neutron moderators

[idobox]

I have been reading a lot of layman-level stuff on nuclear power plants, and I don't really understand the choices of neutron moderators.

Hydrogen makes sense, because it very light, although, it absorbs a lot of neutrons and thus need refined fuel.
Deuterium is a bit heavier, but has a much lower cross-section, so can run on lower grade fuel, but is also less compact and more expensive.
Carbon was used in many experimental reactors, but I don't really see the advantages: it's much less efficient than hydrogen and deuterium at slowing down neutrons, not as good as water to extract heat, and can accumulate Wigner energy and catch fire.

Most commercial reactors use light water, both as coolant and moderator. The main issue is you need to keep it at high pressure to avoid boiling, and large high pressure vessel are expensive and dangerous. Why don't we use high boiling point hydrocarbons to do the same job? They are made of hydrogen and carbon, and possibly oxygen and nitrogen, that are all neutron moderators, they are liquid at the working temperature of the reactor, and would work at ambient pressure.
Of course, using flammable coolant is a risk, but refineries handle large amounts of heated oil every day, and you could have safety features, like a containment unit filled with nitrogen, that would be much cheaper than the current ones because they wouldn't need to be able to withstand an explosion. Also, they are reactors using molten sodium or fluorides which are much more difficult to handle, and much more dangerous in case of a leak.

I'm pretty sure there is a good reason people don't use molten paraffin, I just don't see it.

[oxoiron]

There is an essentially infinite supply of water available for virtually no cost.

[thoughtfully]

It isn't exactly cheap to build, maintain, and assume liability of a lot of high pressure plumbing.

[Seraph]

Wouldn't the hydrocarbons fall apart (or polymerize, or something else equally unpleasant) under the bombardment of ionizing radiation they'd be exposed to?

[Izawwlgood]

Whoa, oxoiron is still around?

For what it's worth, I think Wired had a really cool article about the dangers posed to divers who are tasked with repairing the cooling systems of nuclear plants. It doesn't sound like a terribly cheap endeavor.

[Zamfir]

Wouldn't the hydrocarbons fall apart (or polymerize, or something else equally unpleasant) under the bombardment of ionizing radiation they'd be exposed to?

Yep, that's the main one. Not just the radiation, also simply the temperature.

A moderator gets hot, since it's absorbing most of the energy of the neutrons. Hydrocarbons are usually bad heat conductors, making them hard to cool. So using them as solid moderator (like graphite*) is out.

The alternative is to use them like water in light-water reactors: as liquid coolant and moderator at the same time. But with the temperature and radiation effects, you end up with a liquid that is constantly changing its chemical composition in a deeply complicated way, a bit like like the sludge in oil refineries. Note that oil refineries are expensive machines to build and run.

It's not a crazy idea, just one that appeared to have more complications that advantages. Here's wiki http://en.wikipedia.org/wiki/Organically_moderated_and_cooled_reactor


* Which immediately gives you the attraction of graphite: it's one of the few materials that stay solid up to high temperatures, without introducing neutron-absorbing elements. It's also a fairly good heat conductor, so this allows you to decouple moderation and cooling, usually to allow gas cooling. Metallic beryllium would be the other choice, but that's expensive and nasty stuff.

[oxoiron]

It isn't exactly cheap to build, maintain, and assume liability of a lot of high pressure plumbing.

I guess I was thinking the cost of high pressure plumbing would be offset by the coolant savings. Filling a reactor with a purified metal, salt or organic compound will not be cheap. However, I've never taken the time to calculate which is more expensive, engineering a high pressure containment system or gathering the required metal, salt, etc., so I'm just speculating wildly about costs (i.e. talking out of my ass).

Clearly, there are advantages and disadvantages to BWRs, PWRs, MSRs, LMFRs and OMRs and the fact that have all been built tells me that not one type is the be-all and end-all of nuclear power sources.

Whoa, oxoiron is still around?

Yes, I am alive, but nowadays I spend more time lurking than participating. Thanks for the shout out. It's nice to know someone noticed I haven't been contaminating the fora.

For what it's worth, I think Wired had a really cool article about the dangers posed to divers who are tasked with repairing the cooling systems of nuclear plants. It doesn't sound like a terribly cheap endeavor.

Do you have a link to that?

[PM 2Ring]

Which immediately gives you the attraction of graphite: it's one of the few materials that stay solid up to high temperatures, without introducing neutron-absorbing elements. It's also a fairly good heat conductor, so this allows you to decouple moderation and cooling, usually to allow gas cooling. Metallic beryllium would be the other choice, but that's expensive and nasty stuff.

