Fusion power (fission welcome too)

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Rob Lister
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Re: Fusion power (fission welcome too)

Post by Rob Lister »

Both the SSR and the LFTR fuels develop the same decay products as a solid fuel reactor, including the poisons Xenon and Krypton. The difference is that in SSR's and LFTR's the reprocessing must be done on-the-fly, constantly. Not so with solid fuels. And because all the decay products are intermixed, with a huge percentage of U233 not much found in solid fuel, the reprocessing procedure is significantly more hazardous and complex. With spent solid fuel they can't take it out and wait a few years for it to become relatively cool. So, as much as I like LFTR designs, plumbing is a real issue.

So the really great thing about LFTR's is that they can pretty much burn all the decay products, but with some restrictions. The Xenon and Krypton are fairly easy to remove, as they are gases. A 100mw reactor would create about a coke can's worth of each per year. But in that year, a cubic yard of Neptunium will also be created, which doesn't have to be removed since it burns too, just slower and cooler. That can become an issue. I'll pull some numbers out of my stinky ass.

Assume a cubic foot of U233 yields 1mw/h of energy. Assume a cubic foot of Neptunium yields 100kw/h. After a year, 10% of that original U233 is now Np236. Your reactor output is now roughly 90% of what it could be. It is more economical to remove it and store it than to waste valuable vessel space trying to burn it.

So maybe the best solution is to have a half-dozen or so LFTR's burning the waste from a hundred or so LWR's. I'll store the Neptunium in my garage.

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sparks
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Re: Fusion power (fission welcome too)

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Well, that's all very interesting. But LFTRs are supposed to breed U233 from Th232. In order to do that, one must pay very close attention to the neutron economy. If you leave fission products in the fuel, you'll quickly end up fucked in that regard. SSRs are not attempting to breed new fuel, and so the fuel components must be replaced on a regular basis. And since you did not start with the typical mix of 5% U235/95% U238, you're not going to be creating all the really nasty stuff like Pu 239. I'd consider that a good thing even in light of the fact that if you can arrange to leave the Pu in the reactor, it will burn up and very nicely too.

Facts remain: Solid fuel reactors aren't efficient burners and in a lack of cooling scenario, the fuel can melt. As mentioned before, this SSR thing sounds intriguing, but that fuel must at some point be reprocessed, not as tricky as solid fuel, granted, but it still must be done if you want a high burn up of the fuel or the fission products will fuck the reactivity. Best thing that can be said about SSRs is that they don't need active cooling in the event of a loss of power to move coolant through the core. Bravo on that if true (and why would they lie about a thing like that?) And the individual cores are small which makes them much more manageable to start with.
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Rob Lister
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Re: Fusion power (fission welcome too)

Post by Rob Lister »

sparks wrote:
Fri Dec 20, 2019 2:16 pm
And since you did not start with the typical mix of 5% U235/95% U238, you're not going to be creating all the really nasty stuff like Pu 239.
The Pu 239 created in a LWR is thoroughly mixed with Pu 240 and so is quite useless for nuclear booms. They cannot me chemically separated. To separate them, you'd have to use one of the same processes you use to separate U 235 from U 238, only much more of it because of the mass difference is less. And uranium is easy to get, Plutonium, not so much. The get pure PU 239 the reactor must be specifically designed to that end. U 233 however, can be made into a bomb directly, and LFTR's produce tons of that. And hazard-wise, U 233 makes Pu 239 look like a cute puppy.

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Re: Fusion power (fission welcome too)

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All reactors, including naval reactors, produce very dangerous and very radioactive fission products. A smaller safer reactor still has the same dangers all reactors pose.
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Re: Fusion power (fission welcome too)

Post by Abdul Alhazred »

OK catching up on this thread.

Question for sparks. You mentioned problems with solid fuel in fission reactors.

