The probability of a nuclear exchange remains zero until the sovereignty of a nuclear capable state is threatened. Tens or hundreds of millions would have to die for such a threat to emerge. Therefore, there is virtually no risk of it happening at this point by any nuclear capable power, except by the terrorist state in the middle east with its stolen highly enriched American uranium (NUMEC affair), because their modus operandi is to watch the world burn and take over the ashes (patagonia fires, et. al.)