7 ms·

“NuScale's design doesn't depend on pumps or generators that could fail in an emergency because it uses passive cooling. The reactors would be in a containment

“NuScale's design doesn't depend on pumps or generators that could fail in an emergency because it uses passive cooling. The reactors would be in a containment vessel, underground and in a huge pool of water that can absorb heat. That means that even a reactor that fails would still be safe. "It doesn't require any additional water," says Feldman. "It doesn't require AC or DC power. It doesn't require any operator action. And it can stay in that safe configuration for as long as is needed."” What if there’s a leak? What if for some reason the temperature inside the reactor is slight enough to vaporise the water?

vages 7y ago
About the water vaporizing: I'm guessing the pool is so "huge" that the sheer amount of energy required to vaporize the water can't be found in the fissile fuel alone. That doesn't account for the possibility of a leak, though.
- rwcarlsen 7y ago
  In an accident scenario, the heat that is problematic is not from the fissile material (i.e. U235, Pu239) but from the fission products' decay heat - the isotope soup remaining after the fissile atoms split. It is much less intense than the heat/energy produced during normal operation and drops off significantly after a couple days. So the pool's purpose in terms of accident mitigation is to keep things cool enough during the period of time after reactor shutdown and before radioactive isotopes left over from split atoms decay/cool-down enough.
  - petschge 7y ago
    While you are correct that it is less than the nominal heat output, we are still talking about 50MW a minute after shut down and 5MW one day after the shutdown of a 700MW reactor. So even a day after the shutdown you are able to boil of 2kg of water every second.
    - rwcarlsen 7y ago
      Yes, it is still a lot of heat. But to a lay reader, it might be easy to believe that the pool needing to be able to cool the reactor means that the pool has to cool an operating plant during an accident which is incorrect. I was just trying to make the distinction here. Also the drop from (your very approx numbers) 50 MW to 5 MW is mostly exponential - and so very quickly drops away from 50 MW. After that, any remaining heat is diffuse enough to not actually boil away water from the pool. Fuel-melting risk (which is not the chain reaction restarting for any who may be limited in fission plant understanding) is generally only a concern for the first couple days after reactor shutdown.
      - petschge 7y ago
        Funny little detail that might interest you: Due to the presence of a large number of different fission products (with different half lifes), the drop does not really follow an exponential but is better described by P(t) = P_0 * a * (t^-0.2), i.e. a power law, where P_0 is the power before the shutdown, a is a couple of percent and t is the time since the shutdown in seconds.
acidburnNSA 7y ago
Leaks are designed out because the entire reactor vessels are surrounded by a giant swimming pool. The temperature stays low enough because this swimming pool is passively cooled. Lack of relible decay heat[1] cooling caused both Fukushima and three mile island accidents. Having passive decay heat removal usually decreases the probability of core damage by about 100x. 1 https://whatisnuclear.com/decay-heat.html https://whatisnuclear.com/decay-heat.html
- benj111 7y ago
  "Having passive decay heat removal usually decreases the probability of core damage by about 100x." 100x isn't 100% though. 100 of these small reactors has the same risk as one large reactor. Perhaps higher, because like you, everyone goes around saying the risk has been designed out. Though obviously one small reactor going boom is preferable to a big one doing the same.
  - acidburnNSA 7y ago
    100% safe is never the metric. Nothing is 100% safe. Every form of energy generation has risks that have to balance the good the energy source brings. Nuclear is nearly unique in that it can produce carbon-free energy (unlike fossil and biomass), continuously for 2 years on end between reloads (unlike wind/solar/coal), and it can be deployed in a wide variety of geographic locations (unlike hydro and geothermal). The risk is that there are occasionally nuclear accidents: Three mile island and fermi 1 meltdowns, both of which killed zero people. Chernobyl, which killed ~50 from ARS and up to 4,000 early deaths over the next several decades (on top of a few million cancers that would have happened anyway in the population). Fukushima, which killed up to one person. Can anyone name any fatal natural gas explosions? Any hydro dam failures that killed 100,000? How many people per year does coal kill by operating normally (~200,000). We as a society accept certain risks from our energy infrastructure because of all the good we get from it. Here is a plot of how many thousands of days of life are lost by various things [1]. Note that nuclear is almost ridiculously low. This chart doesn't include the impact of climate change on human lives. Nuclear fission is a climate superpower, and we cannot be justified in neglecting this natural resource because we're afraid of something going boom. TLDR: A factor of 100x improvement in nuclear safety goes from tiny risk to supertiny. Current plants are already safer than almost everything else we know. Some studies have more people dying from installing solar on their roofs than getting killed by radiation from nuclear fission plants. [1] https://i.imgur.com/ClgbpA4.png https://i.imgur.com/ClgbpA4.png

vages 7y ago

About the water vaporizing: I'm guessing the pool is so "huge" that the sheer amount of energy required to vaporize the water can't be found in the fissile fuel alone. That doesn't account for the possibility of a leak, though.

