Radioactivity in cooling water of Musashi Reactor

The replacement of Al cladded fuel elements with stainless steel ones was undertaken in the Musashi Reactor (TRIGA-II, 100 kW) in 1985. The change of radioactivity in the cooling water before and after the replacement has been discussed in this report. The radioactivity of 19 nuclides in the cooling water has been measured in the present study. The activity of 51Cr, 54Mn, 56Mn, 51Co, 60Co, 122Sb and 124Sb increased by about 100 to 10 times and 24Na, 72Ga, 99mTc and 187W decreased by about 0.1 times that before the replacement. The increase and decrease of their nuclides, except for 99mTc and 187W were related to the amount of each element, which produced the radioactive nuclides, in the clad of fuel elements. Fission products such as 131I, 132I, 133I, 134I, 135I and 132Te showed only slight decreases by about 0.5 times in spite of the large difference of U contents in the cladding material between new and old fuel elements. Release processes have been considered based on two mechanisms, chemical dissolution and direct recoil of activated atoms, using the radioactivity data. (author)