Criticality Safety Research Group

In Japan, there are various kinds of nuclear material, including spent nuclear fuel, that must be handled safely. Appropriate criticality safety control and attendant regulations are necessary to guarantee safety of workers and the public.

Reasonable safety margins will be taken into account by criticality analysis with less uncertainty, which makes the criticality safety control more effective. Introduction of “burn-up credit,” a typical example of such improvements, considers reactivity reduction by fuel burn. An essential study of criticality safety is necessary if the safety margin is not clear. A typical example is the criticality safety control of fuel debris resulting from the core-meltdown accident at Fukushima Daiichi Nuclear Power Station (1FNPS).

Our research subjects are:

(1) Establishment of the risk-assessment standard of fuel-debris criticality

The database designated “Criticality Map,” to be used to judge subcriticality of fuel debris taking fuel debris conditions as input parameters will be built using well-proven methods of modeling and computation.

(2) Verification of the “Criticality Map” by critical experiments

The criticality map will be validated by criticality experiments to enable judgments of shallow subcritical conditions of fuel debris. The Static Experiment Critical Facility (STACY) of the Japan Atomic Energy Agency (JAEA) will be converted to a general-purpose heterogeneous assembly built with fuel rods and water moderator that can produce various neutron spectra.

(3) Development of the analytical model to evaluate the criticality accident

An analytical model is prepared for evaluation of temperature reactivity coefficients of the criticality accident of uranium solution.

(4) Improvement of the criticality safety analysis method including new fuel types

Evaluation methods of the criticality characteristics are developed for high-burn-up fuels of the next-generation light-water reactor that include uranium whose initial 235U enrichment is more than 5 wt.%.

Figure 1
Figure 1:Experimental equipment for simulated fuel debris samples

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