rwcarlsen 7y ago
In an accident scenario, the heat that is problematic is not from the fissile material (i.e. U235, Pu239) but from the fission products' decay heat - the isotope soup remaining after the fissile atoms split. It is much less intense than the heat/energy produced during normal operation and drops off significantly after a couple days. So the pool's purpose in terms of accident mitigation is to keep things cool enough during the period of time after reactor shutdown and before radioactive isotopes left over from split atoms decay/cool-down enough.
- petschge 7y ago
  While you are correct that it is less than the nominal heat output, we are still talking about 50MW a minute after shut down and 5MW one day after the shutdown of a 700MW reactor. So even a day after the shutdown you are able to boil of 2kg of water every second.
  - rwcarlsen 7y ago
    Yes, it is still a lot of heat. But to a lay reader, it might be easy to believe that the pool needing to be able to cool the reactor means that the pool has to cool an operating plant during an accident which is incorrect. I was just trying to make the distinction here. Also the drop from (your very approx numbers) 50 MW to 5 MW is mostly exponential - and so very quickly drops away from 50 MW. After that, any remaining heat is diffuse enough to not actually boil away water from the pool. Fuel-melting risk (which is not the chain reaction restarting for any who may be limited in fission plant understanding) is generally only a concern for the first couple days after reactor shutdown.
    - petschge 7y ago
      Funny little detail that might interest you: Due to the presence of a large number of different fission products (with different half lifes), the drop does not really follow an exponential but is better described by P(t) = P_0 * a * (t^-0.2), i.e. a power law, where P_0 is the power before the shutdown, a is a couple of percent and t is the time since the shutdown in seconds.

acidburnNSA 7y ago

Leaks are designed out because the entire reactor vessels are surrounded by a giant swimming pool. The temperature stays low enough because this swimming pool is passively cooled. Lack of relible decay heat[1] cooling caused both Fukushima and three mile island accidents. Having passive decay heat removal usually decreases the probability of core damage by about 100x. 1 https://whatisnuclear.com/decay-heat.html https://whatisnuclear.com/decay-heat.html

benj111 7y ago
"Having passive decay heat removal usually decreases the probability of core damage by about 100x." 100x isn't 100% though. 100 of these small reactors has the same risk as one large reactor. Perhaps higher, because like you, everyone goes around saying the risk has been designed out. Though obviously one small reactor going boom is preferable to a big one doing the same.
- acidburnNSA 7y ago
  100% safe is never the metric. Nothing is 100% safe. Every form of energy generation has risks that have to balance the good the energy source brings. Nuclear is nearly unique in that it can produce carbon-free energy (unlike fossil and biomass), continuously for 2 years on end between reloads (unlike wind/solar/coal), and it can be deployed in a wide variety of geographic locations (unlike hydro and geothermal). The risk is that there are occasionally nuclear accidents: Three mile island and fermi 1 meltdowns, both of which killed zero people. Chernobyl, which killed ~50 from ARS and up to 4,000 early deaths over the next several decades (on top of a few million cancers that would have happened anyway in the population). Fukushima, which killed up to one person. Can anyone name any fatal natural gas explosions? Any hydro dam failures that killed 100,000? How many people per year does coal kill by operating normally (~200,000). We as a society accept certain risks from our energy infrastructure because of all the good we get from it. Here is a plot of how many thousands of days of life are lost by various things [1]. Note that nuclear is almost ridiculously low. This chart doesn't include the impact of climate change on human lives. Nuclear fission is a climate superpower, and we cannot be justified in neglecting this natural resource because we're afraid of something going boom. TLDR: A factor of 100x improvement in nuclear safety goes from tiny risk to supertiny. Current plants are already safer than almost everything else we know. Some studies have more people dying from installing solar on their roofs than getting killed by radiation from nuclear fission plants. [1] https://i.imgur.com/ClgbpA4.png https://i.imgur.com/ClgbpA4.png