Yield of gaseous fission products from defective fuel elements in the BOR-60 reactor

Conclusions

• 1. In the BOR-60 reactor the formation of defects in the fuel-element cans is accompanied by the escape of radioactive gaseous fission products. The extent of this escape for a burnup of over 5% equals the proportionate yield from the fuel, and amounts to (5.3±1.0)% for¹³³Xe. The yield of the remaining gaseous fission products is approximately proportional to the square root of the halflife, in agreement with the laws of the diffusion model describing the escape of gases from under the can. The yield of solid fission products directly after the formation of a defect is negligible.

• 2. Owing to the low yield of gaseous fission products at the instant of defect formation and their considerably lower yield on subsequent irradiation, the radioactivity in the gas system of the reactor does not limit the permissible number of defective fuel elements in the active zone. In developing fuel elements which allow the escape of gaseous fission products into the coolant, it is essential to ensure the retention of the solid fission products (¹³¹I,¹³⁷Cs,⁹⁵Zr−⁹⁵Nb,¹⁴⁰Ba−¹⁴⁰La); the period of holding the gases under the can may be made very short.

• 3. The can hermeticity-monitoring system based on delayed neutrons is not capable of detecting the appearance of defects in the fuel elements, but its readings may serve as an indication of any substantial opening of the can. Constant γ-spectrometric monitoring of the gas radioactivity enables us to estimate the number of defective fuel elements in the active zone of the reactor at any moment of time.