Rokkasho Update: Earthquake safety and criticality design flaws Nuke Info Tokyo 118
Step 3 of “active tests” at the Rokkasho reprocessing plant were completed on April 26th. Although numerous problems have arisen, Japan Nuclear Fuel Ltd (JNFL) has not officially changed the November 2007 scheduled date for the commencement of commercial operations. Nevertheless, there is no doubt that there will be further delays.
Incident during denitration of mixed uranium/plutonium solution
On March 11th, during uranium/plutonium denitration tests, a second batch of uranium/plutonium solution was accidentally poured into a tray which had not been emptied of the previous batch. The operator noticed an irregularity after denitration of the first batch (7.3 liters) had been completed and denitration of the next batch commenced. In order to prevent criticality accidents, these trays are designed to contain only one batch of plutonium solution at a time. However, the first batch had become a solid as a result of the denitration process, so the volume was less than that of the original solution. It was therefore possible to add a second batch to the tray. The ease with which design measures aimed at preventing criticality accidents were circumvented on this occasion demonstrates the inadequacy of those measures at the Rokkasho reprocessing plant.
Under an agreement between the US and Japanese governments, the Rokkasho reprocessing plant is designed such that plutonium and uranium are separated, then remixed in a 1:1 solution, which is denitrated and stored as a uranium/plutonium mixed oxide (MOX) powder. The denitration process used at the Rokkasho reprocessing plant was originally developed at the Tokai reprocessing facility. It involves heating with microwaves, rather like a microwave oven.
JNFL employs an interlock system, based on weight, to prevent trays containing denitrated MOX powder from being returned to the denitration machine. However on this occasion, due to an equipment problem, the machine had been switched to manual mode and the interlock was not operable. JNFL foresaw the possibility of pouring two batches of solution into a single tray, but it did not envisage the possibility of a second batch being poured onto a solid mixture of plutonium and uranium. JNFL blamed the operator for not paying sufficient attention to the movement of the tray and not checking carefully enough when adding the solution. It says it will adjust the process and amend the operating manual.
Earthquake safety design flaws
In April, deficiencies were discovered in the earthquake safety design of over ten types of equipment, including the channel box shearing machine and fuel handling equipment in the spent fuel pool, and equipment in both the separation building and the low-level waste processing building. The equipment in question was designed in 1993 by Hitachi Engineering and Services. In 1996 an employee noticed that an incorrect calculation had been made in regard to earthquake safety, but he did not report the mistake. A recalculation showed that the equipment failed to meet earthquake safety design standards and that it would not withstand the type of earthquake envisaged by these standards. If a strong earthquake struck Rokkasho, the equipment could fall and smash the spent fuel in the pool.
The discovery of the mistake at this time was a complete accident. For many years citizens have been criticizing the Nuclear Safety Commission in regard to earthquake safety. Last year, after a five-year process, the earthquake safety guidelines were finally revised and earthquake safety assessments are now being carried out for all nuclear facilities. The design flaw at Rokkasho was discovered during these back checks. JNFL is still piecing together the full picture, but clearly design and construction work will have to be redone. However, in regard to the fuel handling equipment above the spent fuel pool (a crane used to move the spent fuel), JNFL said that it would “carry out calculations using a more realistic analysis model” and continued to use the equipment in order to complete Step 3 of the active tests. Clearly the new calculation will downplay the impact of earthquakes. Work to bring the plant up to earthquake safety standards will require an extended shutdown, but JNFL would rather sacrifice safety than change its operating schedule.
“Active tests” at Rokkasho are now on hold because of the earthquake safety design flaws. The Nuclear Industrial and Safety Agency has indicated that it will not allow JNFL to proceed to Step 4 until these flaws have been rectified. But will this really assure the plant’s safety in this earthquake-prone zone? We are skeptical.
Masako Sawai (CNIC)