Reprocessing and clearance as ways for reducing radioactive waste from fusion reactors

The irradiated material chosen to investigate this concept is V-4Cr-4Ti, used as in-vessel structural material of PM-1, one of the three reactor models of SEAFP-2 assessments. The analysis performed deserves the following comments: in general, the modified concentrations of the parent isotopes have been assumed equal to the actual detection limits of the concerned elements. The nuclides investigated are those having the higher activities in the irradiated alloy (dominant nuclides). The contribution to the clearance index of other nuclides, present in the irradiated alloy and not yet examined should also be assessed. Even having reduced the concentrations of the impurities to the very low levels hypothesized, the ratios (A/Lc)m of some nuclides are too great. Beside K-42 and Ar-42, deriving from titanium, this happens for nuclides deriving from niobium, silver, nickel, copper, strontium. Hence the clearance index of the modified alloy is greater than the unity and clearance conditions are not achieved. Additional purification process have to be envisaged after the irradiation, with an elemental dilution of the noxious nuclides and then performing a purification process. As an example, it is found from table 2 that the sum of the ratios (A/Lc)m of Nb-93m, Nb-91 and Nb-94, nuclides deriving from a concentration of 2 x 10-2 ppm of niobium, is 7.75. An addition of 5/10 ppm of natural niobium to the molten irradiated material and a further purification of niobium to levels near to the initial one could achieve a 100-fold reduction of the ratios (A/Lc)m. 10000 t of V-4Cr4Ti, representing the total amount (maintenance + decommissioning) arising from the in vessel structures of a power reactor should be reprocessed with 100 kg of niobium, in the highly conservative hypothesis that all material is irradiated in first wall conditions. The secondary waste arising from the purification processes would not exceed a few ton which could be disposed of in an extremely safe way.