Half a croissant, on a plate, with a sign in front of it saying '50c'
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Gas Centrifuge Nuclear Reactor

A Uranium-fueled power/breeder reactor
  [vote for,

(Note: for convenience' sake, "top", "bottom", "sides", "gravity" etc. are all referred to in the pseudogravitional sense imparted by a centrifuge)

The Basics:
A ceramic drum containing nuclear reaction elements (fuel, coolant, byproducts, etc) is spun around. Coolant is introduced at the bottom, quickly rising to the top where it exits out one end to run a turbine (probably indirectly, given the ridiculously high temperatures involved), in a closed cycle, which is where the power comes from.

The working temperature varies from bottom to top, obviously, but operation counts on the Uranium fuel, coolant and byproducts being gaseous and the Thorium shielding (we'll get to that in a minute) liquid. In this light, we can see that due to the heavy gravity the major components will form distinct layers: liquid Thorium on the bottom, then gaseous Uranium, then gaseous Xenon (used coolant). The nuclear layer is mostly U235 and U238, not differentiated much since their atomic weights are pretty close, but still the U235 (the good stuff) will be more concentrated in the upper portion of the nuclear layer.

A chain reaction starts when the U235 is compressed enough that a number of naturally emitted neutrons start banging into other U235 atoms, causing more and more neutrons to be emitted. The reaction is kept under control by introducing coolant, which dilutes the Uranium as it passes through. Byproducts are mostly Iodine and Xenon (which is also the coolant), which rise quickly into their own layers, getting out of the way. Compression is achieved by gravity as well as overall system pressure.

(A number of side-reactions also occur: escaping fast neutrons impinge on the U238, occasionally creating Plutonium. Some will make it as far as the Thorium, and create more Plutonium. Some of the created Plutonium will get turned into U236. These will mostly stay out of the way of the main reaction).

The bottom-most layer is liquid Thorium: it's there not only to soak up neutrons that are trying to make a run for it, but to provide some thermal shielding between the nuclear reaction and the container. Coolant bubbles up through the Thorium then permeates through the gaseous nuclear layer. Coolant is also pumped directly in through the sides which not only provides direct cooling, but locally keeps the nuclear layer subcritical.

There are many simple (in context) ways to stop the reaction: reducing the pressure by pumping the coolant out quicker, or simply stop the spinning which mixes up all the gases, quenching the reaction immediately. In that respect, if the temperature is kept up, the reactor can be restarted simply by spinning it back up.

To refuel just toss in more uranium, which will evaporate and join the nuclear layer. For "garbage" collection, just drain the appropriate layer.

Physical construction could have either just the contents of the static drum spinning (by magnetics or having the coolant do all the work of spinning it up by being pumped in tangentially), or the entire mess could spin (it is, after all, self-contained). with the only stationary bit being the stator of the generator.

FlyingToaster, Mar 20 2015

Plasma Power Plants Plasma_20Power_20Plants
The present Idea has some similarities to this old Idea. [Vernon, Mar 20 2015]


       Given that compressing solid U235 sufficiently to produce a reaction in a less than critical mass requires some extremely high forces, do you have any idea of the numbers required to make gaseous U235 fiss? I suspect you're getting above it's critical point, and that's going to cause problems.
MechE, Mar 20 2015

       I used to work for an outfit that built centrifuges used for making pharmaceuticals, skin care products, chemical goodies and the like. We sold new and refurbished old.   

       This nuke stuff is not my specialty, but based on my experience I see a bang coming. Scale this thing up slowly and far away from the big city, okay?
normzone, Mar 20 2015

       What kind of bearings do you have to use for these things?
RayfordSteele, Mar 20 2015

       //A chain reaction starts when the U235 is compressed enough that a number of naturally emitted neutrons start banging into other U235 atoms// As [MechE] pointed out, that is a fearsomely fearsome amount of compression, I think.
MaxwellBuchanan, Mar 20 2015

       [RayfordSteele], big ones, but nothing exotic, just heavy duty industrial. The equipment cladding materials were varying grades of stainless, titanium, or hastelloy, sometimes coated with various polymers. The machines ranged in size from a desktop model for prototyping to units the size of a Ford truck.   

       One customer kept a bunch of machines going, rotating them in and out for refurbishing, while the patent on Nutrasweet was approaching sundown - they were thrashed. They'd run those machines 7/24/365, and the wear patterns were interesting.   

       Ah - a flashback. We took a unit apart, and these were supposed to be all decontaminated when they were sent to us. We pulled out half a cubic yard of this white substance and sent them a nastygram asking how did they want us to deal with their industrial waste.   

       They responded eagerly. That was valuable product, please ship soonest. Probably cocaine or something.
normzone, Mar 20 2015

       Verbing the word fiss was a stroke of genius.
Voice, Mar 20 2015

       [Mech] The idea's nift lies in making the materials naturally separate themselves, which could be done with fluids, and in having the coolant(/reaction-mitigant) flow directly through the reaction, which requires both to be gases or supercritical fluids, so there's no surface tension: it permeates rather than bubbles.   

       Oddly there's very little information on the 'net concerning supercritical Uranium.   

       There are, however, various Uranium-based molecules, which critical point isn't totally unreasonable: UF6 for instance @ 232C and 45.5bar.
FlyingToaster, Mar 20 2015

       Right, how's this then...   

