5 ms·

I'm a nuke proponent but it's very worth pointing out that a nuclear reactor that has been recently shutdown will need days of cooling to be available in order

by mpyne 5y ago

I'm a nuke proponent but it's very worth pointing out that a nuclear reactor that has been recently shutdown will need days of cooling to be available in order to safely avoid damage to the reactor and potential meltdown, pressure spikes, etc. New-ish designs can do the cooling completely passively but not all active nuclear reactors are so equipped (Fukushima's Daiichi's ancient BWR design needed active cooling, which failed in the tsunami, while the slightly newer BWR design at Fukushima Daini survived the tsunami damage without further core damage).

Isthatablackgsd 5y ago
Is that why Chernobyl reactor core kept burning after the blowup? I am not familiar with cycle of the nuclear core in the plant. I assumed once the control rod is in, the core will stop "combusting"?
- garmaine 5y ago
  Once the control rods are in, the reactor is losing more energy than it is producing and will eventually cool down. However in many old (e.g. 60's era) designs including Chernobyl and Fukushima, the reactor is losing that heat via active water cooling. If you turn off those heat pumps, then the core is still energy-positive even with rods inserted. In Chernobyl the core literally blew up, so it's a different situation. But in Fukushima the pumps lost power and so even though the control rods were fully inserted, there was a real risk it might get hot enough to melt down anyway. That was thankfully avoided.
- kibwen 5y ago
  Reactor designs differ, and they tend to have a large number of emergency backup and contingency plans. Here's a decent overview for a typical reactor: https://interestingengineering.com/nuclear-meltdown-what-would-happen-in-the-worst-case-scenario https://interestingengineering.com/nuclear-meltdown-what-wou... (scroll down to "Preventing a nuclear meltdown"). TL;DR: the control rods slow the reaction, but the fuel rods are still hot, and the space is enclosed (to prevent radioactive material from casually escaping) so it gets hotter and hotter without intervention. If the fuel rods get hot enough they start melting, which produces hydrogen, which can explode.
- mpyne 5y ago
  The very short story is that unlike with something like gasoline combustion, where the energy output basically happens nearly instantaneously, a nuclear fission event releases energy both directly (from the fissioning) and indirectly (from the subsequent radioactive decay of the fission products). This indirect energy output is small in comparison to the reactor's output, but a small percentage times a very large power output is still sufficient to meltdown a reactor (and boil away the coolant in the process) if there's no way to remove the heat being generated. Imagine what you'd have to do with a car engine if every car trip of 4 hours or more produced 4% power output in the form of heat for the next day after your trip was over.

by mpyne 5y ago

Isthatablackgsd 5y ago

Is that why Chernobyl reactor core kept burning after the blowup? I am not familiar with cycle of the nuclear core in the plant. I assumed once the control rod is in, the core will stop "combusting"?

garmaine 5y ago
Once the control rods are in, the reactor is losing more energy than it is producing and will eventually cool down. However in many old (e.g. 60's era) designs including Chernobyl and Fukushima, the reactor is losing that heat via active water cooling. If you turn off those heat pumps, then the core is still energy-positive even with rods inserted. In Chernobyl the core literally blew up, so it's a different situation. But in Fukushima the pumps lost power and so even though the control rods were fully inserted, there was a real risk it might get hot enough to melt down anyway. That was thankfully avoided.
kibwen 5y ago
Reactor designs differ, and they tend to have a large number of emergency backup and contingency plans. Here's a decent overview for a typical reactor: https://interestingengineering.com/nuclear-meltdown-what-would-happen-in-the-worst-case-scenario https://interestingengineering.com/nuclear-meltdown-what-wou... (scroll down to "Preventing a nuclear meltdown"). TL;DR: the control rods slow the reaction, but the fuel rods are still hot, and the space is enclosed (to prevent radioactive material from casually escaping) so it gets hotter and hotter without intervention. If the fuel rods get hot enough they start melting, which produces hydrogen, which can explode.
mpyne 5y ago
The very short story is that unlike with something like gasoline combustion, where the energy output basically happens nearly instantaneously, a nuclear fission event releases energy both directly (from the fissioning) and indirectly (from the subsequent radioactive decay of the fission products). This indirect energy output is small in comparison to the reactor's output, but a small percentage times a very large power output is still sufficient to meltdown a reactor (and boil away the coolant in the process) if there's no way to remove the heat being generated. Imagine what you'd have to do with a car engine if every car trip of 4 hours or more produced 4% power output in the form of heat for the next day after your trip was over.