A loss of criticality ensures that you don't end up with a positive feedback cycle that lets the problem spiral out of control faster than you can react, and possibly faster than you can comprehend.
A pressurized water reactor relies on the pressure vessel not being breached in order for the passive decay cooling to work, if that happens we're right back to Fukushima/Chernobyl style problems.
I am talking about short-term "everyone evacuate the building" types of disaster scenarios. You can do precisely that with a LFTR because of the passive decay heat cooling combined with the loss of criticality that happens when the freeze plug melts.
On a longer term the fission products might be a problem with an abandoned LFTR but because of the loss of criticality and the passive decay it's 100% acceptable to just wait for everything to cool down, fix the problem and restart. Or if things are so broken you can't fix them at least you don't have to keep putting people in harm's way to try and prevent a wider-scale disaster. Once things have cooled you go collect the nuclear material and send it to another plant to be used.
The best way for me to explain this is with analogies using potential energy.
1. The nuclear power plants we have now are a big heavy rock precariously balanced on top of a fairly narrow peak. Small movements to one side or the other can be recovered from but at some point that rock has started to move and will move aggressively. It will eventually reach the bottom of the hill but not before crushing everything in its path. We don't really even know how bad the damage can get.
2. A LFTR is like a big heavy rock perched about 10 feet up a shallow hill. It requires some energy to keep it there or else it'll roll down hill. You can't really push it up further so it's safe from disturbances in that direction. If you push it down hill (or cease holding it up) it only rolls 10 feet before it naturally hits the bottom and comes to a stop.
The majority of the things in the world act more like 2 than 1 and thus we aren't terribly scared of them. Yes there are a great many things you can do to ensure that 1 doesn't get away from you, but ultimately you're still fighting gravity.
Let me reiterate, I'm not against nuclear power. I just REALLY don't like the idea of the balancing act that has to be performed in 1 and far prefer the kind of intrinsic safety that you get from 2. I'm all for building plants like 2 even if they're not LFTR based. I don't have religion about the form, I just want as much safety as I can get.
A pressurized water reactor relies on the pressure vessel not being breached in order for the passive decay cooling to work, if that happens we're right back to Fukushima/Chernobyl style problems.
AFAIK the pressure vessel was never breached at Fukushima. They had plenty of problems whose primary cause was the absence of cooling water (including a hydrogen gas explosion), but there was never an uncontrolled criticality because of it. (Also see further comments below on criticality.)
I am talking about short-term "everyone evacuate the building" types of disaster scenarios. You can do precisely that with a LFTR because of the passive decay heat cooling combined with the loss of criticality that happens when the freeze plug melts.
Ok, this makes it clearer where you are coming from. I certainly agree that the LFTR is a big improvement over the standard PWR design.
We don't really even know how bad the damage can get.
It's true that Chernobyl could have been worse, so we can't really judge the worst case from what happened there. However, these worst-case scenarios have been simulated in great detail; the physics is actually pretty simple.
With any design that isn't Soviet-built, a loss of coolant will not cause an uncontrolled criticality; losing coolant decreases the reaction rate, causing loss of criticality. The problem with the Chernobyl design was that it had a "positive void coefficient of reactivity", meaning that the reaction rate increased on loss of coolant. That feature, as I said, is not present in any non-Soviet design, which means it's not present in any commercial design currently operating (since all the old Soviet reactors have been shut down).
I just REALLY don't like the idea of the balancing act that has to be performed in 1 and far prefer the kind of intrinsic safety that you get from 2.
So do I. Now that we have such designs, we should certainly be building them, and should not be building the old designs that lack those passive safety features.
Yes that's true that there was never uncontrolled criticality. But that's because people were in there restoring some way to get water into the cores. Had that not happened, they likely would have melted down. Once all the water boils the heat doesn't have anywhere to go so temperatures go up. If that happened that's a melt-down and the core liquefies and then may or may not have been stopped by the containment shell. http://www.nbcnews.com/science/if-theres-meltdown-then-what-...
In my mind the worst-case scenario is that the core melts, the containment shell is cracked and the whole mess goes through the floor and into the ground. Has that been simulated at all? I'd love to read a paper if it has. I haven't seen anything with a cursory search.
Yeah, absolutely. We need reactor designs that don't melt down when they fail and lose power. LFTR and perhaps pebble-bed reactors would be far safer than what we have now, and safer than coal in many ways.
Unfortunately, it's complicated to explain that to people. People don't really want anything to do with nuclear after all the issues of the past, and it's hard to blame them. Nuclear really needs a rebranding and safer designs.
Some common sense would help too. We should shut down nuclear power plants that are on earthquake faultlines.
A pressurized water reactor relies on the pressure vessel not being breached in order for the passive decay cooling to work, if that happens we're right back to Fukushima/Chernobyl style problems.
I am talking about short-term "everyone evacuate the building" types of disaster scenarios. You can do precisely that with a LFTR because of the passive decay heat cooling combined with the loss of criticality that happens when the freeze plug melts.
On a longer term the fission products might be a problem with an abandoned LFTR but because of the loss of criticality and the passive decay it's 100% acceptable to just wait for everything to cool down, fix the problem and restart. Or if things are so broken you can't fix them at least you don't have to keep putting people in harm's way to try and prevent a wider-scale disaster. Once things have cooled you go collect the nuclear material and send it to another plant to be used.
The best way for me to explain this is with analogies using potential energy.
1. The nuclear power plants we have now are a big heavy rock precariously balanced on top of a fairly narrow peak. Small movements to one side or the other can be recovered from but at some point that rock has started to move and will move aggressively. It will eventually reach the bottom of the hill but not before crushing everything in its path. We don't really even know how bad the damage can get.
2. A LFTR is like a big heavy rock perched about 10 feet up a shallow hill. It requires some energy to keep it there or else it'll roll down hill. You can't really push it up further so it's safe from disturbances in that direction. If you push it down hill (or cease holding it up) it only rolls 10 feet before it naturally hits the bottom and comes to a stop.
The majority of the things in the world act more like 2 than 1 and thus we aren't terribly scared of them. Yes there are a great many things you can do to ensure that 1 doesn't get away from you, but ultimately you're still fighting gravity.
Let me reiterate, I'm not against nuclear power. I just REALLY don't like the idea of the balancing act that has to be performed in 1 and far prefer the kind of intrinsic safety that you get from 2. I'm all for building plants like 2 even if they're not LFTR based. I don't have religion about the form, I just want as much safety as I can get.