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[en] The Nuclear Safety Research Center (NSRC) of Japan Atomic Energy Agency - JAEA is conducting computations of criticality characteristics of the fuel debris of the Unit 1-3 reactors of Fukushima Daiichi Nuclear Power Station, and building a database of their results. The database will be useful to evaluate criticality control parameters when samples of fuel debris are taken and analyzed. Further computations will be performed to evaluate uncertainty of neutron multiplication factors due to random distribution of fuel debris composition using the new Monte Carlo solver, Solomon. Mathematical models and voxel geometry have been implemented in the solver to handle the randomness. The computation of new materials such as fuel debris mixed with steel or concrete modeled by the new techniques will be validated by critical experiments using the new Static Experiment Critical Facility (STACY). A risk analysis method supported by computer codes will be provided for estimation of criticality risk of the fuel debris retrieval, which will be useful for study and selection of retrieval work designs to be proposed in a few years.
[en] The paper presents a method to evaluate the potential re-criticality issues in severe accident configurations. This method is developed by Tractebel ENGIE, using respectively MCNP and a dataset of MELCOR calculations. The work has focused on the in-vessel phase and more particularly on a TMI-2-like configuration. A model based on a corium sphere approximation associated to the detailed WIMS10 fuel characterization has allowed us to highlight some degraded configurations presenting re-criticality risks associated to the intact fuel rods that remain in the core. The resulting distributions of the multiplication factor (keff) in function of several parameters (core degradation fraction, boron concentration in water, core burnup, moderator and fuel temperature,...) has allowed us to identify the more sensitive parameters regarding the re-criticality risk. The core degradation fraction is the major parameter, even for a high level of degradation, which highlights the influence of the intact peripheral fuel rods. The boron concentration in the water injected after the accident also plays an important role: even with a relatively high core degradation fraction (about 60%) and a consequent boron concentration (1500 ppm) the re-criticality risk cannot be excluded. These conclusions being well dependent on the assumptions and conservatisms of the study Finally a 'surrogate model' - regression model based on artificial neural network - is presented for the prediction of the multiplication factor for TMI-2-like configurations.
[en] The fuel debris produced by the accident of the Fukushima Daiichi Nuclear Power Station is probably in a state of mixture of spent fuels with different burnups each other. In such a case, the mixing ratio of spent fuels in fuel debris would affect its criticality. This report shows computation results of criticality characteristics of fuel-debris compositions prepared by mixing nuclide compositions of spent fuels in various patterns based on a fuel loading pattern. The results indicate that fuel debris is potentially subcritical when 1-cycle fuels, whose average burnup is several GWd/t, are included homogeneously in fuel debris because remaining 155Gd and 157Gd in 1-cycle fuels works to reduce neutron multiplication. The results also indicate that 155,157Gd/235U ratio well characterize criticality of fuel debris. (authors)
[en] Difficulty and importance of determining possibility of occurrence of criticality in the accidents with destruction and even melting of nuclear fuel at NPP differ significantly whether this determination occurs immediately or after a long period of time since accident. Immediately after the accident, when situation is not yet stabilized, there is no sufficient amount of data on the state of fuel and moderator and this requires additional efforts to assembly conservative computational models. Current report presents an analysis of the experience of evaluations of criticality that were performed at the Kurchatov Institute immediately after accidents at the 4. block of the Chernobyl NPP in 1986, in the fuel assembly washing tank at the Paks NPP in 2003 and at the Fukushima NPP in 2011. Based on the experience of the first two accidents, there was developed the specialized complex SAPFIR-2006, designed for evaluation the criticality in severe beyond design basis accidents. The operative criticality evaluations made by this complex during the course of the accident did not significantly differ from the conclusions of subsequent analyzes performed quite a long time after the accident. However, earlier criticality evaluations may influence the management of the accident progression and the determination of optimal strategy of emergency response. After the accident at the Paks NPP, results of computational evaluation of the criticality were taken into account when preserving the subcritical state of tank with destroyed fuel and when removing fragments of fuel assemblies from the tank. In emergency response during Chernobyl accident, such results were not taken into account. In the accident at the Fukushima NPP, there was no opportunity to inform emergency workers with results of computational evaluation of the criticality. (authors)