nuclear power plant

[whitepointer23]

frostys thread on economy got me thinking, if it was possible to build a small nuclear power plant for boats that produced around 300 horsepower , what size would it be . just wondering if anyone knows. imagine how economical it would be.

[daiquiri]

There is a device called RHU (Radioisotope Heater Unit) which was used for heating the instruments on board some spacecraft: http://saturn.jpl.nasa.gov/spacecraft/safety/rhu.pdf
It is of a very simple construction, basically a cylindrical shell containing a small pellet of a radioactive fuel, which produces heat while decaying. The heat is then transferred to the surrounding ambient. No moving parts. It is imho probably a simplest and cheapest thing you can come up with for producing heat with nuclear fuel.

You could, for example, immerse an RHU in a waterflow, and produce steam. In order for this to work you need to add, at least:
- a pressure-resistant sleeve with a heat exchanger
- a water pump with backups
- a turbine
- reduction gears
- high pressure steel piping
- power regulation devices and electronics
- safety devices
- one or more external radiation and pressure-containment vessels
- a thing or two which you only realize should be there when it's too late.

The RHU alone, like the one shown in the above NASA pdf file, weighs 40 grams (or 0.04 kg) and gives around 1 W of thermal output. By scaling this number you get a weight of 30 kg/HP.
Or 9 tonnes for 300 HP, plus all the said additional equipment.

[whitepointer23]

what about a reactor that powers warships, can these be scaled right down or is it not possible.

[daiquiri]

Perhaps someone else will know the answer - I don't.

[whitepointer23]

thanks for the interest daquiri, i know its a bit of an out there topic but there will have to be something to replace fossil fuels eventually. hydrogen looks promising , i can't see electrics pushing large planing hulls , biofuels are good but how could they ever grow enough crops to power everything, i don't think there is enough farm land available .

[daiquiri]

Originally Posted by whitepointer23

hydrogen looks promising

Not to me, at least in a near future (but I might be wrong). Hydrogen is just a mean for energy storage. It doesn't come for free but has to be produced in some way, with all the inefficiencies of the process.

Actually, fossil fuels are pretty much the same thing, though we consider them like they've always been under the soil, waiting for someone to extract and burn it. In reality, fossil fuels have been slowly produced by the nature during a time period of milions of years, with the use of sun energy, gravitational pressure and microbic action.

We have managed to burn it all in little more than 200 years.

So, more than anything else we need to slow down and start turning off unecessary things, imho.

[whitepointer23]

We have managed to burn it all in little more than 200 years.

So, more than anything else we need to slow down and start turning off unecessary things, imho.

interesting statements, i have been trying to talk my family into moving out of town onto a bush block where we can set ourselves up as self sufficent with power and water. i want to build an energy efficent house from recycled materials . with the quality of solar and wind generators plus led lights and invertor technology we should be able to do it for a fairly low cost.

[cthippo]

Originally Posted by whitepointer23

what about a reactor that powers warships, can these be scaled right down or is it not possible.

Nuclear reactors scale, but only to a point. The smallest in-service reactor I'm aware of is the Navy's NR-1 research submarine which is 150' long and over half of that length is the reactor compartment. Even this minimal craft required three officers and eight enlisted crew, most of them to run the reactor.



The reason for this is not so much the size of the reactor itself, but all the ancillary components it takes to convert the heat produced by the reaction into usable force to move the boat. A basic reactor design includes not only the reactor itself, but also a heat exchanger, a turbine, and various control and safety systems. The image below shows the basic components of a nuclear power plant, but the concept holds for propulsion plants. just replace the electrical generator with a gearbox and it works the same.



There was some work done in the 1950's on reactors for aircraft propulsion, but in order to get the reactor into the largest combat aircraft in the world at the time (The B-36 bomber) they had to cut down on shielding to the point where the crew had to enter the aircraft from a special shielded tank because it wasn't safe to stand outside of it. As you can see, even with the compromises made to get it in an aircraft, this was not a small plant. They did flight test the reactor at Idaho, but by the time they got it sort of working the era of the manned nuclear bomber was largely passed.



The systems Daquiri refers to are Nuclear Thermal Generators or NTGs. They are not true reactors at all since an ongoing reaction does not take place within them, but rather they are powered by natural decay heat of the material inside. This heat is converted to electricity by thermocouples which are notoriously inefficient. Due to the fact that they aren't reacting they don't produce as much radiation as a real reactor, thereby greatly reducing the mount of shielding required, however they also cannot produce much power.

