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  1. Murkie

    Murkie Active Member

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    Nuclear Reactors for Laymen

    Discussion in 'Research' started by Murkie, Nov 1, 2020.

    I'd love to get some opinions on this paragraph. I'm hoping that I've got just enough of a grasp from googling nuclear reactors to understand the basics of the process and how I can apply it to a future setting in a stranded spaceship.

    His mind ran over the problem once more - there had been a loss of pressure in the reaction chamber causing super-heated water to burst through a pipe and immediately expand to steam. The fault could have ended their lives right there. The water in the reactor should have exploded into the ship with the force of a nuclear detonation, but somehow the computer had intervened in the hundredths of seconds it took to analyse the situation and vented the expanding liquid to space, simultaneously scramming the reactor and ending the process of nuclear fission. The ship had been saved but the two men inside were left with the problem of how they were going to get their crippled vessel home without any means of propulsion.​
     
  2. SethLoki

    SethLoki Retired Autodidact Contributor

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    Erm, as I understand it, there’s no nuclear detonation with a runaway reactor core; they just overheat and melt as their decay goes unchecked. The explosions you mention are most likely from the pressures of the cooling water being superheated. The radioactive material’s then swept up and dispersed by the explosion.
     
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  3. Thundair

    Thundair Contributor Contributor

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    To power a ship would not be a water-cooled reactor. There are two basic uses for reactors on a spacecraft the first and most popular is to provide electric power, next would be the not so popular propulsion. Check out The Orion nuclear pulse drive. I worked at WPPS reactor in the Tri-cites of Washington, which was huge. Almost everything you see of the structure above ground is mirrored below. I also worked on the Pueblo reactor when I was at Gulf Atomics, which gives me a little insight to the complex nature of the beast. To begin with, the water is only one stage of the coolant. It starts out with sodium and ends up with air through a bunch of dedicated heat exchangers. All of that process would go away in space because removing heat is easier with a minus 455 degree F temperature just outside the door. Beside water is around eight pounds a gallon and is really too heavy to use for a coolant.
     
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  4. JuliaBrune

    JuliaBrune Member

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    If you're going for scientific accuracy, you have a fundamental problem here and it's Newton's First Law. A ship without propulsion will not be stranded, it will keep going at current speed in a straight line* forever. Basically optimum space travel is constant acceleration up to the mid point then you spend the rest of the journey slowing back down.

    If your astronauts are around the middle of their journey their main issue isn't that they're stranded it's that unless they slow down immediately they're going to either miss their target or crash into it. Once they're off course their target is moving along and it's not yet at the point that they calculated as their destination.

    Also note that if water is dumped off into space it would change their trajectory in the opposite direction.

    So basically they're very dead. You're gonna have to put in a lot of scifi brilliance to save them !


    *according to some German dude that straight line might appear curved if the underlying space is deformed by big stuff and I don't think he would joke about matters of such gravity
     
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  5. Thundair

    Thundair Contributor Contributor

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    One last point about removing heat without the multistage heat exchangers is to use something similar to a gas refrigerator. When I was at Rocketdyne on the ISS, we had a power supply that generated tons of heat. What we attached to it were tubes that were connected to the heat source and routed out of the structure into space. The coolant would phase to travel from the cold to the hot side.
    https://www.carthage.edu/space-sciences/microgravity/coolant/
     
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  6. DriedPen

    DriedPen Member

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    If you do go with a typical nuclear power plant as propulsion (and I am not sure that you should), you would also have the issue of the safety valve, and on a nuclear reactor, MANY of them.

    You are right, raising the pressure of a boiler increases the boiling point of water significantly. However, if the pressure is lost, the boing point goes back to 212 degrees (f) and since the boiler was at 900 PSI before, it instantly flashes to steam at 212 degrees at 0 pressure.

    Steam expands at 700 times to 1, and has an amazing amount of power.

    In the industry, we jokingly refer to that as the Squish-Bang Factor!

    For this reason, boilers all have a safety valve on them, sized to vent the boiler so it cannot explode if it loses pressure, or feed water (coolant water). Depending on the size, it may have several of them.

    I have worked with boilers all my life, and the power of steam absolutely amazes me. However, it probably is not going into space.
     
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  7. Murkie

    Murkie Active Member

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    Looks like I need to come up with either a different fault or a different propulsion system! Thanks for the insight, everyone!
     
  8. newjerseyrunner

    newjerseyrunner Contributor Contributor

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    Are you implying that the reactor was in a state of near-meltdown? If that is the case, then you are far beyond the energy where steam physics are dominate. A pipe breaking because the pressure got too high is a bad thing, but not a catastrophic failure of the reactor. It certainly was never in danger of going full meltdown. But reactors are built to be extremely durable, and handle pressures approaching meltdown levels, so I think you are trying to describe a meltdown. If that's the case, perhaps a quick overview of what a meltdown actually is, and how it'd happen.

    First, a nuclear reactor can never stop fission, the fuel itself is constantly undergoing fission, that's what makes radioactive stuff radioactive. During the fission, a nucleus splits almost in half, with two neutrons left over, which fly off at high speed, and two gamma rays. These gamma rays then interact with everything around them and covert into heat, which then heats the water, which spins the turbine. This splitting process happens randomly, but it can be initiated by hitting the nucleus with a neutron at high speed. This is why it can chain react, splitting atoms send off neutrons which then hit other atoms and split them, geometrically expanding. Normally, nucleus are too far apart for this to go on uncontrolled, which is where the reactor comes in. It can slow the process by absorbing those neutrons in heavy water, or speed it up by wrapping the rods in a neutron reflector, sending more back into it, triggering more splits.

    A meltdown happens when the rods overheat. This sends the temperature skyrocketing which tends to then weld the reflectors in place, making the process then completely uncontrollable. Everything will go so hot that it'll just melt through anything. This process happens so fast that by the time the tank has ruptured, it's well past the temperature where steam is even a thing. The water molecules themselves break, creating hydrogen and oxygen. This then creates a hydrogen explosion, which causes the majority of the damage in every meltdown.

    A nuclear explosion is never an option, that can only happen when the uranium is past a certain mass and shape. These rods are no where near that mass and can only get as hot as they do with the reflecting material in place. The temperatures get so high that nothing can withstand them, and the shielding that created the runaway effect in the first place will burn off very quickly.


    So going back to your description, if all you had was a small puff of steam, you were no where near the point at which you'd have to jettison the reactor. Also wait, what does the reactor have to do with propulsion? I mean, they'd have no power to light the engine, but I'd think the loss of the primary energy source creates way more urgent problems than propulsion. If you have backup power for life support, I'm sure you could find a way to rig a way to connect the engines to the battery.
     

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