Radiation shielding for ITER to allow for hands-on maintenance inside the cryostat. Methodology for estimating shutdown dose rate in a complex geometry

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. (author)