By now, in Russia more than 70 000 fuel assemblies (FA) have operated their service life at NPPs with RBMK–1000 (11 units); the discharged FAs have been stored in water of cooling ponds and in spent nuclear fuels (SNF) storage facilities for 25 years.

During their operation, more than 3 000 spent fuel assemblies became leaky while claddings of some integer fuel rods were subjected to a substantial wear (fretting-corrosion up to 400μ in depth, nodular corrosion to a depth of 380μ — both kinds of the wear proceeded under spacer grids). In this case, the claddings revealed a higher hydrogen uptake and degradation of mechanical properties. During the wet storage in cans, spent FAs show general corrosion proceeding at the rate of 3–5 μ/ year.

The paper points out that as applied to a long-term (up to 50 years) dry storage of RBMK-1000 spent nuclear fuel, consideration is to be given to the following degradation mechanisms proceeding in stainless steel fuel rod claddings and FA components:

Oxidation of Zr-1%Nb alloy (fuel rod claddings, central channel) in the atmosphere of storage (air, nitrogen, argon and impurities).

• Creep of a fuel rod cladding material.

• Delayed hydride cracking (DHC) of a fuel rod cladding.

• Corrosion induced cracking of FA components fabricated from stainless steels.

The first results obtained in assessing the creep rate of standard fuel claddings effected by the internal pressure are under consideration.

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