Nuclear Reactors and Natural Disasters

[Ted C]

This one's obviously inspired by current events.

Does anyone know much about the design of the Japanese power plant that's currently out of control? I'm assuming that it's running on enriched fuel, much like the reactors used in the US. I live in the mid-South, and there are various reactors in the area. Authorities have been reassuring the public here by saying the reactors in the area have all been built to withstand the strongest earthquakes ever recorded for the region. Of course, I'm assuming that the Japanese had the same design goals for their reactors, but they still have a major problem.

I'm wondering if the sort of problem we're seeing in Japan would even be possible in the CANDU style reactors that Mike has described. Could it happen in a "pebble bed" reactor?

[GrandMasterTerwynn]

Based on the description of the CANDU reactor, I'd have to say "No, it wouldn't be possible." The design is apparently highly seismic-tolerant, the design of the core and fuel density are such that with the control rods in place and a suitable neutron poison introduced into the system, the fuel is not apt to wind up as molten lump at the bottom of the containment vessel should things to pear-shaped (if the fuel pellets do melt within their little bundles, the geometry is such that the only way they can flow is in such a manner as to dramatically increase the surface area presented to the surrounding water, cooling them off.)

The Fukushima-I nuclear power plant, that's in the process of transforming itself into the world's most expensive pile of scrap, is what's called a Boiling Water Reactor. It's a conventional light-water design using enriched fuel.

[Crossroads Inc.]

As for as safe plants go, if Money, and poltical will, were no limit, thwen what WOULD be the safest Reactor to go with? I know "safe" is a subjective term because what can be safe in one area might be vulnerable in another. So over all in terms of backup power, cooling, chance of meltdown, and ease of cooling in an emergancy, what is the best choice to go with?

And on a similar topic, which companies have the best records of safe Nuclear plants? Not just in terms of overall record, but of modernization and efficacy?

[Ted C]

Looking at those BWR diagrams, it looks like the control rods would fall out of the reaction chamber in the event of a failure in their mechanism: quite the opposite of the CANDU design. If that occurred at Fukushima, no wonder they're having trouble keeping it cool.

[General Zod]

As for as safe plants go, if Money, and poltical will, were no limit, thwen what WOULD be the safest Reactor to go with? I know "safe" is a subjective term because what can be safe in one area might be vulnerable in another. So over all in terms of backup power, cooling, chance of meltdown, and ease of cooling in an emergancy, what is the best choice to go with?

One of these reactors would be fairly safe. But money is a relatively small factor for this sort of thing.

[Ted C]

The pebble-bed design looks very safe. According to the Wikipedia article, it supposedly can't get too hot, even in a disaster scenario. Not sure how it compares to light water reactors or CANDU reactors for power generation, though.

[Sea Skimmer]

The GE MK1 BWR designs breaking in Japan are by some of the oldest and cheapest reactor designs still allowed to be operated in the western world. Building new ones like this was banned in the US in 1972, though a very similar design with better containment was still approved. On top of that some of them are loaded with MOX fuel which is hotter. So just about anything else would have been safer... and for all that it still looks like this will kill no one. Pebble Beds can't melt down, but that doesn't mean other things can't go wrong; as we are seeing in Japan if your plant workers forget about checking water levels in the spent fuel pool that can blow up too.

[aerius]

I'm wondering if the sort of problem we're seeing in Japan would even be possible in the CANDU style reactors that Mike has described.

It would be pretty much impossible. CANDU nuke plants have a vacuum building for dealing with radioactive steam releases. If they have to release steam from the reactor vessel into the reactor containment building, it gets sucked into the vacuum building and condensed. You don't get the buildup of hot steam and hydrogen gas in the containment building which went kaboom at Fukushima. The reactor core is also larger and has more redundant cooling loops so it'll take longer to heat up. It's also designed so that it has natural convective cooling in the event of a total power loss, the water will move itself through the pipes to a degree even if all the pumps are down. In the event of a core shutdown with total power loss the water will still circulate and remove residual heat from the reactor core.

