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

Abstract

Molybdenum is a potential material for future nuclear fusion experiments and power plants. It has good thermo-mechanical properties and can be readily fabricated, making it attractive as an alternative material to tungsten (the current leading candidate) for high neutron flux and high thermal load regions of fusion devices. Unfortunately, exposure to fusion neutrons is predicted to cause significant radioactivity in elemental Mo for decades and centuries after exposure, which would be a problem during maintenance and decommissioning operations. Simulation predictions indicate that Mo activation could be reduced by isotopic adjustment (biasing). If these predictions are proven and validated, and if isotopic adjustment is technically and economically feasible, then Mo could be used in future demonstration and commercial reactors without significantly increasing the amount of long-term, higher-level radioactive waste. Transmutation (inventory) simulations used to predict activation rely on nuclear reaction data. The quality of these data impact on the confidence and uncertainty associated with predictions. Recently, UKAEA has developed benchmarks to test and validate the FISPACT-II inventory code and the input nuclear data libraries. Verification of molybdenum inventory simulations is performed against experimental decay-heat measurements from JAEA’s fusion neutron source (FNS) facility and using new data acquired from γ-spectroscopy measurements of Mo irradiated in the ASP 14 MeV facility in the UK. Results demonstrate that FISPACT-II predictions (with TENDL-2019 nuclear data) for Mo are accurate on the short-timescales (minutes, hours of irradiation and minutes, days, weeks of cooling) of these laboratory experiments. However, these kinds of experiments are limited in their coverage of the important radionuclides for decay radiation from Mo on the years, decades and beyond timescales. Further experiments with fusion relevant conditions and timescales, potentially with alternative measurement techniques, are still needed.

Nuclear data for fusion: inventory validation successes and future needs

“…Figure 4 shows the C/E comparison performed in [37] for the radionuclides identified by their peaks in the eight experiments performed for Mo (the eight experiments differ in irradiation times and average fluxes-see [37] for details). Mo is of particular interest for fusion applications as a potential alternative armour material in high heat-flux regions of a reactor [37,38]. There are relatively large errors shown as vertical bars for each data point, which include contributions from statistical count errors, leading to uncertainties in counts for both radionuclides of interest and the radionuclides uses to estimate flux values using Fe and Al foils.…”

“…Further work is underway to analyse the remaining ∼300 experiments (starting with recent analysis of 14 tungsten experiments to test predictions of 185 W production [39]) from the UKAEA-ASP campaigns, with the final aim being to turn them into an experimental benchmark that can be used to test the performance of nuclear libraries for a variety of fusion materials-in much the same way that the FNS-decay-heat benchmark described earlier is now used. However, the analysis for Mo [37] confirms the drawbacks of the experiments; they are only able to interrogate the production of short-lived radionuclides due to insufficient counts and consequently low signal-to-noise ratios for radionuclides with longer half-lives. The longest-lived of those identified in figure 4 is 97 Nb, with a half-life of only T 1/2 of 1.23 h, which has limited relevance to fusion reactor operations (except maybe to identify the cooling requirements immediately after shutdown).…”

Discovery, nuclear properties, synthesis and applications of technetium-101

“…The exchange of Mo for W, its heavier chemical homolog, in component materials has been a particular design strategy to reduce induced radioactivity, although the presence of Mo at limited concentrations is considered acceptable; this has been the basis of reduced activation ferritic-martensitic steels (RAFMs) 137 , 138 . Alternatively, the use of Mo enriched in specific Mo isotopes that do not significantly activate, i.e., 96 Mo and 97 Mo, has been proposed; however, because of enrichment costs, better economic options will need to be developed for commercial applications 139 .…”

First direct determination of the 93Mo half-life

“…The main source of 93 Mo is the neutron capture of 92 Mo in molybdenum rich materials, typically steel, Iconel alloys or cables used for constructions within nuclear power plants 10 – 12 . Molybdenum is also a potential material for nuclear fusion devices where high neutron fluxes unavoidably lead to the production of 93 Mo as one of the main long-lived radioactive products 13 . Other sources of 93 Mo in nuclear waste are particle accelerator facilities where the neutrons are emitted due to nuclear reactions.…”