# Mini Nuclear Power Plants

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I split this off from the GW thread.

By the way, what's the story on these micro-nuclear power plants?

http://www.guardian.co.uk/environment/2008/nov/09/miniature-nuclear-reactors-los-alamos

That seems to be the only story on the topic at the moment, and it's sparse on detail.

edit: more articles. Very little, if any, more information. Some is even conflicting

http://www.off-grid.net/2008/10/31/micro-nuclear-plants-for-local-power/

http://gigantico.squarespace.com/336554365346/2008/11/9/nuclear-reactor-home-edition.html

One article says 200 kW, another says 25 MW. 10,000 homes one place, 20,000 homes another. But those numbers indicate 25MW is probably correct. The typical home uses 1 kW on average (i.e. somewhere, roughly, around 24 kWh per day), but this varies throughout the day. This raises the question of whether the system can keep up with peak demand.

Edited by swansont

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The Toshiba plant is 200 kW, the Hyperion plant is 25MW.

Sounds good for remote industry such as mines where they will have fairly constant and predictable demand. A community would would have to be designed with its limitations in mind, i.e. reduce peak space heating and cooling, if it were to be off the grid,

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Perhaps off-peak power could be used to charge plug-in hybrids or the like to help even the load?

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How about a modified version of the plant to power a large ship?

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What are the similarities and differences in the two, just the enrichment level of the uranium?

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Just read Hyperions own write up.

http://www.hyperionpowergeneration.com/

Seems to me it must be neutron reflector technology. A small amount of fissionable fuel is inside the module, and surrounded by neutron reflectors designed to make it react. Presumably, something in the reflector design makes it passively fail safe. If it starts to flare, the reflectors fail.

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How about a modified version of the plant to power a large ship?

They're doing it backwards in Russia, a few places in the north they have old atomic icebreakers or subs tied up providing power for industry or homes.

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How about a modified version of the plant to power a large ship?

Like a nuclear powered aircraft carrier?

What are the similarities and differences in the two, just the enrichment level of the uranium?

Near as I can tell size is the main difference.

I would like to know how electricity is produced with no moving parts, is it DC? In a typical reactor the only moving parts are the control rods and they are not required in some designs

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typically there is the turbine which is a really really big moving part.

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They're doing it backwards in Russia, a few places in the north they have old atomic icebreakers or subs tied up providing power for industry or homes.

A fine example of Russian reversal.

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How about a modified version of the plant to power a large ship?

Been there, done that.

Just read Hyperions own write up.

http://www.hyperionpowergeneration.com/

Seems to me it must be neutron reflector technology. A small amount of fissionable fuel is inside the module, and surrounded by neutron reflectors designed to make it react. Presumably, something in the reflector design makes it passively fail safe. If it starts to flare, the reflectors fail.

Small reactors tend to have negative reactivity coefficients. If the water heats up, the fission rate slows because more neutrons leak out. Power naturally follows steam demand, because as the load goes up the temperature drops, which causes an increase in the fission rate. Do the flow with convection and there are no moving parts in the primary. But a turbine is definitely a moving part, though it's not part of the reactor, per se.

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To npts and swansont

Sorry, I was a bit ambiguous in my statement about ships. I meant, how about using it in a commercial ship, as opposed to a navy ship? If the system is that good, it means that a large container vessel could run nuclear, and avoid burning oil. International trade goes carbon emission free.

I agree that the 'no moving parts' would refer only to the reactor. The heat generated has to be turned into electricity, and the most likely way to do this is via a steam turbine, which definitely does have moving parts.

The key to this reactor has to be the passive failsafe for the reactor. Anyone got any better ideas on how this will operate? I could imagine a system with neutron reflectors held against the reactor against powerful springs with electricity. Anything that goes wrong would interfere with electricity production, causing the springs to push the reflectors out, stopping the fusion????

Is it possible that we will see even smaller reactors in the future? Imagine being able to power every commercial vessel with a similar nuclear system, including small coastal freighters. Perhaps even trains might go nuclear??? If the inbuilt safety systems are good enough, this would be a major step forward.

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http://en.wikipedia.org/wiki/List_of_civilian_nuclear_ships

IIRC there was a nuclear powered "Hindenburg" proposed in the 60s... yeah, that one didn't get too far.

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The key to this reactor has to be the passive failsafe for the reactor. Anyone got any better ideas on how this will operate? I could imagine a system with neutron reflectors held against the reactor against powerful springs with electricity. Anything that goes wrong would interfere with electricity production, causing the springs to push the reflectors out, stopping the fusion????

Fission, not fusion.

I mentioned this above — here's more detail. You have a water-moderated reactor, and as water heats up it expands, reducing the ability to thermalize the neutrons before they leak out of the core. U-235 has a much smaller cross-section for higher-energy neutrons ("fast" neutrons), so the only appreciable fission is from neutrons that have become thermal.

Fission heats up water, and the hot water is how one delivers energy to the turbine, subsequently cooling the water down. If the electrical demand goes up, more energy is taken from the water and it cools further, making it more dense. This, in turn, means neutrons are thermalized more efficiently, which boosts the fission rate. Hence, the power of the reactor naturally responds to the electrical demand. Should the water boil away, or there was a leak, you would not be thermalizing neutrons, and the fission rate would drop to a very low level. This design supposedly does not have a problem with fission products escaping if the temperature gets pretty high, so there would not be core damage in such an instance.

