August 14, 2022, 05:48:23 PM
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Topic: Tokamak produces radioisotopes  (Read 15597 times)

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Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #30 on: July 31, 2022, 11:34:34 AM »
Figures about aluminium coils at 20K for MRI. 1.5T, the rest is arbitrary just to assess the feasibility.

A uniform induction needs more current density at the ends. I take two currents for simplicity. Solving for 1.5T at two points gives 1.81MA and 1.17MA, as illustrated.

Real cases refine that. Dividing more the current distributes the induction better. Choosing some reasonable axial induction distribution, outside the coils too, and deconvolving by the distribution of a current loop using a Kálmán filter would do it too. Surely dozens of methods exist.

Each of the four coils is L=1m Ri=0.87m Ro=1.13m big on the example. Aluminium fills only 70% because other coils need voids to vary the flux locally. So length=6.28m and S=0.182m2 for 1 turn.
  nvlpubs.nist.gov (5MB) page 20 - lss.fnal.gov
the resistivity is 6.3pΩ×m at zero field, but magnetoresistivity adds 4.6× at 15kOe (1.5T in air). This extreme value happens at the inner face, so as a mean 2.3× for 21pΩ×m. 0.72nΩ/turn2 dissipate then 2.4+1.0+1.0+2.4=6.7kW at 20K. Heat leaks can be much smaller. 30% of the 6.2% Carnot limit towards +50°C=323K let evacuate 360kW in the air. If the machine operates for 4h/day and 250 days/year, the electricity bill is 300k€ in 4 years. I suspect superconducting coils cost more, and they need cooling power too, at 4K.

0.23kg/s = 0.10m3/s gaseous helium from 19K to 21K shall remove the 2.4kW from the outer coils thanks to 10.5kJ/kg and 2.4kg/m3. An axial flow through 10% of the coil section or 0.16m2 needs mean 0.6m/s helium speed, so easy.

That's a part of the picture. For instance the mechanical stability of the coils matters. Open MRI apparatus, more generally a vertical main field, saves power with shorter induction lines in air, while cold aluminium coils create a longer observation zone than permanent magnets do.

A thought for Sapo.
Marc Schaefer, aka Enthalpy

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #31 on: August 06, 2022, 11:43:38 AM »
Sources of γ rays sterilize objects, measure thicknesses without contact, display the contents of closed containers, etc. But radionuclides like 60Co are also a danger. A switchable γ source could be useful. As usual, I didn't check what is already done or abandoned. Nor am I reliable on the topic.

Bombardment by protons or deuterons lets some targets emit γ.
  • It often produces β+ emitters. When the 511keV photons from positron-electron annihilation are a drawback, shielding them away lets waste many 2MeV photons. Very few nuclides disintegrate by electron capture without β+.
  • Some reactions create β- emitters, typically by deuteron absorption and proton emission. The β braked by surrounding matter (Bremsstrahlung) creates photons rarely useful that can be minimized. The disintegration often emits γ rays of single or several energies by internal transition. If the β- emitter is short-lived, the γ emission stops some time after the protons or deuterons beam. But seconds would be better than minutes.
  • A beam could first produce neutrons whose absorption creates β- emitters. But neutrons take decimetres to brake, and a wide source makes fuzzy γ images. Neutrons tend to activate all materials, possibly for a long time. And they need strong protons but the double conversion is inefficient. I didn't insist.
  • A few reactions just absorb a weak proton or deuteron and emit a strong photon, immediately at human scale. Most promising.
========== Example

Among these fourth-type reactions, in this message I consider
  19F(p,γ)20Ne. For chemists: 19F + proton :rarrow: 20Ne + γ
Data is gratefully taken from the Janis books
  protons (8MB) page 62 - deuterons (3MB)

Natural fluorine is pure 19F. 20Ne is stable. I understand the γ carries 12844keV reaction energy plus 19/20 of the incident proton contribution: an energy not available from radioactivity, twice as penetrating as 1.33MeV from 60Co, for instance to measure thicker red-hot steel plates when rolled.