[phongn]

Apparently many BWRs were modified after TMI to be able to safely vent (and much more battery power available, along with manual operation of valves). Had those features been installed we might not be seeing the mess we see now in Japan.

AP1000 and ESBWR are designed to be passively safe; EPR has quad-redundant cooling systems.

[Sea Skimmer]

AP1000 also has a pretty thin containment dome on the grounds that its so passively safe... bad logic. As for the Vacuum building, I'm fairly sure that needs emergency power to operate and contain more then a modest amount of steam by running condenser sprays.

[phongn]

AP1000 also has a pretty thin containment dome on the grounds that its so passively safe... bad logic. As for the Vacuum building, I'm fairly sure that needs emergency power to operate and contain more then a modest amount of steam by running condenser sprays.

Yeah, the containment cylinder is only 1m thick on AP1000 (though the steel is thicker than normal). IIRC the pressure vessel is designed to withstand fairly enormous loads of steam, though. It should be inherently safe from 'internal' threats, earthquakes, tornadoes, etc, but the NRC has noted some questions about withstanding, say, airplanes.

[His Divine Shadow]

Maybe this is a good thread to bring up the LFTR or Liquid Fluoride Thorium Reactor. I've seen people push these as the next coming of jesus. Being able to use thorium and even burn up nuclear waste and inherently safer than uranium reactors.

From what I've seen the theory seems sound, but nobodys built one commercially yet and theres no thorium support network either. I dunno if there are any really huge technical issues though.

It's been hard to find good solid articles about it, mostly a bunch of stuff that has the same vibes as cold fusion and free energy BS, which so far is the biggest con of this to me, the lack of serious people backing it.

A seemingly good link I found:
http://debatepedia.idebate.org/en/index ... ear_energy

[aerius]

A CANDU nuke plant can already do all that, it can use a thorium cycle and reuse spent fuel from PWR nuke plants. It can burn pretty much any fuel that fissions.

[His Divine Shadow]

The CANDU reactor can do some of the things that the LFTR can, though as cheaply and easily I do not know.

[someone_else]

Potentially stupid idea that hit me when I saw people eyeballing the thermal output of the spent fuel rods in the thread about the quake/tsunami/nuclear incident in japan:

Why don't use the spent fuel rods as a RTG-like emergency generator?
Would they generate enough energy to be useful?

Probably not since the people designing nuclear plants, that know much more than me, didn't do it, but it's still worth asking.

[General Zod]

Potentially stupid idea that hit me when I saw people eyeballing the thermal output of the spent fuel rods in the thread about the quake/tsunami/nuclear incident in japan:

Why don't use the spent fuel rods as a RTG-like emergency generator?
Would they generate enough energy to be useful?

Er, no. It has to be reprocessed in order to be used in nuclear reactions. By definition spent fuel cannot be used for generating power.

[Sky Captain]

Maybe this is a good thread to bring up the LFTR or Liquid Fluoride Thorium Reactor. I've seen people push these as the next coming of jesus. Being able to use thorium and even burn up nuclear waste and inherently safer than uranium reactors.

From what I've seen the theory seems sound, but nobodys built one commercially yet and theres no thorium support network either. I dunno if there are any really huge technical issues though.

Biggest issues probably the corosive nature of molten salts requiring exotic materials and automated fuel reprocessing plant that can work continously in high radiation envinroment although there were small experimental molten salt reactor operated 30 years ago so some of those issues appearently were adressed. Single biggest problem is lack of will to allocate few billion$ to start actual R&D process. If no one builds prototype molten salt reactor plant then we will never know how it actually performs.

[Imperial528]

Potentially stupid idea that hit me when I saw people eyeballing the thermal output of the spent fuel rods in the thread about the quake/tsunami/nuclear incident in japan:

Why don't use the spent fuel rods as a RTG-like emergency generator?
Would they generate enough energy to be useful?

Er, no. It has to be reprocessed in order to be used in nuclear reactions. By definition spent fuel cannot be used for generating power.

Zod, RTGs get energy from the heat of radioactive decay. There's no processing needed.

