Proposal of shutdown dose estimation method by Monte Carlo code

Nuclear performance analyses for HCPB test blanket modules in ITER

“…Typically 20 million source neutron histories were tracked in the MCNP-calculations to ensure a sufficient statistical accuracy for the calculated nuclear responses. Shielding calculations were performed to assess the shielding efficiency of the TBM in terms of the transmitted neutron radiation and the gamma dose rate around the test blanket port 10 days after shutdown making use of the direct one step (D1S) computational approach [5] and automatically generated weight windows. The spatial neutron source distribution was sampled in a special routine linked to MCNP.…”

Evaluation of Shutdown Gamma-ray Dose Rates around the Duct Penetration by Three-Dimensional Monte Carlo Decay Gamma-ray Transport Calculation with Variance Reduction Method.

“…Figure 1 shows the Monte Carlo decay gamma-ray transport calculation procedure applied in the present study. It is possible to conduct the neutron and prompt By replacing a prompt gamma-ray spectrum with a decay gamma-ray spectrum in the nuclear data library and also a modest change in the original MCNP code, 9,10) we conduct the neutron and decay gamma-ray transport calculation simultaneously for the radioisotope production rates during operation in the revised MCNP. By multiplying each correction factor, which is ratio of the actual activation level after shutdown to the production rate during operation, with each decay gamma-ray flux due to each radioisotope, the decay gamma-ray dose rate after shutdown is evaluated.…”

“…The actual activity (decay rate) of each radioisotope after shutdown is usually smaller than the production rate during operation, and it depends on the machine operation scenario and time after shutdown. 5,9) The ratio of the actual activation level to the production rate is shown in Table 1 for each radioisotope at 10 6 s after shutdown based on the ITER operation scenario. Their ratios are different from each radioisotope as shown in Table 1, therefore we separate the contribution to the total dose level from each other using a time bins.…”

“…The different time delays are given for different decay gamma-ray productions. 9) We can obtain the decay gamma-ray flux due to each radioisotope, and multiply each ratio with each gamma-ray flux to estimate the total decay gamma-ray dose rate.…”

“…Valenza et al have proposed the Monte Carlo calculation method replacing the prompt gamma-ray spectrum with the decay gamma-ray spectrum to conduct the Monte Carlo decay gamma-ray transport calculation and evaluate the decay gamma-ray dose rates after shutdown with good accuracy. 5,9) In this method, decay gamma-ray can be generated at the exact location where the radioisotopes are produced by the nuclear reaction induced by the neutron, and the neutron and decay gamma-ray transport calculations can be simultaneously conducted. This method does not require a very fine (small) cell definition, though it may a little fine cell definition to reduce the statistical error by the weight window importance.…”