July 04, 2022, 07:19:36 AM
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Topic: Tokamak produces radioisotopes  (Read 12773 times)

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Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #15 on: June 24, 2022, 05:56:23 PM »
Replacing 98Mo by a fissile material as a target for the tokamak's neutrons would seemingly ease the separation of the Mo produced then by fission. Just chemical separation of Mo would provide a big proportion of 99Mo, and the other isotopes disappear quickly or are stable. Alas, this attempt is impractical.

The mini-tokamak itself is nonproliferating. It would produce too little Pu or 3H to make bombs. Keeping this advantage excludes targets of 235U and Pu. Natural U or 238U or 232Th can make nonproliferating targets, but they need fast neutrons to fission, like 14MeV, and then only very thick target blankets intercept the neutrons, and very little 99Mo must be extracted from much fissile material.

A fission cross-section like 1b needs 4t/m2 of 238U to intercept 14MeV neutrons with fair probability. It also needs more neutrons, as the yield to 99Mo is like 5%.

Fission yields, cross-sections and more data is available there, thanks
  iaea.org - bnl.gov , also iaea.org - wikipedia

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #16 on: June 29, 2022, 05:56:00 PM »
All data sources indicate the mean energy of thermalized neutrons as kT.
I believe it should be 1.5×kT, for instance 39meV at 300K.

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #17 on: July 02, 2022, 11:57:17 AM »
[...] So 2.1×1015/s neutrons as well. [...]
Wrong! Every second D+D reaction produces T+p, but T has 1MeV recoil energy while the magnets confine only mean 20keV hydrogen. Tritium escapes the reaction zone.

2.1×1015n/s need a machine 8×8×8× smaller than Iter rather. Or better magnets, as these progress.

And the neutrons have 2.45MeV as emitted by the D(D, n)T reaction, not 14.1MeV as in T(D, n)4He.

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #18 on: Yesterday at 05:09:38 PM »
Natural uranium blankets and thermal neutrons could produce 99Mo after all, but this takes a bigger tokamak, and the little molybdenum must be extracted from much uranium. At least, no isotopic separation is needed.

300K neutrons see 450b section for 235U fission, 67b for 235U absorption, 2.2b for 238U absorption. 0.72% and 99.28% abundances give natural uranium mean 5.9b with 55% chances to fission 235U. Natural C provides 4.9b for collisions (22mm mean free path) and 3.2mb for absorption. Accepting 1/3 neutrons lost in C, 923 moles C pass the neutrons many times through each mole U in a Brownian motion. Mean 510 collisions before absorption let a neutron spread by very roughly 0.5m rms. The uranium load is a bit over 1t.

Neutrons leaving a blanket inwards shall serve at opposite blankets. U absorbs too strongly between 1keV and 3eV so most neutrons must survive in well separated C. I didn't check deuterated polyolefins, light water nor the proliferating heavy water. Liquid oxygen could replace graphite, save a little bit uranium, and bring many drawbacks. Colder moderators bring little and liquid deuterium has big drawbacks. And the tokamak's coils must sit somewhere but will catch some neutrons.

Materials that absorb neutrons little: ethylene and propylene glycol carbonates and their eutectic, oxalates of alcohols or Be, including deuterated variants.

Some 30mg 99Mo made in a day must be extracted from >1t U. As the fission destroys the U molecule, maybe the chosen environment could build a new Mo molecule easy to separate.

6.1% of the fissions yield 99Mo. Maybe every second neutron misses the blankets or is absorbed elsewhere. 1/3 is lost in C. Among U events, 55% produce Mo. All this takes 100 neutrons per Mo atom, so the tokamak is only 1.7×1.7×1.7× smaller than ITER, alas. But producing no energy and needing no tritium, it's simpler. Adding U blankets at the top and if possible the bottom would reduce the size.

Marc Schaefer, aka Enthalpy

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