Commercial LWR spent nuclear fuel contains on average (excluding cladding) only four percent plutonium, minor actinides and fission products by weight.
234U readily absorbs thermal neutrons and converts to fissile 235U, which needs to be taken into account if it reaches significant proportions of the fuel material.
Some variations of the DUPIC fuel cycle make deliberate use of this by including a voloxidation step whereby the fuel is heated to drive off semi-volatile fission products or subjected to one or more reduction / oxidation cycles to transform nonvolatile oxides into volatile native elements and vice versa.
The direct use of recovered uranium to fuel a CANDU reactor was first demonstrated at Qinshan Nuclear Power Plant in China.
[5] The first use of re-enriched uranium in a commercial LWR was in 1994 at the Cruas Nuclear Power Plant in France.