[atomic128]

Neutrons make hardware radioactive.

Many on Hacker News fantasize about fusion (not fission) reactors. These fusion reactors will be an intense source of fast neutrons. All the hardware in a fusion reactor will become radioactive. Not to mention the gamma rays.

If you have to deal with radioactive materials, why not just use fission? After 70 years of working with fission reactors, we know how to build and operate them at 95%+ efficiency. Fission can provide all the power we need.

Today there are 440 nuclear fission reactors operating in 32 countries. 20% of America's grid power comes from nuclear fission. If you want to develop energy technology, focus on improving fission. For example, TRISO fuel (https://news.ycombinator.com/item?id=41898377) or what Lightbridge is doing (https://www.ltbridge.com/lightbridge-fuel). Hacker News is hostile to fission and defeatist (unable to contemplate innovation in fission technology) but this attitude will gradually change.

Quoting John Carmack: "Deuterium fusion would give us a cheap and basically unlimited fuel source with a modest waste stream, but it is an almost comically complex and expensive way to generate heat compared to fission, which is basically 'put these rocks next to each other and they get hot'."

[jylam]

I'm not a specialist but here is what I think I know (I'm talking with the point of view of a Frenchman, who consumes most of his electricity from (fission) nuclear power plants):

1/ Uranium is not a renewable (quite the opposite), needs to be mined and treated (which is expensive and very polluting), and not present at the required concentrations in most of the world (this creates geopolitical issues).

2/ Fission nuclear plants require a well functioning [state|government], and no war. A (conventional) strike on a nuclear power plant can have devastating and lasting consequences. Even a random terrorist group can do that.

3/ I've read that "Ultimately, researchers hope to adopt the protium–boron-11 reaction, because it does not directly produce neutrons, although side reactions can" (that's a wikipedia quote, but I've read that already from other sources).

So fusion doesn't seem the best option on the short term, because of the complexity and cost of research, but definitely seems to be the very best option in the middle and long term. And we made the short term catastrophic choice already with coal and oil, it'll be good to learn from that.

Or maybe I'm totally wrong.

[adrian_b]

Deuterium is also not renewable, even if it is more abundant than uranium.

The H1-B11 reaction would be a much better energy source than anything else, but for now nobody knows any method to do it. There is no chance to do it by heating, but only by accelerating ions, and it is not known how a high enough reaction rate could be obtained.

[rnhmjoj]

I'm curious, what are you considering for stating that deuterium is not renewable? AFAIK there's an essentially limitless supply in the form of HDO in the oceans[1] and there are cost effective methods[2] to isolate it.

[1]: https://en.wikipedia.org/wiki/Semiheavy_water

[2]: https://en.wikipedia.org/wiki/Girdler_sulfide_process

[adrian_b]

If you are able to say that there is a limitless amount of deuterium in the oceans, than you can say the same about the amount of uranium in the oceans, even if the amount of dissolved uranium is about one thousand times less.

Both the amounts of deuterium and of uranium in the solar system are finite and smaller than of the abundant elements. Moreover, the natural processes that create deuterium and uranium within a normal stellar system are slower than those that destroy them, so there is no chance of their quantities ever increasing.

Unlike using other chemical elements to make some stuff, using deuterium or uranium for producing energy destroys them without any means to regenerate them, so it is by definition a non-renewable process.

The hydrogen (protium) in the Sun is also non-renewable, but its quantity is enormous in comparison with the amount of deuterium existing on Earth (and the amount of energy that the Sun produces per proton is greater than the amount of energy that can be produced per deuteron).

Like deuterium is extracted from sea water, uranium can also be extracted from sea water, where it is one of the most abundant metals, except for the alkali metals and the alkaline earth metals. However the energy required for extracting uranium is significantly higher, due to its much lower concentration than deuterium (though deuterium is difficult to separate due to its similarity with the lighter isotope of hydrogen, while for the uranium ions much more efficient chemical reactions would be possible, which would bind uranium ions without being affected by the other dissolved ions).

[perihelions]

Then wind power is not renewable either! The saturation wind power potential of this planet (250 terawatts?), integrated from now until this planet ceases to exist, is a finite number—and it is actually a smaller number than this planet's deuterium resource.

[ben_w]

> Deuterium is also not renewable, even if it is more abundant than uranium

Technology correct, in that after around a hundred trillion years even the red dwarf stars will have stopped burning hydrogen.

But last I checked as yet there is no known way to harness the only (and even then merely suspected) infinitely renewable energy source: the expansion of the universe.

