The irradiation of tungsten and tungsten alloys with energetic neutrons (14MeV) resulting from the D-T reaction causes radiological hazards that were already discussed in Section 4.17.3.2.6. In addition, the neu­tron irradiation affects the material composition by transmutation of tungsten to Re and subsequently osmium (transmutation of W isotopes to Ta and Hf are negligible222). The amount of transmutation strongly depends on the applied neutron wall load and neutron spectrum223 and for the W to Re transmu­tation reaction reaches values between 0.3 and 5 at.% per dpa.222 The subsequent transmutation of Re to Os is expected to occur faster than the production of Re from W resulting in a steadily proceeding burnup of Re. The neutron fluence on the first wall varies strongly with location. For the full lifetime of ITER a maximum of ~0.3MWam-2 is achieved22 («1.35 dpa in tungsten225). As the divertor PFCs will be exchanged 3 times and only the last three will operate in a D-T environment, a neutron fluence of ~0.1 MW m~2 is expected during the lifetime of each PFC. For DEMO, an average neutron wall load of 2 MW m~2 is assumed for the main chamber, which would result in ^45 dpa after 5 full power operation years. These conditions yield a transmutation of 100% W into 75% W, 12% Re, and 13% Os.36 For geometrical reasons, that is, larger surface to angular extension ratio, it will be roughly a factor 2 less in the divertor region.

Furthermore, neutron irradiation damages the material properties by the formation of vacancies and interstitials (see Chapter 1.03, Radiation-Induced Effects on Microstructure). Their behavior including analysis of displacement cross-sections,226,227 diffu­sion, mutual recombination, and clustering are being assessed by atomistic modeling.228-231

Both transmutation and defect generation influ­ences the material properties and subsequently the material response to steady state and transient thermal loads.