Fig. 4-6 The critical levels of fuel solutions The critical levels of 10% enriched uranyl nitrate fuel solutions were measured in STACY experiments.

Fig. 4-7 Verification of the JACS code system by the STACY experiments The results of analysis of experimental data show that the criticality limit used in the safety review of nuclear facilities is conservative.

Fig. 4-8 The core tank of TRACY In TRACY, a supercritical condition of fuel solutions can be obtained by feeding fuel beyond the critical level or withdrawing the transient rod inserted in the center of the core tank.

Criticality safety research using the Static Experiment Critical Facility (STACY) and the Transient Experiment Critical Facility (TRACY) are carried out in the Nuclear Fuel Cycle Safety Engineering Research Facility (NUCEF) for safe handling of plutonium and uranium solutions in reprocessing plants. The STACY performs critical experiments using the various core tanks that simulate equipment in reprocessing plants. In Feb. 1995, the first criticality of STACY was achieved. Critical experiments of 10% enriched uranyl nitrate solution using the cylindrical core tank (60 cm in diameter) are now in progress. Figure 4-6 shows critical levels of fuel solutions using a water reflected or unreflected core tank. Figure 4-7 shows an analysis of the STACY experimental data. The calculated value of the effective neutron multiplication factor in the STACY experiments is larger than the estimated lower limit criticality value of homogeneous low-enriched uranium by using the JACS code system in the criticality safety handbook. This data shows that the criticality limit values in the criticality safety handbook are conservative (on the safe side). In TRACY, a supercritical condition of low-enriched uranyl nitrate fuel solution is simulated to investigate the behavior of fuel solution and the released radioactive material. The first criticality of TRACY was achieved in Dec. 1995. Transient critical experiments have now started.

Reference Proceedings of the First NUCEF International Symposium NUCEF95, JAERI-Conf 96-003 (1996).

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