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 (DIS) [2,3]. The DIS 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. The activation libraries are produced in a preliminary phase considering the relevant radioactive isotopes. The decays gammas emitted as prompt are scored and binned according to the parent nuclide. The different contributions to the dose rate are multiplied by a proper factor, time dependent, which takes into account the build-up, the decay of the radioactive isotope and on the irradiation history. It can be derived analytically or by means of an activation code, i.e. FISPACT . Previous benchmarks performed at the Frascati Neutron Generator (FNG, Italy ) and the Fusion Neutron source (FNS, Japan) facilities have shown the effectiveness of the method to predict dose rate [5,6], but the application to more complex machine like JET, the biggest European Fusion machine ITER-like, resulted less satisfactory [7-10]. One of the probable reasons of the problems is the fact that it was not a dedicated experiment. 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). Here the neutron flux is lower (almost two order of magnitude in terms of neutron fluence) and the dose rate to be measured is not very far from the residual background. Passive detectors are used for in-vessel measurements: the high sensitivity TLDs GR-200A (natural LiF), which ensure measurements down to environmental dose level. An active detector of Geiger-Muller type is used for out of vessel dose rate measurement. Before their use the detectors were calibrated in a secondary gamma-ray standard (Cs-137 and Co-60) facility in term of air-kerma. Last JET operations occurred in March 2004 and the background measurements have been performed in the period July -September 2005 in the outside position EX using the GM tube and in September 2005 inside the vacuum vessel using TLD detectors located in the 2 Upper irradiation end IE2. In the present work, experimental background data are compared with the same quantities calculated using DIS 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). 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.
|Publication status||Published - 2006|
|Event||American Nuclear Society's 14th Biennial Topical Meeting of the Radiation Protection and Shielding Division - , United States|
Duration: 1 Jan 2006 → …
|Conference||American Nuclear Society's 14th Biennial Topical Meeting of the Radiation Protection and Shielding Division|
|Period||1/1/06 → …|
All Science Journal Classification (ASJC) codes
Villari, R., Angelone, M., & Petrizzi, L. (2006). Benchmarking of shutdown dose rate calculation with direct one step method applied to JET. Paper presented at American Nuclear Society's 14th Biennial Topical Meeting of the Radiation Protection and Shielding Division, United States.