Laser fusion put on slow burn | Hacker News Reader

Laser fusion put on slow burn

nature.com

30 pointsananyob27 comments

Relevant graph:

http://commons.wikimedia.org/wiki/File:U.S._historical_fusio...

Here's the paper the graph refers to. The original graph is on page 12:

http://fire.pppl.gov/us_fusion_plan_1976.pdf

wow, that’s pretty sad. the 'moderate' path would have required an average expenditure of ~$2.5 billion a year, and projected the completion of a reactor by 2005. a pretty small investment when you consider the size of the federal budget [1] and where most of the funding goes.

even more sad, when considering the likelihood that many graphs relating to other potentially transformative technologies probably look much the same..

[1] http://en.wikipedia.org/wiki/Expenditures_in_the_United_Stat...

Uh... I would not take that graph as some sort of gospel. The truth is that even now we do not have an effective known path to economically viable fusion power generation, and if nothing else our computer simulations are indescribably better than anything they had in the late 70s. Had we blown billions more in the 80s, we probably would simply have blown billions more in the 80s and still have no fusion today.

that may be true, but getting it (or any new technology for that matter) to the point where it’d be economically viable requires an investment at some point..

and ‘blowing’ those billions in the 80s may very-well not have paid-off with economically viable fusion power today, but i highly doubt it would have left us with nothing to show for it.

We don't really have a hell of a lot to show for what we have spent. Non-zero, sure, but a lot of the lessons we learned could certainly have been learned for a lot more cheaply if we hadn't tried to leap to the end of the process with grandiose projects built more on hope than science.

$2.5B is roughly on par with the combined 2012 salaries of all NFL teams( $90MM - $118MM per team, at the cap ).

Which is as it should be, since football is far more important to humanity than something as silly as cheap, safe, and clean energy.

One thing I take from this graph is that doubling fusion funding would still leave us at "fusion never", so we shouldn't bother doubling it. :(

I'm all for fusion research. Love the idea of clean, inexhaustible power. However, the NIF has consistently over promised and under delivered, and they have no idea _what_ their problems are [1]. They could use a little restraint when making schedule predictions.

[1] http://fire.pppl.gov/NIF_NIC_report_rev5_koonin_2012.pdf

Yep. NIF is such an old project that it was used as the "digitizer" in the original TRON movie. Still no progress.

What a silly thing to say -- of course there has been progress, even if it's only a better understanding of what doesn't work.

Only the US bothers with lasers because everyone else understands that lasers don't work for fusion. Tokamaks are where it's at.

The only real use of NIF is to better understand fusion so as to better understand how nuclear weapons work. I just want them to be honest about it.

Only the US bothers with lasers because everyone else understands that lasers don't work for fusion.

https://en.wikipedia.org/wiki/Laser_Mégajoule

I personally think lasers are much more promising, if only because of how much room there is for disruption with solid-state lasers. Look at how horrible the NIF lasers are: ~1% efficient [0], billions of dollars, the size of a warehouse [1]. And the fusion end seems to work fine.

[0] https://lasers.llnl.gov/about/nif/about.php

[1] https://en.wikipedia.org/wiki/National_Ignition_Facility#Dri...

Uh, no. There have been multiple lasers built at LLNL of increasing power over the decades - NIF is the replacement for NOVA, which was the replacement for SHIVA - which was used in TRON. Each successive laser has been least an order of magnitude more powerful than the previous I believe.

And to say "no progress" is to belittle the efforts of a lot of people that are pushing the boundaries of physics and the associated engineering problems.

one thing i never understood about fusion is how you can capture all the released energy efficiently enough. isn't it released in many ways that cannot really be harnessed directly eg: gamma radiation, x-rays, emf. that makes me think that the input:output ratio would need to be not just marginally better, but sufficiently better. does anyone know what the current best input:captured number is?

Almost all of the energy comes out as kinetic energy of the fusion byproducts, which are mainly atomic nuclei and neutrons. Sadly, a great deal of the energy of the easiest to achieve fusion reactions (D-D and D-T) tends to exist in the neutrons released by the reactions. And those neutrons tend to fly right out of the charged plasma quite readily. This makes creating a self-sustaining magnetically confined fusion reaction more difficult but it also makes building a fusion reactor slightly challenging. But far from impossible. Neutrons can penetrate fairly deeply through many materials but fortunately ordinary water serves as a robust shield. So by surrounding a fusion reaction with water you can quite easily capture most of the energy of the fusion reactions and can use an ordinary steam turbine generator to produce electricity.

