For criticality safety assessment, investigation is now underway on injecting boric acid based on conservative assumptions, preliminarily injecting non-soluble neutron absorbers, and overseeing work for subcriticality level based on neutron measurements. However, it is necessary to show the basis(e.g., compatibility of neutron absorbing materials with U, the amount of U, and neutron absorbing materials, etc.) for the existence/absence of possibility of fuel debris criticality in each unit and part through analysis of fuel debris samples and rational evaluation based on the analysis.

In heat generation and cooling countermeasures, the heat generation countermeasure can be evaluated analytically by evaluating changes in temperature distribution through on-site cooling water injection stop tests. However, when a temperature increase is observed in a specific part, it is important to obtain knowledge on the heat source, such as the U isotopic ratio and the FP composition. In the cooling countermeasure, on the other hand, it is important to evaluate the effect of a change in the chemical environment caused by changing the cooling method on the characterization of fuel debris and also evaluate changes in the surface and the deposition state of the fuel debris caused by the oxygen incorporation in the atmosphere during negative pressure control.

In the hydrogen generation countermeasure, it is necessary to evaluate the radiolysis of the cooling water by beta and gamma nuclides (especially, the influence of low energy radiation is large), which is considered to be a hydrogen generation source and the reaction between the cooling water and the unoxidized materials caused by the exposure of new surfaces during the fuel debris retrieval.

For the confinement of alpha dust, etc. during cutting, currently assuming laser cutting and mechanical cutting as cutting methods, rough evaluation of dust dispersions is in progress based on cutting simulation tests using simulated debris and past experience in hot cells. For laser cutting, it is important to evaluate changes in the oxidation degree caused by higher temperature of fuel debris (if the oxidation degree is low, low-order oxides may be formed, leading to a concern about re-evaporation) and the degree to which the volatile materials contained will be dispersed. For mechanical cutting, it is important to understand the mechanical properties (including hardness, brittleness, and melting point, etc.) and chemical properties (including average properties of a mixture or aggregate of phases and compounds) of fuel debris.

The handling of fuel debris needs to be investigated from the viewpoint of safety and workability and from the viewpoint of radiation dose. From the viewpoint of safety and workability, the handling of chemically active metallic debris, local chemical reactions and FP leaching caused by new surface exposure of fuel debris during retrieval work, and clarification of the persistence and precipitation of hard materials (residual B4C and borides) are the issues. In the case of air-cooling, fine particles adhering to the surface of the fuel debris may scatter, so it is necessary to take into account the surface adhesion during drying. As for the radiation dose, information on the dose rate of fuel debris is important as the basic data for the worker exposure control and the lifetime evaluation of the equipment used. In addition, it is important to understand the source information necessary for occasional dose rate evaluation by analysis.

In the investigation of a side access method for fuel debris retrieval, it is important to improve the accuracy of the evaluation of the condition inside the RPV, where a considerable amount of fuel debris remains. For this purpose, it is important to improve the evaluation accuracy of the fuel debris accumulation in the lower plenum of the RPV and the damage condition of the lower head, by the inverse problem analysis of the accident progression based on the analysis of samples containing fuel debris obtained from the pedestal, etc. The analysis items that are important in the inverse problem analysis are the chemical properties of the main components(including U, Zr, Fe, and B) of the fuel debris and deposits and the items related to their evaluation and among them, chemical properties of phases and particles containing U are specifically important.

In the case where the concentration of U, Pu, burnable poison, FP radionuclides in the fuel debris is not known, for countermeasures such as criticality safety, cooling, and radiation shielding of transport/storage container of fuel debris, radiation resistance of equipment installed and hydrogen generation, the safety assessment is considered to be proceeded by assuming the maximum (or minimum) burnup and making conservative assumptions depending on the fuel debris handling such as no combustible Gd, no residual FP (or all FP remaining), etc.. Extremely conservative assumptions may delay the fuel debris retrieval with high handling volumes and may generate new risks. In order to allow for more reasonable conservative assessments, it is desirable to analyze burnup indices (148Nd (or alternative nuclides or elements) mass/U mass in fuel debris, and 235U mass/U mass in fuel debris).

