Abstract: Purification-efficiency of the coolant purification system, decay of the key nuclides, effect of the activation products, effect of the energy-adjacent fission products, etc. are the main influencing factors in the fuel element rupture detecting by measuring the activity of the key nuclides in the primary coolant. These factors were calculated and analyzed systematically, and the results indicate that ⁹⁹Mo has a prominent effect on the activity measuring of the key nuclides.