What kind of fission does not have solid fuel?
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Re: Fusion power (fission welcome too)

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answering for the awol sparks, molten salt reactors (MSR or SSR) and liquid fluoride thorium reactors (LFTR) to name two. Both are walk-away safe; they are both self-regulating in that they require neither control rods nor external cooling. Essentially, the fissile products are suspended in a high temperature liquid bath of sodium or fluoride.

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Re: Fusion power (fission welcome too)

Post by Abdul Alhazred »

But the fuel is in solid form until the reactor fires up?
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Re: Fusion power (fission welcome too)

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correct, but not kept in quantities or geometries that would facilitate decay products

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Re: Fusion power

Post by Witness »

Anaxagoras wrote:
Fri Dec 20, 2019 7:23 am
I changed the title to broaden the discussion.
Thank you, kind sir.

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Re: Fusion power (fission welcome too)

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"The Pu 239 created in a LWR is thoroughly mixed with Pu 240 and so is quite useless for nuclear booms. "

Question for you then: How did the reactors at Hanford and What the fuck was it down South make Pu239 for Fat Man? These were not LWRs to be sure, but where's the difference? (If memory serves, they were graphite moderated air cooled??)
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Re: Fusion power (fission welcome too)

Post by ceptimus »

They separated the isotopes by, I think, pumping compounds of uranium or plutonium through miles of tubing. The lighter isotopes diffuse slightly faster and you can slightly separate them into two streams with different concentrations. The streams go around and around getting a bit more concentrated with each pass, until you reach the desired purity. It's a bit like chromotography but on an industrial scale.

They also experimented with ion beam separation - like a mass spectrometer - but found it to be too slow.

The modern method is to use pipes spinning at high speed so that the heavier isotope centrifuges towards the outer wall of the pipe as it passes through. Again, the gas or liquid compounds are only slightly separated into two streams of different isotope concentrations with one pass through - but with whole banks of centrifuges and the correct plumbing, you can eventually reach the desired separation after enough passes.

But I don't think they used centrifuges at Hanford in the early days.

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Rob Lister
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Re: Fusion power (fission welcome too)

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sparks wrote:
Sat Dec 21, 2019 2:48 am
"The Pu 239 created in a LWR is thoroughly mixed with Pu 240 and so is quite useless for nuclear booms. "

Question for you then: How did the reactors at Hanford and What the fuck was it down South make Pu239 for Fat Man? These were not LWRs to be sure, but where's the difference? (If memory serves, they were graphite moderated air cooled??)
For an indepth look:
https://en.wikipedia.org/wiki/Plutonium-239#Production

Nutshell: The fuel rods contain U238. Lots of neutrons are flitting about. After X amount of time:
Image
There's your weapon's grade plutonium. But, that p239 is also exposed to the same neutron source so after another X amount of time you start to get:
Image
Which fucks your devious plans to make a goodly bomb.

So what they did was insert a sample of U238 into the core, wait X time, but not 2X time, shut the reactor down and remove the sample. Rinse, repeat. Your samples now have a tiny degree of P239 and almost no P240. I suppose the tricky part is finding X.

The reactor can be made in such a way as to not have to shut down the core to insert new samples. CANDU is one such, I think.

And I don't see any reason why inserting a sample in an LFTR wouldn't work just as well. Probably wrong though.

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Re: Fusion power (fission welcome too)

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Rob Lister wrote:
Fri Dec 20, 2019 5:58 pm
molten salt reactors (MSR or SSR) and liquid fluoride thorium reactors (LFTR) to name two. Both are walk-away safe; they are both self-regulating in that they require neither control rods nor external cooling.
That's hilarious. The molten salt reactor in Japan was anything but walk-away safe

https://en.wikipedia.org/wiki/Monju_Nuc ... k_and_fire

These are pipe dream physics projects, where the exact same problems exist for all reactors.

