Nuclear's next generation • Hacker News Board

Re: Nuclear's next generation

#21

Post by sketerpot » Mon, Dec 14, 2009, 5:27 AM UTC

bjelkeman-again wrote: post #15

dubcomesaveme wrote: post #13

u.s. nuclear reactors have always been safe: if a loss of coolant failure occurs the reaction is designed to slow down rather than speed up soviet cheronobyl style. the concern is what to do with all the junk from processing\using fuel rods and depleted uranium besides dumping it on Iraqis via munitions.

That doesn't always seem to work though: http://en.wikipedia.org/wiki/Three_Mile_Island_accident

In the case of TMI, the steam formation did slow down the reaction -- just not enough to prevent damage to the fuel rods. Of course, the reactor's pressure vessel acted as a passive heat sink once the fuel melted and easily prevented the fuel from escaping, but it was still costly and scary, and the response of the operators at the time can only be called a ridiculous clusterfuck.

This sort of thing is why I really like pebble bed reactors: you can just shut off the coolant and walk away, and they'll sit tight. (The operators of China's HTR-10 research reactor actually do this.) Everything is designed to withstand the maximum temperatures they could possibly achieve. Light water reactors have an impressive safety record, and the modern versions aren't susceptible to the problems that led to TMI, but inherently self-moderating reactors are just really aesthetically pleasant.

Re: Nuclear's next generation

#22

Post by sketerpot » Mon, Dec 14, 2009, 5:34 AM UTC

Daniel_Newby wrote: post #4

I like the sodium fast reactor: low pressure, moderate temperature, sodium is dirt cheap, high burn-up of non-uranium/plutonium products ("waste"), and plutonium need never be separated from other reaction products. The article doesn't say, but because molten salt reactors use liquid fuel, they release the radioactive xenon gas produced by uranium fission. (In fact this is sold as a benefit, since xenon absorbs neutr…

In the case of liquid fluoride thorium reactors, I don't think the xenon-135 is what you need to be worrying about; worry about the fluorine gas instead! It's very reactive. Xenon, on the other hand, is chemically stable because it's a noble gas. And it's larger than helium, so it's easier to contain.

Also, I'd like to point out that pebble-bed reactors, while usually cooled with helium, could also be cooled with nitrogen that we just pull out of the air. It's more reactive than helium and it forms carbon-14 when you expose it to neutron flux, but those issues are both fairly minor. I think they're going with helium now because they want to get something working as quickly as they can, and they don't want to go mucking with what works.

Re: Nuclear's next generation

#23

Post by sketerpot » Mon, Dec 14, 2009, 5:36 AM UTC

dubcomesaveme wrote: post #13

u.s. nuclear reactors have always been safe: if a loss of coolant failure occurs the reaction is designed to slow down rather than speed up soviet cheronobyl style. the concern is what to do with all the junk from processing\using fuel rods and depleted uranium besides dumping it on Iraqis via munitions.

We can use the U-238 in breeder reactors. Or make a subcritical blanket of U-238 and some of the longer-lived radioactive elements in spent fuel rods, and put them around a fusion reactor. The fusion reactor doesn't have to generate a net positive amount of usable energy; it just has to provide a lot of neutrons, which it does. This is the basis for fusion-fission hybrid systems, which can be marketed as nuclear-waste annihilation systems.

Re: Nuclear's next generation

#24

Post by sketerpot » Mon, Dec 14, 2009, 5:38 AM UTC

jeremyw wrote: post #3

One form of very high temperature reactor , the pebble bed reactor, was theorized in the 1950s and has been under development at various locations for more than a decade -- China begins commercial construction this year. PBRs are one of several inherently safe designs wherein shutoff of circulating coolant raises core temperature and natural processes serve to choke criticality. http://en.wikipedia.org/wiki/Pebble_be…

Pebble beds are also compatible with thorium, in case anybody was wondering.

Re: Nuclear's next generation

#25

Post by rainier » Mon, Dec 14, 2009, 8:55 AM UTC

jeremyw wrote: post #3

One form of very high temperature reactor , the pebble bed reactor, was theorized in the 1950s and has been under development at various locations for more than a decade -- China begins commercial construction this year. PBRs are one of several inherently safe designs wherein shutoff of circulating coolant raises core temperature and natural processes serve to choke criticality. http://en.wikipedia.org/wiki/Pebble_be…

The pebble bed reactor has inherently safe features concerning loss of coolant, but it is not inherently safe: There are other sever accidents not known in conventional reactors, like graphite fire (as it happened in Chernobyl, too) and water ingress into the core with subsequent explosions.

Re: Nuclear's next generation

#26

Post by dmfdmf » Mon, Dec 14, 2009, 11:09 AM UTC

The current reactor designs are safe enough, let's build them now. Debating the pros and cons of advanced designs is dumb and accepts the enviros false claim that current BWR or PWR light water reactors are "unsafe". Moreover, significant real-world safety comes from operational experience on a fleet of the same or similar designs. It would be foolish to throw away 50 years of knowledge and experience on light water reactors to start over on sodium, fluoride, pebble-bed, etc.

