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PostPosted: Nov 07, 2018 7:49 pm 
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I am curious if primary neutron sources such as:

Californium-252
Plutonium-238
americium-241
polonium-210
radium-226

Specifically Radium 226 could be used to boost the output of a LFTR such that it generated more U-233 than it consumed.


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PostPosted: Nov 08, 2018 4:17 pm 
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From https://en.wikipedia.org/wiki/Neutron_source#Radioisotopes_which_decay_with_alpha_particles_packed_in_a_low-Z_elemental_matrix

As an example, a representative alpha-beryllium neutron source can be expected to produce approximately 30 neutrons for every one million alpha particles.

So for each 33,000 atoms that decay with an alpha particle, one may expect to breed one atom of fissile material. Not much help!

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PostPosted: Nov 09, 2018 8:14 am 
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https://en.wikipedia.org/wiki/Startup_neutron_source

It looks like these are used to start up reactors.

Does what you posted still apply?

Thanks much.


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PostPosted: Nov 09, 2018 5:10 pm 
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It really only takes one neutron to start a reactor. Neutron sources are used to generate a measurable flux of neutrons so one can tell how close to critical the reactor is in order to start it safely. Once the reactor is started, the neutrons from fission completely overwhelm the neutrons from the neutron source.

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PostPosted: Nov 10, 2018 3:45 am 
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Joined: Nov 14, 2013 7:47 pm
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Location: Iowa, USA
matthewwight wrote:
I am curious if primary neutron sources such as:

Californium-252
Plutonium-238
americium-241
polonium-210
radium-226

Specifically Radium 226 could be used to boost the output of a LFTR such that it generated more U-233 than it consumed.


I'm not an expert here but I'm just thinking this through from what I've read. I don't know much of anything on radium but I could find a bit of data on Pu-238 so I'll use that as an example.

Pu-238 does undergo the rare spontaneous fission, producing a neutron, but for the most part it decays by alpha emission. In a thermal neutron flux, like that in LFTR, it has a fission cross section of about 18 barns and a capture cross section of 400 barns. This makes it a neutron poison, it grabs more neutrons than it produces. I can imagine that Pu-238 is used in thermal fission reactors as a startup source of neutrons because it is something readily available from "waste" or "ash" produced from other fission reactors. After capturing a neutron it becomes Pu-239, which if it happens to fission from another neutron then 1.9 neutrons are produced. So, 2 neutrons in and 1.9 neutrons out. As far as neutron poisons go this is pretty mild, again by my relatively uneducated opinion.

Oh, and the U-234 produced by the alpha decay of Pu-238 is also of interest. If I'm reading the charts I found correctly there's a roughly 50/50 chance it will capture or fission if hit by a neutron. The U-235 produced by a neutron capture will fission most of the time if hit by a neutron and produce 2.5 neutrons, if it doesn't fission then it can go on to grab more neutrons. So, by my back of the envelope math this could be a minor poison as well.

The heavier isotopes you listed, Am-241, Cf-252, and Pu-238, are created by successive failed attempts at fission. In an ideal world these would not even be produced in a reactor because they'd have fissioned long before they "stole" so many neutrons from the core. This is why many designs for a molten salt reactor has a means to filter out neptunium, after uranium grabs enough neutrons to decay to neptunium then the thermal spectrum performance isn't so great. It's best to just remove the Np and sell it to NASA so they can use it for Pu-238 production to make RTGs for their deep space missions.

Perhaps I missed an important detail but I'm just not seeing any of these isotopes as all that beneficial to boosting the output of a LFTR.

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Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


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