Criticality experiments: analysis, evaluation, and programs. 4. Benchmark Experiments with Silicon Dioxide Waste Matrix

Experiments involving both highly enriched uranium and plutonium interspersed in a typical waste matrix were performed at the Institute of Physics and Power Engineering (IPPE) in Obninsk, Russia. The National Spent Nuclear Fuel Program (NSNFP) at the Idaho National Engineering and Environmental Laboratory sponsored the experiments. Data are being used to validate criticality-safety calculations in support of NSNFP efforts to store highly enriched spent nuclear fuels at the U.S. geologic repository at Yucca Mountain, Nevada. However, these data are applicable to numerous other criticality-safety operations involving fissile waste. Preliminary and final design parameters were published previously. Complete documentation of these two series of experiments is given in the 'International Handbook of Evaluated Criticality Safety Benchmark Experiments' as HEU-MET-MIXED- 005 and PU-MET-MIXED-001. Several experimental measurements were made in addition to the critical state. The purpose of this paper is to highlight all measurements made in these two series of experiments. Two series of criticality-safety benchmark experiments involving highly enriched uranium and plutonium interspersed in a typical waste matrix, silicon dioxide, were recently performed on the BFS-1 facility at IPPE. Experiments with highly enriched uranium fuel (designated BFS-79) were completed in the fall of 1999. Experiments with plutonium fuel (designated BFS-81) were completed in the spring of 2000. The BFS-1 critical facility and the individual core components are described in detail in Ref. 3. Basically, the assemblies are constructed from aluminum tubes containing fissile pellets and SiO2, arranged in a tight-fitting hexagonal lattice inside a cylindrical steel vessel. Hydrogenous moderation was provided for some cores by inserting polyethylene. Five configurations were assembled with highly enriched uranium and six with plutonium. Calculated spectra data indicate that the percent of the total fissions occurring in the intermediate energy range (0.625 eV to 100 keV) varies between 20 and 68% for the uranium systems and 18 and 64% for plutonium systems. Computer-code validation efforts typically focus solely on the critical configuration. However, good agreement between calculated and measured critical states does not necessarily ensure agreement between calculations and measurements of other important parameters. In addition to critical states, the following measurements were made for selected configurations within each series: 1. the reactivity worths of the regulatory rods; 2. the reactivity worths of various core components (tubes and dowels); 3. central reactivity-worth measurements for uranium, plutonium, boron, polyethylene, silicon dioxide, and graphite 4. spatial (axial and radial) distribution of fission rates in BFS-79/5 (no hydrogenous moderation present) and BFS- 81/5 (significant amounts of hydrogen moderation present); 5. average cross-section ratios. Calculations were performed (keff only) for each critical system using various codes and cross-section data. A new evaluation of the silicon cross section, based on new capture measurements and reevaluation of existing ENDF/B data, is being prepared at ORNL; however, results using these new data are not yet available. Given in Tables I and II are comparisons of the results of calculations that were performed by various ICSBEP participants with the experimental values. Definite trends can be observed in the results given in Tables I and II for some code/cross-section combinations while others show no trends across the entire series. Furthermore, trends are sometimes reversed for uranium and plutonium systems. In addition to the critical configurations, numerous other measurements were performed and are also available to aid in the evaluation of the basic nuclear data for uranium, plutonium, and silicon. The results from these other experimental measurements provide important information that may be helpful in resolving the discrepancies reflected in the calculated keff values. (authors)