The Japan Atomic Energy Agency is conducting critical experiments to clarify the criticality characteristics of fuel debris using the Static Experiment Critical Facility (STACY). The geometric shape of fuel debris may change, such as joining or separation, due to the unintentional falling of fuel debris or processing during retrieval work. To confirm the validity of simulating these conditions by computations, a series of critical experiments was carried out with two core configurations close to each other. The geometrical change in the fuel debris was simulated by changing the relative distance between the two cores in a core configuration where a small core consisting of 3 & times; 5 or 5 & times; 3 fuel rods was close to a larger core with a pit shape to receive it.An experimental analysis was carried out using the Monte Carlo calculation codes MCNP and MVP and the Japanese evaluated nuclear data library JENDL. It was shown that the effective multiplication factor can be evaluated using a simple calculation model, with the calculation results showing that the values of the effective multiplication factor of MCNP were smaller than those of MVP and that the values of the effective multiplication factor of JENDL-4.0 were smaller than those of JENDL-5. In particular, in JENDL-5, it was shown that it is effective to correctly calibrate the temperature point of the thermal scattering law (TSL) used.
During the fabrication of fuel elements for research reactors, uranium silicide alloys are melted at temperatures close to 1800 degrees C using porous ZrO2-MgO crucibles. The controlled porosity required to withstand thermal shock also promotes infiltration of approximately 1% to 2% of the molten alloy into the pore network, retaining uranium within the ceramic matrix. In the present work, the interaction between a molten U-Si alloy and porous stabilized zirconia was investigated through microstructural, crystallographic, and diffusion analyses. The ZrO2-MgO matrix exhibited a total porosity of similar to 18%, while molten alloy infiltration reached nearly 50% of the crucible wall thickness. X-ray microtomography revealed the preferential filling of larger pores, reducing the maximum equivalent pore diameter from similar to 400 to similar to 250 mu m after infiltration, with a geometric tortuosity of tau = 1.907. The uranium concentration profiles obtained by energy-dispersive X-ray spectroscopy allowed for the estimation of an effective diffusion coefficient on the order of 10(-5) cm(2)/s. X-ray diffraction and Rietveld refinement revealed the formation of a previously unreported disordered cubic pyrochlore-type structure crystallizing in the Fd3m space group with stoichiometry Zr-2(Zr-0.8125 U-0.1875)(2)Si3O4. The results indicate that uranium infiltration is primarily governed by metallostatic pressure and pore connectivity, while simultaneous diffusion and chemical interaction promote the formation of uranium-containing pyrochlore/defective fluorite-type structures relevant for actinide immobilization applications.
Accurate and fast formulations are of particular relevance in applications of multiplicity counting in nuclear safeguards. That is, the final goal is to determine the parameters of the item, primarily the mass of the fissile component. This may be achieved through an inverse procedure by unfolding the parameters of the item from the measured multiplicity rates. However, these rates are directly related to the calculation of multiplicity moments. In a recent work, the forward transport approach, in particular the analytical discrete ordinates (ADO) method, was used to efficiently evaluate factorial moments as an alternative form of the backward integral-type formulation available in the literature. The technique was extended in a subsequent work to consider the mean number of secondary neutrons per collision c > 1, and the first results were established for the multiplicity moments. In both works, the results were compared with the collision number (CN) expansion method. In the present work, seeking to improve even more the speed of calculation, we introduce to the ADO formulation the use of the so-called explicit eigenfunctions to evaluate the factorial moments, and subsequently, the multiplicity moments. Furthermore, thinking of establishing benchmark solutions for the problem, in addition to the CN expansion approach, we present a comparison with additional numerical results available in the literature for the multiplicity moments, including scattering effects, based on an integral formulation. Our method provided very accurate solutions, and it was found that our method is superior in terms of speed in comparison with the originally used CN expansion, for which we have data available. Furthermore, the use of explicit solutions also showed a time computational gain in comparison with the numerical eigenfunctions formulation.
A criticality incident occurred at the Windscale Works in the United Kingdom in 1970 by the mixing of an aqueous solution and an organic solvent; however, the detailed criticality scenario remains unclear. In this study, a multiphysics analysis was performed to clarify a possible criticality mechanism, focusing on the time variation of reactivity. Computational fluid dynamics (CFD) calculations were carried out considering the effects of a short-duration inrush and the presence of a stable emulsion, which were not taken into account in the previous study. Based on the space- and time-dependent material distributions obtained from the CFD results, Monte Carlo neutron transport calculations were performed to evaluate changes in the effective multiplication factor. The results show that both the inrush and the stable emulsion might have influenced the criticality, leading to conditions in which the effective multiplication factor exceeded 1. Furthermore, neutronic kinetics analyses using simplified wave-like reactivity variations demonstrated that both the total number of fissions and the duration of criticality are consistent with historical records under certain conditions. These findings support the hypothesis that the Windscale Works criticality incident was governed by a wave-like reactivity variation.
In this paper, we introduce a predictor-corrector quasi-static (PCQS) algorithm in which the conventional point kinetics model is replaced by a coarse mesh finite difference (CMFD) solver serving as the corrector. By eliminating the need to compute adjoint flux-weighted point kinetics parameters, the proposed algorithm becomes more computationally efficient and is particularly advantageous for Monte Carlo applications, where the adjoint flux calculation is inherently challenging. Numerical analyses are provided to examine the convergence, stability, and consistency of the CMFD-based PCQS algorithm (referred to as CMFD-PCQS), offering insights into its performance and potential advantages for reactor kinetics simulations. Numerical results are presented to demonstrate the effectiveness of the proposed algorithm.
