The SL-1 Accident 52 Years Later

Remembering the SL-1 Accident 52 Years Later

By Ken Moore

July 19, 2013

I was a young engineer working at the MTR (Materials Testing Reactor) for Phillips Petroleum Company, a contractor to the U. S. Atomic Energy Commission, in the research group headed by Deslonde deBoisblanc and my direct supervision was Bill Byron. As I now remember, this was the time era when I was programming the IBM 650 computer to solve the reactor kinetics equations for hypothetical rod ejection accidents. When the SL-1 reactor accident occurred the night of January 3, 1961, I had been working at the National Reactor Testing Station (NRTS) for about four years.

The morning after the SL-1 accident I was on a site bus going from Idaho Falls to the NRTS where the MTR and other reactors are located. The bus actually passed the side road that leads to the SL-1 area. As I remember there was some activity at this area where vehicles being directed to quickly move past. Most people on the site busses either sleep or read for the trip that takes about 45 or so minutes and I was asleep and remember only some activity there. I do not remember when I learned that a reactor accident had happened. It may have been on the bus but most probably it was when I reached my work location at the MTR facilities. I think my friend Dwight Parry was working at the ML-1 facility which was very near the SL-1.

We definitely heard all the various rumors involving love affairs, etc. One of the men that assisted in the rescue efforts was an acquaintance named Sid Cohen who was reported to have entered the reactor building to help extract one of the men. Sid received a fatal dose of radiation and died sometime later. It was also reported that one of the bodies was impaled with the reactor control rod in the ceiling of the building. I do not know if this gory description was true. The bodies were very radioactive when retrieved from the building. I heard that the bodies were taken to the Chemical Processing Plant (CPP) and placed in vats of ice and alcohol until it could be decided as to just how to prepare them for burial. After some time (several weeks?), the level of radiation decreased significantly and it was said that the alcohol mixture probably leached out surface contamination. I think they were eventually buried in lead lined caskets.

I was assigned the job of calculating just how far the control rod had to be withdrawn to bring the reactor into critical operation. This involved obtaining the operating history of the SL-1 with many ups and downs as well as the history control rod positions for criticality. SL-1 had boron loaded polyethylene strips mechanically attached to the fuel plates. Over time the mechanical attachments failed and strips were falling off into the bottom of the reactor vessel. Boron is a neutron absorber or what is called a poison to the chain reaction process. Thus when a boron strip fell off, the distance that the control rod needed to be raised to make the reactor critical was less and less as more strips fell off.

The other significant item is the operating history. When a reactor is operating, some of the products from uranium fission are also poisons to the fission process itself. The main ones to consider are xenon Xe-135 and samarium Sm-149. The amount of these poisons produced depends on the reactor power level and amount of time that the reactor is operating. These poisons are also radioactive and decay when the reactor is in a shutdown condition.

From what I remember, the SL-1 had one control rod that had to be lifter up manually and hooked onto a drive mechanism that could be electrically operated to withdraw the rod to “go critical”.  While I do not have any of the original records of my calculations (done on an IBM-650 computer?), I remember that it was only a matter of inches that the rod could be withdrawn for criticality. This was obviously a very touchy situation. The incident certainly could have been a simple accident where a “stuck” rod was jerked way past the critical position. Conversely the suicide scenario and personal problems may have also been the cause. I certainly do not know.

Later that year, I published a report “Preliminary Study of a Xenon Predictor for the MTR”, PTR-531, May 23, 1961. This PTR is a “Phillips Technical Report” for the U. S. Atomic Energy Commission. I have my copy of this report and believe it was an outgrowth of the work that I did for the SL-1.

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