It depends on the type of fuel used. The holy grail is a boron + hydrogen reaction, which produces three helium nuclei.

Since the nuclei are positively charged and moving quickly, the net voltage is around 2 million volts. When done correctly, fusion will create electricity directly. At that point it's just a matter of stepping it down to a usable voltage, but we have lots of ways to do that.

Usually you are trying to catch neutrons in a lithium blanket, and using the fission products to extract energy. At least that is what they are doing at ITER. The electromagnetic energy goes back into the plasma and helps with keeping things hot. The energy extraction is one of the biggest challenges with sustained fusion.

isn't it released in many ways that cannot really be harnessed directly eg: gamma radiation, x-rays, emf.

Yes, just like nuclear fission! All this radiation gets absorbed by surrounding matter, and its energy becomes heat. Powers a steam or gas turbine -- just another heat engine (tm).

With fusion reactors, the radiation absorber is a blanket of pipes filled with molten metal, several meters thick and several tens of meters in inner diameter. There's a critical second purpose to this: creating fusion fuel. The molten metal contains lithium, which on absorbing neutrons transmutes to tritium (hydrogen-3), which can be scrubbed out and recycled. Tritium is fusion fuel; fusion reactors must create it like this in self-sufficient quantities.

About the fraction of energy captured; it's very close to 100% (only neutrinos escape), with the subsequent conversion from heat to electricity being some 30-50% efficient.

http://www.iter-industry.ch/wp-content/uploads/2010/01/Pr__s... (particularly first few slides)

http://aries.ucsd.edu/raffray/publications/JNM/ICFRM_10_Raff...

Misleading title, from the article NIF is a weapons research project.

to help weapons scientists to care for the ageing US nuclear stockpile

Simulating explosions helps determine maintenance for nuclear warheads?

I would bet they were more likely trying to make a "nuclear" weapon without radiation/fallout and failed.

I wonder what the world would be like with really cheap power though - I'd like to think a better place to live but more likely there would be a lot more war since the energy to do it would then be cheap.

Simulating explosions helps determine maintenance for nuclear warheads?

It's not completely unreasonable: they need to validate the computational models that are used to check if an aged nuclear weapon will explode.

Second order effects can be quite surprising: as gasoline replaced whale oil as a fuel source, it also increased the demand for cars. This accelerated the practice of whaling because whale oil remained extremely desirable as the lubricant inside transmissions.

One of the things with maintenance is that, although you do not want to use your them, you must make sure that your warheads work if you ever need them.

The traditional and easiest way of doing that is to frequently build new warheads, and test explode the older ones. That is expensive and, nowadays, forbidden/frowned upon (I think the test ban treaty hasn't been ratified)

So, one stops building new ones and ends up with thirty+ year old warheads. The radioactive materials inside them have aged, steel may have become brittle, etc. To ascertain whether these devices still work, tests are needed. You could do them on the devices themselves, but it is easier and cheaper to do an experiment with a few grams of radioactive materials than to work with a kg or so from a warhead (all IIRC; corrections welcome)

"Simulating explosions helps determine maintenance for nuclear warheads?"

Yes. Radioactive material is, by definition, slowly becoming non-radioactive by decaying into other atoms. You can think of those other atoms as 'contaminants' in your plutonium soup.

What this means is if you create a nuclear warhead, and put it on the shelf, if you did nothing you would have a very sophisticated box holding some lead. But long before that it stops working as a nuclear warhead.

So what you do is you take put one on the shelf in the lab and you sample it periodically to see how its content has changed, and run your simulations to see if plutonium or uranium with those makeups would still work within the boundaries of the device, and you try to test it periodically under conditions that simulate, as much as possible, an actual explosion.

They used to do this at the test site, basically explode one warhead from a batch every 2 - 3 years to insure they still worked. But they stopped when we banned such testing.

That's not the only problem. Plutonium has numerous allotropes, and the allotropes have different material properties such as hardness and density. Given that creating super-criticality in a fission weapon is highly dependent on density and that the implosion dynamics are dependent on the properties of the fissile material this has a significant impact on weapons design. Given that an allotrope of Plutonium can change into another over time this makes keeping an eye on Plutonium based nuclear warheads quite necessary.

Additionally, radioactive decay releases Helium gas (alpha particles) over time which will lodge in the crystalline structure and cause disruptions over time.

Either of these factors can result in a change in the reliability and yield of nuclear warheads kept in storage for a long period of time.

Studying fusion reactions and high energy ablation reactions definitely help understand nuclear weapons dynamics.

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