2 ms·

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 pa

by 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.
      - deleted 5y ago
        [deleted]
      - Retric 5y ago
        I can only assume my original point wasn’t clear. The normal amount of decay heat is the best case possibility and should be handed just fine by any reasonable design. I don’t think there’s any reason to assume a design has that kind of fatal flaws. “quickly lose multiple GW of heat in an emergency and as much as 200+MW of heat for days after a shutdown.” Was in reference to something compounding the issue of which their’s two main issues either it didn’t shutdown quickly or it didn’t shutdown completely. I am objecting to is the assumption that safety systems should assume things are fine in an emergency. Chernobyl had multiple compounding issues, many other accidents where less serious because X and Y happened but Z didn’t happen. Depending on such trends continuing results in a false sense of security. A passively safe system doesn’t mean there isn’t damage. It’s perfectly reasonable for a design to say in the event of X, Y, and Z stuffs going to break. Causing a billion dollars in damage is a perfectly reasonable trade off, losing containment isn’t. PS: Part of that is acknowledging bad designs are going to happen, we engineers are going to make mistakes. Which means not all assumptions hold.

by mlyle 5y ago

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.
    - deleted 5y ago
      [deleted]
    - Retric 5y ago
      I can only assume my original point wasn’t clear. The normal amount of decay heat is the best case possibility and should be handed just fine by any reasonable design. I don’t think there’s any reason to assume a design has that kind of fatal flaws. “quickly lose multiple GW of heat in an emergency and as much as 200+MW of heat for days after a shutdown.” Was in reference to something compounding the issue of which their’s two main issues either it didn’t shutdown quickly or it didn’t shutdown completely. I am objecting to is the assumption that safety systems should assume things are fine in an emergency. Chernobyl had multiple compounding issues, many other accidents where less serious because X and Y happened but Z didn’t happen. Depending on such trends continuing results in a false sense of security. A passively safe system doesn’t mean there isn’t damage. It’s perfectly reasonable for a design to say in the event of X, Y, and Z stuffs going to break. Causing a billion dollars in damage is a perfectly reasonable trade off, losing containment isn’t. PS: Part of that is acknowledging bad designs are going to happen, we engineers are going to make mistakes. Which means not all assumptions hold.