[Herra Tohtori]

One of the posters on slashdot had an interesting point about that: Deuterium is abundent and easy to find, but tritium has a very short half life and so is rare in nature. That being said there are huge amounts of it being generated by fission power plants. Assuming that is accurate, we wont completely replace fission, just mostly replace it. That being said fission isn't bad either, but they have so many people scared ****less about it I have no doubt more than a few people will take their claims about fusion seriously.

Tritium can be produced by neutron activation of Lithium-6 isotope.*

Lithium 6 has natural abundance of about 7.5 % of the element occurring in Earth, which means you can just cover the insides of the reactor with lithium, and the neutrons coming from the nuclear reaction will produce more tritium that way. In fact, Lithium deuteride (basically lithium hydride LiH but with H-2 isotope aka deuterium) is used as the fusion fuel in thermonuclear weapons. As far as availability in general goes, Lithium is the 25th most abundant element in Earth's crust, about the same as nickel or lead.

As a bonus, you get to use it as a neutron absorbing material to protect other elements in the reactor. This is beneficial because neutron activation normally can make things radioactive by changing them to unstable isotopes, which also affects things like structural integrity of the reactor's parts.


*Actually, with fast neutrons you can get tritium out of Li-7 as well, but the reaction is endothermic whereas ⁶Li + N -> ⁷Li -> ³H + ⁴He is exothermic which means that with proper calibration, the thermal energy of the reactions can be added to the thermal output of the primary fusion reaction.

Aside from that, there are other fusion reaction candidates than ³H + ²H -> ⁴He + N; most promising example would be:
³He + ²H -> ⁴He + p⁺

which, as you might notice, uses deuterium and Helium-3 isotope which is unfortunately scarce on Earth but slightly more abundant on the surface of the Moon...

See more examples in here.

[Klaustrophobia]

i think this is what they were thinking of when they said that, because tritium is not a byproduct of fission reactions.  fission produces heavy elements.  the only possible source of tritium would be absolutely miniscule amounts of hydrogen in the moderator getting activated (twice).  and mind you, we're talking not enough to even find here.

[Herra Tohtori]

i think this is what they were thinking of when they said that, because tritium is not a byproduct of fission reactions.  fission produces heavy elements.  the only possible source of tritium would be absolutely miniscule amounts of hydrogen in the moderator getting activated (twice).  and mind you, we're talking not enough to even find here.

Fission products are always lighter than the fissile material. A nucleus of the atom can go through a number of fission reactions, and the end results are not always the same.

However, neutron bombardment typically activates elements, which means the stable isotopes can become unstable isotopes, which means radioactivity. And the neutron bombarded nuclei can react in a number of ways depending on bounding energy of the nucleus, bounding energy with the new neutron introduced, energy of the neutron and so forth. Slow and fast neutrons react in a different way.

For example, in a typical fission chain reaction, Uranium-235 is impacted by a neutron, which can have two end results: Neutron absorption to Uranium-236 which, if it stays as such, is a bloody nuisance, or in a very short-lived U-236 nucleus that promptly decays through fission into daughter nuclei (fission products) and additional neutrons which will initiate new fission reactions. There is no way to know what the fission byproducts of a single reaction will be, but in a larger mass, the proportions of fission products can be predicted with significant accuracy and precision. U-235 mostly fissures into iodine, cesium, strontium, xenon and barium, and these radioactive isotopes start decaying immediately, which causes the radioactivity of nuclear waste. Usually, the fission results in two nuclei of relatively same atomic number, but sometimes one of the daughter nuclei happens to be tritium - about one in ten thousand reactions results in tritium, and other light elements/isotopes have similar yield rates. This is enough that the tritium accumulated in used fuel rods can be worth extracting.

There are also fission reactors specifically built for production of tritium, seeing how it is a valuable element for scientific and military applications alike.

