Based on the current monitoring of the concentration of noble gas (Xe-135) (the concentration is sufficiently low compared to the criticality criterion of 1 Bq/cm3) and the status of the expected fuel debris, the possibility of criticality in the fuel debris at Fukushima Daiichi NPS is low from an engineering point of view. However, it is necessary to understand the conditions under which criticality may occur, such as changes in the shape of fuel debris during fuel debris retrieval, and to prevent criticality in a rational and reliable manner. In the past, organization of the methods have been proceeded to evaluate the likelihood and impact level of criticality of fuel debris based on information to be obtained from internal investigations and at each stage of fuel debris retrieval. In order to evaluate them, it is planned that information on parameters that have a large impact on the criticality evaluation can be obtained in the process of internal investigations and fuel debris retrieval.

In cases where the Uranium content rate of fuel debris is unknown, it is assumed that the safety evaluation and safety measures for criticality management and off-premises transportation are conservatively investigated using the Uranium content rate of 97-98%, which is the rate in the fuel assembly before the accident. From the calculation results of the severe accident code and the video images of the PCV internal investigation, it is highly likely that the Uranium content rate has decreased due to melting and mixing with the surrounding structural materials, though since there is no value to be used for evaluation, an excessive margin should be included in the safety measures.

In the early stage of fuel debris retrieval, a method that does not significantly change the shape of fuel debris is used, including grasping and suction. In addition, the processing volume is limited based on the estimated reactivity change. In the stage of expanding the scale of fuel debris retrieval and cutting, measures such as subcriticality measurement before the work and neutron absorber feeding, are considered. In addition, it is thought that fuel debris retrieval can be performed while evaluating the criticality of fuel debris by checking the amount of fluctuation of the neutron signal around the fuel debris due to the retrieval operation, thereby ensuring the prevention of criticality by combining the response by design and the monitoring and judgment by the operator.

There are various types of neutron detectors currently available, such as fission chamber, B-10 proportional counters, semiconductor detectors, depending on the application, and it is important to select a neutron detector based on its characteristics.

The required specifications of a neutron detector for criticality monitoring are as follows: : The neutron detector shall
1. be able to maintain the lifetime (accumulated dose (Gy)) corresponding to the work period;
2. be mountable on the target equipment (size, weight, cable diameter) or installable in the work site (size, weight, cable routing); and
3. meet the required detection efficiency (time, accuracy).

In order to prepare for a case which the criticality of the fuel debris is found to be high, investigation on the evaluation of the required boron concentration and the feasibility of filling the debris with sodium pentaborate during normal fuel debris retrieval, while the environmental impact of the leakage and the coexistence with concrete as a structural material are being evaluated. In addition, non-soluble neutron absorbers, which can limit the effect on the PCV circulation cooling system locally, are being developed. Basic physical property tests and radiation resistance tests were conducted so far and found that sintered B4C metal, B/Gd glass, Gd2O3 particles, and water glass/Gd2O3 granulated powder are candidates for nonsoluble neutron absorbers. For these candidate materials, effect of long-term irradiation on the storage canister integrity during fuel debris storing have confirmed, in addition to the method and the effect of spraying on fuel debris corresponding to debris processing. In FY2020, the feasibility study on a boric acid conditioning facility in the circulating water system was conducted, and technical development for on-site operation combined with neutron detectors will be promoted in the future.

In order to prevent criticality during fuel debris retrieval, criticality monitoring and criticality management systems are being developed. Although the likelihood of criticality in most operations is considered fairly low, measures are being investigated in case of re-criticality, for example, storing neutron absorbent in a container when removing structures with fuel debris attached.