Reply to: UK joins laser nuclear fusion project

[minestrone]

Conversion for the Hiroshima device was about 1.5grams out of the 50 odd pounds

Choose the ounce or pick the kilogram.

[TimberWolf]

The energy input to produce the 40 - 50lbs of 235 in the Hiroshima device must have been far in excess of the 12 - 15 kt yield.

Ah, now we're on the same wavelength. That was my first thought, though looking at the yield of that big bomb OwlHoot mentioned, it is hard to see how they could have used more energy in building it than the staggering yield.

[TimberWolf]

Conversion for the Hiroshima device was about 1.5grams out of the 50 odd pounds of 235.

So pretty tulipty really.

Oddly, the energy release for fusion is a good deal less than the release for fission IIRC, it's just that you can scale the device by multistaging etc. to produce really enormous yields. (57Mt for the Tsar Bomba 3 stage for instance, which would have been 100Mt with a 238 tamper).

The yield is higher for fusion bombs than fission bombs per unit mass too, but that's not what I meant by efficiency. I meant in a way comparable to the efficiency of a nuclear fusion reactor, I suppose economic efficiency might be a better description: the total energy used in making a bomb (or pellet for a reactor) versus the yield. Bombs may be inefficient too.

[OwlHoot]

Has this tech ever been theoretically "proven" as much as theory can be? ...

Yes it has

[TimberWolf]

It's been 10 years away for the last 50 odd years, so I'm not holding my breath.

And there've been fusion boosted nukes since the 1940s*, the difficult bit was figuring out how to create a genuine bedouine H bomb**. Which are actually fission fusion fission as currently implemented.


*Alarm clock/layer cake

**Teller/Ulam staged design.

It would be quite interesting to know how efficient nuclear bombs are, i.e. yield compared to the energy used to make them. Maybe they aren't all that efficient either. The yield is known to a good accuracy of course.

[doodab]

I afre with putting money into fusion.

With current rates of inflation it won't be long before we can cut out the middle men and just burn the money directly.

[BrilloPad]

I afre with putting money into fusion. Along with nuclear reactors. Lets stop our dependancy on oil for energy.

But no way to joining forces with USA. Look what happened over race to sound barrier. We gave them our data with promise of getting theirs in return. It never materialized.

[doodab]

I meant specific theories about fusion reactions, not high-level theory. We know fusion works because of the sun but that doesn't constitute a working plan.

We are quite capable of creating fusion reactions. Hydrogen bombs are fusion reactions triggered by a fission reaction.

Generating power is a different kettle of fish. It's like the difference between discovering fire and building an internal combustion engine.

[TimberWolf]

Yes, Einstein proved it donkey's years ago. In theory the energy needed to bring fusing atoms close enough together is much less than the energy released, and in theory the energy released could be used to help along more fusions and a self sustaining reaction continue (the odds of fusion happening is related to temperature, pressure and some other variables). In practise, who knows.

Actually Einstein probably didn't know about the force, or energy, needed to squash atoms together (Coulomb barrier - Wikipedia, the free encyclopedia), though he would have known how much energy would be released when fused.

Looking further into theoretical efficiency:

Using deuterium-tritium fuel, the resulting energy barrier is about 0.01 MeV.[citation needed]...The (intermediate) result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV.[citation needed] The recoil energy of the remaining 4He nucleus is 3.5 MeV,[citation needed] so the total energy liberated is 17.6 MeV.[citation needed] This is many times more than what was needed to overcome the energy barrier.
Nuclear fusion - Wikipedia, the free encyclopedia

So if I understand correctly (50% odds):
For one fusion reaction, 0.01 MeV in, 17.6 MeV out, or 1760 times as much out as in. But the energy out could instead be used to liberate further reactions and so the efficiency could approach 100%, minus the manufacturing the fuel, etc and other theoretical inefficiencies. I think they hope for 10 time as much out as in.

[d000hg]

Yes, Einstein proved it donkey's years ago. In theory the energy needed to bring fusing atoms close enough together is much less than the energy released, and in theory the energy released could be used to help along more fusions and a self sustaining reaction continue (the odds of fusion happening is related to temperature, pressure and some other variables). In practise, who knows.

I meant specific theories about fusion reactions, not high-level theory. We know fusion works because of the sun but that doesn't constitute a working plan.

[TimberWolf]

What's so funny?

You only need to reproduce the the conditions at the centre of a star here on earth, how hard can it be?

And even then, under those conditions, reactions are painfully rare. The average atom in our sun takes over a billion years to get around to fusing. Luckily there are lots of them. If it were much quicker our sun would have burnt out by now.

[gingerjedi]

Nah.

What's so funny?

You only need to reproduce the the conditions at the centre of a star here on earth, how hard can it be?

[TimberWolf]

I am not sure what bankers have got to do with this, other than the fact that 60% of their earnings go to government who should be supporting these sorts of projects.

Yeah, but their earnings come from the government [or rather, the tax payer]

Perhaps you're right, maybe when all the pluses and minuses are cancelled out they do actually end up with a pluses. Not at the moment though, it's minuses all the way, and bonuses?

[doodab]

They were talking about this on R4 this morning, apparently they're not too far off break even on energy in/out.

Could be the dawn of a new age?

Yes, a new age of profitability for the utility companies who will charge us all extra for using their new "green" energy.

[gingerjedi]

They were talking about this on R4 this morning, apparently they're not too far off break even on energy in/out.

Could be the dawn of a new age?