Production of tritium directly in a fusion reactor still requires a "start-up" amount of tritium in the reactor each time the reactor is ignited. While the reaction goes on, neutron flux will impact the lithium-coated walls/floor of the reactor chamber, and tritium nuclei are produced. Problem, then, would be to induce those nuclei into the plasma where fusion is ongoing. If that works out, then you need to figure out if the production of tritium can be equal or larger than the consumption of tritium, which will decide whether it's possible to sustain the reaction with lithium-originating tritium once it gets going.

Of course, helium-3 would be a much less problematic fusion fuel combined with deuterium, since that sort of reactor would have predominantly no neutron flux at all - it would produce Helium nuclei and protons. The reason why neutron flux is annoying is because it makes the reactor itself radioactive via neutron activation as explained before - and that means that even though the reaction products are harmless and even useful, the parts of the reactor will become a problem, their service time will be shortened because radioactive decay changes the structural properties of alloys, and the reactor as a whole becomes a total ***** to service because it becomes a radioactive environment. It won't be anywhere near the amount of radioactive waste produced by fission power plants, but it'll be a problem nevertheless, as long as the fission reaction itself produces a strong neutron flux.

So, the solution? We need to figure out an abundant and reliable source of Helium-3.

Which means, hopefully, that we'll eventually go and start mining the Moon and the asteroids. In which case it would be a win-win scenario...

EDIT: of course, tritium naturally decays into helium-3, an electron and electron's antineutrino... so if we can get production of tritium to high enough levels, that would mean it would also be a source of He-3.

But that would mean no reason to become a Moon Miner...

sadface

[ChronoReverse]

Neutron bombardment also tends to make the walls brittle over time.  It's something they'll have to engineer around =/

[Herra Tohtori]

That's what I said.

...parts of the reactor will become a problem, their service time will be shortened because radioactive decay changes the structural properties of alloys

...

 

And, basically, they just need to factor that into the service time of the reactor components.

[ChronoReverse]

Heh, it's the power of tl;dr

[Nuclear1]

'Nuclear = Bad'

[Klaustrophobia]

Herra, i know that stuff, i'm a nuclear engineer.  i didn't check up on the fission yeild for tritium before i made that post, my bad.  but the point remains, you don't get any appreciable amount of tritium from an ordinary fission power reactor.  the ones that do breed tritium irradiate lithium, not get it from the fission products.

you don't necessarily need to startup with tritium in a fusion reactor, there are plenty of other reactions, D-D being the most common.  i don't know about ITER specifically, but most research fusion reactors (the ones not looking to produce power) breed tritium with lithium coating if they need it.  my design team is going to be considering this for our neutron generator.  

radiation damage in tokomaks is almost a moot point for materials concerns, next to the heat and the plasma itself if it perturbs into the walls.  the main reason for wanting neutrons flying around is so you don't have to shield it.

[perihelion]

I once was fortunate enough to take a course on plasma engineering from a professor who had worked on the Z-Machine at Sandia National Lab.  From what I was taught in that class, the issue of what nuclear fuel you start with in a fusion reactor is largely a moot.  Yes, some reactions have lower "ignition temperatures" than others.  Yes, some reactions (i.e. He-3 + D) have a yield that is almost entirely charged particles that can lend themselves more easily to confinement.  But you aren't going to be dealing with just that one ideal reaction by itself for more than a few nanoseconds.

Let's take the perfect ideal fusion reaction everyone talks about.  He-3 +D --> He-4 + p(+) (each with a corresponding amount of energy).  So, your initial fuel is He-3 and deuterium.  Your initial reaction will undoubtedly be the one you want because its reaction rate at lower temperatures is higher than the other possible reactions.  However, once fusion starts, the deuterium in your fuel will start other much less friendly reactions almost immediately.  D+D can fork a couple different ways, one of which brings tritium into the picture, while the other starts throwing off neutrons.  We're just in the 3rd generation of reactions now, and we already have gone from just He-3 and D to He-3, He-4, D, T, p(+), and n(0).  Because these reaction rates are so fast, in no time at all, you are losing huge amounts of energy to neutron flux, which you cannot confine, and so all that energy leaves the reaction mass, cooling it, and turning your reactor structural materials into a radioactive nightmare as they leave.  Couple that with  Bremsstrahlung radiation and the myriad other mechanisms for energy dissipation and you've cooled your reaction mass to the point where fusion cannot be maintained any further in far less time than it takes to blink.

