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16 March 2011 @ 04:03 pm
Risk calculations  
This whole situation with the nuclear reactors in Japan is really frustrating. Everyone is freaking out about it and a lot of analysts are convinced that it's going to have a serious impact on the political/popular feasibility of the use of nuclear power in the future, especially in terms of building new reactors.

Certainly the situation sucks, but there's a couple huge fallacies in the ZOMG nuclear power = BAD! line of thought. The first, that this is what one can usually expect from reactors. The reactors having the biggest problems are about 30-40 years old. The ones that are faring slightly better but still having some problems are 25-30 years old. They were hit with the largest recorded quake in Japanese history, and the fourth largest recorded quake in the world, after which they were doing fairly well until they were then also hit by the tsunami. Yes one has to account for that kind of disaster happening from time to time, but given their age these reactors have fared that badly and newer, more safety conscious designs, would have performed much better.

Second, everyone is focusing on the danger from the nuclear power plants while completely ignoring the problems posed by other methods of generating power and/or heat. Like many earthquakes the one in Japan was followed by the outbreak of numerous fires, many caused by broken gas mains, and more than one major oil refinery burst into flames as well. It's hard to say exactly how big a factor the fires have been in the over ten thousand dead and missing, to say nothing of the property damage, but everyone accepts that because gas and oil are understood by everyone and fires are "expected" after an earthquake. A fire doesn't have "invisible scary magic" numbers associated with it like how many microsieverts you can be exposed to before you need to start worrying.

That said, i'm certainly concerned about and interested in the processes going on at the Japanese nuclear plants right now. Obviously these plants were not designed to be passively safe like the newer models, but i'm curious about what issues are giving them so much trouble. Nuclear fission generally works by bringing pieces of uranium (or some other similar element) "close together" in a relative sense such that the neutrons released by one atom decaying will trigger the decay of other atoms. You stop the reaction by taking the pieces of uranium "apart" from each other. This may or may not involve physically moving the pieces, it's just as possible (and actually more usual) to modulate the space between the pieces of uranium in order to prevent (some of) the neutrons from passing through to the next piece of uranium.

So what i wonder, in short, is what's stopping them from just trashing the reactor core? Do they think they still have some hope of salvaging them and getting them back online at a later date? Or have they given up hope of that and it's only technical problems that are getting in the way? I would have thought that in any kind of worst-case scenario short of actual meltdown the most surefire way to stop the reaction would be to dump a bunch of molten lead into the reactor. Surely lead has got to be at least as absorbent as the materials normally used as control rods in such reactors? Assuming you do so before the reaction gets hot enough to vaporize lead you ought to be able to just fill it up, wait for it to cool, and separate the uranium elements later.

Alternately (and admittedly more risky) why don't they just drop a hand grenade or two in? Or at least go in there (presumably with a robot) and smash the thing with a hammer? If you can knock enough elements out of the array presumably the feedback reaction would stop?

Are those plans too risky somehow? Or would they just not work? Or would it make it "too costly" to clean up the mess afterwards?
 
 
 
Beth Leonardbeth_leonard on March 17th, 2011 12:20 am (UTC)
You stop the reaction by taking the pieces of uranium "apart" from each other.

So, this they did within 5 seconds of the initial quake by inserting the control rods. It takes the reaction down from 100% to 7% energy levels. The problem is cooling the remaining 7%. In some sense, just letting it all blow up and sit in the concrete pot that was designed to hold it while all the workers watch safely from miles away should that happen is a viable alternative, BUT if for some reason that doesn't work, then they're in a world of hurt.

Also, the clean-up gets more complicated, because while nothing (or nearly nothing) is released into the environment, instead of having some nice "spent fuel" control rods to move around in large buckets of water, we've got a massive pile of radioactive sludge that can only be touched by remote control robots nobody trusts.

Another problem they're having at the plants right now (ok, as of yesterday when I was current) is that the spent fuel rods are usually kept in an enormous tank of water and kept cool. At one of the plants, the cooling systems for that are not working properly, and while there's nothing cateastrophic with boiling off all that water and burning up the building in which they're housed, as the radioactive decay rate of the particles which are spun off is really fast and the other part which is harmful to humans (again) just sits their and travels as far as solids travel in a day, it's still a bigger pain to clean up while keeping your cleanup crew safe.

--Beth
Sister Atom Bomb of Courteous Debateakiko on March 17th, 2011 01:27 pm (UTC)
A guy on my facebook feed said (paraphrased): those 50 people at Fukushima are going to leave some very nice medals of honor to their next of kin.

The problem with nuclear is that when it goes wrong, it goes VERY wrong. And even if nothing goes wrong, you still have radioactive waste to put somewhere.
DonAithnendonaithnen on March 17th, 2011 02:27 pm (UTC)
The radioactive waste from nuclear plants is a lot easier to deal with than the radioactive waste from coal plants. Or at least it would be if the government wasn't so insistent on putting political blocks in the way of reprocessing.

And like i said, the "it goes very wrong" part is that old design thing. The new models you can just cut the power and walk away and it will naturally shut down safely by itself.
Beth Leonardbeth_leonard on March 17th, 2011 06:18 pm (UTC)
No one predicted a disaster of this magnitude. Lots and lots of people died, in addition to the future potential deaths of the people working at the nuclear plant. The news isn't focusing on the oil refinery fire, which is putting quite a lot more contaminates into the air from what I can see. Last I read, that fire still isn't out either, and I can only guess at how many fire fighters are having smoke inhalation or other medical issues as a result, or how many workers lost their lives. That fire isn't news when there's NUCLEAR REACTORS to report on. I can't find many details on it at all. The nuclear energy has not yet gone as wrong as the conventional energy has already gone, but it has more press appeal.

--Beth
DonAithnendonaithnen on March 17th, 2011 02:28 pm (UTC)
So even in total isolation a single fuel element is able to boil water by itself? Or am i missing something?
Beth Leonardbeth_leonard on March 17th, 2011 06:11 pm (UTC)
It's able to evaporate water by itself. It doesn't have to make the water go to boiling temperature for the water level to drop. I'm still trying to sort out how much is FUD and how much is of actual concern.

--Beth
landfishydragon: Sleepylandfishydragon on March 17th, 2011 03:02 am (UTC)
The radioactive core throws off neutrons that strike the fissionable fuel material :D These neutrons are slowed by what is called "heavy water" surrounding the fuel rods. As the neutrons strike the heavy water, the molecules get all excited and emit a blue glow in the process. So dropping a grenade into the core might not be the best option of continuing to restricting the reaction as the neutrons would only have an easier time of speeding up the reaction :D Fun fact: The measurement unit of the diameter (or cross section) of a uranium is a called a "barn," named for the scientists who discovered how easy it was to hit it with neutrons and cause a reaction. Go go nuclear engineering class!
DonAithnendonaithnen on March 17th, 2011 02:24 pm (UTC)
Um, the Fukushima plants are light water reactors, not heavy water reactors, and even if they were heavy water reactors they've dumped so much seawater into them by this point that there wouldn't be much heavy water left, and in any case heavy water actually absorbs _less_ neutrons than regular water unlike what you describe above, and finally i don't believe conventional explosives perform any differently in the presence of heavy water than they do in the presence of regular water.