By January 3, 1961, the SL-1 had been producing power for over two years, and the experiment was going well. This was more than just a test to see if a small boiling-water reactor would produce power. There was not really anything to learn on this point. The purpose of the test was to see if such a plant could be run and managed by minimally trained soldiers without the advantage of expert scientists looking over their shoulders. Could the inhabitants of a remote station, even more cut off from civilization than the NRTS, maintain a power reactor no matter what happened?
There had been a few problems. Water seals and gaskets were leaking, but it was nothing fatal. Those boron strips tacked onto the fuel turned out to be a bad idea. The boron, when it was changed into carbon-14, would curl up and crack. Pieces of it were falling into the bottom of the vessel, and what was left on the fuel was binding up the controls. Under any normal reactor operating conditions, the words “binding up the controls” would result in a quick shutdown, a study, a redesign, assignment of blame, papers written, operating rules changed, and so on. In this case the problem was treated as an interesting perturbation thrown into the exercise. It was just the sort of unexpected problem that could show up on the glacier in Greenland. The Army said “Let’s see how it plays out,” while the AEC and Combustion Engineering kept a detached, mildly interested stance. Any time the reactor was down and apart for maintenance, the peripheral rods had to be jacked up and down by hand to clear out the bent boron at the bottom of the core, or the motors would not be strong enough to move them against the resistance. The main rod, weighing a hefty 100 pounds, was big enough to take care of itself, crashing its way through the twisted metal and having no particular trouble moving with the motor.
It was a clear night and bitterly cold. The graveyard shift consisted of Senior Reactor Operator Jack Byrnes, an Army private; Assistant Operator Dick Legg, a Navy Seabee; and trainee Richard McKinley, Air Force. Byrnes was going through a marriage crisis, he wasn’t making enough money, and he had problems with being managed. Legg was sensitive to comments about his stature and enjoyed playing tricks on his colleagues to pass the time. McKinley was there to learn how to run a reactor plant.
It was time to change out the flux wires, and the previous shifts had done all the heavy lifting. The reactor was basically put back together, with the vessel filled to 9 feet with water and the head screwed down. All the night shift had to do was reconnect the controls and put the big concrete blocks that shielded the top back into place. It was a three-man job. Byrnes and Legg would reconnect the rack to the main control while McKinley would stand off and act like a health physicist, pointing a “cutie pie” ionization chamber at Byrnes and Legg while they worked. The day shift would actually have an “HP” on the staff, monitoring all the activities in the reactor building to constantly check for abnormal radiation, but the night shift was a minimum crew.
The rack that engaged the pinion for up-down motion was screwed to the top of the control, which was at its lowest position in the core, keeping the reactor at cold shutdown status. A C-clamp had been tightened on the rack to hold it in a slightly raised condition, just above the top of the shield plug. This position allowed a three-foot metal rod to be temporarily screwed into the top of the rack so that a man could handle it with the motor disconnected.
The instructions were clearly mimeographed. Byrnes would take hold of the handling rod with all ten digits and lift the 100-pound center control by one inch. With the load off the C-clamp, Legg, crouching over the shield plug, would unscrew it and lay it aside, and then Byrnes would gently lower the control until it rested on the bottom of the core structure. McKinley was standing off the reactor top, in front of one of the man-sized concrete shield blocks, idly watching the show and pointing his radiation detector.
All that we know for sure is that at 9:01 P.M., Byrnes, against the written directions and everything that the instructors had drilled into his head, with one massive heave jerked the master control clean out of the core as fast as he could. If it were lifted four inches, the reactor would go critical, blasting the three workers with unshielded fission radiation. Byrnes managed a full 23-inch pull, and the reactor went prompt critical with a 2-millisecond period, producing a steam explosion in the reactor such as has never been seen before or since. The water covering the reactor core instantly became superheated steam.[108] The four-foot slug of still water over the core, not becoming steam, was pushed with incredible speed to the top of the reactor vessel, through the 4 feet 6 inches of clear space, until it hit the screwed-down top like a very big hammer. The force of the hammer-hit picked up the 13-ton steel vessel and shot it nine feet out of the floor, shearing away its feed-water and steam pipes. The nine shield plugs on top of the vessel shot off like cannon shells, burying control-rod fragments in the ceiling.
Byrnes and Legg were killed instantly, not by the intense radiation surge, but by the explosive shock of two billion billion fissions, 15 megawatt seconds of energy, and an air pressure wave of 500 pounds per square inch. McKinley died two hours later of a massive head wound, inflicted by the concrete shield block as he was thrown backwards. All three men had fission products, built up by the reactor running at full power for two years and turned to nascent vapor in the sudden heat of prompt fission, buried deep in their bodies. There was no way to simply wash away the contamination. It seemed embedded in every tissue.
Legg was pinned to the ceiling with a piece of the master control. The reactor internals were an unrecognizable tangle of twisted parts, and water, gravel, and steel punchings were scattered all over the reactor building floor. The crude steel building, which was meant only to house the equipment and keep the rain off, managed to prevent a scattering of radioactive debris, and outside it was hard to tell that anything had happened. The plant was a total loss, and everything would have to be carefully disposed of, leaving not a trace of radioactive contamination or subjecting any worker to an abnormal dose. The three bodies were so deeply and severely contaminated, they would have to be treated as high-level radioactive waste. Autopsies were performed quickly, behind lead shields with instruments on 10-foot poles.
A commendable job was done analyzing the accident and cleansing the site of any trace of it. It required a great deal of skill and planning to decontaminate the site, as the inside of the reactor building was too radioactive for normal work. A full-scale mock-up of the building was constructed for decontamination practice and to figure out the actions of Byrnes and Legg before and during the accident. Television cameras were inserted into the reactor core using remote manipulators, and a Minox subminiature camera was used on the end of a pole to take photographs through small openings. A seldom-mentioned technique called the “gamma camera” was used to spot where highly radioactive fragments of the reactor had landed in the ceiling and on the floor of the building.
The gamma camera is a variation of the old “pinhole” camera. It is possible to make a picture on a piece of photographic film by mounting it on one inside face of a light-proof, square box. On the opposite face of the box is a tiny pinhole. Light enters the box through the hole, very dimly, and it forms an image on the film without the use of a lens. Light can only travel in a straight line, so individual rays of light from the scene outside the box are organized into an image simply because they all have to come through at the same point, the pinhole. If the box is made of lead, through which radiation cannot pass, then the pinhole on the front face of the box will image gamma rays onto the film the same way it uses visible light. Gamma rays are photons, just like visible light but at a much higher oscillatory frequency and energy. To use the camera, the investigators first made a light image of the ceiling in the reactor building using the open pinhole, then covered the pinhole with a light-tight but gamma-transparent shutter. The gamma ray image was allowed to expose the film for 24 hours through the same pinhole, superimposing a gamma-ray image atop the light image. When the film was developed, it identified radioactive objects in the picture as shining brightly, like points of light.
108
Some accounts say that the rod was pulled 20 inches, but the revised figure of 23 inches is based on careful reevaluation of the data. The rod was pushed out an additional 7 inches by the upward force of the steam blast. The weight of the master control was originally recorded as 85 pounds, but this did not take into account the weight of the rack plus the handling rod.