Yoshinori Miyoshi

Modification of the STACY Critical Facility for Experimental Study on Fuel Debris Criticality Control

For the decommissioning of the Fukushima Daiichi Nuclear Power Stations, fuel debris involving molten structural materials should be retrieved from each reactor unit. The fuel debris, which is of uncertain chemical composition and physical state, needs to be treated with great care from the standpoint of criticality safety. For developing criticality control for the fuel debris, the Japan Atomic Energy Agency (JAEA) has been planning to modify the Static Experiment Critical Facility (STACY) and to pursue critical experiments on fuel debris. STACY, a facility using solution fuel, is to be converted into a thermal critical assembly using fuel rods and a light water moderator. A series of critical experiments will be conducted at the modified STACY using simulated fuel debris samples. The simulated fuel debris samples are to be manufactured by mixing uranium oxide and reactor structural materials with various chemical compositions. This report summarizes a facility development project for an experimental study on criticality control for fuel debris using the modified STACY and simulated fuel debris samples.

Benchmark Critical Experiments and FP Worth Evaluation for a Heterogeneous System of Uranium Fuel Rods and Uranium Solution Poisoned with Pseudo-Fission-Product Elements

A series of critical experiments were performed using heterogeneous cores at the Static Experiment Critical Facility (STACY) of Japan Atomic Energy Agency (JAEA) in order to obtain systematic benchmark data concerning the dissolving process in a reprocessing plant. Focusing on the introduction of the burn-up credit, critical mass measurement was conducted for a combination of uranium dioxide fuel rods (5 wt% 235U) and uranyl nitrate solution (6 wt% 235U) poisoned with pseudo-fission-product (FP) elements—samarium, cesium, rhodium, and europium. Fuel rods were arrayed at a 1.5-cm lattice interval in the poisoned fuel solution in a 60-cm-diameter cylindrical tank. The uranium concentration of the solution was roughly kept at about 320 gU/L, and the FP element concentrations were adjusted to be equivalent to that in a burn-up of about 30GWd/t. The result provides basic experimental data for validation of computational methods to evaluate the reactivity effect of each FP element, as well as benchmark criticality data for validation of neutron multiplication factor calculation for heterogeneous systems of spent fuel. In this report, the details of the experiments and benchmark models will be presented as well as the procedure and the result of separate reactivity worth evaluation for each FP element. The experimental results and the computational evaluation results will also be compared.

Validating JENDL-3.3 for Water-Reflected Low-Enriched Uranium Solution Systems Using STACY ICSBEP Benchmark Models

Abstract Evaluated criticality benchmark data obtained at the Static Criticality Experiment Facility (STACY) account for a large percentage of low-enriched uranium (LEU) solution systems documented in the “International Handbook of Evaluated Criticality Safety Benchmark Experiments.” These data are available for validation of computer codes and nuclear data used for criticality safety analyses of LEU solution systems. The calculated keff’s for the water-reflected STACY criticality experiments have been overestimated with JENDL-3.2 by ˜0.7%. These overestimations were kept in mind while making modifications of the fission spectrum and the fission cross section of 235U, and the (n,p) cross section of 14N in JENDL-3.3. Because of these modifications, the keff’s calculated with JENDL-3.3 were largely improved. The contributions of these modifications in JENDL-3.3 with respect to JENDL-3.2 and ENDF/B-VI.5 were investigated by performing perturbation calculations. The overestimation of the elastic-scattering cross section of 56Fe in the mega-electron-volt range was one of the reasons for the keff overestimations for the STACY experiments with JENDL-3.2. The modification of 56Fe cross sections in JENDL-3.3 reduces keff’s in the STACY experiments by 0.2%. The dependence of calculated keff’s on uranium concentration still exists in JENDL-3.3. The overestimation of calculated keff’s for the STACY experiments with JENDL-3.3 is not insignificant and is as much as 0.6%. These problems are to be resolved in a future evaluation of the cross-section library.