[adrian_b]

The amount of deuterium contained in a planet is a very small fraction of its hydrogen content.

The amount of hydrogen contained in a medium-sized planet like Earth is extremely small in comparison with the amount of hydrogen contained in a star.

The amount of energy that can be produced by fusion per deuteron is smaller than the amount of energy that is produced in stars per proton.

With all these factors multiplied, the amount of energy that could be obtained from all the deuterium contained in Earth is many orders of magnitude smaller than the energy produced by the Sun or by any other star.

Moreover, the energy obtained from fusion could never exceed a very small fraction of the energy received by Earth from the Sun as light, otherwise it would lead to a catastrophic warming of the Earth.

Nuclear fusion reactors are not really useful for solving Earth's energy problems. They could have a crucial importance only for the exploration of the Solar System and for providing energy for human bases established on Moon, Mars or other outer planets.

For Earth the only problems worth solving are how to make better batteries, including very large capacity stationary batteries, how to make other large capacity energy storage devices, e.g. thermal devices, and how to improve the energy efficiency of the methods used to synthesize hydrocarbons from carbon dioxide and water.

Making hydrocarbons at large scale from carbon dioxide would be the best way to sequester carbon dioxide, offering the choice between just storing the carbon in safe products (paraffin like) and using a part of the synthesized hydrocarbons for generating energy in a carbon-neutral way.

[ben_w]

I am already aware of those things.

On earth, there is an estimated 4.85×10e13 tonnes of deuterium; the energy density is 3.4x10e14 J/kg, giving a total yield of 1.649e31 joules. If you deleted the sun, this would be sufficient to maintain the current temperature of the Earth for ~9.5 million years: https://www.wolframalpha.com/input?i=%281.649×10%5E31+joules...

At "merely" the level of current human power consumption, this will last about 43 times longer than C3-photosynthesis, about 26 times longer than the oceans, about 5 times longer than before Andromeda merges with the Milky Way, and 6-3 times longer than when the Earth is currently expected to be absorbed into the outer envelope of the sun as it enters the Red Giant phase: https://www.wolframalpha.com/input?i=%281.649×10%5E31+joules...

Even if the sources I read giving those estimates are off by a factor of 10, deuterium alone, from earth alone, used as a total replacement for the sun, would still last longer than our species is likely to last before even natural evolution would have us speciate.

In the hypothetical future where we had a useful fusion reactor, the gas giants become harvestable, so the fact they're not on earth is unimportant. Likewise, on this timescale, every star in the nearest several galaxies — indeed, even absent novel technology and "merely"(!) massively scaling up what we've already invented, we already 'know'* how to get to places so far away that cosmic expansion is what would prevent a return trip.

As I said, it's technically correct that it is a finite resource. All I'm saying is that this is not a useful point on the scale at which we operate.

I expect it will be a useful point when we're star-lifting, but not now.

> Nuclear fusion reactors are not really useful for solving Earth's energy problems. They could have a crucial importance only for the exploration of the Solar System and for providing energy for human bases established on Moon, Mars or other outer planets.

I agree, however I also hope nobody makes a convenient cheap fusion reactor due to the proliferation impact of an affordable switchable source of neutron radiation.

> For Earth the only problems worth solving are how to make better batteries, including very large capacity stationary batteries, how to make other large capacity energy storage devices, e.g. thermal devices, and how to improve the energy efficiency of the methods used to synthesize hydrocarbons from carbon dioxide and water.

FWIW, I think that — if only we could cooperate better — a global power grid would be both cheaper and better than stationary batteries. Even just made from aluminium, never mind superconductors (and yes, I've done the maths). But we'd still need mobile batteries for transport, so that's fine.

The cheap abundance of PV power even today means I don't think we need to care much about making hydrogen electrolysis more joule-efficient.

> Making hydrocarbons at large scale from carbon dioxide would be the best way to sequester carbon dioxide, offering the choice between just storing the carbon in safe products (paraffin like) and using a part of the synthesized hydrocarbons for generating energy in a carbon-neutral way.

I suspect that carbon sequestration is unlikely to be a great win: there's a very narrow window close to zero loss/profit where on the loss side it's still cheap enough that people do it because it's a vote winner and on the profit side where it's not so profitable that people break photosynthesis a few hundred million years before natural processes do it.

* in the sense that Jules Verne "knew" how to get to the moon: the maths wasn't wrong, but the engineering was only good enough for a story

[DoctorOetker]

Do you have an opinion on using N2O (laughing gas) as an energy carrier?

