تجزیه و تحلیل فنی از یک منبع نوترون همجوشی اولیه مبتنی بر ارتقاء از نمونه اولیه شتاب دهنده IFMIF / EVEDA

In the framework of the Engineering Design and Engineering Validation Activities for the International Fusion Materials Irradiation Facility (IFMIF/EVEDA), three major prototypes have been designed and are being manufactured, commissioned and operated which are firstly the Accelerator Prototype (LIPAc) at Rokkasho, fully representative of the IFMIF low energy (9 MeV) accelerator stage, secondly the EVEDA Lithium Test Loop (ELTL) at Oarai, and thirdly critical components of the High Flux Test Modules to be tested in the helium cooling loop (HELOKA-LP) at Karlsruhe. The present paper analyses possibilities from a technical point of view, for combining, modifying, and enhancing, at limited cost, selected components of the prototypes towards the realisation of an early reduced-flux neutron source, able nonetheless to start the testing of candidate DEMO materials and realising by this a first step towards the construction and operation of a complete IFMIF plant. Various options of deuteron beam parameters, such as energy, current and shape are analysed with respect to their technical challenges and the neutron yield resulting from the nuclear reaction with the Li target. Related requirements for the liquid Li target with respect to jet parameters are evaluated and the neutron mapping in the high flux region is presented underlying an analysis of the available volume for testing reduced activation ferritic martensitic (RAFM) steels at relevant structural damage levels.

The International Fusion Materials Irradiation Facility (IFMIF) is projected to provide an accelerator-based, d–Li neutron source to produce high energy neutrons at sufficient intensity and irradiation volume to simulate as closely as possible the first wall neutron spectrum of future nuclear fusion reactors such as DEMO and Power Plants [1]. Structural damage generated by neutrons in the material is quantified in terms of dpa (displacement per atom), so the performance of materials is qualified in terms of dpa per full power year (dpa/fpy), where for Fe 1 dpa equals 1.5 × 1024 n/m2 (E = 14 MeV) [2]. In conjunction with the development of a DEMO concept, Materials R&D and Breeder Blanket R&D are two high priority issues in nuclear fusion technology [3]. They justify special interest in the following topics: Topic 1: Proof of feasibility of an efficient cooling concept for ITER Test Blanket Modules (TBMs) by studying the cooling performance in small scale mock-ups under DEMO-relevant nuclear heat load and activation conditions (requiring damage rates of typically 1–5 dpa/fpy); Topic 2: Determination of the occurrence of a specific helium effect for early DEMO conditions by measuring high quality data for the ductile-to-brittle transition temperature and fracture toughness (i.e. well-defined irradiation temperature with no significant temperature excursions) around the expected threshold of 30–50 dpa (requiring damage rates of typically 10–15 dpa/fpy); Topic 3: Experimental demonstration that the Reduced Activation Ferritic Martensitic steels have the expected radiation resistance well above the helium threshold level. This experiment calls for exploring “terra incognita” in neutron damage well above 50 dpa (requiring damage rates above 20 dpa/fpy). In the framework of the Engineering Design and Engineering Validation Activities for the International Fusion Materials Irradiation Facility (IFMIF/EVEDA), the IFMIF Engineering Design has been advanced [4] and the IFMIF Intermediate Engineering Design Report (IIEDR) has been completed recently [1]. To validate the developed design features, prototyping sub-projects are continued which consist of the design, manufacturing and testing of the following prototypical systems [5], [6] and [7]: Prototype 1: The Accelerator Prototype (LIPAc) at Rokkasho (Japan), fully representative of the IFMIF low energy (9 MeV) accelerator (125 mA of D+ beam in continuous wave), now under construction and planned to be commissioned by June 2017 (Fig. 1); Prototype 2: The Experimental Lithium Test Loop (ELTL) at Oarai (Japan), integrating all elements essential to the IFMIF lithium target facility, already commissioned in February 2011 and with a test programme running at least until mid-2014; Prototype 3: Critical components of the High Flux Test Modules tested in the helium cooling loop (HELOKA-LP), at Karlsruhe (Germany), with testing in 2014 and possibly beyond. Even though the IFMIF/EVEDA project scope is limited to studies with these prototypes being kept separately, their combination would be appealing, with the potential to obtain an early fusion neutron source, albeit with reduced capabilities in terms of annual fluence, irradiated volume, and overall versatility. There are several options for a step-wise approach towards the final IFMIF plant as well as for the partial use of the infrastructure and components developed for the IFMIF/EVEDA phase. The present paper focuses on an elementary variant of an early neutron source (“ENS”) which can only respond to studies for TBM cooling concepts (Topic 1) and a limited specimen set for materials qualification at 30–50 dpa level (Topic 2). For a more consolidated study of Topic 2 which heads for an engineering and scientific database for DEMO, a variant called “DONES” (DEMO oriented neutron source) is being considered. This variant, which is the subject of another paper [8], is designed to provide a larger available volume for high fluence neutron irradiation, allowing therefore a larger choice of materials and irradiation conditions. The goals set for Topic 3 (investigation of radiation resistance under the extreme damage levels expected in Fusion Power Plants) can realistically only be tackled with the full IFMIF plant.

The elementary option for an early neutron source is conceived on the basis of integrating a maximum of components from the three major prototypes produced for IFMIF in its EVEDA phase and to limit the accelerator stages to a deuteron beam energy of 26.5 MeV. As a consequence, only the two central front rigs will allow irradiations at 10–20 dpa/fpy and these values are only achieved over their half height. Albeit the slight shift in the neutron spectrum, characteristic ratios of the He to dpa generation can be reached. With typical dimensions for this central area 10 cm (width), 1.8 cm (depth) and 4.1 cm, a volume of 75 cm3 will be available for first studies of the occurrence of a specific helium effect in the RAFM steels with fusion neutrons. The volume of two rigs with only half of their height with adequate damage will nevertheless still be sufficient to house a full set of 80 RAFM specimens that can be studied in each of the 12 irradiation rigs designed for the IFMIF plant. For reduced conditions (damage rates higher than 5 dpa/fpy) the number of specimens can be easily tripled. The full volume of the HFTM container is expected to be suited for the blanket R&D requirements.