[en] The computer code ''FPRM-1'' has been developed for calculation of the quantities of fission products gases released from pellets into plenum in a fuel rod. On the assumption that the irradiation tests of plutonium fuel and others under development in an in-pile water loop were performed, FP generations and accumulations in the fuel rods were calculated by the code. The result of measurement of ¹³¹I released from a fuel rod (UO₂ pellets, ²³⁵U 1.5% Enriched) with an artificial hole through cladding in an in-pile water loop was compared with that of calculation by the code; both were in good agreement. (author)