EVALUATION OF SHIELDING ANALYSIS-METHODS IN SPENT-FUEL CASK ENVIRONMENTS

The three-dimensional Monte Carlo code MORSE-SGC, as implemented in th e SCALE system calculational sequence SAS4, is applied to the analysis of a series of simple geometry benchmark experiments and prototypic s pent-fuel storage cask measurements. The simple geometry experiments w ere performed in Japan and at the General Electric-Morris Operation fa cility; the cask measurements were performed at the Idaho National Eng ineering Laboratory. The quantification of uncertainties in a typical shielding analysis process for transport/storage casks can be accompli shed by comparison of consistent trends between calculated and measure d dose rate quantities in both benchmark and prototypic environments. Benchmark results typically measure the validity of cross-section data and computer code adequacy; prototypic environments, however, general ly measure the overall validity of the calculational procedure. A tota l of five storage cask problems and two simple geometry problems were analyzed to determine the expected accuracies of computational analyse s using well-established source-generation and Monte Carlo codes. The general trends seen in this work are in agreement within 30% or better with the measurements for neutron dose rates along the cask side, lid , and bottom. The gamma-ray dose rates with substantial contributions from the top endfitting, plenum, and bottom endfitting regions also ar e in good agreement. Based on the latest results, gamma-ray dose rate calculations with major contributions due to the active fuel region sh ow a consistent factor of 1.6 overprediction of the measured quantitie s for casks with iron and concrete shields. Major uncertainties exist in the quantification of Co-59 concentrations in endfitting hardware m aterials. The results presented support the accuracy of source generat ion methods and dose estimation methods in these regions given accurat e impurity characterizations. Thus, it is felt that the practice of us ing upper bounds for Co-59 initial concentrations should ensure conser vative cask designs. Fortunately, the gamma-ray dose discrepancies see n along the sides of both the iron and concrete cask surfaces are over predictions. The reason for overprediction is not fully known. Even th ough these overpredictions are not clearly understood, the trends obse rved, combined with some degree of code and data testing using these o r similar benchmark measurements, should inspire confidence in the shi elding results for a shipping/storage package.