Nobody actually has any plan (or any solution) for when things go bad.
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Rob Lister
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Re: Fusion power (fission welcome too)

Post by Rob Lister »

robinson wrote:
Sat Dec 21, 2019 3:47 pm
Rob Lister wrote:
Fri Dec 20, 2019 5:58 pm
molten salt reactors (MSR or SSR) and liquid fluoride thorium reactors (LFTR) to name two. Both are walk-away safe; they are both self-regulating in that they require neither control rods nor external cooling.
That's hilarious. The molten salt reactor in Japan was anything but walk-away safe

https://en.wikipedia.org/wiki/Monju_Nuc ... k_and_fire

These are pipe dream physics projects, where the exact same problems exist for all reactors.

Nobody actually has any plan (or any solution) for when things go bad.
Molten-salt-fueled reactors (MSRs) supply the nuclear fuel mixed into a molten salt (Sodium-fluoride, generally). They should not be confused with designs that use a molten salt for cooling only, such as Monju.

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Re: Fusion power (fission welcome too)

Post by sparks »

Rob Lister wrote:
Sat Dec 21, 2019 11:09 am
sparks wrote:
Sat Dec 21, 2019 2:48 am
"The Pu 239 created in a LWR is thoroughly mixed with Pu 240 and so is quite useless for nuclear booms. "

Question for you then: How did the reactors at Hanford and What the fuck was it down South make Pu239 for Fat Man? These were not LWRs to be sure, but where's the difference? (If memory serves, they were graphite moderated air cooled??)
For an indepth look:
https://en.wikipedia.org/wiki/Plutonium-239#Production

Nutshell: The fuel rods contain U238. Lots of neutrons are flitting about. After X amount of time:
Image
There's your weapon's grade plutonium. But, that p239 is also exposed to the same neutron source so after another X amount of time you start to get:
Image
Which fucks your devious plans to make a goodly bomb.

So what they did was insert a sample of U238 into the core, wait X time, but not 2X time, shut the reactor down and remove the sample. Rinse, repeat. Your samples now have a tiny degree of P239 and almost no P240. I suppose the tricky part is finding X.

The reactor can be made in such a way as to not have to shut down the core to insert new samples. CANDU is one such, I think.

And I don't see any reason why inserting a sample in an LFTR wouldn't work just as well. Probably wrong though.
Holy shit. Didn't realize that. Thanks Rob! And Merry Xmas to you and yours.

You weren't by chance a reactor officer were you? Don't reveal anything classified of course.... :)
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Rob Lister
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Re: Fusion power (fission welcome too)

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Oh hell no, I'm a complete amateur.

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Re: Fusion power (fission welcome too)

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Me too Rob. :)
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Re: Fusion power (fission welcome too)

Post by Anaxagoras »

Rob was an avionics guy in the Navy iirc.

I signed up to be a nuclear guy but ended up as an airframer. Probably for the best.
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Re: Fusion power (fission welcome too)

Post by Abdul Alhazred »

A US Army recruiter said my job would be nuclear missile maintenance after giving me an alleged IQ test.

The occasion was my draft physical, which I passed.

I didn't join and I didn't get drafted either. :)
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Re: Fusion power (fission welcome too)

Post by Witness »

There's also Rubbia's proposal (and patent):
Accelerator-driven subcritical reactor

An accelerator-driven subcritical reactor is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton accelerator. It could use thorium as a fuel, which is more abundant than uranium.[1]

The neutrons needed for sustaining the fission process would be provided by a particle accelerator producing neutrons by spallation. These neutrons activate the thorium, enabling fission without needing to make the reactor critical. One benefit of such reactors is the relatively short half-lives of their waste products. The high energy proton beam impacts a molten lead target inside the core, chipping or “spallating” neutrons from the lead nuclei. These spallation neutrons convert fertile thorium to protactinium-233 and after 27 days into fissile uranium-233 and drive the fission reaction in the uranium.[1]

Thorium reactors can generate power from the plutonium residue left by uranium reactors. Thorium does not require significant refining, unlike uranium and has a higher neutron yield per neutron absorbed.
https://en.wikipedia.org/wiki/Accelerat ... al_reactor