Whether or not spent fuel rods meet the criteria for RTGs is debatable.

[Hamstray]

Whether or not spent fuel rods meet the criteria for RTGs is debatable.

SRGs might be a better idea. You would probably still need to separate DU first.

[Sea Skimmer]

RTGs can use a wide ranges of fuels depending on how much power is needed and how weight critical the user is, spacecraft versus a lighthouse for example. Spent nuclear fuel will make a heavy low power RTG just fine, but a high end RTG needs very highly refined fuel which may be so hot it will spontaneously ignite without having cooling fins attached. No RTG setup could be a relevant supply of power on the global scale.

[someone_else]

No RTG setup could be a relevant supply of power on the global scale.

I was thinking about using them to run the pumps in case of emergency using the heat from the spent fuel pools or the reactor's decay heat, or somesuch. I know already that there are much better ways to produce power.

[Sea Skimmer]

I was thinking about using them to run the pumps in case of emergency using the heat from the spent fuel pools or the reactor's decay heat, or somesuch. I know already that there are much better ways to produce power.

That really won't work, not to any useful extent for the cost. The largest RTG I've ever seen figures on only produced 2kw for a communist lighthouse, and weighed something like 10 tons, including the radiation shielding. A nuclear reactor needs many megawatts of power to run its pumps. This is why the Japanese cannot simply show up with a generator on a truck and restore cooling flow. You need a significant fraction of the plant's own output rating to support it, It think usually about 10% but don't quote me on that, and most nuclear reactors produce at least 1 GW of electrical power.... not easily met. That's also why that replacement power cable has taken a week to lay, its a damn big cable.

It seems kind of clear anyway that they had plenty of reserve power sources, but they simply made a mistake of placing them all together. RTGs won’t change that specifically. If half the generators were on one side of the reactor, and half on the other this would not have happened. However the plant was physically carved into a hillside, so any real estate on the inland side of the reactors was going to be very expensive as you have to dig away more hillside. Also I think they wanted the generators as close as possible to the seawater intake pumps, so that the power cable runs would be short. That way less risk of an earthquake breaking an underground cable and causing power failure that way.

[Sea Skimmer]

Also I forgot; some of the plutonium waste compounds materials used for RTGs are so hot that if they become separated from the cooling system the metal can undergo self combustion. This is exactly what Japan has been fearing in some of its MOX fuel rods. The RTG doesn't need active cooling or anything complicated, but they need passive cooling fins like a computer heat sink attached at all times to be safe. This is not good for safety, but that's how you get a high power RTG.

[His Divine Shadow]

Speaking of reactors, I read that the Advanced Candu Reactor will require enriched fuel.

Seems like it lost one of it's major pros thjere to me, I mean not having to process fuel was a big cost saver and it also gave the CANDU-6 the ability to burn all this other stuff and have this nice fuel cycle. The ACR can't do this from what I see, but its cheaper to build, though since it'll require more infrastructure WRT fuel now I dunno if the final cost isn't higher anyway?

[RRoan]

I was thinking about using them to run the pumps in case of emergency using the heat from the spent fuel pools or the reactor's decay heat, or somesuch. I know already that there are much better ways to produce power.

That really won't work, not to any useful extent for the cost. The largest RTG I've ever seen figures on only produced 2kw for a communist lighthouse, and weighed something like 10 tons, including the radiation shielding. A nuclear reactor needs many megawatts of power to run its pumps. This is why the Japanese cannot simply show up with a generator on a truck and restore cooling flow. You need a significant fraction of the plant's own output rating to support it, It think usually about 10% but don't quote me on that, and most nuclear reactors produce at least 1 GW of electrical power.... not easily met. That's also why that replacement power cable has taken a week to lay, its a damn big cable.

I believe he meant to have the spent fuel rods themselves used to power a thermoelectric generator in a manner similar to a RTG, using all that waste heat that they put out while they're sitting in the cooling pond. I'm not sure how it would work, though it would probably serve as a decent backup generator if you could get the concept to work. They are putting out several megawatts of thermal energy, after all.