The first competing reaction is on p63 in the Janis book, with similar sections over the energy range
  19F(p,α)16O
The product is stable and the α stops within the source. If emitting no additional γ, this reaction looks harmless and acceptable.

The next competing reaction is on p59
  19F(p,n)19Ne
which is 4MeV endothermal, so a smaller proton energy prevents it and the others.

The useful reaction has a measured section around 40mb from 1.4 to 0.3MeV. Target fluorine can be AlF3 since the Janis book lists no reaction of Al active at these energies. NaF and MgF2 seem clean too. 13N from (CF2)n would emit 1/1000 γ at 511keV with 10min half-life. Very pure 11B is less convenient. Li and Be emit other radiations.

32%wt Al and 68%wt F brake 1.4MeV protons over 44g/m2, that is 30g/m2 F, as deduced from
  physics.nist.gov
and then 40mb section let 3.8ppm of all protons create a γ, so a 1.4MeV 1.6mA 2.2kW proton beam emits 1Ci. Maybe protons up to 4MeV improve the yield but the Janis book lacks experimental data, and 1.4MeV accepts a small cyclotron, or maybe a linear accelerator with radiofrequency quadrupoles (RFQ).

Aluminium or other, possibly cooled by water, can support AlF3 of which 10-20µm stop the protons. The two transmutations deplete F in few years of continuous 1Ci operation, faster degradation processes are plausible.

Marc Schaefer, aka Enthalpy

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #32 on: August 07, 2022, 09:55:33 AM »
Some more proton-to-γ reactions. Reactions not indicated in the Janis book may ruin the attempts.

Target  Nature  Product  Janis    Q   Avoid  Section  From   To   Yield
           %              page   MeV   MeV      mb     MeV   MeV  Ci/mA
=========================================================================
  7Li    92.50     8Be    007  +17.3   1.64    7?      1.6   0.5  0.5?
  9Be   100       10B      13   +6.6   1.85    0.5?    1.8   1.0  0.02
 44Ca     2.09    45Sc    103   +6.9   4.43    1.8     4.0   2.0  0.2
 51V     99.75    52Cr    147  +10.5   1.53    0.2     1.5   1.2  0.02
 52Cr    83.79    53Mn    162   +6.6   5.49    0.6     4.0   2.0  0.06
 58Fe     0.28    59Co    194   +7.4   3.09    1.4     3.0   2.0  0.03
 59Co   100       60Ni    201   +9.5   1.86    0.6     1.8   1.6  0.002
 62Ni     3.63    63Cu    230   +6.1   4.73    2.4     4.3   3.0  0.06
 64Ni     0.93    65Cu    235   +7.5   2.45    1.1     2.4   1.2  0.02
 63Cu    69.17    64Zn    243   +7.7   4.15    1.8     4.1   2.8  0.1
 65Cu    30.83    66Zn    250   +8.9   2.13    0.6     2.1   1.8  0.004
 68Zn    18.80    69Ga    274   +6.6   3.70    7       3.6   2.8  0.1
 >Zn                     Lacks γ data
=========================================================================


  7Li: No data about 6Li. Does the data distinguish 8Be from α+α? How much energy in the photon vs α+α?
44Ca: Remove well 42Ca 48Ca.
51V : Leave 50V.
52Cr: Remove well 53Cr 54Cr. Produced 53Mn has 3.7My half-life.
58Fe: Remove well 54Fe 56Fe 57Fe.
59Co: No separation.
62Ni: Remove well 58Ni 60Ni 61Ni 64Ni. Can start below 4.3MeV.
64Ni: Remove 58Ni 60Ni 62Ni. Remove well 61Ni.
63Cu: Remove 65Cu and start below 2.13MeV, or remove well 65Cu.
65Cu: Remove 63Cu.
68Zn: Remove well 64Zn 66Zn. Remove 67Zn and start below 2.41MeV, or remove well 67Zn.

