The influence of tritium behaviour on spent fuel pool concrete

This paper investigates the effects caused by Tritium released from spent nuclear fuel (SNF) stored into a spent nuclear pool made of (reinforced) concrete. The release of tritium from fuel elements to coolant during normal LWRs operation is of 1 % while during 6–20 years of wet storage is assessed to 13–45 %. Permanent Tritium concentration in coolant is assessed to n·109 Bq·dm−3. The water contamination first and the filtration of the concrete pool walls then pose important safety problems to face from both radiological and structural point of view: during Tritium filtration, radiation affects significantly the structural and mechanical properties of concrete due to the breaking of atomic bonds. Under the effect of radiation concrete microstructure modifies; collision determines atom dislocation and so a lattice defect. Ionized rays may cause the decay of free or bonded water leading to the formation of H2 and O2 and produce explosive mixture. The damage from “soft” β-decay of Tritium inside the concrete could be harder due to short track of the particle and the resulting “volume” effect. Neither this effect nor to what extent Tritium movement in/through concrete material would take place over long-term are yet completely studied. Due to Helium (He) formation, when Tritium decays, the restructuring of water molecules with generation of reactive free radicals occurs. We have assessed these effects: the irradiation and accompanying volume expansion, formation of free radicals and corresponded chemical corrosion are not considerably influencing on the structure and sustainability of concrete biological shield. The most significant effect is the radiation breaking of van der Waals bonds that within 10 years can lead to the destruction of about 2 % of concrete. Concrete shield does not provide a complete barrier against Tritium release into the environment. The main threat of Tritium diffusion to coolant in SNF storage pools is caused by filtration of tritiated water through the concrete walls that may provide significant health hazards for employees of the nuclear industry. Results show that β-decay of tritiated water adsorbed in 1 m3 of concrete may destroy about of 0.05 mol (11.4 g) of cement stone within 10 years, which is less than 0.001%. Moreover, with 3. With reference to the 5th power unit of the Zaporizhzhia NPP, the volume of Tritiated water released from the pool is 0.67 m3 annually. Results show that the tritium waste management concept needs to be revised.