Japanese Scientists Were Preparing Nuclear Fuel In '99, Then Something Went Very Wrong

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Nuclear power-related disasters can occur on various levels. Among the biggest and most well-known incidents in history, the Chernobyl disaster in 1986 had a widespread negative impact and was caused by a mishandled safety test. Power plants aren't the only places where nuclear operations can go awry, though. In Tōkai-mura, Japan, for instance, the preparation of nuclear fuel went very wrong in 1999, becoming the first criticality accident of its kind in the country.

First of all, the process of uranium enrichment is required because pure, natural uranium can't be used as a source of power. The JCO (formerly Japan Nuclear Fuel Conversion Co.) plant at Tōkai-mura involved in the '99 criticality accident — an uncontrolled nuclear chain reaction — was processing as much as 3 tons of enriched uranium every year. Although dry processes are used at most preparation plants, this one was designed to use a wet process involving multiple stages of chemical conversion. The water in the solution used can expedite reactions, so safety is important. However, the scientists weren't following approved procedures. They used a modified process without regulatory permission and cut corners, one of which included bypassing criticality controls.

When the accident happened on September 30, three workers were enriching the metal to 18.8% U-235, which is considered high-assay low-enriched uranium, for use in an experimental advanced reactor. Another problem was that, while they had experience processing up to 5% U-235, they weren't properly trained for such a high level of uranium enrichment. Eventually, a critical mass was reached in the precipitation tank that triggered a self-sustaining nuclear fission chain reaction, intermittently emitting neutron and gamma radiation for about 20 hours even without an explosion.

Because the Tōkai-mura nuclear accident involved the emission of multiple types of radiation over such a long period, homes within about a 6-mile radius were told to stay indoors until the hazard subsided, and those within about a quarter mile were evacuated. Inside and directly outside the plant, 172 workers were exposed to varied levels of radiation. About 260 accident response task-force members and 235 nearby citizens were also exposed.

Of that total 667 people, most didn't exceed the maximum allowable annual dose of 50 millisievert (mSv). The three workers who were preparing the enriched uranium, though, were exposed to a range of full-body doses: (1) 1,000 to 5,000 milligrays (mGy), (2) 6,000 to 10,000 mGy, and (3) 16,000 to 20,000 mGy. Considering that a fatal dose is 8,000 to 10,000 mGy, the two workers exposed to the highest doses eventually died: Hisashi Ouchi died on December 21, 1999, and Masato Shinohara died on April 27, 2000. "A Slow Death: 83 Days of Radiation Sickness" documents the radiation poisoning and treatments of the workers at University of Tokyo Hospital until their deaths.

Also following the Tōkai-mura criticality accident, the Japanese government made changes to laws to update worker safety education and put resident inspectors in place to monitor procedural compliance. An employee whistleblower system and periodical inspections for fuel fabrication facilities were also implemented. On top of that, the national preparedness and response capabilities were bolstered.