The issue is not primarily technical, it's economic. Operate a nuclear plant at lower capacity factor and the economics becomes even more hopeless.
Subsidy can make sense, but not because shuffling euros around under walnut shells makes the cost disappear. The argument for subsidies is generally that it helps technologies move down experience curves. This improvement is a positive externality that a pure market would not necessarily reward. Unfortunately, nuclear (and nuclear in France) has been showing NEGATIVE experience effects.