       Supercritical-Fluid Nuclear Reactor (Gas Centrifuge Nuclear Reactor v2.0)   

       The idea is to use a fuel and coolant, both operating in a supercritical-fluidic state, to enable cooling and neutron moderation on-the-fly in a simple process.   

       To a reaction chamber containing a nuclear fuel, coolant is introduced at the bottom. Being much lighter than the fuel, it permeates its way upwards through the reaction, and is taken off from the top, exported to a generator, cooled and recycled..   

       Since the elements operate in a supercritical-fluid state, there's no surface tension and permeation is smooth. Due to gross disparities in density, distinct layers will form: nuclear fuel on the bottom, byproducts in the middle, lightweight coolant on top.   

       A certain amount of quenching agent is included in the coolant: the percentage can be varied to speed up or slow down the reaction. Likewise a moderator (a substance that slows down the speed of the neutrons).   

       More fuel can be added, and waste products removed, at their respective layers.   

       If more layer differentiation is required in the design (Brownian motion will have stuff wandering around) the whole thing can be spun up to operate as a centrifuge, inducing a heavy artificial gravity.
FlyingToaster, Mar 20 2015

       And when appropriate you'll drop in the plow and pull out a layer or level?   

       We had a sales guy attempt to check in with a customer, and his coworkers told him that they hadn't seen the guy for a while. Our sales guy persisted, and then they said they couldn't find him.   

       Our sales guy persisted, and they relented. They drove him out onto the vast acreage of the plant, dotted with silos every so often where our machines were used to process exciting materials.   

       The trip ended at a fresh hole in the ground, with some shattered machinery scattered around it. His customer's coworkers said " This is the place where he was working last. We haven't seen him for a while, and we can't find him."
normzone, Mar 21 2015

       //And when appropriate you'll drop in the plow and pull out a layer or level? // I'm not sure I'm parsing that correctly...   

       If you're referring to the high pressure/temperature required for supercritcal-fluidity:
There already exist steam reactors, and a supercritical water reactor is in the works (there's no direct interaction between the water and the uranium in either). High pressure/temperature reactors exist, including ones where the uranium is dissolved in molten salt.

       This idea (ver 2 anyways) is in the ballpark for that.   

       I'd like to be able to mention an existing, known fuel composition, however most reactors' design require a solid fuel that remains solid at high temperature, for instance Uranium Dioxide, which MP is greater than that of Uranium.   

       The requirements are a fuel composition that has a low enough critical point to be able to exist as a supercritical fluid without harming the container. A coolant, moderator, and quench, supercritical within the same pressure and temperature range, and with compatible chemistry, are also necessary.   

       Once those are defined, the actual reactor is much safer than any other design. Hot spots ? no such thing: the supercriticalfluidity means the mediae is uniform: there's no bubbles of coolant, no clumps of uncooled fuel. And, even if one does develop... so what ? There's no pellet shell to break open. The only mechanical situation would involve the wall of the chamber: as previously mentioned, increasing the flow of quench along the walls keeps the fuel there subcritical.   

       If you're referring to the centrifuge bit, that might not even be necessary, depending on how much incident interaction there might be between layers. It might be enough to for instance insert a buffer layer of some intermediate weight substance between the U and the I/Xe byproducts, seeing as how the latter act as a quench, to keep them from wandering into the reaction layer.
FlyingToaster, Mar 21 2015

       For visualization purposes, ver.2 (without centrifuging), would be a long vertical pressure cylinder. Inside the cylinder are layers of supercritical fluids: in order, starting with the greatest density:   

       - a very heavy liquid (I like Thorium but that might require too hot a temperature to be liquid). This is just thermal insulation separating the reaction layer from the bottom of the cylinder.   

       - the nuclear layer: a U compound. Since the coolant coming up the sides is a high quench mixture, the outer edge of the layer remains subcritical and will not be producing heat.   

       - a buffer layer to keep the byproducts out of the reactive layer (may not be necessary)   

       - Iodine (byproduct)   

       - Xenon (byproduct, quench)   

       - water (coolant, moderator)   

       - Helium (coolant)   

       (I realize nobody's actually read this far, but I enjoy typing)
FlyingToaster, Mar 21 2015

       Why Xenon ? Are you aware of "Xenon poisoning" in fission reactors ?
8th of 7, Mar 21 2015

       Yep. Doesn't apply to non-solid fuel reactors, since the Xe can and does escape.
FlyingToaster, Mar 21 2015

       So, you just let all the coolant escape from the reactor ? That's not new, it's been tried ... several times ...   


       [norm], that anecdote has the eerie quality of a disturbingly familiar event retold in authentic detail from a different point of view ...   

8th of 7, Mar 21 2015

       [8th] Bear in mind the idea, rather than providing a complete blueprint for a nuclear reactor, simply proposes using two natural properties of matter to obviate mechanical complexity of design and engineering.   

       No mediator control rods or mechanical heat-exchangers, no multi-substance fuel pellets or rods: density differential between the 4 components (fuel, byproducts, mediator, coolant), all in a gaseous or thermal-supercritical state, means cooling and mediation is done seamlessly and far more efficiently, and byproducts separate themselves out, all due to gravity and lack-of-surface-tension, in one minimally-sized, empty save for the working substances, tank.
FlyingToaster, Mar 24 2015


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