6 ms·

It's also what you get from a reactor with no containment dome, unlike every modern reactor. On top of that, the Chernobyl reactor had a strong positive feedba

It's also what you get from a reactor with no containment dome, unlike every modern reactor. On top of that, the Chernobyl reactor had a strong positive feedback: as the temperature went up, the reaction sped up. Modern reactors do the opposite.

Retric 5y ago
Only up to a point. Modern designs tend to be short term passively safe, but shut off the pumps and radioactive decay alone can be enough to eventually cause a melt down in many modern designs. Which is the core issue, there’s a huge cost trade off for protection vs every possible issue no matter how remote. Spent fuel pools are probably the greatest example of this. They haven’t caused a major issue yet but they’re potentially a much larger risk than the actual reactor.
- koheripbal 5y ago
  No. Modern reactors use their pumps to keep the reactor going. Shut them off and the reactor shuts itself off. They now all have passive safety systems that do not require power.
- i_am_proteus 5y ago
  >shut off the pumps and radioactive decay alone can be enough to eventually cause a melt down in many modern designs The newest designs being built worldwide use natural circulation cooling and do not need cooling pumps in emergencies. Eventually the cooling pool needs to be refilled, but it's external to the containment pressure boundary, so you could refill it with a fire truck.
  - Retric 5y ago
    Natural circulation gets heat from a reactor to X, but now your dependent on X. This often seems like a trivial detail, but Fukushima failed 3 days after the earthquake. The issue is you want several things from a passive system at the same time, don’t lose heat in normal operation, quickly lose multiple GW of heat in an emergency and as much as 200+MW of heat for days after a shutdown. The obvious solution is to have a tank of water that boils if the reactor temperature gets to high, but now you need to keep that tank full. Thus many designs result in a reactor that is passively safe for some number of hours and at risk after that. They describe this as a passively safe reactor even if it’s got external dependencies.
    - i_am_proteus 5y ago
      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.
        5y ago
  - petre 5y ago
    One could use pumps for increased efficiency during normal operation but the idea is that natural circulation should be able to remove all the heat if the reactor is SCRAMed. NuScale's design for instance only uses pumps for the steam generator, the rest is handled by natural circulation and the reactor sits in a water pool that needs to be replenished after two weeks in case of a major accident.
  - mlyle 5y ago
    > so you could refill it with a fire truck. Note you might need a lot of fire trucks, though. If decay heat is 0.2% of nominal power after a week, we need .8mL per nameplate MW (thermal) per second... which doesn't sound bad, but a 2GW reactor needs 5000L firetrucks showing up every 50 minutes, 24/7, after a week. This could be challenging depending upon the underlying emergency. ((0.002 megawatt) / ((2260 + 250) * (kilojoules / kg))) / (997 (kg / (m^3))) = 0.00079921038 liters per second Assumption is that you're raising the temperature of the water 60C and then evaporating it, and that all the heat energy goes into the water. In the real world it can be expected to be slightly better than this, but not much...
    - xyzzyz 5y ago
      You're off in your calculation by 3 orders of magnitude, you meant to write 0.002 gigawatt, not megawatt. Then yes, you get 0.7 liters/second, and 5000 L/hour for 2 GW plant. However, in actual reality, the firetruck would just park there, put one end of the hose into the plentiful source of water that surely must be available next to a power plant, and the other one into the entrance for coolant, and start its pump. Pumping 1 liter/second is quite in range of firetruck pumps abilities.
      - Retric 5y ago