Benchmark Critical Experiments of a Heterogeneous System of Uranium Fuel Rods and Uranium Solution Poisoned with Gadolinium, and Application of Their Results to JACS Validation

A series of critical experiments were performed using heterogeneous cores at the Static Experiment Critical Facility (STACY) of the Japan Atomic Energy Agency (JAEA) in order to obtain systematic benchmark criticality data concerning the dissolving process in reprocessing plants. Focusing on the use of gadolinium as a soluble poison, critical mass was measured for a combination of uranium dioxide fuel rods (5 wt% 235U) and uranyl nitrate solution (6 wt% 235U) poisoned with gadolinium (Gd). Fuel rods were arrayed with a 1.5 cm lattice interval in the poisoned fuel solution in a 60-cm-diameter cylindrical tank. The uranium concentration of the solution was roughly kept at about 330 gU/L, and the Gd concentrations were varied up to 0.1 gGd/L. The other series of experiments were also conducted, as reference cases, varying uranium concentration in the fuel solution without Gd. The results provided benchmark criticality data for validation of neutron multiplication factor calculation on heterogeneous systems such as a dissolver. Validation calculation of JACS based on the newly obtained benchmarks supports the justification of its utilization for the criticality safety analysis.

Evaluation of personal dosimeters and electronic modules under high-dose field

Described in this report are evaluation tests of neutron-induced malfunction of real-time personal dosimeters and electronic modules using the Transient Experiments Criticality Facility, TRACY, at the Japan Atomic Energy Agency. In the event of radiological or nuclear terrorism, radiation levels are expected to be high, especially if nuclear-critical devices are used. In such devices, fission chain reactions develop that may lead to nuclear criticality with strong lethal emissions of both neutrons and gamma-rays. A wide area of radius 400m at least should be cordoned off. In such a situation real-time dosimeters with wireless data transmission capability would provide essential support for first-response teams. The reliability of these apparatuses, however, has not been evaluated sufficiently under such high-dose conditions. We have evaluated these at TRACY, which is a reactor that can realize controlled nuclear excursions using 10% 235U-enriched uranyl nitrate solutions as fuel.

Calculation of Criticality Condition Data for Single-unit Homogeneous Uranium Materials in Six Chemical Forms

Single-unit criticality condition data were calculated for homogeneous uranium materials in six chemical forms for revision of the Data Collection appendix to the Nuclear Criticality Safety Handbook. The calculated criticality condition data were the estimated critical and the estimated lower-limit critical masses and volumes of spheres, diameters of infinitely-long cylinders and thicknesses of infinite-area slabs of uranium materials in six chemical forms encountered in criticality safety evaluation of nuclear fuel cycle facilities. The chemical forms were U-H2O, UO2-H2O, UO2F2, UO2(NO3)2, ADU-H2O and UF6-HF, the first and last ones of which were not dealt with the Data Collection. The calculations were made with the continuous-energy Monte Carlo criticality calculation code MVP and the Japanese Evaluated Nuclear Data Library JENDL-3.2. The values and precision of the present calculations are discussed in comparison with previous results.

Benchmark Evaluation on Single Core System Composed of 10% Enriched Uranyl Nitrate Solution at STACY

Critical experiments on 10% enriched uranyl nitrate solution have been conducted for obtaining benchmark data using a Static Experiment Critical Facility, STACY, in the Nuclear Fuel Cycle Safety Engineering Research Facility, NUCEF. Two cylindrical and one slab tank were used to construct a single core system. Critical solution levels were measured for three core tanks with changing the uranium concentration of uranyl nitrate solution. Fundamental data were obtained for unreflected or water reflected cores. The sensitivities of the fuel composition and tank geometry on the criticality were calculated, and total uncertainty of the experiments are evaluated less than 0.15% Δ k. Calculations of the neutron multiplication factor keff for the critical conditions were performed using a continuous-energy Monte Carlo code MCNP-4B employing JENDL-3.2. This paper presents the core configurations of STACY, their experimental uncertainties and calculation results with above calculation system.