A dose rate experiment at JET for benchmarking the calculation direct one step method

Abstract

Neutrons produced by D–D and D–T plasmas induce the activation of tokamak materials and components. The development of reliable methods to assess dose rate is a key issue for maintenance and operation of nuclear machines, in normal and off-normal conditions. In support of ITER design activity an innovative computational tool based upon MCNP-4C Monte Carlo code has been developed to predict the dose rate after shutdown: it is called direct one step method (D1S). In the D1S approach the decay gammas are coupled to neutrons as in the prompt case and are transported in a single step in the same run. To check the capability of D1S method to predict dose rate in a tokamak a dedicated benchmark experiment was proposed for the 2005–2006 experimental campaign of JET. Two irradiation positions were selected for the benchmark: one inner position close to vessel and not far from plasma, called the two upper irradiation end (IE-2), while the second position is just outside a vertical port in an external position (EX). Passive detectors are used for in-vessel measurements: the high sensitivity thermo-luminescent dosimeters (TLDs) GR-200A (natural LiF), while one active detector of Geiger–Mueller (GM) type is used for out of vessel dose rate measurement. Both TLDs and GM detectors were calibrated in a secondary gamma-ray standard facility in terms of air-kerma. In the present work, residual background and dose rate due to early phase of JET 2005–2006 campaign are compared with the same quantities calculated using D1S approach. The impact of geometrical model and materials impurities to the calculated dose rates is discussed as well. The activity has been performed in the frame of the European Technological Tasks for JET JW5-FT5.20, in collaboration with United Kingdom Atomic Energy Agency (UKAEA) and Forschungszentrum Karlsruhe (FZK).