Shielding analysis of the ITER Collective Thomson Scattering system

Highlights

• A neutronics analysis is presented for the ITER Collective Thomson Scattering.

• The n0 uxes are underestimated by a 2–3 factor by the use of a homogeneous DSM.

• The equatorial port plug 12 will exceed the 100 _Sv/h limit in the port interspace.

• The accurate modelling of the modular DSM is recommended in future studies.

Abstract

The Collective Thomson Scattering (CTS) system will be the ITER diagnostic obtaining the plasma fast alpha-particle velocity distribution and will be implemented in drawer #3 of the Equatorial Port Plug #12 of the reactor. In this work, a neutronics analysis is presented for the in-vessel front-end parts of the CTS system, including neutron and gamma-ray fluxes and nuclear heat loads for the main components of the system calculated with the Monte Carlo radiation transport code MCNP6. In previous analyses the shielding materials were modelled as a homogeneous mixture, a crude approximation which did not consider the small gaps between the different components and between the CTS system and the shielding structure. In this work, a detailed model of the modular Diagnostics Shield Module (DSM) was developed, including all the shielding trays and all the individual boron carbide bricks. The results obtained with this model are compared with the ones obtained using a homogeneous mixture, to assess the effect of this approximation on the estimation of the neutron fluxes in the port interspace. The results show that the total neutron flux reaching the closure plate is estimated to be 2–3 times higher when the shielding is accurately modelled. This shows that the often-used homogeneous mixture approach underestimates the neutron fluxes during operation – a fact that could have great importance in the global shutdown dose rate estimates. On the other hand, the shielding implementation does not affect the heat loads in the front-end components of the system. Simulations to assess the Shutdown Dose Rates are performed using the D1S-UNED code. The results suggest that the entire port plug where the current CTS design is included will exceed the dose rate limit of 100 μSv/h in the port interspace. The contribution from the CTS system alone, however, is not sufficient to exceed the threshold.

Introduction

The Collective Thomson Scattering (CTS) diagnostic system will be the operating diagnostic responsible for measuring the velocity distribution of the alpha particles produced by the D-T reaction at ITER [1,2]. CTS is the only energetic particle diagnostic sensitive to alpha particles below 1.7 MeV [3]. Alpha particles above 1.7 MeV can also be diagnosed by gamma-ray spectrometry [4,5]. Integrated data analysis will allow a reconstruction of the 2D velocity distribution function [3,6]. CTS will further measure the densities of alpha particles [7,8]. Additionally, the CTS will have secondary roles, namely the measurement of the plasma rotation and the ion temperature. This diagnostic, as it was originally proposed [9,10], will operate in the 60 GHz frequency range, which is below the electron cyclotron resonance corresponding to the ITER nominal magnetic field of B₀ = 5.3 T. A 1 MW, 60 GHz gyrotron will produce the CTS probing beam that is injected into the plasma through the apertures in the first wall. The in-vessel components of the CTS system will be placed in drawer #3 of the Equatorial Port Plug #12, and with the exception of the plasma-facing launcher and receiver mirrors they will be protected from the plasma by the diagnostic first wall (DFW) and will be shielded inside the drawer by the diagnostic shield module (DSM). The ITER CTS system is further described in recent publications [[11], [12], [13], [14], [15], [16]].

The current design, shown in different perspectives in Fig. 1, Fig. 2, consists of a series of waveguides and mirrors that deliver the probing beam into the plasma and collect part of the resulting scattered radiation. The system, as shown in Fig. 3, includes 11 mirrors, 2 in the high-power launcher transmission line and 9 in the low-power receiver transmission line. These mirrors, in particular the plasma-facing Launcher and Receiver mirrors, will be fully or partially exposed to plasma radiation, being bombarded by energetic neutrons and gammas, resulting in considerable nuclear heat loads that may affect their performance and durability [17].

A recent concept was proposed for the ITER Diagnostics Equatorial Port Plugs – the modular DSM [18], shown in Fig. 4. The modules in this concept consist of stacks of trays with boron carbide bricks. Fig. 5 shows how the modular DSM concept is implemented in the CTS system. Applying this concept to the CTS, the DSM will have 6 columns with a variable number of trays, each containing 120 B₄C bricks (if the CTS system was not installed in the drawer). The brick dimensions are 54 mm × 40 mm × 40 mm with a 22 mm diameter hole in the centre. Fig. 6 shows the CTS system without the modular DSM, revealing the existence of a steel backfilling structure that fills all the gaps between the waveguides of the system and the modular DSM shielding trays.

This paper presents a nuclear analysis for the main components of the current design of the ITER CTS diagnostic, taking into account the DSM implementation in the CTS system. The current design has been changed compared to the previous CTS design [19]. The new model is more complex and much more detailed. Concrete differences that impact directly some of the estimations include the inclusion of a bulk shielding block around the collecting mirrors and the introduction of the already mentioned backfilling structure shown in Fig. 6. In previous analyses the shielding materials were modelled as a homogeneous mixture of boron carbide (B₄C), stainless steel (SS) and water with different volume fractions, a crude approximation which did not consider the small gaps between the different components and between the CTS system, as well as the shielding structure.

In the present work, an accurate model of the modular DSM is developed, including all the trays and all the individual B₄C bricks. The results obtained with this model are compared with the results obtained using a homogeneous mixture to assess the effect of the homogeneous approximation on the estimation of the neutron fluxes in the port interspace. An assessment of the shutdown dose rates (SDDR) in the port interspace is also performed for the homogeneous mixture shielding.