How the Tokaimura criticality occurred

Definition of criticality:

Criticality in nuclear physics is defined as a self-sustaining chain reaction. In fission reactors, it described the process in which atoms split and their neutrons hit the nuclei of neighboring atoms, leading their neutrons to hit the other nuclei and so forth. In a reactor, it is controlled. A criticality accident occurs when radioactive material (usually uranium or plutonium) achieves criticality. What saves those exposed to criticality accidents is adequate shielding and distance.

General information:

There are many ways to prepare uranium fuel depending on the reactor. What must be remembered is that the Tokaimura fuel production facility had its accident mainly due to the bypassing of mass and shape restrictions. Mass restriction means that in a container, there must be a specific mass of fissionable material, so that there are few enough atoms that fission cannot take place. In case mass restriction is bypassed, shape restriction can prevent fission. Shape restriction means that the shape of the container makes fission extremely difficult. Containers used in fuel processing are tall, narrow, and vertical, as to increase the surface area as much as possible and to allow for many neutrons to be lost in the area around the container. 

The way that uranium fuel is produced -normally- is to purify uranium oxide to increase the concentration of uranium-235 (U-235). Uranium oxide is a powder, and it is dissolved in a dissolution tank by nitric acid. Then, it is moved to a hydrolysis column in order to remove any impurities from the mixture, and then to the extraction column, in order to extract the pure solution of uranyl nitrate, and finally it reached the precipitation tank, where it is mixed with ammonia. The result of that mixture is named ammonium diurinate (ADU) and it is removed via trays, which are placed in a calciner, and then heated up with some oxygen surrounding them, in order to further remove impurities. Afterwards, only the purified uranium powder is left, which is passed through the reduction furnace entrance, and then collected. To produce uranyl nitrate solution, the powder is passed through a dissolution tank yet again with nitric acid, and homogenized by cross-blending through twenty different vessels. That way, it is certain that the fuel is fully at the same enrichment level. 

Information about the facility:

In the Tokaimura facility, due to the fact that uranium could be enriched at levels of 20%, all containers has the correct shape other than a precipitation tank. For whoever used the tank, the rule was that once it reached a specific concentration of fuel, the fuel had to be drained before more was added. The cross-blending process was not implemented as it was not necessary for the company behind the facility, JCO (Japan Fuel Conversion Company), to implement it in order to keep its license for producing reactor fuel. It produced uranium fuel from 25/6/1986 to the day of the accident.

After the sixth request for fuel for the experimental reactor Joyo, the process was changed and turned illegal. The second dissolution happened in a stainless steel bucket of a ten litre size. The solution was poured into only seven buckets, with nitric acid mixed in to dissolve the powder. Then it was moved to ten different product vessels to be cross-blended.

In 1995, the bucket replaced the first dissolution tank. A polyethalane bucket replaced the hydrolysis column and the solution was transferred to the extraction column through a hose. The cross-blending was replaced altogether with the dissolution tank from the first step, and through there the solution would be released in the product vessels. Thus, the mass restriction was violated. As to why nobody put a stop to that, the answer is simple: the inspection element was lacking, and the 11/3/1997 accident at the fuel reprocessing plant, which severely damaged the reputation of the company controlling it.

The building where the fuel conversion was taking place was unknown to JCO's president. Thus, after its supervisor retired, instead of someone experienced, the man sent was Yutaka Yokokawa, whose specialty was the production of uranium oxide solution, and who was very unsure about working there, but soon found himself the building's supervisor. He and his crew would produce the fuel. The so-called special crew was composed of Yokokawa, Hisashi Ouchi, Masato Shinohara, and Hiroshi Watanabe.

Before the accident:

For eighteen days, the crew enriched the uranium. Yokokawa considered Watanabe as someone without any knowledge about the conversion test building, and so Watanabe was absent for the last days. 

When they were supposed to follow the last step, they realised that the piping was very badly installed. On 28/9/1999, Yokokawa suggested using the precipitation tank instead, he was told to ask his supervisor, and the next day he had permission. The process changed again. After re-dissolution, the mixture would be split into two five litre buckets, the contents of which would be poured into the precipitation tank, through a funnel, and fans inside the tank would help with homogenisation. By the end of the day, four batches were prepared and placed in the tank. 

The date is 30/09/1999. The technicians processing the fuel are Hisashi Ouchi (worker A), his supervisor Masato Shinohara (worker B), and Shinohara's supervisor Yutaka Yokokawa (worker C).

The positions of the workers during the accident.

30/9/1999:

The fuel processing procedure was resumed at 10AM. The last three batches of uranium solution werefirst melted in a stainless steel bucket and filtered by Ouchi through a filter called Nucho. Yokokawa placed the funnel in a hole and held it in place as Shinohara poured the solution from a stainless steel beaker into the precipitation tank (a large container that held uranium fuel during its preparation). Then, after the sixth batch was done and Ouchi was done with the mixing, he was called over by Yokokawa to hold the funnel. Yokokawa sat at a desk outside the room in order to fill out paperwork. Shinohara continued pouring. After seven buckets, the mixture in the precipitation tank reached enough mass and achieved criticality. There was a smacking sound and a release of blue light, Cherenkov radiation, as well as gamma and neutron rays. The workers left the building.