3 ms·

>would be the basis for variable power output. This has been done literally forever. Why do you think nuclear plants run at 100% capacity 24/7?

dragontamer 5y ago
> Why do you think nuclear plants run at 100% capacity 24/7? The primary reason would be economic. Fuel costs IIRC are really low but capital-expenses (especially safety engineering / politics / convincing the people nearby that the plant is safe) is so costly. When you have cheap-fuel but expensive machines, it makes sense to run closer to 100%. If you have expensive fuel but cheap machines (ex: Gas turbines), it makes sense to run in a "peaker" fashion. ----- That being said: I've heard that older nuclear plants are "baseload" designs, in that they _CANNOT_ turn off or feasibly regulate their power output. I mean, they obviously "can" turn off, they all have control rods for safety. But they're clearly not using them to regulate the power output.
- Lascaille 5y ago
  >But they're clearly not using them to regulate the power output. No, you heard wrong. They're often the last or second to last type of plant that's called upon to alter supply to match demand, but they don't run 100% 24/7.
  - cinntaile 5y ago
    They want to run them nearly 100% 24/7 for economic reasons though. Also what is the effect of regulating with control rods on the fuel, does it have an impact on how often the fuel needs to be changed?
    - Lascaille 5y ago
      >does it have an impact on how often the fuel needs to be changed Yes, fissonable elements in the fuel are consumed in a manner that's proportional to output. So if you run at 25% output your fuel needs to be changed (example) every 4 years, if you run at 100% output your fuel needs to be changed every year. But it is not 100% proportional, for example, 15% of the plant's output is required simply to run the plant's accessories. So it is advantageous to run at 100% output if you can.
      - cinntaile 5y ago
        Another comment in this thread, I can't find it now, said that lower output past a certain level did lead to having to change the fuel faster. I don't know who's correct but it feels strange that this would be a linear relationship, these kinds of things don't tend to work like that.
- cinntaile 5y ago
  I recall reading that the fuel is 1/3 of the operating expenses, but I could be misremembering.
  - ch4s3 5y ago
    About 28% as of 2018[1], but there have been some advancements since then. Half of that cost is enrichment and fabrication, and disposal is a factor too. It costs a lot less to fuel nuclear plants than coal plants, but maintenance is more costly for nuclear. Pricing in externalities or requiring scrubbers makes coal FAR more expensive. Natural gas is really cheap right now so that is largely what is undercutting Nuclear for base load power. It's worth noting that historically natural gas production/use has been far more dangerous than nuclear and far less scrutinized by regulators. [1]http://large.stanford.edu/courses/2018/ph241/wang-k2/ http://large.stanford.edu/courses/2018/ph241/wang-k2/
jcrawfordor 5y ago
Most nuclear power plants in the US fleet are not variable-output for design reasons. Mostly, both the reactor (see my other comment about neutron poisoning) and turbines are designed for safe operation at a fixed output and cannot operate at "partial" power for extended periods (in the turbines I think the problem is vibration and how it interacts with the bearing designs). A lot of this basically comes from these reactors having been designed without the benefit of computer modeling (everything had to be hand-calculated around a specific target output), newer designs based on computational modeling and optimization techniques achieve much larger safe/sustainable power ranges. Unfortunately we have very few reactors of modern design in the US.

dragontamer 5y ago

> Why do you think nuclear plants run at 100% capacity 24/7? The primary reason would be economic. Fuel costs IIRC are really low but capital-expenses (especially safety engineering / politics / convincing the people nearby that the plant is safe) is so costly. When you have cheap-fuel but expensive machines, it makes sense to run closer to 100%. If you have expensive fuel but cheap machines (ex: Gas turbines), it makes sense to run in a "peaker" fashion. ----- That being said: I've heard that older nuclear plants are "baseload" designs, in that they _CANNOT_ turn off or feasibly regulate their power output. I mean, they obviously "can" turn off, they all have control rods for safety. But they're clearly not using them to regulate the power output.

Lascaille 5y ago
>But they're clearly not using them to regulate the power output. No, you heard wrong. They're often the last or second to last type of plant that's called upon to alter supply to match demand, but they don't run 100% 24/7.
- cinntaile 5y ago
  They want to run them nearly 100% 24/7 for economic reasons though. Also what is the effect of regulating with control rods on the fuel, does it have an impact on how often the fuel needs to be changed?
  - Lascaille 5y ago
    >does it have an impact on how often the fuel needs to be changed Yes, fissonable elements in the fuel are consumed in a manner that's proportional to output. So if you run at 25% output your fuel needs to be changed (example) every 4 years, if you run at 100% output your fuel needs to be changed every year. But it is not 100% proportional, for example, 15% of the plant's output is required simply to run the plant's accessories. So it is advantageous to run at 100% output if you can.
    - cinntaile 5y ago
      Another comment in this thread, I can't find it now, said that lower output past a certain level did lead to having to change the fuel faster. I don't know who's correct but it feels strange that this would be a linear relationship, these kinds of things don't tend to work like that.
cinntaile 5y ago
I recall reading that the fuel is 1/3 of the operating expenses, but I could be misremembering.
- ch4s3 5y ago
  About 28% as of 2018[1], but there have been some advancements since then. Half of that cost is enrichment and fabrication, and disposal is a factor too. It costs a lot less to fuel nuclear plants than coal plants, but maintenance is more costly for nuclear. Pricing in externalities or requiring scrubbers makes coal FAR more expensive. Natural gas is really cheap right now so that is largely what is undercutting Nuclear for base load power. It's worth noting that historically natural gas production/use has been far more dangerous than nuclear and far less scrutinized by regulators. [1]http://large.stanford.edu/courses/2018/ph241/wang-k2/ http://large.stanford.edu/courses/2018/ph241/wang-k2/

jcrawfordor 5y ago

Most nuclear power plants in the US fleet are not variable-output for design reasons. Mostly, both the reactor (see my other comment about neutron poisoning) and turbines are designed for safe operation at a fixed output and cannot operate at "partial" power for extended periods (in the turbines I think the problem is vibration and how it interacts with the bearing designs). A lot of this basically comes from these reactors having been designed without the benefit of computer modeling (everything had to be hand-calculated around a specific target output), newer designs based on computational modeling and optimization techniques achieve much larger safe/sustainable power ranges. Unfortunately we have very few reactors of modern design in the US.