There are exceptions to this rule, but they generally bring nuclear up to Nominal full load, and run it at that load, unless they have a fault, then it may be set to lower loads, but they don't turn nuclear up and down to balance the grid, a nuclear reactor must power up and down slowly, or they get all kinds of problemsI thought nuclear stations were 'throttled' rather than turned 'on and off' due to the nature of how they generate electricity?
Have a look here,
Find the status of our nuclear power stations & see which nuclear reactors are in service & what power they are generating. You can also find which reactors are out of service and for how long.
There is only around 6GW of nuclear power, so they try to run it all the time, as it's cheap and constant.