Shielding analysis of the ITER Collective Thomson Scattering system

The Collective Thomson Scattering (CTS) system will be the ITER diagnostic obtaining the plasma fast alphaparticle 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.