For the assessment of γ -ray doses from short-lived fission products (FPs) under criticality accident conditions, γ -ray exposure rates varying with time were experimentally determined in the Transient Experiment Critical Facility (TRACY). The data were obtained by reactivity insertion in the range of 1.50 to 2.93$. It was clarified from the experiments that the contribution of γ -ray from short-lived FPs to total exposure during the experiments was evaluated to be 15 to 17%. Hence, the contribution cannot be neglected for the assessment of γ -ray doses under criticality accident conditions. Computational analyses also indicated that γ -ray exposure rates from short-lived FPs calculated with the Monte Carlo code, MCNP4B, and photon sources based on the latest FP decay data, the JENDL FP Decay Data File 2000, well agreed with the experimental results. The exposure rates were, however, extremely underestimated when the photon sources were obtained by the ORIGEN2 code. The underestimation is due to lack of energy-dependent photon emission data for major short-lived FP nuclides in the photon database attached to the ORIGEN2 code. It was also confirmed that the underestimation arose in 1,000 s or less of time lapse after an initial power burst.
For the assessment of γ -ray doses from short-lived fission products (FPs) under criticality accident conditions, γ -ray exposure rates varying with time were experimentally determined in the Transient Experiment Critical Facility (TRACY). The data were obtained by reactivity insertion in the range of 1.50 to 2.93$. It was clarified from the experiments that the contribution of γ -ray from short-lived FPs to total exposure during the experiments was evaluated to be 15 to 17%. Hence, the contribution cannot be neglected for the assessment of γ -ray doses under criticality accident conditions. Computational analyses also indicated that γ -ray exposure rates from short-lived FPs calculated with the Monte Carlo code, MCNP4B, and photon sources based on the latest FP decay data, the JENDL FP Decay Data File 2000, well agreed with the experimental results. The exposure rates were, however, extremely underestimated when the photon sources were obtained by the ORIGEN2 code. The underestimation is due to lack of energy-dependent photon emission data for major short-lived FP nuclides in the photon database attached to the ORIGEN2 code. It was also confirmed that the underestimation arose in 1,000 s or less of time lapse after an initial power burst.
An observation system has been developed as a new instrumentation of TRACY (Transient Experiment Critical Facility) in order to observe the behavior of uranyl nitrate solution and radiolytic gas voids under criticality accident conditions. The system consists of a radiation-resistive optical fiberscope, a light source and a radiation-resistive video camera. The severe radiation environment in TRACY and safety functions as the primary boundary of TRACY were considered in the design of the system.The system has been successfully utilized in the recent TRACY experiments, and provided clear color motion pictures showing the behavior of the solution and radiolytic gas voids. As a result, it was visually confirmed that there is the difference in the behavior of the solution and radiolytic gas voids depending on the conditions of the reactivity addition.The system provides detailed information on the behavior of the solution and voids, and will contribute to the development of a computational kinetics model.
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