Predictions of material activity in commercial fusion conditions predominantly rely

on computational methods, due to a lack of data on long-term e ects of high-energy

neutron irradiation on structural steels. Consequently, this could result in a bias

due to uncertainties in nuclear data used. This work focused on modelling neutron

activation of four structural steels in a fusion reactor environment after 20 years of

operation. Eurofer, F82H and G91, were assessed as candidate in-vessel materials,

whereas SS316L(N)-IG was solely modelled in the vacuum vessel. Activation calculations

were performed using the inventory code FISPACT-II using inputs from

Monte-Carlo transport simulations performed with OpenMC. The study employed a

one-dimensional reactor model with a Helium-Cooled Pebble Bed (HCPB) tritiumbreeding

blanket design. With the XSUN-2021 code package, a nuclear data sensitivity

and uncertainty analysis on production cross-sections of relevant radio-nuclides

was carried out.

Eurofer and F82H steels exhibited signi cantly higher resistance to neutron activation

than G91 and SS316L(N)-IG. At 100 years after shutdown, none of the steels

reached UK low-level waste (LLW) activity levels in the  rst wall. In the backsupport

structure (BSS) of the reactor blanket, all assessed steels reached LLW

levels within approximately 30 to 45 years of reactor shutdown. It was found that

the vacuum vessel (SS316L(N)-IG) would not be classi able as LLW for several centuries.

Dominant radio-nuclides for each material were identi ed with FISPACT-II

to the sensitivity analyses. The calculated uncertainties were too small to a ect the

waste disposal options for the  rst wall within 100 years, but the time-to-reach LLW

for BSS and vacuum vessel steel could be uncertain by approximately 6 years.