5 ms·

Nope. Decay heat is below a half percent of operating after about a day. So 200 MW decay heat days later would mean a 40GW (thermal) reactor. That's about ten

Nope. Decay heat is below a half percent of operating after about a day. So 200 MW decay heat days later would mean a 40GW (thermal) reactor. That's about ten times larger than the largest reactors in existence today. Also, watts measure power, not heat.

Retric 5y ago
Watts is joules per second be that electricity, horsepower, or heat. Passive systems can’t assume a successful shutdown.
- mlyle 5y ago
  I can't figure out what you're saying here. You're assuming that the control rods aren't in, days later, for the purpose of calculating heat days after shutdown?
  - Retric 5y ago
    I am saying passive safety can’t assume anything else worked.
    - mlyle 5y ago
      If you're going to apply "passive safety" globally, sure. But what we're talking about here is passive cooling system safety, not that the entire reactor is passively safe. The multiply-redundant shutdown systems suffice to end the chain reaction. If the chain reaction ends, you're pretty much immediately at 7% of decay heat -- so sure, a 1.5GW reactor will put out 100MW of decay heat, still. But this will rapidly fall off. After about an hour, it's more like 15MW; after a day, 6MW. Your statement of "200MW of decay heat" days later assumes either a ridiculous initial condition (an implausibly large reactor) or assumes you still have an operating reactor, which... isn't decay heat anymore.
      - Retric 5y ago
        There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic. Also, Palo Verde Nuclear Generating Station has 3 different 4000MW thermal reactors, 7% of that is 280MW, though sure if everything shuts down properly it should hit ~15MW. Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that.
        mlyle 5y ago
        > There have been multiple cases where reactors haven’t fully shut down safely There's Chernobyl, and a few cases where a scram was delayed by 15 minutes or less. How can this produce hundreds of megawatts days later? > There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic. Assuming that every system fully fails is unrealistic, too. > 7% of that is 280MW, You said hundreds of megawatts days later. > Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that. What, they're going to run it over nameplate for days straight? A small excursion over 4.5GW won't appreciably change the amount of power output days later. Now you're just being silly.
        Retric 5y ago
        Your boiling a fixed pool of water. 15 minutes of nameplate capacity is boiling over 5 days of reserve at an expected 0.2% thermal output. The margins are often measure in minutes not days which is a long way from anything that could be called passively safe. > Assuming that every system fully fails is unrealistic, too. Not if you want to say your system is passively safe. I fully believe nuclear can be operated safety, but a huge part of that is acknowledging every possible failure mode rather than just saying unlikely means impossible.
        mlyle 5y ago
        I think you're trying really hard to salvage a point talking about hundreds of megawatts of "decay heat" days later. An operating reactor isn't making "decay heat". The claim made is that the cooling system is passively safe in shutdown. Fudging the amount of decay heat by a couple orders of magnitude, and then arguing about "what if it doesn't shut down" is a bogus argument. Obviously if you cannot reduce a reactor below nameplate power indefinitely, you have a big problem. Thankfully, we have multiply-redundant protections against this in modern designs: redundant control rod assemblies, neutron poisoning, positive stability, etc. Other than Chernobyl (a clearly bad design), all cases of delayed shutdown experienced so far have been innocuous and we've learned a lot from them.
- i_am_proteus 5y ago
  The watt is a measure of power, heat (or energy) per unit time.

Retric 5y ago

Watts is joules per second be that electricity, horsepower, or heat. Passive systems can’t assume a successful shutdown.

mlyle 5y ago
I can't figure out what you're saying here. You're assuming that the control rods aren't in, days later, for the purpose of calculating heat days after shutdown?
- Retric 5y ago
  I am saying passive safety can’t assume anything else worked.
  - mlyle 5y ago
    If you're going to apply "passive safety" globally, sure. But what we're talking about here is passive cooling system safety, not that the entire reactor is passively safe. The multiply-redundant shutdown systems suffice to end the chain reaction. If the chain reaction ends, you're pretty much immediately at 7% of decay heat -- so sure, a 1.5GW reactor will put out 100MW of decay heat, still. But this will rapidly fall off. After about an hour, it's more like 15MW; after a day, 6MW. Your statement of "200MW of decay heat" days later assumes either a ridiculous initial condition (an implausibly large reactor) or assumes you still have an operating reactor, which... isn't decay heat anymore.
    - Retric 5y ago
      There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic. Also, Palo Verde Nuclear Generating Station has 3 different 4000MW thermal reactors, 7% of that is 280MW, though sure if everything shuts down properly it should hit ~15MW. Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that.
      - mlyle 5y ago
        > There have been multiple cases where reactors haven’t fully shut down safely There's Chernobyl, and a few cases where a scram was delayed by 15 minutes or less. How can this produce hundreds of megawatts days later? > There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic. Assuming that every system fully fails is unrealistic, too. > 7% of that is 280MW, You said hundreds of megawatts days later. > Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that. What, they're going to run it over nameplate for days straight? A small excursion over 4.5GW won't appreciably change the amount of power output days later. Now you're just being silly.
        Retric 5y ago
        Your boiling a fixed pool of water. 15 minutes of nameplate capacity is boiling over 5 days of reserve at an expected 0.2% thermal output. The margins are often measure in minutes not days which is a long way from anything that could be called passively safe. > Assuming that every system fully fails is unrealistic, too. Not if you want to say your system is passively safe. I fully believe nuclear can be operated safety, but a huge part of that is acknowledging every possible failure mode rather than just saying unlikely means impossible.
        mlyle 5y ago
        I think you're trying really hard to salvage a point talking about hundreds of megawatts of "decay heat" days later. An operating reactor isn't making "decay heat". The claim made is that the cooling system is passively safe in shutdown. Fudging the amount of decay heat by a couple orders of magnitude, and then arguing about "what if it doesn't shut down" is a bogus argument. Obviously if you cannot reduce a reactor below nameplate power indefinitely, you have a big problem. Thankfully, we have multiply-redundant protections against this in modern designs: redundant control rod assemblies, neutron poisoning, positive stability, etc. Other than Chernobyl (a clearly bad design), all cases of delayed shutdown experienced so far have been innocuous and we've learned a lot from them.
i_am_proteus 5y ago
The watt is a measure of power, heat (or energy) per unit time.