[ancient kayaker]

Given the events at Fukushima in Japan I am surprised to find anyone interested in nuclear power. It comes in 2 flavors, fission and fusion. Current fission technology is inherently complex as cthippo explained.

Fusion technology should be cleaner since it can be designed so it does not generate radioactive by-products with long half-lives, so such a reactor should theoretically be safe enough to recycle most of its components after a few years shut-down. Fusion is critically dependent on confinement which cannot be provided by any material since it involves a temperatures similar to those at the sun's core, and a magnetic field is the only practical way. Currently magnetic confinement is receiving most of the research. Like fission, it is inherently complex and large - wall-to-wall superconducting magnets etc.

As far as anyone I have met knows, cold fusion does not and will not work. It seems to be a bit like global heating, triggering furious debates but no usable theory. Lots of energy in the hot air but as yet not usable.

Inertial confinement fusion may be down-scalable, though not within our lifetimes. Inertial confinement fusion is referred to by cognizant types as Infernal Con-Fusion which gives you some idea of its current state of development. It consists of triggering a series of microscopic thermo-nuclear bombs. The fuel comes pre-packaged like a wood-pellet stove and is brought to ignition temperature by a laser.

In theory it can be made so small that the inverse square law will provide sufficient attenuation of levels of radioactivity that the weight of shielding can be manageable but the efficiency of current methods of energy conversion at that size would be ludicrously poor. If the energy can be tapped at the nuclear level that problem maybe can be fixed, but the probability of that happening is not much better than making a success of cold fusion. In practice it hasn’t been made to work at all - AFAIK I don’t think an experimental test setup even exists but I am long out of touch. Don’t hold your breath . . .

[EuroCanal]

A Thorium Reactor should do what you want. You will need a solution of liquid beryllium fluoride (1.4kg) and lithium fluoride (2.5kg). Heat it to around 500C to melt it. Dissolve 800g of thorium tetrafluoride. All are easy and cheap to buy.

Next you need a neutron source to start the reaction - uranium-233 fluoride is best, but difficult to obtain. Alternatively build a neutron gun using either cobalt-60 (from medical instruments) or americium-241 (from smoke detectors) with a sheet of aluminium foil. The neutron source will convert thorium to U-233, and once you have enough, you can remove the source.

You will also need some graphite in or near the liquid to slow down neutrons.

The liquid will heat up to, say, 700 - 800C, and produce heat until all the thorium is used (it should last a while), after which you just add more thorium tetrafluoride.

One disadvantage of this reactor is the production of isotopes such as U-232, which (along with its decay products) is very radioactive. You either need to remove intermediate reaction products to prevent this happening (which is tricky), or encase the reactor in several layers of thick metal and concrete shielding.

It all works at normal pressure. You will need to vent some radioactive gases, and fluorine, I guess.

Decommissioning the reactor will either be very expensive or very lucrative, depending which country you are in at the time.

[CatBuilder]

Someone needs to make a better reactor that works like a fuel cell burns hydrogen.

[ancient kayaker]

Originally Posted by CatBuilder

Someone needs to make a better reactor that works like a fuel cell burns hydrogen.

Well, that would probably be fusion power, but we are at about the same stage as the Romans were for developing a hydrogen fuel cell.

[CatBuilder]

Originally Posted by ancient kayaker

Well, that would probably be fusion power, but we are at about the same stage as the Romans were for developing a hydrogen fuel cell.

I guess my short post wasn't easily interpreted unless you really understand fuel cells and/or have a background in physics and could make the jump.

Expanding on it a little, I mean someone needs to do with nuclear power, what the fuel cell did with combustion. Someone needs to create a solid state reactor that can be infinitely controllable at an atomic/molecular level.

Fusion is just the reverse of fission - but both produce heat to boil water and spin turbines. Producing heat is a very tired way to do things. Solid state electricity production would be much more interesting and efficient.

Hmmm... may look into this idea myself when I'm done building the boat.

whitepointer23: Good thought. I've been dreaming of this kind of thing for years, especially while wintering over up north with a foot of snow on the deck. Ahh.... to not have to carry any more fire wood and also have main propulsion and electrical generation - for many years without refueling.

[cthippo]

If you could design a thermocouple that had anything like the efficiency of a turbine and couple that with something like a pebble bed reactor design you could be on to something. Still a lot of waste heat to dump though.

[whitepointer23]

it is good to read all the replys, i learn a lot from this site. so the amount of equipment attached to the reactor is the problem for downsizing. i did watch a documentary on the ronald reagan and the reactor was a fraction the size of the older carriers reactors, about the size of a sea container i think.