Re: Nuclear's next generation

#27

Post by dmfdmf » Mon, Dec 14, 2009, 11:44 AM UTC

sketerpot wrote: post #21

bjelkeman-again wrote: post #15

Earlier quoted context omitted.

That doesn't always seem to work though: http://en.wikipedia.org/wiki/Three_Mile_Island_accident

In the case of TMI, the steam formation did slow down the reaction -- just not enough to prevent damage to the fuel rods. Of course, the reactor's pressure vessel acted as a passive heat sink once the fuel melted and easily prevented the fuel from escaping, but it was still costly and scary, and the response of the operators at the time can only be called a ridiculous clusterfuck. This sort of thing is why I really l…

>the response of the operators at the time can only be called a ridiculous clusterfuck.

This is unfair to the operators. The accident revealed some reactor and control room design flaws plus some equipment out of service that left the operators in the dark re what state the reactor was in during the accident. They knew the info they were getting was bad and took heroic steps to get better data, including sending men down into radioactive zones to read thermocouples manually with a volt meter, among other things. It is the anti-nukes who perpetuate the myth that the reactor operators freaked out and just mindlessly started throwing switches, closing valves willy-nilly. Unfortunately, the operators did make the situation worse but it was not due to stupidity or incompetence. One problem for the operators was their training was based on some assumptions that were not true for this accident. TMI was a pressured water reactor and one of the cardinal sins taught in training was never let the primary coolant system "go solid", i.e., no steam void in the pressurizer. A solid piping system could easily be burst by even a mild pressure transient which was why they opted to drain more coolant from an already overheating reactor. Tragically, the pressurizer was going solid because a steam void had formed in the core, something their training did not adequately address and they could not infer from the info available at the time. For obvious reasons, they had to make critical decisions within the time constraints and the data actually at hand, not 6 months later in an academic study.

Re: Nuclear's next generation

#28

Post by Daniel_Newby » Mon, Dec 14, 2009, 6:21 PM UTC

sp332 wrote: post #9

Daniel_Newby wrote: post #4

I like the sodium fast reactor: low pressure, moderate temperature, sodium is dirt cheap, high burn-up of non-uranium/plutonium products ("waste"), and plutonium need never be separated from other reaction products. The article doesn't say, but because molten salt reactors use liquid fuel, they release the radioactive xenon gas produced by uranium fission. (In fact this is sold as a benefit, since xenon absorbs neutr…

> Xenon is a noble gas and therefore volatile This must be some definition of "volatile" I'm not familiar with.

Evaporating easily. Xenon boils at -108 deg. C. Given the chance, it would tend to rapidly escape from liquid fuel at reactor temperatures.

Re: Nuclear's next generation

#29

Post by Daniel_Newby » Mon, Dec 14, 2009, 6:40 PM UTC

sketerpot wrote: post #22

Daniel_Newby wrote: post #4

I like the sodium fast reactor: low pressure, moderate temperature, sodium is dirt cheap, high burn-up of non-uranium/plutonium products ("waste"), and plutonium need never be separated from other reaction products. The article doesn't say, but because molten salt reactors use liquid fuel, they release the radioactive xenon gas produced by uranium fission. (In fact this is sold as a benefit, since xenon absorbs neutr…

In the case of liquid fluoride thorium reactors, I don't think the xenon-135 is what you need to be worrying about; worry about the fluorine gas instead! It's very reactive. Xenon, on the other hand, is chemically stable because it's a noble gas. And it's larger than helium, so it's easier to contain. Also, I'd like to point out that pebble-bed reactors, while usually cooled with helium, could also be cooled with nit…

They use the relatively-nonvolatile fluoride ion (F-), not the gaseous molecular fluorine (F2). Fluoride is corrosive in higher concentrations, but nowhere near as dangerous as the oxidative toxicity of fluorine gas.

Re. nitrogen, the radiation would turn it into ionized nitrogen radicals. Any info on how much damage that would cause to the graphite and metals? Also, carbon-14 is preferentially concentrated into living things, so a release would provoke a political panic similar to strontium-90.

Re: Nuclear's next generation

#30

Post by cheriot » Mon, Dec 14, 2009, 7:44 PM UTC

dmfdmf wrote: post #26

The current reactor designs are safe enough, let's build them now. Debating the pros and cons of advanced designs is dumb and accepts the enviros false claim that current BWR or PWR light water reactors are "unsafe". Moreover, significant real-world safety comes from operational experience on a fleet of the same or similar designs. It would be foolish to throw away 50 years of knowledge and experience on light water…

It's also about efficiency. Nuclear power plants regularly live beyond their designed lifespan of 30-40 years. If a new reactor can be built with 45% efficiency rather that the 33% of existing light water reactors get, that alone will justify the delay.