Nuclear plants are modestly controllable, but it is rarely done, because the cost savings of ramping down are negligible, so typically nuclear plants only ramp down for operational reasons, or grid acceptance reasons. In countries with large amounts of both nuclear and renewable power, nuclear plants operate in a load-following mode, ramping up and down with demand/renewable supply.Old nuclear plant normally offer ramp rates of 2.5% of nameplate rating per minute, with more modern plants offering 5% or greater. There can be some issues with ramping older plants because of temperature changes in the reactor which can contribute to fatigue and limit the reactor life time. Modern plants are designed for isothermal ramping to prevent reactor thermal fatigue from load following operations.
Large coal plants typically can achieve approximately 2% per minute, with the more modern coal gasification combine cycle plants achieving approximately 3%. The big problem with coal plants is start up time after a shut down. A hot start (48 hours) can incur an 8-12 hour delay.
Most existing combined cycle gas turbines can ramp at approximately 3% of rating per minute, with a 60 minute start up delay from warm, or 3 hours from cold. Modern (new build) combined cycle gas turbines can ramp at approximately 5% of rating per minute (when hot), or approximately 1% per hour from cold start with a 15 minute start delay.
Open cycle gas turbines can ramp at approximately 10% of rating per minute, with a cold start delay of approximately 10 minutes.
The big advantage of OCGT is that they can start from cold with minimal notice, so for short-term peaking, they are excellent.
Modern CCGT has most of the benefits of OCGT, but a very much higher capital cost - so there needs to be adequate baseload demand to make the economic case for CCGT, even though efficiencies can be considerably greater with CCGT (62% for a state-of-the-art CCGT, compared with 38% for state-of-the-art OCGT).
In California, utilities are building OCGT like crazy, because it's the cheapest way to provide rapid start standby capacity when the Spring/Autumn Sun starts to go down, just as demand starts to peak.