Radiation Shielding for ITER to Allow for Hands-on Maintenance inside the Cryostat (Methodology for Estimating Shutdown Dose Rate in a Complex Geometry)

Abstract

In the shielding design of the ITER machine, which is a tokamak fusion experimental reactor and has a very complex geometry, it is important to have a reliable estimation of the dose rate levels after reactor shutdown for realising hands-on maintenance around the torus. The ITER project position is that dose rates inside the cryostat be kept low enough to allow for human access shortly after shutdown for limited periods to provide rescue and/or maintenance activities. The methodology of estimation for dose rates after shutdown in such a complex geometry machine is discussed. The Monte Carlo method is preferable to conduct neutron transport calculations in the ITER geometry. The Conversion Factor method, which was used for dose rate estimation in the 1998 ITER shielding design, is described with an example of dose rate estimation around penetrations in the vacuum vessel. A Full Monte Carlo method is proposed showing the possibility of eliminating uncertainty accompanied with conversion factors from neutron flux or isotope production rate to dose rate after shutdown.

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.

“…14 for the fusion power with 500 MW. Neutron (>1 MeV) fluxes in the space inside the maintenance duct and behind the shield plug-3 are 5×10 5 to 1×10 6 neutrons·cm −2 ·s −1 as shown in Fig. 14, and they are reduced by about 8 orders of magnitude compared with those at the plasma region.…”

“…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.…”

“…In the present study, the decay gamma rays are generated from the radioisotopes of 51 Cr, 54 Mn, 59 Fe, 58 Co and 60 Co, which dominate the dose rates after shutdown. Inside the cryostat, the decay gamma-ray dose rate at 10 6 s after shutdown (typical time point of interest concerning maintenance operation) can be determined by these five radioisotopes, 5) whose production rates can be obtained by MCNP. The decay gamma rays are emitted from the location where the radioisotopes are generated by the nuclear reactions induced by the neutrons in this method.…”

“…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.…”

“…In this case, the uncertainty due to the conversion factor is a critical issue. 5) In order to evaluate the decay gammaray dose rates around the duct after shutdown with good accuracy, the Monte Carlo decay gamma-ray transport calculation is required. The spatial distribution of induced activity, which forms the decay gamma-ray source, is needed for the decay gamma-ray transport calculation, and it can be obtained from the neutron flux distribution by using the activation code such as ACT-4, 6) CINAC 7) or FISPACT.…”