        That’s a great assumption unless loss of water is the underlying emergency and to really mess things up you lost both water and the scram failed. That risk can be minimized by placing them near large natural bodies of water, but not all are ex: https://ejatlas.org/conflict/metsamor-nuclear-power-plant https://ejatlas.org/conflict/metsamor-nuclear-power-plant
      - mlyle 5y ago
        > You're off in your calculation by 3 orders of magnitude, you meant to write 0.002 gigawatt, not megawatt. No... decay heat is about .2% of nameplate power after a week. So for each megawatt of nameplate thermal power, you need to get rid of 2 kilowatts of heat-- or 0.8mL/second of water boiling off. "If decay heat is 0.2% of nominal power after a week, we need .8mL per nameplate MW (thermal) per second..." Was quite clearly said. > put one end of the hose into the plentiful source of water that surely must be available next to a power plant, and the other one into the entrance for coolant, and start its pump. Pumping 1 liter/second is quite in range of firetruck pumps abilities. This is pretty optimistic in many disaster scenarios and doesn't apply to all plants.
- chasil 5y ago
  I know that molten salt reactors have a "salt plug" at the bottom of the tank that will melt if the temperature is too high, dumping the liquid fuel into a boron bath. I think this kind of reactor is safe in a way that no modern reactor is - operators can remove all power and walk away in this shutdown state. This isn't possible with modern reactors, where 6% of the heat that they produce comes from daughter nuclei, and this decay heat requires cooling power for months after a controlled shutdown. I do agree, we have to build these safely, with every conceivable scenario, such that walking away is possible. Converting to thorium fuel would also be far better, as there is only one stable isotope in nature, so no refing is necessary (beyond high-purity smelting), and no centrifuges.
- godelski 5y ago
  The Soviets were the only ones that built reactors that way. As in, having the ability to blow up. > Spent fuel pools are probably the greatest example of this. They haven’t caused a major issue yet but they’re potentially a much larger risk than the actual reactor. I think you're also over estimating the danger here. We've been operating over 500 reactors for over 70 years. That's some pretty good statistical power.
  - Retric 5y ago
    Edit2: Blowing up isn’t the only risk from a melt down, a major meltdown on a river could contaminate millions of people’s drinking water without any boom. First pools are shared between reactors so there’s probably only around 200 that have ever been built. Also, the risk isn’t simply in year X for pool Y, it’s for every pool and every year. At best we can estimate the risk of a pool over it’s lifetime is probably under 2% and the risk from all pools is also under 2% in any given year. The risk of any pool over the next 50 years, now that we have very little hard data on. At least in terms of real world data ir could be 0.05% or 50% and we just don’t have enough real world data to validate. Edit: You can scroll through this list but it looks like on average there’s around 3 reactors per location. https://en.wikipedia.org/wiki/List_of_commercial_nuclear_reactors https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...
    - godelski 5y ago
      > At best we can estimate the risk of a pool over it’s lifetime is probably under 2% and the risk from all pools is also under 2% in any given year How are you getting this number? > The risk of any pool over the next 50 years, now that we have very little hard data on What are you talking about? We have plenty of data. We literally have decades of physical testing and hundreds of millions (if not billions) of dollars worth of simulation testing. What do you think Sierra[0] (#3 super computer in the world) is doing all day? It is a classified machine at a DOE lab. Even a substantial portion of Summit does this same research (it also does a lot of climate research). But come on, 3 of the top 10 super computers are at US DOE labs, and the 3 exascale machines being built are also targeted for DOE labs (Aurora, Frontier, El Capitan (classified)). LLNL and ORNL spend significant resources on nuclear research. [0] https://top500.org/system/179398/ https://top500.org/system/179398/

