EXFOLIATION AND DIFFUSION FOLLOWING HELIUM ION-IMPLANTATION IN FLUORAPATITE - IMPLICATIONS FOR RADIOCHRONOLOGY AND RADIOACTIVE-WASTE DISPOSAL

The properties of fluorapatite, both a useful radiochronometer and a p otential storage matrix specific for minor actinides produced by the r eprocessing of spent nuclear fuel, have been investigated with emphasi s to its response to alpha decay. Exfoliation, which occurs after impl antation of high doses of 1.6-MeV He-ions (> 1.4 x 10(17) ions cm(-2), corresponding to 5% atomic proportion), could set an upper limit to t he concentration of imbedded actinides (about 2 atoms % corresponding to 20 wt. %) or storage age unless significant diffusion of radiogenic He intervenes. This process has been studied by combining He implanta tion, thermal treatments in the temperature range 124-250 degrees C an d measurement of the resulting He profile by an ion beam technique (ER DA) using 8.5-MeV C ions. The diffusion coefficient follows an Arrheni us' law with an activation energy of 120 (+/-2) KJ/mole and a frequenc y factor of 14.5 (+/-7) x 10(-3) cm(2) sec(-1) in agreement with liter ature data. The inferred closure temperature which validates the U,Th- He radiochronological method also fits previous values: 97 (+/-10)degr ees C for grain size 165 mu m. With respect to radwaste disposal. He v olume diffusion is too small to exclude the occurrence of exfoliation unless diffusion at grain boundary is much higher and a fine-grain mat rix is deliberately chosen. (C) 1998 Elsevier Science Ltd. All rights reserved.