Activation properties of tungsten as a first wall protection in fusion power plants”, Fusion Engineering and Design

Abstract Tungsten is a candidate material for a protective armour on the plasma-facing first wall of a fusion power plant. In order to assess the radiological implications of this use of tungsten, two power plant models from the European Power Plant Conceptual Study (PPCS) have been extended by the addition of a 2 mm tungsten layer on the ferriticmartensitic first wall. Neutronics and activation modelling have shown that there is no significant impact on the favourable conclusions regarding long-term disposal of materials, but that earlier activity levels could potentially be of importance on the timescale of maintenance operations. The short-term activity and decay are expected to have only a minor impact on the consequences of postulated accidents. Keywords Neutron activation, tungsten, first wall armour, dose rate, decay heat, fusion power plant 2 1 Introduction Studies of future fusion power plant design concepts in the European Power Plant Conceptual Study (PPCS) This material is employed as blanket structure, including the plasma-facing first wall surface. It may be desirable to protect this Eurofer surface from erosion by provision of a protective armour layer, either as a coating or as tiles. One candidate for such an armour is tungsten, attractive by virtue of its expected low sputtering and erosion rate. But exposure to a high flux of 14 MeV neutrons from the plasma, as well as lower energy neutrons returning from the blanket after moderation, raises the issue of activation of this tungsten layer and the possibility of radiological consequences. In this work the neutron activation has been calculated in a tungsten armour layer added to two of the PPCS plant models, and the implications for safety, environmental impact and operation are assessed. The two models, labelled Plant Models A and B in the PPCS study, are chosen to represent a water-cooled and a helium-cooled concept, in order that any influence of the different blanket neutron spectra can be observed. Calculations performed The activation behaviour of a 2 mm-thick layer of tungsten has been studied by extending existing neutronic models of PPCS Plant Models A and B. Both assume Eurofer structure, which will have a similar influence on the neutron spectra in the vicinity of the first wall for both models, but whereas Model B has helium-cooled first 3 wall and blanket, Model A is water-cooled which could lead to a softer neutron spectrum through greater moderation. The neutronics calculations were performed with the MCNP code (version 4C3) and ENDF/B-VI cross section data, using 3-D models previously generated for PPCS safety and environmental analyses using the in-house HERCULES code system Special care was taken to correctly calculate the contribution of the reaction 186 W(n,γ) 187 W, which has a giant resonance at about 20eV. Due to resonance selfshielding, the use of multigroup cross sections and group fluxes is likely to lead to an over-estimation of 187 W production via this resonance capture. To ensure a correct evaluation, the 186 W(n,γ) reaction rate was computed directly in the MCNP continuousenergy calculation, and this value transferred to the FISPACT code for the inventory calculation. The magnitude of the effect is geometry-dependent, with thicker tungsten regions leading to more pronounced self-shielding; nevertheless, even in a 2 mm layer the effect was found to be significant. 4 A comparison was made of the total activation results for tungsten with this correct procedure and with a purely multigroup calculation. In the FISPACT calculations, the irradiation history assumed that the first wall lifetime is 5 full power years -the armour was also assumed to be exposed for this period. At the end of the first 2.5 years operation there is a shutdown for 2 months for divertor replacement, followed by a further 2.5 years operation. After the end of this lifetime, the inventory, activation and all related quantities were calculated at decay times from 1 second up to 10,000 years. Results Activation results The specific activity of the tungsten material after the end of operation is shown in dose rate at early times could also have an impact on maintenance operations. The contact gamma dose rate is above 10,000 Sv/hr for around 6 months after shutdown, which could be a challenge for some remote handling equipment. Although this result for tungsten is initially around three times that for Eurofer, beyond this 6-month point it falls to lower values -however, this is too late to be of benefit for maintenance procedures. The decay heat density of the tungsten after the end of operation is shown in Waste categorisation Based on the results of the activation calculations presented above, it is possible to categorise the active tungsten according to the criteria adopted in PPCS and earlier European studies [5]. These define categories for non-active waste (below Clearance limits), simple and complex recycling material, and permanent disposal waste. The Clearance level is never reached by the tungsten armour material, and the simple recycling level only after several hundreds of years. So the only relevant categories are 7 complex recycling material and permanent disposal waste. The material is categorised for permanent disposal if the contact gamma dose-rate is above 20 mSv/hr or the decay heat is above 10 W/m 3 . These limits are somewhat arbitrary, and the 20 mSv/hr limit is now believed to be conservative, but it has been adopted by PPCS in common with earlier European fusion safety and environmental assessments [5], pending a reevaluation. The tungsten armour falls below the 20 mSv/hr level after about 75 years ( The total mass of tungsten, less than 300 tonnes, is only a relatively minor addition to the total masses of material arising from Plant Models A and B. The presence of the 2 mm tungsten armour has an insignificant effect on the activation levels in other components due to neutron absorption in the tungsten itself; comparison of the MCNP results with and without the armour layer showed that the total neutron flux at the back of the first wall is reduced by just 0.7% when the tungsten is added. Conclusions The addition of a 2 mm tungsten armour layer to the plasma-facing surface of the Eurofer first wall has been analysed for its neutron activation behaviour. The decay 8 heat in the short term (~1 day) following shutdown is modest, and unlikely to give rise to any problems in postulated accidents. The relatively high activity at these early times may be significant for accident scenarios in which in-vessel dust, formed mainly of material from the plasma-facing surface, is a potential source term. The dose rates from tungsten armour, both direct and through inhalation, are initially around three times higher than those from an equivalent mass of Eurofer first wall exposed without tungsten armour. For the first few months after shutdown the contact gamma dose rate remains relatively high, but falls below that of Eurofer after six months. The high dose at the times that maintenance operations would be in progress could conceivably be an issue for some remote handling equipment that is sensitive to operation in a radiation field. After the end of plant life and up to 50 years later, the activation properties of the tungsten material are such that only a small proportion is categorised as suitable for recycling. But some 75 years after end of plant life, all of the tungsten material has a contact gamma dose-rate that has fallen below the 20 mSv/hr level, allowing it to be categorised as complex recycling material. A key outcome of the earlier studies in PPCS was that for all four Plant Models there is no material requiring permanent disposal after 100 years