J.-Ch. Sublet

An integrated model for materials in a fusion power plant: transmutation, gas production, and helium embrittlement under neutron irradiation

The high-energy, high-intensity neutron fluxes produced by the fusion plasma will have a significant life-limiting impact on reactor components in both experimental and commercial fusion devices. As well as producing defects, the neutrons bombarding the materials initiate nuclear reactions, leading to transmutation of the elemental atoms. Products of many of these reactions are gases, particularly helium, which can cause swelling and embrittlement of materials.This paper integrates several different computational techniques to produce a comprehensive picture of the response of materials to neutron irradiation, enabling the assessment of structural integrity of components in a fusion power plant. Neutron-transport calculations for a model of the next-step fusion device DEMO reveal the variation in exposure conditions in different components of the vessel, while inventory calculations quantify the associated implications for transmutation and gas production. The helium production rates are then used, in conjunction with a simple model for He-induced grain-boundary embrittlement based on electronic-structure density functional theory calculations, to estimate the timescales for susceptibility to grain-boundary failure in different fusion-relevant materials. There is wide variation in the predicted grain-boundary-failure lifetimes as a function of both microstructure and chemical composition, with some conservative predictions indicating much less than the required lifetime for components in a fusion power plant.

Neutron-induced transmutation effects in W and W-alloys in a fusion environment

W and W-alloys are among the primary candidate materials for plasma-facing components in the design of fusion reactors, particularly in high-heat-flux regions such as the divertor. Under neutron irradiation W undergoes transmutation to its near-neighbours in the periodic table. Additionally He and H are particles emitted from certain neutron-induced reactions, and this is particularly significant in fusion research since the presence of helium in a material can cause both swelling and a strong increase in brittleness. This paper presents the results of inventory burn-up calculations on pure W and gives quantitative estimates for He production rates in both a fusion-reactor environment and under conditions expected in the ITER experimental device. Transmutation reactions in possible alloying elements (Re, Ta, Ti and V), which could be used to reduce the brittleness of pure W, are also considered. Additionally, for comparison, the transmutation of other fusion-relevant materials, including Fe and SiC, are presented.

Neutron-induced dpa, transmutations, gas production, and helium embrittlement of fusion materials

In a fusion reactor materials will be subjected to significant fluxes of high-energy neutrons. As well as causing radiation damage, the neutrons also initiate nuclear reactions leading to changes in the chemical composition of materials (transmutation). Many of these reactions produce gases, particularly helium, which cause additional swelling and embrittlement of materials. This paper investigates, using a combination of neutron-transport and inventory calculations, the variation in displacements per atom (dpa) and helium production levels as a function of position within the high flux regions of a recent conceptual model for the ‘next-step’ fusion device DEMO. Subsequently, the gas production rates are used to provide revised estimates, based on new density-functional-theory results, for the critical component lifetimes associated with the helium-induced grain-boundary embrittlement of materials. The revised estimates give more optimistic projections for the lifetimes of materials in a fusion power plant compared to a previous study, while at the same time indicating that helium embrittlement remains one of the most significant factors controlling the structural integrity of fusion power plant components.

Energy spectra of primary knock-on atoms under neutron irradiation

Materials subjected to neutron irradiation will suffer from a build-up of damage caused by the displacement cascades initiated by nuclear reactions. Previously, the main “measure” of this damage accumulation has been through the displacements per atom (dpa) index, which has known limitations. This paper describes a rigorous methodology to calculate the primary atomic recoil events (often called the primary knock-on atoms or PKAs) that lead to cascade damage events as a function of energy and recoiling species. A new processing code SPECTRA-PKA combines a neutron irradiation spectrum with nuclear recoil data obtained from the latest nuclear data libraries to produce PKA spectra for any material composition. Via examples of fusion relevant materials, it is shown that these PKA spectra can be complex, involving many different recoiling species, potentially differing in both proton and neutron number from the original target nuclei, including high energy recoils of light emitted particles such as aparticles and protons. The variations in PKA spectra as a function of time, neutron field, and material are explored. The application of PKA spectra to the quantification of radiation damage is exemplified using two approaches: the binary collision approximation and stochastic cluster dynamics, and the results from these different models are discussed and compared.

Inventory Simulations Under Neutron Irradiation: Visualization Techniques as an Aid to Materials Design

Abstract The simulation of neutron irradiation–induced transmutation using inventory codes is an important part of the research into materials in various nuclear environments, including fusion, fission, medical physics, nuclear security, and astrophysics. These simulations, even in their simplest form, such as the neutron irradiation of a single pure element, generate large time-dependent data sets of complex results. For each nuclide in the inventory, as a function of time, the output data will include the number of atoms and its contribution to a variety of radiological quantities including total or specific activity, gamma dose, heat output, and ingestion and inhalation hazards. A key challenge when performing inventory calculations is thus to represent the full complexity of the results in a visual and understandable format. This paper discusses two different approaches to visualizing inventory data: (a) nuclide maps, which allow the concentrations or activity contributions from all nuclides in the inventory to be displayed and also for the variation to be traced in time under a specific irradiation scenario, and (b) importance diagrams, which are a neutron spectrum–independent representation of the dominant nuclides that contribute to the activity of an irradiated material. Finally, these techniques are applied in parallel to investigate how the activation response of molybdenum can be improved via isotopic tailoring (enrichment or depletion), which could make it a more viable alternative armor material in the design of fusion reactors.

