Benchmarking of shutdown dose rate calculation with direct one step method applied to Jet - 048

In nuclear fusion reactors, neutrons produced by the D-D and D-T reactions induce the activation of materials and of components. Even when the machine is not operating significant dose levels can be achieved due to the decay of the radioactive nuclides. The development of reliable methods for the assessment of dose rates is one of the key issues for maintenance and operating nuclear machines, in normal and off normal conditions. In the frame of Fusion Technology a computational tool based on MCNP Monte Carlo code has been developed to predict the dose rate after shutdown: it is called Direct One Step Method (D1S). The D1S is an innovative approach in which the decay gammas are coupled to the neutrons as in the prompt case and they are transported in one single step in the same run. The coupling of the decay gammas to the neutron generating the associated radioactive isotope is carried out using special libraries and modified routines of MCNP. A dedicated benchmark experiment has been thus proposed for the 2005-2006 campaign of JET. Two irradiation positions have been selected for the benchmark: one inner position inside the vessel, not far from the plasma, called the 2 upper irradiation end (IE2), where neutron fluence is relatively high. The second position is just outside a vertical port in an external position (EX). In the present work, experimental background data are compared with the same quantities calculated using D1S approach. Dose rate has been calculated multiplying the gamma fluxes with air-kerma factors to be consistent with calibration procedure of the detectors. The transport of both neutrons and decay gammas emitted as prompt was carried out in a single step, but distinct calculations were needed for DD and DT neutron source, that have different yields and irradiation histories. The calculated doses have been multiplied by time correction factors calculated using FISPACT, after a proper description of DD and DT JET irradiation histories since 1993 to 2004 (total neutron yield 2.37 10²⁰ and 2.22 10²⁰ for DT and DD respectively). An optimal agreement between calculation and measurement has been obtained: the ratio between calculated over measured air-kerma rate (C/E) is 0.89±0.08 in IE2 and 0.75±0.31 in EX. It should be noted that the calculated quantities are generally lower with respect to the measured ones, especially in the external position. The impact of key parameters to the dose rate calculated values is very important to be determined, so the paper beside the C/E values for the background dose rate, will address the following issues: the impact of: details of JET model in the surrounding of external position (EX) around the GM tube and the tube itself, concentration of Co in the steel composition. The use of different activation cross section should be investigated as well