Characterization of JET neutron field in irradiation locations for DD, DT and TT plasmas
Introduction
A new Deuterium-Tritium campaign (DTE2) is planned at JET in 2020, accompanied also by campaigns with Tritium and Deuterium plasmas. In JET several diagnostic components such as the KN2 neutron activation detectors [1] and long term irradiation stations [2] are located at the inner edge of the first wall and are going to be exposed to neutron fluence comparable with that occurring in the rear part of the ITER port plug. In preparations for the upcoming campaigns preliminary characterizations of neutron fluxes and spectra are needed for determination of viable activation experiments in the diagnostic components. In addition accurate flux and spectra will be needed for comparison with experimental results [3].
Several materials with useful activation reactions in the thermal neutron energy region are used for neutron diagnostics in JET, one example is cobalt [2]. For this reason reaction rate calculations with low statistical uncertainty in the whole neutron energy spectra, including the thermal part, are desired. Due to the commonly high statistical uncertainties in Monte Carlo calculations for fusion applications in the present work the weight window variance reduction method was used for three different plasma sources (DD, DT and TT) employing the hybrid Monte Carlo/deterministic ADVANTG code [4] to generate weight windows for acceleration of MCNP [5] simulations.
The generation of weight windows with the ADVANTG code for variance reduction in the MCNP code and results of acceleration are presented for each studied irradiation position and plasma source. In the last section the characterization of results is performed.
Weight windows are often employed for MCNP calculations, however in the majority of cases they are optimized for calculations of the total flux or a low energy distribution, the first being the default setting also in ADVANTG [4]. In the present work special attention was given on the statistical uncertainties of energy bins in the entire range of neutron spectra, which turned out to be very important in case of capture reactions.
Section snippets
Irradiation positions
Four different irradiation positions located close to the plasma were chosen for neutron flux characterization; two neutron activation irradiations ends KN2 in octant 3 and octant 6 [1] and two long term neutron irradiation stations I-LTIS and O-LTIS [2].
An existing MCNP model of JET was used for simulations with new implementations of all four irradiation positions [6,7]. A detailed model for both long term irradiation stations I-LTIS and O-LTIS was used constructed from the CAD models [8].
ADVANTG hybrid code
All analyzed irradiation positions are close to the plasma source. Cobalt activation foils are frequently used in JET in order to target also thermal neutrons [1]. Lengthy analog Monte Carlo simulations are needed to obtain statistically significant results in the thermal region of the neutron spectra and use of variance reduction methods is desired. Hybrid Monte Carlo/deterministic calculations using the ADVANTG code were used for characterization of the neutron fluxes. The fundamental concept
Results analysis
To ensure no bias was introduced due to variance reduction the 10 statistical tests in MCNP were studied and results of the hybrid calculations compared longer analog MCNP calculations.
Conclusion
In the upcoming experimental campaigns at JET DD, DT and TT plasmas will be used. In their preparation the neutrons fluxes and spectra were characterized for two KN2 detectors and two long term irradiation stations close to plasma. The goal of the characterization was to obtain statistical significant results in the entire neutron energy spectrum by the study of variance reduction techniques. A hybrid method was used with ADVANTG generated weight windows to accelerate MCNP Monte Carlo
Acknowledgments
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors acknowledge the support of the Slovenian Ministry of Education, Science and Sport (project codes J2-6756, Analysis of material damage and activation in large scale fusion reactors -
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