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Whoops. I meant fission, but the keyboard obviously had a mind of its own!

Swansont

As I understand it, you are suggesting that cold water thermalises or slows neutrons more effectively than hot water. Since slower, or lower energy neutrons are needed to stimulate fission, their absense leads to the reactor slowing. Any incipient disaster with fission increasing, will heat the water, leading to fewer low energy neutrons, and thus a reduction in fission.

Have I got it right?

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Whoops. I meant fission, but the keyboard obviously had a mind of its own!

Swansont

As I understand it, you are suggesting that cold water thermalises or slows neutrons more effectively than hot water. Since slower, or lower energy neutrons are needed to stimulate fission, their absense leads to the reactor slowing. Any incipient disaster with fission increasing, will heat the water, leading to fewer low energy neutrons, and thus a reduction in fission.

Have I got it right?

Yep. That's the basic idea behind it.

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Whoops. I meant fission, but the keyboard obviously had a mind of its own!

Swansont

As I understand it, you are suggesting that cold water thermalises or slows neutrons more effectively than hot water. Since slower, or lower energy neutrons are needed to stimulate fission, their absense leads to the reactor slowing. Any incipient disaster with fission increasing, will heat the water, leading to fewer low energy neutrons, and thus a reduction in fission.

Have I got it right?

yep. The U.S. Navy has had a training reactor that uses exactly that method of controlling the fission reaction for many years now.

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I live out here in Washington near Hood Canal where the Bangor submarine base is located. All of those subs are powered by small nuclear reactors, and I've often wondered how long it will take that localized-power concept to catch on at the consumer level. Mexican or otherwise, it's only a matter of time before we reinvent our energy infrastructure. I think this idea has legs.

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Out of curiosity, when they describe the dimensions, I assume they only mean the heating component? You still need the water to cycle, and the turbines - how much more cost and volume/weight would these account for? Does the water require much space to cool before recycling?

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I live out here in Washington near Hood Canal where the Bangor submarine base is located. All of those subs are powered by small nuclear reactors, and I've often wondered how long it will take that localized-power concept to catch on at the consumer level. Mexican or otherwise, it's only a matter of time before we reinvent our energy infrastructure. I think this idea has legs.

The problems with doing this that I see are as follows;

1) Expense, reactors are very expensive to design, license, build, maintain, and decommision. The more reactors that are built, the more expensive they will become (at least this has held true in the past and I see no reason for it to change).

2) Political opposition, proposing a new nuclear power plant is guaranteed to bring out more NIMBY's than almost anything else someone could come up with.

3) Security and proliferation, the more use you have of a technology, the more people who have access and knowledge the easier it is for some malcontent to concieve of a way to use it for harm. Also materials are more difficult to secure if kept in different places as would be required for even moderately widespread use.

4) Waste, every nuclear power plant will produce some of the most toxic and radioactive waste ever produced by humans. Furthermore, that waste will be around for many times longer than human civilization has even been in existence and nobody has come up with a politically acceptable solution for their disposal.

5) Better alternatives, unlike the case of reactors, alternatives like wind, solar, tides, or geothermal are likely to decrease in cost as more are built. These methods also require less regulation and have more fixed costs for their power sources (basically free).

Merged post follows:

Consecutive posts merged
Out of curiosity, when they describe the dimensions, I assume they only mean the heating component? You still need the water to cycle, and the turbines - how much more cost and volume/weight would these account for? Does the water require much space to cool before recycling?

IIRC the reactor and power generation systems on a submarine use about 1/3 of the total space on board. The rest is completely dependent on the output of the heat source, efficiency of use of the energy, temperature gradient to your heat sink, etc.

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The problems with doing this that I see are as follows;

1) Expense, reactors are very expensive to design, license, build, maintain, and decommision. The more reactors that are built, the more expensive they will become (at least this has held true in the past and I see no reason for it to change).

One of the advantages this system is trying to exploit is that there is only one design.

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One of the advantages this system is trying to exploit is that there is only one design.

I realize that, but will believe costs decrease at all when I see it. IMO by the time safety, security, and disposal costs (which never seem to be adequately addressed in design and operation estimates) the idea of cheap energy from nuclear power will vanish. The only positive thing I can see about doing it, that you can't accomplish any other way, is having fuller employment without retraining the reactor technicians getting out of the Navy.

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Out of curiosity, when they describe the dimensions, I assume they only mean the heating component? You still need the water to cycle, and the turbines - how much more cost and volume/weight would these account for? Does the water require much space to cool before recycling?

You'll need a cooling tower, or a place to dump waste heat.

How big?

Consider the (approx.) 1 Gigawatt coal powered plants, and their gigantic cooling towers. Then realize that 25 MW is about 1/40th of that... and scale it down 40 times (in cooling power, not height). Sorry if I'm not doing all the math (and heat and mass balances).

And in case you wonder why you need a cooling tower in the first place:

The steam cycle is a Carnot cycle.

$\eta=\frac{W}{Q_H}=1-\frac{T_C}{T_H}$

where the Tc and Th are the cold and hot temperatures in Kelvin, and the $\eta$ is the efficiency.

So, you need not only a source of heat, but also a source of cold (water). That's not really an issue on a submarine in the middle of the ocean... but on land, it becomes much more of an issue.

Edited by CaptainPanic
damned edit button - I never completely finish. Always adding a comment to my own posts after I clicked "post".

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I don't know, can't you just let it radiate out into the open while on land?

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