Homogenization of the Solution and PNC
After the plant had passed a safety review, PNC requested JCO to homogenize uranyl nitrate solution up to one lot (40 liters). In such circumstances, JCO invented the so-called cross-blending method to homogenize the entire solution by using multiple stainless cans, and began manufacturing the solution in 1986. PNC requested blending the solution to a uniform density, due to the approved conditions established for transporting the solution from the facility to the PNCs Tokai plant. If PNC had manufactured the solution on its own, it would have avoided such licensing procedures. Thus, the product quality data -- one unit to be considered as 10 cans, each containing 4 liters -- had to be submitted to authorities for inspection prior to shipping.
Yet, if the quality of the solution was different for each batch (considered as one unit) it would require significant inspection time for testing. Normally ti takes several days for one safety analysis. Therefore, homogenizing the solution in a uniform way to one lot of 40 liters would require only one test, reducing time and effort substantially.
During the 1980s, JCO used the cross-blending method as described above to make the density of the solution consisted and started to fabricate the solution by using the storage column in the facility during 1990s. Seen in this way, it can be said that the transformation of manufacturing process (from cross-blending to the storage column, and then to the precipitation tank) made criticality more likely to happen. Among the three methods, only the precipitation tank had a structural design likely to trigger criticality.
It is also known that workers at the JCO facility carried out re-dissolution work by using stainless containers (bucket). But, the direct cause of the accident was in the process used to make the density of the solution consistent. To summarize these points, neither the blending methods performed at the precipitation tank and storage column nor the cross-blending was checked during the safety inspection of the stainless-steel maker (Conversion Test Building in 1984.
Moreover, the Conversion Test Building was originally designed to handle uranium powder and only later, as a result of Japan-United States agreements, was it used to manufacture uralyl solution. In short, JCO was forced to develop many different manufacturing processes in order to make use of the facilitys equipment originally intended to produce power products.
In principle, the manufacturer should have designed a different way for manufacturing the solution, separate from processes used for power products. However, the safety review of the Conversion Test Building approved adapting the equipment so that it could be used for making the uranyl nitrate solution. Furthermore, even several years after the safety review, PNC requested JCO undertake a process to make the solutions density consistent, knowing that such work was not approved by the explicit conditions of the license.
Limitation of the Court Trial
Yet, such factors external to JCO were used as the defendants strategy to establish extenuating circumstances at the trial. Lawyers for the prosecutions or lawyers of the accused made efforts to investigate the cause of the accident thoroughly and comprehensively.
The Court did not investigate significant people involved in the accident, including the JCO Accident Examination Committee organized by the Nuclear Safety Committees. Nor were important people called as witness at the trial. The list would include the governments Nuclear Safety Committee that approved the safety review without checking claims by JCO and governmental officers; the PNCs officer responsible for requesting JCO to create a manufacturing method to assure the solutions consistent density, and the STA official responsible for administering JCO.
There were many uncertainties and inconsistencies in the testimony of accused -- for example, the claim that nobody has any knowledge of criticality, or again it was believed that solution was less likely to cause criticality than a powdered form. (Actually, criticality is more likely to occur in a solution form.) Crucial questions regarding why the precipitation tank was invented were not to answered and detailed discussion on how it was used did not take place.
Everyone involved in this trial protected them self and so it ended as though there were no direct causes for the accident, no one to accuse, no one to provide answers. It can be said that the ruling was a kind of negotiated outcome devised to work for prosecutors, accused, the PNC, and the government. Thus our unstinting efforts to reveal the truth in the accident are still necessary.
On April 18 -- next month after the ruling -- JCO announced that it would not to resume the operation of the facility. At the same time, JCO also announced that it planned to dismantle the interior of the Conversion Test Building -- the scene of the accident. Since the investigation of the cause of the accident is still premature, the JCO facility should be preserved as an important historical monument, instead of clearing it as an excuse for the end of trial.