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Swiss Roll Nuclear Reactor

Yummy
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The fissile materials Uranium and Plutonium have very different thermal expansion coefficients.

It's possible to build a thermometer out of a bimetallic strip by rolling it up into a spring shape. As the temperature changes, the coil unwinds.

Therefore, we should take a large sheet of plutonium, and bond it to a large sheet of uranium, and roll it up into a deliciously radioactive swiss roll.

The swiss roll will then be dunked in a nice cup of liquid moderator. If the temperature changes, the geometry of the roll changes in step, and the ratio of moderator to fuel will adjust, changing the speed of the reaction.

Some careful engineering will be required to get the numbers right, but it should be possible to arrange things so that we end up with a completely automatic, constant-temperature reactor.

mitxela, Aug 29 2019

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       Nifty! [+] Perhaps a plurality of 1 cm or less bimetallic radiation swiss rolls could be piled into a heap, or assembled in tubes to make a more predictable reactor (many nuggets has a more predictable distributional behavior than one roll).   

       Another possibility is a cheap Uranium or even Thorium reactor where the bimetal is a cheap nonradioactive element; it still furls and unfurls but without the less affordable isotope.   

       Another possibility is a nuclear reactor based on 10-W40 motor oil. This hydrocarbon rolls (scrunches) into little low viscosity balls when it gets warm, causing the uranium to concentrate next to each other when it gets warm. If it did not concentrate at 100 degrees C then when spilled it would be harmless, and only people that concentrated it and used it above its concentration temperature would make fission.   

       There are lots of other options as well like macroscopic (.1mm blobs) of polymer as well.   

       Nifty!
beanangel, Aug 30 2019
  

       I do have to compliment you on the logic...   

       "it is possible to safely build a simple measuring tool, so that technique should be used to combine deadly materials. All that is required is engineering"   

       Or words to that effect. Highly appropriate for the Halfbakery. A real expert will be along shortly.
normzone, Aug 30 2019
  

       Why do the Swiss get to have all the fun?   

       So, to recap ...   

       You're going to take large sheets of highly radioactive heavy metals, one of which is pyrophoric and the other extremely toxic,place them in close contact, and roll them up unto a cylinder so as to significantly reduce the surface area available for cooling.   

       Good game, good game. Please do let us know when and where you're going to try this innovative approach to self-immolation as we want to watch - from a safe distance of course; probably low Earth orbit ...
8th of 7, Aug 30 2019
  

       I think it's people like [mitxela] that are the reason the US Patent Office no longer requires working samples of inventions.
lurch, Aug 30 2019
  

       "You're back soon from the evaluation visit."   

       "Yeah, went to the address, but the police had cordoned off the area and there were these guys in plastic suits driving this cute little tracked robot around the edge of this smoking crater ..."
8th of 7, Aug 30 2019
  

       They were NOT plastic suits. They were positive-pressure actively ventilated biohazard suits. And I'd like to point out that they declared the area completely free of several other viruses. Sometimes these things get entirely out of proportion.
MaxwellBuchanan, Aug 30 2019
  

       [8th of 7]; it's not rolled up tightly, it's loose so there's room for coolant/moderator. Slightly less surface area due to the bonded faces, but it could be done as a series of narrow strips instead of 1 large "sheet" to increase the relative external area. I'm not sure about "completely automatic" as the coolant/moderator will need to be circulated (maybe thermo-siphon?).
neutrinos_shadow, Sep 01 2019
  

       The problem with thermisyphon for anything other than emergency cooling is that it intrinsically limits the power you can extract from the core.   

       For safety reasons, you need to construct your core with a negative void coefficient.   

       You can raise the boiling point of your coolant by pressurization but if the core gets too hot and starts to boil, the void coefficient will decrease reactivity.   

       So your thermosyphon must remove the energy as fast ad it's produced to maintain a steady output.   

       But the volume of the core then becomes important. To get enough coolant through the core then needs to be physically large. For lots of reasons, particularly if the core is pressurized, small is better.   

       The RBMK design tried to engineer round this by putting the fuel in pressurized pipes surrounded by coolant with the moderator outside. This unfortunately gave a positive void coefficient and the resut was inconveniently loud and expensive ...   

       Pumped cooling - with thermosyphon as the emergency backup - and a modestly sized core with a negative coefficient is definitely the way to go for the best compromise (and there is always a compromise) between safety, efficiency, and cost.
8th of 7, Sep 01 2019
  

       Hence the question mark. I don't know enough about such matters to ponder the details towards a logical conclusion (or even an illogical conclusion). I hoped some who Knows Stuff (such as your good self) would do that part of the thinking for me.
neutrinos_shadow, Sep 02 2019
  

       No problem; in fact, if you like, we can do all your thinking for you. Just sign here.   

       We can remember it for you wholesale, too (for s small extra charge).
8th of 7, Sep 02 2019
  

       Hmm... tempting. My recall isn't total.
I'm just a bit concerned about the lag; what with us being on opposite sides of the planet. Perhaps shift your Cube into a polar orbit?
neutrinos_shadow, Sep 02 2019
  

       The Collective links together by subspace radio, so communication - even over interstellar distances - is, for all practical purposes - instantaneous.   

       Join us ... don't be afraid. You'll wonder why you rver hesitated ...
8th of 7, Sep 02 2019
  

       // For safety reasons, you need to construct your core with a negative void coefficient. //   

       That's exactly the kind of straight-forward, sensible thinking we've come to never expect from you, [8th].   

       The moderator will also function as the coolant, but if we stop pumping it through then the roll will adjust to become subcritical.   

       The Bimetallic Swiss Roll's inherent stability means that the reactor could be built with a positive void coefficient and still be considered "safe"!
mitxela, Sep 02 2019
  

       OK, we're convinced. We'll come and see your proof-of-concept demonstrator when it's been running smoothly for a few months.   

       Until then, low orbit is fine. We'll just watch from here.
8th of 7, Sep 02 2019
  
      
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