of a new core assembly and I believe he has also requested Oak Ridge to consider building a replacement. I think he is wise to look into the possibility of a commercial supplier for this assembly, and at first glance it would appear that both Babcock and Wilcox and North American Aviation are excellent choices. Personally, I think that North American is parti- cularly apt in view of their past and current experience with water boiler reactors.

Dr. Beck is writing detailed report of this incident to be sent to Dr. Roth, and his agreed to send us a copy

cc: Dr. C. K. Beck N. C. State College Raleigh, N. C.