Very high. Lithium deuteride has been in use since the Castle Bravo test, and almost certainly was adopted by the Soviets. Them doing things the "quick and dirty" way means safety oversights (Chernobyl, etc) and espionage (the Rosenbergs for example), it by no means implies they didn't do everything in their power to duplicate or exceed the US's capability (Tsar Bomba for example) or that those abilities disappeared after the wall fell. Granted they still use liquid propellant ICBMs (RS28) that pose storage issues vs the solid propellant ICBMs the US has (Minuteman/Trident).