The relative efficiencies to detect fission products by using a Φ76.2mm ×76.2mm NaI detector have been calculated for the real--time fuel element rupture detection and analysis.The calculated results show that the optimum gamma energies for the element rupture detection are 81keV(133Xe)
196.3keV(88Kr)
220.9keV(89Kr)
249.8keV(135Xe)
402.6keV(87Kr) and 529.8keV(133I).Ganuna ray energy higher than 800 keV is not suitable for the detection due to the interference of gamma rays from the activated products. The calculated results have successfully been applied to analysis of the element safety for a pressurized-water reactor.