Diamond's a much better conductor of heat than graphite; pity it's so expensive. But who knows, one day synthetic diamond (or diamonds from space) may be cheap enough to use as a moderator. And it's a pity that ¹¹C isn't stable in this universe.

As for beryllium:

The toxicity of beryllium depends upon the duration, intensity and frequency of exposure (features of dose), as well as the form of beryllium and the route of exposure (i.e. inhalation, dermal, ingestion). According to the International Agency for Research on Cancer (IARC), beryllium and beryllium compounds are Category 1 carcinogens; they are carcinogenic to both animals and humans.

Chronic berylliosis is a pulmonary and systemic granulomatous disease caused by exposure to beryllium. Acute beryllium disease in the form of chemical pneumonitis was first reported in Europe in 1933 and in the United States in 1943. Cases of chronic berylliosis were first described in 1946 among workers in plants manufacturing fluorescent lamps in Salem, Massachusetts.[2] Chronic berylliosis resembles sarcoidosis in many respects, and the differential diagnosis is often difficult. It occasionally killed early workers in nuclear weapons design, such as Herbert L. Anderson.

[...]

Early researchers tasted beryllium and its various compounds for sweetness in order to verify its presence. Modern diagnostic equipment no longer necessitates this highly risky procedure and no attempt should be made to ingest this highly toxic substance. Beryllium and its compounds should be handled with great care and special precautions must be taken when carrying out any activity which could result in the release of beryllium dust (lung cancer is a possible result of prolonged exposure to beryllium laden dust).

This substance can be handled safely if certain procedures are followed. No attempt should be made to work with beryllium before familiarization with correct handling procedures.

[idobox]

It's not a crazy idea, just one that appeared to have more complications that advantages. Here's wiki http://en.wikipedia.org/wiki/Organicall ... ed_reactor

I'm happy to see my idea is good enough to be actually considered by India.

And there aren't any simple compounds that are liquid in this range of temperature that contain a lot of carbon or hydrogen?
Have people considered dissolving moderators in non-moderating fluid?
Or adding something in the water than increases the boiling temperature, to allow lower pressure operation?

I guess I was thinking the cost of high pressure plumbing would be offset by the coolant savings. Filling a reactor with a purified metal, salt or organic compound will not be cheap.

Oil is not expensive (compared to the cost of a nuclear power plant), and the better moderation implies smaller cores, so less coolant.

Diamond's a much better conductor of heat than graphite

Even better and more expensive than diamond: carbon nanotubes. I don't know how they react to fast neutrons, though.

And why were so many early reactors or piles using graphite?
My guess is that it can work with lower grade fuel than light water reactors and is cheaper than heavy water ones.

[Zamfir]

And there aren't any simple compounds that are liquid in this range of temperature that contain a lot of carbon or hydrogen?

Sure, but radiation and temperature will unavoidably create some complexer, larger molecules, some of them solid or highly viscous. And some of those will foul your systems, get deposited everywhere, get into moving parts. A nuclear reactor is a silly amount of pipework, pumps and valves, and it's a pain to keep that running realiably. Even a water-based reactor already has lots of systems to keep the water clean and pure enough.

Have people considered dissolving moderators in non-moderating fluid?

That's basically a molten salt reactor. The salt is a mixture of fluoride of the fuel, with berylliumfluoride or lithiumfluroide as moderator. The you create a geometry such that the mixture is only critical in the reactor zone, and you pump it around. There was an experimental twist on those in the Netherlands in the 1970s, with little spheres of thorium suspended in water. One big downside of such systems is that the operating fluid is highly radioactive and toxic, so it's an incredible pain to do maintenance on your systems.

An alternative is to use molten salt only as coolant. So less radioactivity otuside of the reactor zone, but you lose the attraction of the thorium-cycle. And the suitable salts are still very chemically reactive stuff, that eat away metals etc.

Or adding something in the water than increases the boiling temperature, to allow lower pressure operation?

Not sure about this. That would basically be a kind of thick salt water sludge, right? At temperatures of more than a few hundred degrees, I guess that would be basically molten salt again.

Oil is not expensive (compared to the cost of a nuclear power plant), and the better moderation implies smaller cores, so less coolant.

Yeah, cost of the material is a factor, but not an unsurmountable one. I've read that the heavy water in Canadian reactors costs something like 20% of the entire power plant.

And why were so many early reactors or piles using graphite?

Again, it's a solid, even at very high temperatures. You can shape it, it adds to the structural integrity of the core. Often it simply is the structural material of the core. That's hardly something from the past, most high-temperature designs proposed for the future are also based on graphite moderation, with gas or salt to cool.