For the analysis of criticality safety in storing and management, it is important to understand the composition and isotope ratios of nuclear fuel materials in the fuel debris as basic data. In addition, it is necessary to have such basic information on the composition and isotopic ratios of major neutron absorbing materials, the density of fuel debris, and water content ratio, etc., and information on indicators for fuel debris analysis and evaluation, such as 148Nd, etc. is important in evaluating the contribution of FP. The heterogeneity should be taken into account because it is assumed that nuclear fuel materials with various compositions are heterogeneously mixed in the fuel debris.

In the analysis of nuclides and radioactivity in storing and management, it is necessary to obtain information necessary for the evaluation of hydrogen generation amount and calorific value. For hydrogen generation, it is necessary to estimate the nuclides (alpha, beta, and gamma radiation sources) that may contribute to hydrogen generation and their radioactivity levels, the physical properties(including particle shape) of the radiation sources, and the water content ratio in packaged material or water amount in the storage container. As for the calorific value, it is necessary to have data on various radioactivity measurements of nuclides (137Cs, 90Sr) contributing to heat generation during the first decades after packaging, as well as data on the measurement of concentrations (vs. uranium) of actinide radionuclides in fuel debris.

As for the analysis of aging during storing and management, the evaluation has been in progress so far on aging behaviors using simulated fuel debris, though in the future, it is important to demonstrate using actual fuel debris. Specifically, it is necessary to confirm and verify the characteristics of fuel debris (especially the presence and chemical state of key elements such as actinide elements) and the verification of physical, chemical, and biological mechanisms (including weathering due to temperature changes, leaching due to contact with water, and microbial degradation).

For chemical stability during storing and management, it is important to collect knowledge and information on corrosion evaluation of the storage container. It is desirable to analyze the pH, chloride ion concentration, and nitrogen oxide concentration in the liquid phase in and around the fuel debris.

As for the rationalization of the storing facility, it is desirable to analyze the nuclides and their radioactivity of the fuel debris to be packaged, evaluate the calorific value of the storage canister, and confirm the cooling capacity.

The inventory of radionuclides in the fuel debris and the values of radionuclide migration parameters are used to assess the safety of processing and disposal. The analytical research facility in Okuma is planning to analyze 38 nuclides that are considered important for disposal (3H, 14C, 36Cl, 41Ca, 60Co, 59Ni, 63Ni, 79Se, 90Sr, 93Zr, 94Nb, 93Mo, 99Tc,107Pd, 126Sn, 129I, 135Cs,137Cs, 151Sm, 152Eu, 154Eu, 233U, 234U, 235U, 236U, 238U, 237Np, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu,241Am, 242mAm, 243Am, 244Cm, 245Cm, 246Cm).

In the safety assessment of disposal, in order to understand the variation of environmental conditions achieve a highly reliable assessment, it is important to understand the chemical composition and amount of the contained substances (influence substances) that affect a criticality safety assessment of the fuel debris, the leaching characteristics of radionuclides from the fuel debris (including the waste body after processing), the environmental conditions for disposal (liquid properties affecting radionuclide migration parameters (including solubility, sorption distribution coefficient of the barrier, and a diffusion coefficient, etc.)), and various properties of the engineered barrier. In addition, along with understanding those influence substances, it is also important to understand the chemical composition and amount of the of hazardous substances that affect the environment.

Example 1 Application to radiation measurement technology : In the decommissioning of 1F, unlike the decommissioning of an existing nuclear plant, the contamination situation is unknown. Therefore, it may be difficult to analyze the radioactivity concentration of each nuclide with radiation measurement results (e.g., spectra), and more detailed measurement and analysis are required, which is an enormous task. Thus, it should be developed a technology to analogize true values from measurement results for unknown materials by having AI machine-learn the measurement results and simulation results for known materials. If this becomes possible, the work and time required for analysis can be greatly reduced.

Example 2 Application to radioactive waste and fuel debris analysis technology : Radioactive wastes whose contamination situation is unknown. Fuel debris which is composed of various dissolved materials. Since the distribution of contamination, components, and concentrations are non-uniform and inhomogeneous, total analysis, rather than sample analysis, is required to accurately understand the totality of the contamination. By introducing AI into this process, it is aimed to analyze the entirety with high accuracy from few samples. If this becomes possible, the work and time required for analysis can be significantly reduced, and the amount of secondary waste generated can also be significantly curbed.

The above is just one idea for an example of AI application, though it is expected that other AI applications will contribute to solving issues in 1F decommissioning.

To this end, it is important to clarify issues and needs in 1F decommissioning, and to collaborate and co-create between the needs of 1F decommissioning and the seeds of AI technology.