- #1
Enthalpy
- 667
- 4
Hello nice people!
As we all know, nuclear fusion energy in a tokamak power plant has to regenerate the tritium it burns, if fusion is to replace most fission reactors.
We all now as well that this implies a neutron multiplier at the blankets, for which lead seems to be the only candidate.
Sadly, nuclear fusion including tritium regeneration is as dirty as fission, because neutrons bombarding lead make radioactive products.
Here it goes.
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In the desired reaction, one 14MeV neutron produced by D-T fusion hits a lead nucleus, two neutrons emerge, and are absorbed at low energy by one 6Li each to make two tritium atoms. The neutron-lithium reaction also adds its own 5MeV, so one tritium cycle brings 25MeV now.
Tables of reactions, neutron cross-sections, radioisotopes there:
- Handbook of Chemistry and Physics, sure
- The Inaugural-Dissertation by Peter Reimer (= PhD thesis, as English Pdf on the web)
- http://www.matpack.de/Info/Nuclear/Nuclids/P/Pb.html
You guessed, the shaken lead atom leaves other products... Here are just two undesireable reactions, roughly estimated by hand; investigating more would bring more cases.
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204Pb makes 1.4% of natural Pb. The neutron doubling process is efficient (2.1b over 5.3b at 14MeV) and leaves 203Pb, which decays in 2.2 days by electron capture of 0.97MeV with gamma emission.
206Pb makes 24% of natural Pb. When hit by a 14MeV neutron, it sometimes emits an alpha to leave 203Hg, which decays in 47 days by beta- with a 0.28MeV gamma. Section for this production is only 0.7mb over 5.3b at 14MeV but 206Pb is abundant.
1.4% abundance or 0.7mb reaction section may look rather small, but:
- 235U gives 131I in 2.8% and 137Cs in 6.1% of the fission events;
- Fission of 235U brings 200MeV. It takes 8 times more D-T and n-Li reactions to produce as much heat and electricity.
Combine both, you get as much 203Pb as 131I per MW of electricity output.
Now, one may argue that isotopes 204 and 206 could be removed from Pb...
- Well, no. Never completely. Changing a concentration by a factor of 10 is already a big effort, but 10 times less pollutants is still far too much.
- I'm confident other pollutants are produced by the isotopes 207 and 208, like 204Tl.
- I only checked neutrons with 14MeV as they're emitted. As they thermalise before being used by 6Li, more reactions occur.
- Such reactions look inherent to tritium regeneration.
In a leak of hot coolant, a 16% Li / 84% Pb eutectic, I imagine lithium ignites in air (or doesn't it?), with the fire releasing in the atmosphere the contained pollutants.
-----
This pollution is known from fusion researchers, as for instance the PhD thesis by Peter Reimer concentrates on such reactions - but maybe this aspect hasn't been publicized enough up to now...
So we should reach the consensual conclusion:
Stop wasting the valuable time of precious researchers on ITER! Fusion needs unavailable fuel or is dirty. Instead of unrealistic dreams, concentrate on serious technology, like renewable energy.
Marc Schaefer, aka Enthalpy
As we all know, nuclear fusion energy in a tokamak power plant has to regenerate the tritium it burns, if fusion is to replace most fission reactors.
We all now as well that this implies a neutron multiplier at the blankets, for which lead seems to be the only candidate.
Sadly, nuclear fusion including tritium regeneration is as dirty as fission, because neutrons bombarding lead make radioactive products.
Here it goes.
-----
In the desired reaction, one 14MeV neutron produced by D-T fusion hits a lead nucleus, two neutrons emerge, and are absorbed at low energy by one 6Li each to make two tritium atoms. The neutron-lithium reaction also adds its own 5MeV, so one tritium cycle brings 25MeV now.
Tables of reactions, neutron cross-sections, radioisotopes there:
- Handbook of Chemistry and Physics, sure
- The Inaugural-Dissertation by Peter Reimer (= PhD thesis, as English Pdf on the web)
- http://www.matpack.de/Info/Nuclear/Nuclids/P/Pb.html
You guessed, the shaken lead atom leaves other products... Here are just two undesireable reactions, roughly estimated by hand; investigating more would bring more cases.
-----
204Pb makes 1.4% of natural Pb. The neutron doubling process is efficient (2.1b over 5.3b at 14MeV) and leaves 203Pb, which decays in 2.2 days by electron capture of 0.97MeV with gamma emission.
206Pb makes 24% of natural Pb. When hit by a 14MeV neutron, it sometimes emits an alpha to leave 203Hg, which decays in 47 days by beta- with a 0.28MeV gamma. Section for this production is only 0.7mb over 5.3b at 14MeV but 206Pb is abundant.
1.4% abundance or 0.7mb reaction section may look rather small, but:
- 235U gives 131I in 2.8% and 137Cs in 6.1% of the fission events;
- Fission of 235U brings 200MeV. It takes 8 times more D-T and n-Li reactions to produce as much heat and electricity.
Combine both, you get as much 203Pb as 131I per MW of electricity output.
Now, one may argue that isotopes 204 and 206 could be removed from Pb...
- Well, no. Never completely. Changing a concentration by a factor of 10 is already a big effort, but 10 times less pollutants is still far too much.
- I'm confident other pollutants are produced by the isotopes 207 and 208, like 204Tl.
- I only checked neutrons with 14MeV as they're emitted. As they thermalise before being used by 6Li, more reactions occur.
- Such reactions look inherent to tritium regeneration.
In a leak of hot coolant, a 16% Li / 84% Pb eutectic, I imagine lithium ignites in air (or doesn't it?), with the fire releasing in the atmosphere the contained pollutants.
-----
This pollution is known from fusion researchers, as for instance the PhD thesis by Peter Reimer concentrates on such reactions - but maybe this aspect hasn't been publicized enough up to now...
So we should reach the consensual conclusion:
Stop wasting the valuable time of precious researchers on ITER! Fusion needs unavailable fuel or is dirty. Instead of unrealistic dreams, concentrate on serious technology, like renewable energy.
Marc Schaefer, aka Enthalpy
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