The JAEA research program includes computation of criticality characteristics covering a wide range of fuel debris conditions and validation of the computation by critical experiments. In the former activity, several data sets will be systemat­ically obtained by calculation to establish new criticality safety standards for fuel debris. The new standards will be provided as “criticality maps” that indicate subcritical and critical conditions. The maps also show supercritical conditions that would likely lead to a significant threat of human injury [7]. In the latter activity, the new standards (including computation models) will be validated regarding reactivity worth, coefficients of reactivity, and critical mass by critical experiments with simulated fuel debris samples. A criticality monitoring method­ology will also be studied to improve the criticality control measures for fuel debris.

To pursue the aforementioned critical experiments, the core of the modified STACY has a widely distributed neutron energy spectrum between thermal reactor spectra and intermediate reactor spectra. The neutron energy spectrum of the core can be varied by the lattice pitch of the fuel rods, which range from 10.9 to 25.5 mm, corresponding to a moderator-to-fuel volume ratio ranging from 0.9 to 11. Typical neutron energy spectra of the modified STACY are shown in Fig. 22.2 [8].

This figure also shows typical spectra of hypothetical fuel debris of a BWR fuel pellet (3.7 wt.% 235U, 27.5 GWd/t, 5-year-cooled), for comparison. Both spectra were calculated using a burn-up code, ORIGEN2 [9], and a Monte Carlo code, MVP2

[10] , with a nuclear data library, JENDL-3.3 [11]. It can be seen in Fig. 22.2 that the core spectrum with a lattice pitch of 10.9 mm is equivalent to the debris spectrum in 50 vol.% water. The core spectrum of the modified STACY can cover relatively hard spectra of the fuel debris likely to become critical.

For the measurement of the neutronic characteristics of fuel debris, two sets of experimental equipment should be prepared: one includes reactor material struc­tures simulating fuel debris (zircaloy, stainless steel, concrete, etc.), which are pin-, plate-, or box type and are loaded between fuel rods. The other is a sample-loading device to measure its reactivity and which is installed at a test region in the core tank. The experimental equipment is shown in Fig. 22.3.