Assessment of the Operational Dose Rate in Polymer Insulators in the Test Cell of the IFMIF Neutron Source

Insulators for electrical connections of the test modules in the test cell of the International Fusion Materials Irradiation Facility (IFMIF) neutron source will be exposed to intense ionizing radiation even if they are located away from the neutron source. The aim of this work was to check whether radiation-hard polymers, for example polyimide, would be sufficiently long-lasting in such an environment. The calculations presented here were survey calculations to aid with the decisions on possible insulator candidates. The calculations have been done with the McDeLicious code and a detailed 3-D neutronics model of the IFMIF test cell. Mesh tallies were utilized to calculate dose rate maps 10 cm below the ceiling of the test cell where the connectors would be located. The calculated dose deposited within one full power year would be higher than 200 MGy nearly everywhere in the test cell. This value excludes most polymers as candidates for insulators, but even polyimide would reach its limits. It was found that the calculated dose contribution from gamma rays was considerably dependent on the nuclear data library used for describing the lithium in the lithium loop. We have also checked the effect of an additional lead shield of 10 cm thickness which reduced the deposited dose by a factor of 2-5. A new neutronics model has become available recently which was derived from the latest IFMIF test cell design. We have performed preliminary calculations for dose rates in anticipated locations of connector insulators. The results confirm the high dose rate values calculated with the previous neutronics model.