Shutdown dose rate benchmarking using modern particle transport codes

Abstract

Shutdown dose rate calculations provide an essential input to the design and research of fusion power plant technology. They allow the estimation of dose to personnel and equipment during planned and unplanned maintenance. The mesh coupled rigorous 2 step (MCR2S) methodology used at Culham Centre for Fusion Energy (CCFE) was originally developed to link the MCNP particle transport code and the FISPACT-II inventory code. As new particle transport codes are developed there is a strong motivation to move towards a code agnostic approach. This paper details the integration of MCR2S with the FISPACT-II API and two other transport codes, Serpent 2 and OpenMC. Two benchmarks, the FNG shutdown dose rate experimental benchmark and the ITER computational benchmark, have been performed and compared to results produced with MCNP. In general, the results show that MCNP, Serpent 2 and OpenMC give shutdown dose rate results similar to the FNG experiment for both experimental campaigns. However, all codes appeared to slightly overestimate the dose rates for Campaign 1 (all results had a C/E between 1 and 1.5) and underestimate the dose rates for Campaign 2 (all results had a C/E between 0.6 and 1). Differences were seen between OpenMC and MCNP for the ITER port plug benchmark, where the lack of variance reduction in OpenMC meant that the neutron flux estimates at the rear of the model were not converged. This led to differences of up to 13% in the shutdown dose rates. It was shown that Serpent 2 and MCNP, where variance reduction was used, gave shutdown dose rates within 3% of each other. Although some areas for development of the Serpent 2 and OpenMC transport codes have been highlighted, overall the comparisons give confidence that the implementation of these two transport codes into the MCR2S work-flow has been carried out successfully.

Development and validation of fully open-source R2S shutdown dose rate capabilities in OpenMC *

“…The (n,p) reaction rates show C/E values close to 1 particularly when using FENDL-3.2b data, while the calculated (n,2n) reaction rates generally underpredict the experimental values regardless of which data library was used. These results are consistent with previous results in the literature [22,44], which also show a statistically significant underprediction of the (n,2n) reaction rates regardless of the choice of nuclear data library and MC code used for transport.…”

Oak Ridge National Laboratory Shutdown Dose Rate Code Suite

“…The ITER SDDR benchmark was selected to demonstrate the application of the MS-CADIS R2S method. This benchmark, which simulates an ITER upper port plug, has been presented in several papers [1,2,6,7,8,9].…”

“…The geometry of the benchmark problem is shown in Figure 3 of [1], Figure 2 of [2], Figure 2 of [6], Figure 10 of [7], Figure 7 of [8], Figure 1 of [9] and Figure 2 of this report. The geometry includes a steel cylindrical ring with an outer radius of 100 cm, an inner radius of 50 cm, and a length of 550 cm [2,7].…”

“…This source emits 14 MeV neutrons, which is uniformly distributed within the source cell. The irradiation history is given in Table 1 [1,2,6,7]. The steel and steel/water material compositions are given in Table 2 [6].…”

Experimental validation of inventory simulations on molybdenum and its isotopes for fusion applications

“…Similarly, the γ-dose-rate from Mo is predicted to be many orders of magnitude higher than that from W from 10-years after the operational life of blanket and divertor components in DEMO. Figure 1(b) charts the time-evolution in γ-dose-rate, in units of Sv/h (Sieverts per hour), one metre from an idealised 1 g 'point source' of either Mo or W-this approximation of dose calculated by FISPACT-II is more conservative (lower) than the alternative, default, 'contact' dose approximation and is more relevant for radiation workers working in a nuclear environment and wearing protective clothing (a full γ-transport simulation is required for a more reliable prediction of γ-dose, see for example [26]). The UK's Health and Safety Executive (HSE) recommends, based on the UK's regulations for ionising radiation [27], a limit of 7.5 µSv/h (shown in the plot as a horizontal dashed line) for areas where workers will be exposed to ionising radiation [28].…”