5 ms·

Actually their only primary fuel is deuterium. The fusion reactions are 2/3 of D+D and 1/3 of D+He3 D+D fusion reaction yields 50% of T+p (tritium and a proton

by joak 4y ago

Actually their only primary fuel is deuterium. The fusion reactions are 2/3 of D+D and 1/3 of D+He3 D+D fusion reaction yields 50% of T+p (tritium and a proton) and 50% of He3 + neutron So in total their fusion reaction does not consume He3. He3 is here like a catalyzer. They still need some He3. The D-D reaction produces tritium, and the tritium decays into He3. Half life of T is 12.5 years. Done, no issues with the helium 3. Actually they will probably produce T in excess and could sell it.

ncmncm 4y ago
Running deuterium-rich enough to net significant 3He out would seem to emit enough neutrons to render the reactor vessel strongly radioactive, and require the sort of cooling and shielding that makes the pure D-D reactors being pursued by others grossly impractical.
- joak 4y ago
  As I understand the neutrons of the D+D->He3+n reaction are slow neutrons (2.45MeV) that won't damage too much the materials. These are not the fast neutrons of the DT fusion (14.06MeV) They claim it won't be worse than waste neutrons of radiation therapy medical devices. To be seen... But yes, agree this is not exactly aneutronic fusion. Around 10% of energy is lost as slow neutrons and this is definitely a problem to address.
  - DennisP 4y ago
    6% as neutron radiation is the figure I've seen, from Helion and other sources. According to the head of MIT's fusion department, even D-T neutrons only mean you'll have to bury the activated materials for a few decades. A much lower amount of lower-energy neutrons isn't going to cause any major problems.
    - joak 4y ago
      Exactly, the neutron induced radioactivity is orders of magnitude lower than in fission by-products and is a minor issue. The problem is more with neutron embrittlement that weakens the materials exposed to fast neutron radiations. For DT fusion in tokamaks the idea is to replace the inner core every year or so. Helion claim embrittlement is not much an issue for them. First, because their neutrons are relatively slower and second, because they can choose neutron resistant materials since they do not fear plasma contamination thanks to the pulsed nature of their operation: in between pulses they vacuum the vessel before injecting the exact mix needed. In tokamaks, the materials facing the plasma are designed primarily to avoid emissions that might contaminate the plasma, embrittlement comes as an afterthought. Also in DT fusion, it's not 6-10% of the energy released as slow neutrons, but 100% as fast neutrons. Conclusion: there is a neutron problem but it seem minor and fairly solvable
ncmncm 4y ago
I am corrected. Operated just D-rich enough to breed its own 3He (plus tritium, presumably for sale), the material damage, waste heat, and radioactivation from neutron flux produced by the D-D side reactions are much more manageable than for a plant relying on capturing those neutrons for their heat. The waste heat from a 2 MW plant, ~200 kW, would not be negligible, but capturing and handling 200 kW in warm (guessing?) lithium-deuteride saturated heavy water is different in kind from extracting 20 GW (probably the minimum size of a the D-T plant) from thousands of tons of molten metal.

by joak 4y ago

ncmncm 4y ago

Running deuterium-rich enough to net significant 3He out would seem to emit enough neutrons to render the reactor vessel strongly radioactive, and require the sort of cooling and shielding that makes the pure D-D reactors being pursued by others grossly impractical.

joak 4y ago
As I understand the neutrons of the D+D->He3+n reaction are slow neutrons (2.45MeV) that won't damage too much the materials. These are not the fast neutrons of the DT fusion (14.06MeV) They claim it won't be worse than waste neutrons of radiation therapy medical devices. To be seen... But yes, agree this is not exactly aneutronic fusion. Around 10% of energy is lost as slow neutrons and this is definitely a problem to address.
- DennisP 4y ago
  6% as neutron radiation is the figure I've seen, from Helion and other sources. According to the head of MIT's fusion department, even D-T neutrons only mean you'll have to bury the activated materials for a few decades. A much lower amount of lower-energy neutrons isn't going to cause any major problems.
  - joak 4y ago
    Exactly, the neutron induced radioactivity is orders of magnitude lower than in fission by-products and is a minor issue. The problem is more with neutron embrittlement that weakens the materials exposed to fast neutron radiations. For DT fusion in tokamaks the idea is to replace the inner core every year or so. Helion claim embrittlement is not much an issue for them. First, because their neutrons are relatively slower and second, because they can choose neutron resistant materials since they do not fear plasma contamination thanks to the pulsed nature of their operation: in between pulses they vacuum the vessel before injecting the exact mix needed. In tokamaks, the materials facing the plasma are designed primarily to avoid emissions that might contaminate the plasma, embrittlement comes as an afterthought. Also in DT fusion, it's not 6-10% of the energy released as slow neutrons, but 100% as fast neutrons. Conclusion: there is a neutron problem but it seem minor and fairly solvable

ncmncm 4y ago

I am corrected. Operated just D-rich enough to breed its own 3He (plus tritium, presumably for sale), the material damage, waste heat, and radioactivation from neutron flux produced by the D-D side reactions are much more manageable than for a plant relying on capturing those neutrons for their heat. The waste heat from a 2 MW plant, ~200 kW, would not be negligible, but capturing and handling 200 kW in warm (guessing?) lithium-deuteride saturated heavy water is different in kind from extracting 20 GW (probably the minimum size of a the D-T plant) from thousands of tons of molten metal.