Nuclear power plants are most profitable when run at full capacity. They have massive sunk costs, but the marginal cost is low. At a grid level you certainly

Nuclear power plants are most profitable when run at full capacity. They have massive sunk costs, but the marginal cost is low. At a grid level you certainly can change output based on demand. But the markets may need to be adjusted to keep them profitable through mechanisms like a capacity market. Or by finding new uses for the power such as making hydrogen. With peaker plants fuel costs are a factor, but also the cost of the machine. An old fashioned reciprocating engine maybe the cheapest as a last resort that is only used when other peakers are already utilized. Fuel efficiency becomes more important as the plant is more utilized.