Retric 5y ago

Only up to a point. Modern designs tend to be short term passively safe, but shut off the pumps and radioactive decay alone can be enough to eventually cause a melt down in many modern designs. Which is the core issue, there’s a huge cost trade off for protection vs every possible issue no matter how remote. Spent fuel pools are probably the greatest example of this. They haven’t caused a major issue yet but they’re potentially a much larger risk than the actual reactor.

koheripbal 5y ago
No. Modern reactors use their pumps to keep the reactor going. Shut them off and the reactor shuts itself off. They now all have passive safety systems that do not require power.
i_am_proteus 5y ago
>shut off the pumps and radioactive decay alone can be enough to eventually cause a melt down in many modern designs The newest designs being built worldwide use natural circulation cooling and do not need cooling pumps in emergencies. Eventually the cooling pool needs to be refilled, but it's external to the containment pressure boundary, so you could refill it with a fire truck.
- Retric 5y ago
  Natural circulation gets heat from a reactor to X, but now your dependent on X. This often seems like a trivial detail, but Fukushima failed 3 days after the earthquake. The issue is you want several things from a passive system at the same time, don’t lose heat in normal operation, quickly lose multiple GW of heat in an emergency and as much as 200+MW of heat for days after a shutdown. The obvious solution is to have a tank of water that boils if the reactor temperature gets to high, but now you need to keep that tank full. Thus many designs result in a reactor that is passively safe for some number of hours and at risk after that. They describe this as a passively safe reactor even if it’s got external dependencies.
  - i_am_proteus 5y ago
    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.
        5y ago
- petre 5y ago
  One could use pumps for increased efficiency during normal operation but the idea is that natural circulation should be able to remove all the heat if the reactor is SCRAMed. NuScale's design for instance only uses pumps for the steam generator, the rest is handled by natural circulation and the reactor sits in a water pool that needs to be replenished after two weeks in case of a major accident.
- mlyle 5y ago
  > so you could refill it with a fire truck. Note you might need a lot of fire trucks, though. If decay heat is 0.2% of nominal power after a week, we need .8mL per nameplate MW (thermal) per second... which doesn't sound bad, but a 2GW reactor needs 5000L firetrucks showing up every 50 minutes, 24/7, after a week. This could be challenging depending upon the underlying emergency. ((0.002 megawatt) / ((2260 + 250) * (kilojoules / kg))) / (997 (kg / (m^3))) = 0.00079921038 liters per second Assumption is that you're raising the temperature of the water 60C and then evaporating it, and that all the heat energy goes into the water. In the real world it can be expected to be slightly better than this, but not much...
  - xyzzyz 5y ago
    You're off in your calculation by 3 orders of magnitude, you meant to write 0.002 gigawatt, not megawatt. Then yes, you get 0.7 liters/second, and 5000 L/hour for 2 GW plant. However, in actual reality, the firetruck would just park there, put one end of the hose into the plentiful source of water that surely must be available next to a power plant, and the other one into the entrance for coolant, and start its pump. Pumping 1 liter/second is quite in range of firetruck pumps abilities.
    - Retric 5y ago
      That’s a great assumption unless loss of water is the underlying emergency and to really mess things up you lost both water and the scram failed. That risk can be minimized by placing them near large natural bodies of water, but not all are ex: https://ejatlas.org/conflict/metsamor-nuclear-power-plant https://ejatlas.org/conflict/metsamor-nuclear-power-plant
    - mlyle 5y ago
      > You're off in your calculation by 3 orders of magnitude, you meant to write 0.002 gigawatt, not megawatt. No... decay heat is about .2% of nameplate power after a week. So for each megawatt of nameplate thermal power, you need to get rid of 2 kilowatts of heat-- or 0.8mL/second of water boiling off. "If decay heat is 0.2% of nominal power after a week, we need .8mL per nameplate MW (thermal) per second..." Was quite clearly said. > put one end of the hose into the plentiful source of water that surely must be available next to a power plant, and the other one into the entrance for coolant, and start its pump. Pumping 1 liter/second is quite in range of firetruck pumps abilities. This is pretty optimistic in many disaster scenarios and doesn't apply to all plants.
chasil 5y ago
I know that molten salt reactors have a "salt plug" at the bottom of the tank that will melt if the temperature is too high, dumping the liquid fuel into a boron bath. I think this kind of reactor is safe in a way that no modern reactor is - operators can remove all power and walk away in this shutdown state. This isn't possible with modern reactors, where 6% of the heat that they produce comes from daughter nuclei, and this decay heat requires cooling power for months after a controlled shutdown. I do agree, we have to build these safely, with every conceivable scenario, such that walking away is possible. Converting to thorium fuel would also be far better, as there is only one stable isotope in nature, so no refing is necessary (beyond high-purity smelting), and no centrifuges.
godelski 5y ago
The Soviets were the only ones that built reactors that way. As in, having the ability to blow up. > Spent fuel pools are probably the greatest example of this. They haven’t caused a major issue yet but they’re potentially a much larger risk than the actual reactor. I think you're also over estimating the danger here. We've been operating over 500 reactors for over 70 years. That's some pretty good statistical power.
- Retric 5y ago
  Edit2: Blowing up isn’t the only risk from a melt down, a major meltdown on a river could contaminate millions of people’s drinking water without any boom. First pools are shared between reactors so there’s probably only around 200 that have ever been built. Also, the risk isn’t simply in year X for pool Y, it’s for every pool and every year. At best we can estimate the risk of a pool over it’s lifetime is probably under 2% and the risk from all pools is also under 2% in any given year. The risk of any pool over the next 50 years, now that we have very little hard data on. At least in terms of real world data ir could be 0.05% or 50% and we just don’t have enough real world data to validate. Edit: You can scroll through this list but it looks like on average there’s around 3 reactors per location. https://en.wikipedia.org/wiki/List_of_commercial_nuclear_reactors https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...
  - godelski 5y ago
    > At best we can estimate the risk of a pool over it’s lifetime is probably under 2% and the risk from all pools is also under 2% in any given year How are you getting this number? > The risk of any pool over the next 50 years, now that we have very little hard data on What are you talking about? We have plenty of data. We literally have decades of physical testing and hundreds of millions (if not billions) of dollars worth of simulation testing. What do you think Sierra[0] (#3 super computer in the world) is doing all day? It is a classified machine at a DOE lab. Even a substantial portion of Summit does this same research (it also does a lot of climate research). But come on, 3 of the top 10 super computers are at US DOE labs, and the 3 exascale machines being built are also targeted for DOE labs (Aurora, Frontier, El Capitan (classified)). LLNL and ORNL spend significant resources on nuclear research. [0] https://top500.org/system/179398/ https://top500.org/system/179398/