Technically, this type of potential accident builds an impromptu nuclear reactor, and not a nuclear weapon. There is no way to stack up pure uranium bricks fast enough for them to explode as a bomb, because an entire explosion takes place in about a microsecond, much faster than anyone can lay down blocks. But such a situation of stacking enriched uranium bricks would still be extremely dangerous, as it would make a basic nuclear reactor without bio-shielding and without even rudimentary controls. It runs wild until the heat generation is sufficiently severe to wreck the stack and make it subcritical by virtue of shape.
The behavior of the water-solution stack and the solid-metal stack are significantly different. In a water stack, the power generation is dependent on neutrons slowed to thermal speed. Not only are the neutrons slowed down, they are separated by distance from uranium nuclei in the dilute water solution. The reactor is spread out over a large volume, the size of a garbage can. The metal reactor has more uranium in it, but it is extremely compact. In the optimum configuration, a sphere, it is the size of a grapefruit, and it is extremely sensitive to its environment. If you have a barely subcritical sphere of uranium-235 sitting on a tabletop, not fissioning or causing any radiation to speak of, then simply walking by it or waving a hand over it will cause it to go supercritical, raising its temperature and spewing out radiation in all directions, increasing exponentially.[36] This happens because your body consists of about 70 percent water. Random neutrons, born of spontaneous fissions and escaping off the surface of the sphere, will hit your hand occasionally and slow down in your water component. Those occasional neutrons are knocked in all directions. A scant few wind up drifting back toward the sphere from which they came. They re-enter the fissile material, and this extremely slight increase in the number of available slow neutrons can set off a chain reaction. The neutron population bursts into high production, and it’s off to the races.
Incredible as it seems, the difference between the subcritical neutron population in a uranium mass, making no fission, and supercritical, making wild, increasing fission, is a very small number of available neutrons out of trillions: all it takes is just one neutron.
A similar possibility of accidentally assembled reactors existed at the Hanford Works, built a year after the Oak Ridge facility out in the desert in the middle of Washington State. It was another instantly derived city, a bit larger than Oak Ridge, having 50,000 people. Its product was plutonium-239, an artificially produced isotope made by subjecting uranium-238 to neutron bombardment. The fissile material was nearly 100 percent pure, and having low enriched material was never a problem. In water bottles or stacked in bricks, it was as problematic as pure uranium-235 and much more plentiful.
At Oak Ridge in 1944, batches of enriched uranium began to accumulate, and a memo arrived at Los Alamos from a plant superintendent, expressing concern about the possible peril of having bottles of uranium-water solution neatly stacked in a corner. Would it be advisable to install a special fire extinguishing system? This memo set off alarms on multiple levels, and J. Robert Oppenheimer, head of the scientific mission to develop the A-bomb, dispatched Emilio Segré to Tennessee to assess the situation.
Segré was a typical worker at Los Alamos, in that he was a brilliant physicist and a recent immigrant from fascist Europe, having been driven away by the enforcement of official anti-Jewish regulations. He would eventually win the Nobel Prize in physics and discover two new elements and the antiproton, but in 1938 he was a refugee stuck in a $300-per-month job as Research Assistant at Ernest Lawrence’s Berkeley Radiation Lab in California. When Dr. Lawrence, who believed strongly in fiscal responsibility, figured out that Segré had nowhere else to go, he dropped his salary to $116 per month. The talented Segré felt fortunate to have been grabbed by the U.S. Government to work on the bomb program in New Mexico. As head of the experimental division’s radioactivity group, Oppenheimer thought he could spare him for a few days to see what was going on in Tennessee.
Examining the situation at Oak Ridge, Segré found that no workers knew that they were making an explosive, much less that it was a very tricky one, and only a few top officials were aware of the problem of bringing together a critical mass. They had been given the talk, but it had mentioned only the problem of stacking metal bricks, and they had no idea that water diluting the active substance only made it easier to produce a runaway reaction. The accumulating stores of wet uranium at Oak Ridge were on the verge of disaster. Oppenheimer responded to Segré’s grim report by dispatching his best man, Richard Feynman, immediately to the scene.
Feynman was only 27 years old, the youngest group leader in the mass of heavy thinkers gathered at Los Alamos. Working under the director of the theoretical division, Hans Bethe, he was one of the few natural-born Americans on the T-section payroll. He grew up in Far Rockaway, New York, and earned his physics degrees at MIT and Princeton. He had quickly established a reputation as a quick mind with brilliant insights and an ability to find the problem in any aspect of the complex bomb development. He also gained fame by an apparent ability to crack any combination security lock at the lab. Everyone was impressed, particularly Oppenheimer, who was not disappointed by Feynman’s sharp analysis of the problem.
It was even worse than Segré had reported. There were storage drums of different sizes stored in dozens of rooms in many buildings on site. Some held 300 gallons, some 600 gallons, and some an eye-opening 3,000 gallons of uranium oxide dissolved in water, in a range of uranium-235 enrichments from raw, natural uranium to nearly critical concentrations. Some were on brick floors, which was fine, but some were on wooden floors. Wood is an organic compound, and it contains hydrogen, which would moderate the speed of leaking neutrons and reflect them back into a drum, enhancing the conditions for fission. In some cases large drums were segregated into adjoining rooms, but if two drums were backed up against the same wooden wall, the two subcritical nuclear reactors were capable of coupling into one critical assembly, using the wall as a neutron-moderating connection.
Atop all those problems, the very shape of a drum encouraged fission. Drums were made to minimize the amount of metal needed to build a container of a given size, so the volume-to-surface-area ratio was optimized. Feynman examined the floor layouts of the agitators, evaporators, and centrifuges used in the sequential processing of the uranium. From the blueprints of the buildings, he could tell that the architects did not have nuclear physics in mind when they drew the floor plans. The entire industrial complex of the Clinton Works was a disaster under construction. The potential meltdown, in which a nuclear reactor could be unintentionally assembled and run up to power, was given a name: the criticality accident.
There was no set rule for how much uranium water could be stored in one location, or how close two drums could be located. There were simply too many variables at work to be able to look in a room and say, “Put one more drum in here, and it will take an aerial photo to see the entire crater.”[37] Feynman relished the task of mathematically solving the impossibly complex interactions of bricks of metal near steel drums scattered in random locations in connected rooms, but the problem boiled down into one fact: the workers in this production facility could not be kept unknowing of what was going on. Some raw knowledge was the key to preventing a nuclear disaster. Oppenheimer gave him the go-ahead. The rabid security measures were now working against the project, and this would have to be an exception to the total ignorance policy, or the uranium and plutonium production could self-destruct. Feynman prepared a series of lectures for workers and supervisors, starting with the simple basics of nuclear physics. This action probably saved many lives, but in the next few decades the lesson would have to be learned over and over, continent by continent.
36
This problem is only hypothetical. During the war, there was never enough uranium-235 existing at Oak Ridge to make a purely metallic, highly enriched critical mass. As soon as a piece of it the size of a silver dollar accumulated, it was sent off to Los Alamos, where every scientist knew of the potential hazard of collecting the metal in one place.
37
A blatant exaggeration, but you get the point. As will be chronicled in a later chapter, this accident has happened on many occasions. When the contents of the drum go supercritical, the water boils vigorously. Just the boiling action changes the configuration enough to throw the reactor into subcriticality, and the reaction stops. The radiation burst of an uncontrolled approach to criticality in an unshielded assembly is deadly to any nearby organism.