The activation and transmutation of tungsten as a PFM is a critical issue, particularly concerning long-term storage and recycling times. Different studies on activation issues have been performed. These comprise the analysis of cross-sections for high-energy neutrons,1 , 2 studies on the helium — cooled lithium lead divertor for DEMO,113 the inertial fusion devices,109 other benchmark experiments,114,115 and modeling issues, for example, on the self-shielding ability of tungsten.116,117

Furthermore, it was shown that the long-term activation behavior is dominated by activation products of the assumed material impurities while the short­term behavior is due to the activation of the stable W isotopes.113 For a short period of a few weeks, the latter causes a huge amount ofdecay-induced afterheat that has to be removed by continued active cooling.67 On the other hand, the accumulation of the highly radioactive transmutation product 186mRe was deter­mined to be most critical, limiting the component lifetime to a maximum of 5 fpy when using pure W or to 2 fpy when using Re-doped W before the limits for storage by shallow land burial could be exceeded.109 The dose rate limits for recycling after different applications are expected to be reached within 5 years115 to 50 years113 of storage or up to 75 years after end of plant life.118 Fischer etal.113 take a limit of 100 mSv h-1 for remote handling into account, which might be a problem at the times when maintenance operations would be in progress. Taylor and Pampin118 give a value of 20mSvh-1 as the limit for allowing tungsten to be categorized as a recyclable material. The hands-on limit for tungsten should be achieved after about 200 years.115

Besides waste management, tungsten has also been investigated and evaluated according to characteristic radiological hazards that might occur when using it as PFM in tokamak fusion reactors. It was found that the tritium permeation into tungsten does not, in contrast to CFC, appear to be a major problem. However, due to neutron activation, the mobilization of activation products, for example, by forming volatile oxide spe­cies in the presence of steam and air, has to be limited by establishing shutdown requirements to avoid melting of tungsten in case of an accident. The poten­tial exposure from mobilized activation products from the tungsten divertor may be modified by vary­ing the operating conditions of fusion power and change-out time as well as the thickness of the divertor armor. The dose can be reduced by selecting shorter change-out times. However, the total life­cycle waste volume will be increased accordingly. A thinner divertor will produce less mobilized acti­vation products while suffering a more restrictive shutdown requirement.11