2
$\begingroup$

The National Ignition fusion recently announced the achievement of nuclear fusion "ignition", i.e. more energy released from a sample undergoing nuclear fusion reactions than was directly input into the sample by lasers (not the total energy required to power all the lasers, which was still much higher than the produced energy).

My understanding is that one of the main design decisions for a hypothetical practical nuclear fusion reactor is whether to use deuterium or tritium isotopes (or both) as the hydrogen fuel source. Tritium has the advantage that it undergoes nuclear fusion at a lower temperature than deuterium, so it is a "better" fuel source in terms of intrinsic physical properties, but it has the disadvantage of being much, much more expensive and challenging than deuterium to produce in large quantities.

How much deuterium and tritium did the NIF's hohlraum contain in its ignition demonstration? This is the only source I could find that discussed the hohlraum's fuel contents, and it just said that the hohlraum contained a "deuterium–tritium fuel", but didn't give the relative proportions. Was it a roughly even mix of both isotopes?

(I'm also curious what pressure the "peppercorn" of hydrogen fuel was kept at before the lasers were activated, and what phase of matter it was in at that pressure at room temperature. Presumably it was a plasma when the lasers caused ignition.)

$\endgroup$
2

2 Answers 2

1
$\begingroup$

The fusion cross section for DT is much larger than for DD. To optimize the fusion reaction I expect they used equal amounts of D and T but I do not know for sure.

In an actual fusion energy machine, the T will probably be produced using a lithium blanket. The DT reaction produces a neutron and the neutron can react with the lithium to produce tritium.

T has a 13 year half life. T for various components used in a nuclear weapon (e.g. the boosting system) is produced from irradiating Li with neutrons in the Watts Bar nuclear reactor.

A thermonuclear weapon fusion secondary contains lithium deuteride. The deuteride provides deuterium and the lithium provides T once exposed to neutrons from the fission primary. The DT reaction dominates the energy released from the fusion secondary.

$\endgroup$
6
  • $\begingroup$ It needs to be Li-6 to get tritium… $\endgroup$
    – Jon Custer
    Jan 2, 2023 at 1:03
  • $\begingroup$ Yes. Thanks for the clarification. $\endgroup$
    – John Darby
    Jan 2, 2023 at 1:10
  • $\begingroup$ @JonCuster It needs to be Li-6 to get tritium from low energy neutrons, but energetic neutrons can fission Li-7 to D and T. Infamously in the Castle Bravo nuclear test. $\endgroup$
    – John Doty
    Jan 2, 2023 at 1:29
  • $\begingroup$ @JohnDoty - no, the Li-7 does not yield tritium. See en.wikipedia.org/wiki/Castle_Bravo#High_yield for the relevant yield-enhancing reactions. Note also that ENDF has no cross sections for neutrons on Li-7 creating tritium. $\endgroup$
    – Jon Custer
    Jan 2, 2023 at 1:59
  • $\begingroup$ @JonCuster Your reference shows $^3H$ as a product of the first reaction under "But when lithium-7 is present...". $\endgroup$
    – John Doty
    Jan 2, 2023 at 2:07
1
$\begingroup$

Was it a roughly even mix of both isotopes?

Yes, as close to 50-50 as they can get.

I'm also curious what pressure the "peppercorn" of hydrogen fuel was kept at before the lasers were activated

The pellet, which in this case IIRC was diamond, is a hollow sphere that is as round as 60 years of effort can make it. The fuel is deposited by drilling a hole and injecting it as a gas. The entire assembly is then placed in a cryogenic facility that causes the gas to freeze onto the inner surface of the shell. To smooth it out, the frozen gas is warmed with an infrared laser and re-frozen (perhaps more than once, references are unclear). The center is then filled with a small amount of additional gas which is there primarily to control the implosion dynamics and might be entirely D. The hole is then filled and off it goes. There are a number of ways one can do this without the hole, like infusing the gas right through the shell under pressure (as developed by KMS Fusion), but I'm not sure whether this is used in NIF.

So "pressure"... well it's mostly solid and the small amount of gas that's there is likely pretty close to 1ATM.

I seem to recall that end-to-end it is around 55 man-hours per capsule.

$\endgroup$
2

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.