Nuclear analysis and shielding optimisation in support of the ITER In-Vessel Viewing System design

TL;DR

This paper evaluates neutron shielding configurations for the ITER In-Vessel Viewing System, analyzing their effectiveness in reducing radiation and dose rates to ensure safety and compliance during plasma operations.

Contribution

It presents a comprehensive analysis of shielding options for the IVVS, including the use of different materials and configurations, to optimize radiation protection and system longevity.

Findings

Shielding blocks at probe level reduce dose rates but may still exceed requirements.

Shielding at the bioshield plug effectively meets dose rate limits.

Alternative low-activation steels are being considered for improved safety.

Abstract

The In-Vessel Viewing System (IVVS) units proposed for ITER are deployed to perform in-vessel examination. During plasma operations, the IVVS is located beyond the vacuum vessel, with shielding blocks envisaged to protect components from neutron damage and reduce shutdown dose rate (SDR) levels. Analyses were conducted to determine the effectiveness of several shielding configurations. The neutron response of the system was assessed using global variance reduction techniques and a surface source, and shutdown dose rate calculations were undertaken using MCR2S. Unshielded, the absorbed dose to piezoelectric motors (PZT) was found to be below stable limits, however activation of the primary closure plate (PCP) was prohibitively high. A scenario with shielding blocks at probe level showed significantly reduced PCP contact dose rate, however still marginally exceeded port cell requirements. The addition of shielding blocks at the bioshield plug demonstrated PCP contact dose rates below project requirements. SDR levels in contact with the isolated IVVS cartridge were found to marginally exceed the hands-on maintenance limit. For engineering feasibility, shielding blocks at bioshield level are to be avoided, however the port cell SDR field requires further consideration. In addition, alternative low-activation steels are being considered for the IVVS cartridge.