Fig. 1 shows the image of proposed applications of activity measurements of fission products with short half-life ⁸⁸Kr and ¹³⁵Xe to estimate the neutron multiplication factor (k_eff) of a primary containment vessel (PCV) and a fuel debris storage canisters. This method collects gas from various places in the fuel debris storage canister (Fig. 1 top) or the PCV (Fig. 1 bottom). An additional advantage of gamma-ray emitting gas (⁸⁸Kr and ¹³⁵Xe) measurement is that it can be performed remotely in a place with relatively low gamma-ray background. Furthermore, the criticality at each location is evaluated from the relationship between the activity ratio of ⁸⁸Kr/¹³⁵Xe and the k_eff (Fig. 2 and Fig. 3) obtained in advance.
 We applied a subcritical depletion calculation code developed previously by ourselves to various fuel debris conditions in the decommissioning process in the fuel debris storage canister and in the PCV. We modelled the fuel debris storage canister to investigate the correlation between the k_eff and the activity ratio of ⁸⁸Kr/¹³⁵Xe (A(⁸⁸Kr/¹³⁵Xe)) for a wide range of fuel debris compositions. We also modelled the PCV to study the k_eff -A(⁸⁸Kr/¹³⁵Xe) correlation in different geometries.
 We obtained that the correlation between fuel debris storage canister criticality and A(⁸⁸Kr/¹³⁵Xe) was linear and the k_eff -A(⁸⁸Kr/¹³⁵Xe) correlation was not sensitive to geometry such as in the fuel debris storage canister or in the PCV. These results indicated that the activity ratio A(⁸⁸Kr/¹³⁵Xe), which could be measured at a place with relatively low gamma ray background, could be employed for monitoring criticality.