Benchmark Experiments of Fusion Neutron Induced Gamma-Ray Radioactivity in Various Structural Materials

The fusion reactor inventory code FISPACT, together with the European Activation File EAF, is the European reference software for calculating the neutron-induced activation of fusion reactor relevant materials. Experimental verifications (benchmarks) of the code predictions have been performed at ENEA Frascati by means of an irradiation facility consisting of a D-T neutron generator and a moderator/reflector structure which is employed to mimic the neutron spectrum at the a fusion device first wall. Various materials (vanadium alloy, SiC, AISI 316, martensitic steel F82H, copper, tungsten, iron, niobium), candidates to be used in a fusion reactor, have been exposed to neutrons produced in the facility (about 109 n cm−2 s−1) and the short and medium-lived induced radioactivity has been measured by gamma-ray spectroscopy. The experimental results have been used to validate the inventory code FISPACT, the physical database EAF, including its uncertainty predictions, and the composition of the material irradiated in particular for its minor elements and impurities. The comparison between calculated (C) and experimental results (E) is reported as C/E values and shows a satisfactory agreement for almost all radionuclides. Radionuclides for which there is not agreement between calculations and experiments are also discussed and an analysis of the causes of the lack of agreement is carried out.

Maintenance of a commercial fusion power station and its implications for safety

A recent study was undertaken (D.V. Sherwood et al., Fusion Eng. Des. 22 (1993) 367-378) to identify a maintenance scheme which would lead to an acceptable level of availability for a future commercial fusion power station. It was thought that a scheme involving the servicing of the divertor through dedicated maintenance ports provided in each sector of the torus, and the sub-division of the blanket in both the radial and poloidal directions to allow it to be handled through two opposite dedicated radial access ports, could lead to a plant availability in the order of 80% (including 7% unplanned shutdown), i.e. comparable with that expected from APWRs. The method of blanket replacement proposed is substantially different from that of the ITER CDA. Although still conceptual, the maintenance scheme has now been developed sufficiently (NNC Rep. FR/E/004399A, 1993) to allow the main potential hazards requiring control during maintenance and the main feasibility issues to be identified. Taking these into account, a more accurate assessment of the plant availability has been possible which confirms that 80% should be possible. The scheme is considered to present little risk to the maintenance staff and with proper safeguards there would be little potential for the spread of contamination.

Spatial heterogeneity of tungsten transmutation in a fusion device

Accurately quantifying the transmutation rate of tungsten (W) under neutron irradiation is a necessary requirement in the assessment of its performance as an armour material in a fusion power plant. The usual approach of calculating average responses, assuming large, homogenised material volumes, is insufficient to capture the full complexity of the transmutation picture in the context of a realistic fusion power plant design, particularly for rhenium (Re) production from W. Combined neutron transport and inventory simulations for representative {\it spatially heterogeneous} models of a fusion power plant show that the production rate of Re is strongly influenced by the local spatial environment. Localised variation in neutron moderation (slowing down) due to structural steel and coolant, particularly water, can dramatically increase Re production because of the huge cross sections of giant resolved resonances in the neutron-capture reaction of \(^{186}\)W at low neutron energies. Calculations using cross section data corrected for temperature (Doppler) effects suggest that temperature may have a relatively lesser influence on transmutation rates.

Self-shielding effects in a tungsten layer in a fusion device

Abstract The impact of geometrical modelling and energy treatment on neutronics and activation results for materials with giant resonances, such as tungsten, is significant. Three-dimensional (3-D) neutronics calculations have been performed using the Monte Carlo codes MCNP and TRIPOLI to determine the self-shielding effect in tungsten layers in fusion environment. While excellent agreement exists between the two Monte Carlo results, the 187W production rate is overestimated by about a factor of 4 when a homogenised model is used with multi-group sampling. To correctly predict reaction rates in tungsten, 3-D continuous energy Monte Carlo calculations with layered heterogeneous modelling should be used. An effective reaction rate derived from the Monte Carlo pointwise results, which accounts accurately for resonance absorption effects, which should be used in the subsequent activation calculation.

Fusion activation of ferrous alloys - dependence on flux, irradiation time and fluence

Abstract The induced activity, contact dose-rate and decay heat of a number of ferrous alloys have been calculated using the European Activation SYstem EASY for the first wall position in a number of different conceptual reactor designs, namely CCTR, EEF, DEMO, STARFIRE, ITER, and NET. In the first instance all neutron fluxes were normalised to 1 MW m−2 to permit direct investigation of the influence of the different spectra on the predicted activation. The activation levels of ferrous alloys predicted for the various reactor designs at a standardised flux are found to vary by orders of magnitude at cooling times relevant to reactor decommissioning. The ways in which the predicted activation properties of a material scale with the duration of the irradiation, the first wall neutron loading and the corresponding fluence are systematically examined. The activation behaviour of candidate structural materials is governed by a relatively limited number of specific radionuclides and can best be understood by following the production pathways for these nuclides and by examining their dependence on the flux and irradiation time. These pathways can be strongly modified by changes in the irradiation conditions and the amount of a particular radionuclide generated cannot be simply scaled if multiple step reactions are involved in its production. The cases of selected ferrous alloys are considered in some detail; the production pathways for dominant radionuclides are identified and the way in which the pathways evolve as irradiation proceeds is described. The results demonstrate that an appreciation of the main generation routes for radionuclides produced by multiple-stage reaction chains is essential to a proper application of the flux-time scaling relationships in safety and environmental studies.