9Be, 51V, 59Co need no isotopic separation. 63Cu, 65Cu accept limited isotopic purity and emit lower energy γ. Insufficient data to tell about 7Li. The others need strong isotopic purity for cleanliness.

Energetic protons make more gammas but need a bigger cyclotron. All nuclides accept weaker protons.

Marc Schaefer, aka Enthalpy

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #33 on: August 09, 2022, 02:17:55 PM »
A 1.4MeV protons cyclotron for a switchable gamma source is D=0.8m H=0.6m small and can be added to a production facility or an imaging apparatus. Illustration appended. Please remember I didn't consider beam focussing.

0.89T give protons 13.56MHz cyclotron frequency, 6 sectors receive power at the 40.68MHz ISM frequency. If the energy must vary, move the extraction electrode radially?

The DC magnet coils provide 2×14.2kA×turn. 90% fill factor
  chemicalforums
gives them 2.4µΩ/turn2 so they consume 2×0.5kW.

1.36T and 2×20.34MHz would shrink the cyclotron to D=0.7m but need 2×1kW in the coils.

6 sectors all fed with slightly over 5kVpk provide 60keV/turn. Each sector has stray 50pF to the iron compensated by a 0.3µH coil each. These coils can consist approximately of 2.5 turns of 6mm gold-plated wire on D=37mm air. They dissipate roughly 6×65W, so oil and even blown air can isolate and cool them.

A linear accelerator for 1.4MeV using radiofrequency quadrupoles (RFQ) is shorter than 2m and sleek. Its beam can be bent, even by permanent magnets. Serious competitor.

Marc Schaefer, aka Enthalpy

Offline Enthalpy

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Re: Tokamak produces radioisotopes
« Reply #34 on: Today at 03:37:41 AM »
More reactions for a switchable γ source, as alternatives to proton capture.

Data is from the Janis book.

========== β+ and ε

Proton impact makes naturally proton-rich nuclides that decay by β+ emission or by electron capture ε. The impact often emits neutrons too, but surrounding boron can absorb them with little added radioactivity.

The β+ decay can emit a useful γ, but the e+e- annihilation emits also two 511keV γ. If the 511keV are usable, many radionuclides and reactions are possible and known. Removing undesired 511keV is difficult and wastes the other γ even at 2MeV. I didn't insist.

A few proton-rich nuclides decay by ε without β+, hence without 511keV γ. The emitted neutrino takes some energy, and at least a part of the γ spectrum is continuous, less valued for imagery. Maybe some nuclide exists with useful γ energies and credible production reactions, but I didn't find any in a limited time.

========== β-

No 511keV γ here, and some nuclides have a clean γ spectrum. Neutron-rich nuclides are harder to produce by impact. The reactions I found get a deuteron and emit a proton. I checked for unwanted reactions only in the Janis book, they exclude for instance 18O to 16N and 18O to 19O.

Target  Nature  Product  Janis  Avoid  Barn  From   To   t1/2  Gamma   %  Yield
           %              page   MeV          MeV   MeV         MeV       Ci/mA
================================================================================
  15N      0.37    16N      33     ?     0.7   2.6+  1.2  7.1s  6.13   67    19
  27Al   100       28Al     46    4.1    0.2   4.0   2.4  2.2mn 1.78  100    10
================================================================================


The β- emits few photons by braking, even in Al. At 1.5MeV it's around 2%.

The choice is again narrow and implies diverse drawbacks:
  • Isotopic separation, uneasy compounds for 15N. Yield for pure N!
  • Minutes half-life is good for transport and disposal, less for development and use.
  • Deuterons need a cyclotron as big as protons of double energy. Linear accelerator!
Maybe some applications prefer these drawbacks to a permanent radioactive source.

Marc Schaefer, aka Enthalpy

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