I remember when I was first learning all this stuff, I boggled at how insanely "clever" the plasmas seemed to be at dissipating energy faster than it could be pumped in.  Stars have it easy.  They have density and extreme size on their side.  Energy has to get a lot more creative to work its way out of the reaction mass.  We do not have that benefit here on the Earth.

I'd been a huge proponent of fusion research up until I took that class.  Now, I honestly cannot see break even fusion being achieved in our lifetimes.  We'd be better off focusing our efforts on fission, which can be both clean and safe if done right.

[Kosh]

While the reaction goes on, neutron flux will impact the lithium-coated walls/floor of the reactor chamber,

Interesting post, as usual. Does the ITER use the above described technique? I can't seem to find too many details about its design.

I'd been a huge proponent of fusion research up until I took that class.  Now, I honestly cannot see break even fusion being achieved in our lifetimes.  We'd be better off focusing our efforts on fission, which can be both clean and safe if done right.

The ITER is supposed to have a output input ratio of 10-to-1, looks a lot more than break even to me.

[Klaustrophobia]

I'd been a huge proponent of fusion research up until I took that class.  Now, I honestly cannot see break even fusion being achieved in our lifetimes.  We'd be better off focusing our efforts on fission, which can be both clean and safe if done right.

and if we don't work on it, it will NEVER be acheived.  it was never going to be viable in our lifetimes, no one ever claimed it would be.  the most optimistic estimates i've ever heard have been 50 years.

[ChronoReverse]

Yup, except it's a sliding window.  It's one of those 50-years projects that are always 50-years away.

[Klaustrophobia]

Now that's not completely true.  We've made MASSIVE progress from where we've started.  Enough that materials are starting to be the limiting factor, not the fusion reactions.

[Flipside]

'Nuclear = Bad'

LOL Don't worry, it's only environmentalists who haven't been keeping up that think that way

[Klaustrophobia]

No, not really.  Greenpeace swapped sides.  Now the biggest organization i've seen opposing is something called the "Union of Concerned Scientists."  I'm not sure what kind of scientists they are, because every statement I've seen from them has been either fear-mongering or grossly incorrect.  The remainder of opposition is pretty much the government that doesn't want to fund research into advancements and infrastructure for closing the fuel cycle, and ordinary joe who is simply thinks The Simpsons is what nuclear power is really like.

[Kosh]

'Nuclear = Bad'

LOL Don't worry, it's only environmentalists who haven't been keeping up that think that way

No, not really.  Greenpeace swapped sides.

No, they haven't. They are just as stupid as they were before.


Speaking of nuclear power being good for the environment, recently there was a report about that from the International Energy Association

A study by the International Energy Agency, which seeks to coordinate energy policies in industrialised nations, and the Organisation for Economic Cooperation and Development described such a target as "ambitious but achievable."

"Nuclear is already one of the main sources of low-carbon energy today," said Luis Echavarri of the OECD's Nuclear Energy Agency.

"If we can address the challenges to its further expansion, nuclear has the potential to play a larger role in cutting CO2 emissions."

While no major technological breakthroughs will be needed to reach the goal, "a clear and stable policy commitment (by governments) to nuclear energy as part of an overall energy strategy is a pre-requisite," the report said.

Equally critical will be efforts to win greater public acceptance of nuclear energy programs, it added.

Nuclear power at present provides 14 percent of global electricity.

[Klaustrophobia]

Ok, so the message didn't get around to everyone, but the spokesman or president or someone came out on the pro side a few years back.  That article raises some eyebrows about the security level at the Sweedish plant.  I'm sure they could never have done anything dangerous, but the fact they needed a dog to check if any were still there means they've got some serious blind spots.  If that had happened in the US we'd probably be reading and article about how Greenpeace activitsts were shot.