You can reduce the energy output of a nuclear reactor both through the moderation of the fission reaction (via control rods, coolant temperature variation, neutron poisoning, etc), and through moderation of the electrical generator turbine (slowing down the turbine speed, venting steam, etc).

There is no such thing as "excess power" for crytomining. Even if there was then that energy could just be stored using batteries or gravity storage or something, or it could be sold to neighbouring energy users, or it could be used in the grid and other more polluting sources could be scaled down instead.