Russia has given many expliantions for their recent exit from the MOX pact. Overall it is clear that MOX is a “good idea gone bad”. For more see the links below.
Citing “the threat to strategic stability posed by US hostile actions against Russia”. ref
Russia’s Lavrov: Russia’s MOX pact exit is a signal to Washington that: “speaking in the language of sanctions & ultimatums won’t work”
The Russian Non-Proliferation Department’s official reason: The US did not officially inform on planned change of PU disposal method (from MOX plant to WIPP disposal) as required in 2000 pact.