The proposed International-Fusion-Material- Irradiation-Facility (IFMIF) is an accelerator-driven neutron source that is based on the proton-stripping reaction.58,59 Neutrons are generated by a beam of 40 MeV deuterons that undergo a proton-stripping reaction when they interact with a flowing liquid lithium jet target. The resulting neutron beam has a spectrum with a high-energy tail above a peak around 14.60 As in the case of D, T, and spallation reactions, these neutrons are well above the thresh­old energy for n, a reactions; thus, IFMIF produces fusion like He/dpa ratios at high dpa rates. The nuclear reaction kinematics and limited neutron target source dimensions result in an IFMIF irradia­tion volume with large gradients over a high-flux region just behind the target. Two 125 mA beams on the Li target produce an «500 cm3 region with dpa rates of 20-50 fpy 1 (full power year) at He/ dpa «12appmdpa~. The medium flux region, from 1.0 to 20dpafpy~ , is much larger with a volume of «6000 cm3.

The Materials Test Station is a new spallation neutron source, proposed by Los Alamos National Laboratory, that is primarily intended to irradiate fast reactor materials and fuels.61 The LANSCE linear accelerator will produce a 1-MW proton beam to drive the spallation neutron source with a fast reactor like spectrum and a high-energy tail up to 800 MeV. The high-energy tail neutrons produce a He/dpa«6-13appmdpa~ close to that for a fusion first wall. The dpa rates are «7.5-15 fpy~ in a 200 cm3 irradiation volume and 2.5-12.5 fpy_1 in an additional volume of 450 cm3. An accelerator upgrade to 3.6 MW would increase these dpa rates to 20-40 fpy1 and 5-16 fpy, respectively.

In both cases, the limited volume for high-flux accelerated irradiations presents a great challenge to developing small specimen mechanical test meth — ods62,63 and experimental matrices64 that can pro­duce the database needed for materials qualification. The database will require irradiations over a range of temperatures for tensile, fracture toughness, fatigue, and creep property characterization. Indeed, it is clear that qualifying materials for fusion ap­plications will require a new paradigm of linking comprehensive microstructural characterization and physically based predictive modeling tools to multi­scale models and experiments of structure-sensitive properties as input into engineering models of mate­rials performance.

A variety of proposals have been made to develop volumetric D-T fusion devices such as the Fusion Nuclear Science Facility (FNSF), which would pro­vide a basis to test components and materials.65 In some cases, these devices would address a much broader array of issues, such as tritium breeding and extraction. In most cases, the fusion source would be driven by external energetic D beams. Discussing the details of such proposed devices is far beyond the scope of this chapter. However, we note that from a materials development perspective, such devices would be useful to the extent that they are steady state, operate with very high-duty factors, and pro­duce sufficient wall loading to deliver high He and dpa exposures.