Nuclear Plants Do Need Backup Generation, Just Like All Other Power Sources
A power utility has to have the capability of calling into service enough power to make up for the loss of its largest generator on short notice. For a nuclear generator, this would be between 300 and 2000 MW. In our home province of Saskatchewan, the largest single generator SaskPower has to keep backup capacity for currently is a 300MW coal unit. However, if the entire Boundary Dam facility went down, the Saskatchewan system could be missing as much as 813 MW of power.
Most nuclear reactors currently in operation need to be at least partially shut down in order to refuel. Most nuclear units are several hundred megawatts so this is the minimum amount of generation capacity that would have to be supplied to make up for a partially idle nuclear plant. Some plants would require a full shutdown, meaning as much as 1-2 GW could be required to cover the plant during refueling. Some newer designs such as the ACR-1000 have the ability to keep producing 2/3 to 3/4 of their power output while they are being refueled. Designs exist on paper for reactors that do not need to reduce their output at all in order to refuel.
Additional capacity is often available from more costly sources such as natural gas, as well as from neighboring jurisdictions. Most major power grids in the world are connected with their neighbors. A failure on one system can be compensated for by another part of the system, for a price. Importing electricity can sometimes be extremely expensive.
During a heat wave, drought, or other event causing an entire nuclear facility to be turned off, multiple gigawatts of capacity would have to be available on short notice, and this would require a large amount of additional capacity to be built by SaskPower above and beyond the costs associated with the nuclear facility. Another option would be trying to import enough power in such a case. This would leave SaskPower in a relatively weak bargaining position unless agreements had been made much earlier for trading power in such a circumstance. With a functioning nuclear plant, Saskatchewan would likely be a huge exporter of power. A neighbor such as Manitoba could use Saskatchewan’s power when the Nuclear Plant is running, and they would turn on more of their hydroelectric generators if the nuclear plant had to go down. This is thanks to the energy storage capability of hydroelectric reservoirs. They can keep the energy waiting until it is needed.
All in all, this is not a myth, but it must be understood that this is a property not just of nuclear power, but of all forms of electricity generation. This is because no good method yet exists for storing electricity cheaply on a large scale. This is an issue that has been worked on for many decades. The possible solutions are many, and the most likely result is a patchwork of: reducing demand, buying power from neighbors, and ramping up extra generation.