3 ms·

How long does it take a pressurized water reactor to start up again? Is there some kind of "idle neutron balance" they can use when coolant flow is interrupted

How long does it take a pressurized water reactor to start up again? Is there some kind of "idle neutron balance" they can use when coolant flow is interrupted, or do they have to shut down the pile?

shagie 5y ago
From https://www.eia.gov/todayinenergy/detail.php?id=45956 https://www.eia.gov/todayinenergy/detail.php?id=45956 > About 25% of U.S. power plants can start up—going from being shut down to fully operating—within one hour, based on data collected in EIA’s annual survey of electric generators. Some power plants, especially those powered by coal and nuclear fuel, require more than half a day to reach full operations. The time it takes a power plant to reach full operations can affect the reliability and operations of the electric grid. https://engineering.stackexchange.com/questions/7394/why-does-it-take-so-long-to-restart-a-nuclear-power-plant https://engineering.stackexchange.com/questions/7394/why-doe... goes into it a bit more.
mnw21cam 5y ago
It's complicated. Nuclear fission generates a load of by-products, and some of those are known as fission "poisons", mainly Iodine-135 decays to Xenon-135, which very easily absorbs neutrons and inhibits fission. During normal operation, there are plenty of neutrons flying around, so there are enough for some to be absorbed by Xenon-135, and the rest to continue the fission reaction. However, when the power output is reduced, the Xenon-135 can inhibit the nuclear reaction. This can make it less safe to restart the reactor, because the operator would need to increase the reactivity of the system - if they increase it enough to overcome the inhibition caused by the Xenon-135, then the increased nuclear activity can burn off the Xenon-135, and then you have excessive reactivity and the reactor could go out of control. For that reason, when a reactor is shut down, you typically need to wait several hours for the Xenon-135 to decay by itself - this is called the "Xenon dead time". Failure to do this was partially the cause of the Chernobyl disaster.

How long does it take a pressurized water reactor to start up again? Is there some kind of "idle neutron balance" they can use when coolant flow is interrupted, or do they have to shut down the pile?

shagie 5y ago

From https://www.eia.gov/todayinenergy/detail.php?id=45956 https://www.eia.gov/todayinenergy/detail.php?id=45956 > About 25% of U.S. power plants can start up—going from being shut down to fully operating—within one hour, based on data collected in EIA’s annual survey of electric generators. Some power plants, especially those powered by coal and nuclear fuel, require more than half a day to reach full operations. The time it takes a power plant to reach full operations can affect the reliability and operations of the electric grid. https://engineering.stackexchange.com/questions/7394/why-does-it-take-so-long-to-restart-a-nuclear-power-plant https://engineering.stackexchange.com/questions/7394/why-doe... goes into it a bit more.

mnw21cam 5y ago

It's complicated. Nuclear fission generates a load of by-products, and some of those are known as fission "poisons", mainly Iodine-135 decays to Xenon-135, which very easily absorbs neutrons and inhibits fission. During normal operation, there are plenty of neutrons flying around, so there are enough for some to be absorbed by Xenon-135, and the rest to continue the fission reaction. However, when the power output is reduced, the Xenon-135 can inhibit the nuclear reaction. This can make it less safe to restart the reactor, because the operator would need to increase the reactivity of the system - if they increase it enough to overcome the inhibition caused by the Xenon-135, then the increased nuclear activity can burn off the Xenon-135, and then you have excessive reactivity and the reactor could go out of control. For that reason, when a reactor is shut down, you typically need to wait several hours for the Xenon-135 to decay by itself - this is called the "Xenon dead time". Failure to do this was partially the cause of the Chernobyl disaster.