Elsevier

Fusion Engineering and Design

Volume 146, Part B, September 2019, Pages 1967-1970
Fusion Engineering and Design

Characterization of JET neutron field in irradiation locations for DD, DT and TT plasmas

https://doi.org/10.1016/j.fusengdes.2019.03.078Get rights and content

Highlights

  • Calculation of neutron spectra in irradiation positions to be used during the upcoming DD, DT and TT campaigns at JET.

  • Evaluation of the hybrid Monte Carlo by use of ADVANTG for improvement of results.

  • Determination of the acceleration ratio in case of thermal neutron flux calculations.

Abstract

The neutron fluxes and spectra were characterized for four locations close to the plasma and related to activation experiments, as the preparation for the upcoming experimental campaigns in JET. The focus was on the study of a variance reduction technique in order to obtain statistically significant results in the parts of the neutron energy spectra, important for irradiation experiments. The DD, DT and TT plasmas were studied with the Monte Carlo hybrid method and the use of the ADVANTG program for generation of weight windows as variance reduction method to accelerate Monte Carlo simulations. The calculations were optimized to obtain low statistical uncertainties for all energy bins in the 640 energy group structure and for all three plasma sources. This included the acceleration of calculations for reaction rates of capture reactions, i.e. in the thermal flux region in irradiation positions on the first wall. Speed-ups due to use of the hybrid method in excess of two orders of magnitude were found with respect to analog calculations despite the vicinity of the plasma source.

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 -

References (14)

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    Citation Excerpt :

    A set of 415 fusion spectra were taken from theoretically simulated data for the JET, ITER and DEMO reactors and were chosen to represent a wide range of fusion environments, fuel types and positions within a device. These were comprised of reference spectra [10,23] generated using models of the JET, ITER and DEMO fusion devices and simulated using the Monte Carlo N-Particle transport code (MCNP) for a range of device configurations. Each spectrum was folded with the response matrix to form a set of synthetic reaction rates, with these and an a priori spectrum given to each subroutine.

1

See the author list of X. Litaudon et al. 2017 Nucl. Fusion 57 102001.

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