Nuclear Power/Nuclear power plant
How long does a standard nuclear reactor take to start up and produce power from being completely offline, but with a radioactive source at the ready
There's four parts to the startup. First, the plant has to heatup to normal operating temperature. Then the reactor itself has to be started (brought critical). Once the reactor is critical it has to be increased in power until the turbine can be started. Finally, the turbine is loaded from 0% to 100% power.
The heatup is done by running the reactor coolant pumps without drawing off any heat from the plant. At a standard Westinghouse reactor, each reactor coolant pump will heat up the plant at 10 degrees per hour. Assuming 4 pumps (some plants have 2 and some 3) you will heat up at 40 degrees an hour. So, to get from 100 degree shutdown temperature to 550 degree operating temperature, that's about 11 hours.
Once the plant is at normal operating temperature, the reactor is started by withdrawing the control rods. In the Navy with the small reactor cores, that can be done in about 20 minutes. Commercial plants will take anywhere from 1 to 3 hours, depending on the operator and the procedural restrictions.
After the reactor is critical, it will take about 10 minutes to get the plant to the point that the turbine can be started. Again, this can vary depending on the operator and the procedures.
The turbine will take about another 10--15 minutes to start. Once started and the generator is synchronized to the electrical grid, the startup to 100% power begins. Because a lot of monitoring instrumentation and other equipment is checked and calibrated during this startup, it will take 24 to 48 hours to do this. With no restrictions, it could be done in as little as an hour. But all commercial plants will have these restrictions.
So, the total somewhere around 60 hours, assuming a slow, controlled startup. But Navy reactors have to start much quicker and can do that in about 8 hours.