Regulating them down is done by more aggressively cooling the reactor. The thermal energy output is unchanged, but less of that thermal energy is transformed into electricity. This doesn't wear down any materials, but it does waste fuel. Grated, fuel is a tiny faction of nuclear's cost. Furthermore, the gap between peak and minimum electricity consumption isn't that large. It's usually 10-20%.
Which type of reactor are you talking about here? I’m not a nuclear expert, but the wonderful Chernobyl TV series doesn’t align with this. That reactor put out 3mw usually, peaked at 33kmw, and was idled down to 700kw using control rod insertion rather than more aggressive cooling.
Any reactor. This modulating is done with the cooling system, not the reactor itself. To clarify, the thermal output of the reactor remains unchanged, while aggressive cooling reduces the electrical output. It's deliberately reducing the efficiency of the heat engine to reduce electrical power output with the same thermal power output. That's why it's wasteful of fuel. Modulating the thermal output of the reactor is more efficient, but not all reactor designs can do this quickly.
The blast at Chernobyl was due to a positive void coefficient that meant over-heating led to a positive feedback loop (plus some other process failures that led to that initial overheating). Also those figures for the Chernobyl reactor were for thermal output, not electrical output. The figures are an order of magnitude off, the reactor would normally put out 3GW, not 3 MW and was idled down to 700MW for the test. 700 kW is, like, a beefy muscle car.
