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Is it true you can't adjust the output of a nuclear plant? I thought I heard something about being to slide the cooling rods from "minimal output" to "Chernobyl

Is it true you can't adjust the output of a nuclear plant? I thought I heard something about being to slide the cooling rods from "minimal output" to "Chernobyl", but I'm not a nuclear person.

jabl 5y ago
Sure they can adjust output. Some are not licensed to do so, but that's more a paperwork hindrance than any actual physical reason. They can ramp about as fast as other generators using a steam cycle, like ccgt or coal. There are some situations like when you're at the end of the fuel cycle if you ramp down quickly you might have trouble starting quickly again due to Xe poisoning. But, if there was a reason to, the control system can be designed to provide sufficient excess reactivity to power through events like that (e.g. naval reactors). That being said, since most of the cost of a nuclear power plant is the capital cost, every time you're not generating at full power is time you're generating less revenue to cover the capital costs. So even in a hypothetical lowest cost zero carbon system with very high penetration of variable renewables (solar, wind), the nuclear plants would be mostly running flat out.
- unknown_error 5y ago
  Thank you for this detailed explanation!
jjk166 5y ago
Nuclear plants can adjust output, but not quickly. As reaction rates change the composition of the core itself changes, which further modifies reaction rates. Ramping up to full power might take a plant 12 hours. While this is fine for dealing with seasonal variation in demand, or compensating for another plant going down for maintenance, it can't deal with the short term spikes in demand that happen throughout any given day. There are some designs that can ramp up faster, but they are uncommon at least in the US. All plants can decrease output quickly (critical for safety) but without that quick ramp up its still not suitable for dealing with short term dips. That being said, fuel represents only a tiny fraction of the cost of nuclear power, so it's fine to run a nuclear reactor at close to max power 24/7 and just use excess power for tasks like water desalination or simply vent off steam instead of passing it through the turbines when demand is lower.
- ethbr0 5y ago
  > use excess power for tasks Or sink it into stored hydrogen via electrolysis.
  - jjk166 5y ago
    Exactly. Fresh water, clean fuels, carbon capture, waste decomposition, battery charging, there are plenty of productive uses for excess power.
    - bipson 5y ago
      But then there is no argument left for nuclear in the first place! Once you have scalable storage for excess energy, renewables and their fluctuations are simply cheaper, safer and greener, right?
      - jjk166 5y ago
        Having something useful you can do with excess energy is not the same as scalable storage sufficient to deal with all fluctuations in renewables. Sure if you have a solar plant producing too much power, you can use that power to generate hydrogen, and that hydrogen can be burned later as a clean energy source. But generating and storing and using enough hydrogen to consistently get you through the night is substantially harder. Further, renewables are not simply cheaper, safer, and greener. From a safety perspective nuclear is by orders of magnitude safer. Green-ness is a nebulous concept but in terms of damage to the environment nuclear is way better than solar or hydro and comparable to wind and geothermal. Solar and Wind are cheaper than nuclear in terms of nameplate capacity, but their load factor is dramatically lower and their lifetimes are much shorter, to replace a 1 GWe nuclear plant you basically need 3 GW of solar capacity, which are going to have to be replaced twice over the same lifetime, and 12,000 MWh of storage capacity.
        pfdietz 5y ago
        > From a safety perspective nuclear is by orders of magnitude safer. Citation please. And make sure it's the renewable we mostly talk about for this conversation, not hydro that cannot be scaled up much more.

Is it true you can't adjust the output of a nuclear plant? I thought I heard something about being to slide the cooling rods from "minimal output" to "Chernobyl", but I'm not a nuclear person.

jabl 5y ago

Sure they can adjust output. Some are not licensed to do so, but that's more a paperwork hindrance than any actual physical reason. They can ramp about as fast as other generators using a steam cycle, like ccgt or coal. There are some situations like when you're at the end of the fuel cycle if you ramp down quickly you might have trouble starting quickly again due to Xe poisoning. But, if there was a reason to, the control system can be designed to provide sufficient excess reactivity to power through events like that (e.g. naval reactors). That being said, since most of the cost of a nuclear power plant is the capital cost, every time you're not generating at full power is time you're generating less revenue to cover the capital costs. So even in a hypothetical lowest cost zero carbon system with very high penetration of variable renewables (solar, wind), the nuclear plants would be mostly running flat out.

unknown_error 5y ago
Thank you for this detailed explanation!

jjk166 5y ago

Nuclear plants can adjust output, but not quickly. As reaction rates change the composition of the core itself changes, which further modifies reaction rates. Ramping up to full power might take a plant 12 hours. While this is fine for dealing with seasonal variation in demand, or compensating for another plant going down for maintenance, it can't deal with the short term spikes in demand that happen throughout any given day. There are some designs that can ramp up faster, but they are uncommon at least in the US. All plants can decrease output quickly (critical for safety) but without that quick ramp up its still not suitable for dealing with short term dips. That being said, fuel represents only a tiny fraction of the cost of nuclear power, so it's fine to run a nuclear reactor at close to max power 24/7 and just use excess power for tasks like water desalination or simply vent off steam instead of passing it through the turbines when demand is lower.

ethbr0 5y ago
> use excess power for tasks Or sink it into stored hydrogen via electrolysis.
- jjk166 5y ago
  Exactly. Fresh water, clean fuels, carbon capture, waste decomposition, battery charging, there are plenty of productive uses for excess power.
  - bipson 5y ago
    But then there is no argument left for nuclear in the first place! Once you have scalable storage for excess energy, renewables and their fluctuations are simply cheaper, safer and greener, right?
    - jjk166 5y ago
      Having something useful you can do with excess energy is not the same as scalable storage sufficient to deal with all fluctuations in renewables. Sure if you have a solar plant producing too much power, you can use that power to generate hydrogen, and that hydrogen can be burned later as a clean energy source. But generating and storing and using enough hydrogen to consistently get you through the night is substantially harder. Further, renewables are not simply cheaper, safer, and greener. From a safety perspective nuclear is by orders of magnitude safer. Green-ness is a nebulous concept but in terms of damage to the environment nuclear is way better than solar or hydro and comparable to wind and geothermal. Solar and Wind are cheaper than nuclear in terms of nameplate capacity, but their load factor is dramatically lower and their lifetimes are much shorter, to replace a 1 GWe nuclear plant you basically need 3 GW of solar capacity, which are going to have to be replaced twice over the same lifetime, and 12,000 MWh of storage capacity.
      - pfdietz 5y ago
        > From a safety perspective nuclear is by orders of magnitude safer. Citation please. And make sure it's the renewable we mostly talk about for this conversation, not hydro that cannot be scaled up much more.