2 molecules of N2O exothermically react to form 2 x N2 and 1 x O2 molecules, approximately the same composition as our atmosphere.

It is a very potent greenhouse gas, so quite disturbing on that front.

I've been making calculation for designing earth suits, where the suit replaces the home, internal showering, ventilation, heat recovery etc. Using N2O for heating looks rather promising because with fossil fuels one is forced to lose heat by inefficient heat exchange or forced to be exposed to the exhaust fumes; laughing gas decomposed is just warm atmosphere like air.

[fragmede]

Just tie the end of an infinitely long string to the edge of the universe and have it pull on a generator to spin it.

[DoctorOetker]

I would certainly like to see serious critical analysis and calculations of such hypothetical setups by physicists.

Would such a setup slow down the local expansion (action and reaction)?

Since iron is essentially a nuclear ground state, a steel cable being lengthened seems like the least worse mass loss imaginable.

[to11mtm]

1: Well if society could get at least some of their shit together we could do breeders. Alas, someone shot an RPG at Superphenix and that put a damper on a lot of things...

But it's not impossible. Japan seems to do most things decent from a 'security' standpoint, also interestingly for all of the other 'grey-market' stuff out there in the category of "shouldn't be radioactive but is" I have yet to find anything about AliExpress selling fissiable materials.

2: Yes and no and how much do you want to spend to improve the breach/damage ratio. i.e. PBRs have relatively low risk under a number of circumstances but have higher operating/etc costs.

I should also possibly question, what are the potential failure modes of 'not short timeframe fusion reactions'? I honestly have no clue whether they would quickly cease or if there are other potential side effects.

3: Agreed that neutron stuff can be solved in many ways, I do have some questions about maintaining that across various fusion designs. Big challenge is that we aren't 'there' yet.

> So fusion doesn't seem the best option on the short term, because of the complexity and cost of research, but definitely seems to be the very best option in the middle and long term. And we made the short term catastrophic choice already with coal and oil, it'll be good to learn from that.

Agreed that Fusion is the ideal long term, hopefully my comments didn't cause thoughts otherwise. I think we need more funding into it, and maybe even research as to how to have other renewables (e.x. solar) help feed into the initial startup/restart process for plants. We have had decades without sufficient funding of research.

I will say however, especially in relation to my other point-comments, that other countries (re?)embracing fission in the meantime will likely still lead to discovery of better techniques to deal with 'shared' concerns between fission/fusion such as neutrons/weigner engergy/etc

[cosmic_quanta]

> Neutrons make hardware radioactive

True, but two caveats:

1. Neutron bombardment due to fusion makes hardware radioactive for less than 10 years, which isn't great but does not compare to fission waste;

2. Some fusion processes don't emit neutrons (aneutronic fusion). As I understand it, these processes aren't as efficient, but there is the possibility of a tradeoff between generation of ratioactive waste vs. efficiency.

[adrian_b]

> Neutron bombardment due to fusion makes hardware radioactive for less than 10 years

Very false. The current design target for fusion reactors is that the materials taken out of the reactor should become "low-level radioactive waste" after being stored for one hundred years.

It is acknowledged however that it is likely that a small fraction of the materials will not satisfy the criteria for "low-level radioactive waste" even after one thousand years.

For example it is extremely difficult to avoid using carbon in the reactor. Besides various kinds of steels used in reactor components there are now some proposals to replace the tungsten used in the plasma-facing surface with some carbides, for increased endurance. Carbon 14 remains radioactive for thousands of years.

There are many commonly used materials for which substitutes must be developed, e.g. new alloys, because otherwise they would produce radioactive isotopes with lifetimes of tens of thousands of years, e.g. there are efforts to develop some stainless steels with chromium and tungsten as a replacement for the normally used steels with chromium and molybdenum, which would generate long-lived radioactive waste.

See e.g. the UK governmental report:

https://assets.publishing.service.gov.uk/media/61ae4caa8fa8f...

[cossatot]

There is a trade off between the half life and the intensity of radiation (i.e. the number of particle emissions per unit time), correct? So even if waste products are radioactive for thousand of years, they can be more easily handled than materials with a faster decay rate, even if they need to be stored for longer.

[lazide]

Not if total flux at the beginning is still higher - and some elements will get very high flux.

[cosmic_quanta]

Ah well I'm sorry, I must misremember

[khuey]

Aneutronic fusion is even harder than regular fusion so it's not a realistic solution in any near-term scenario.

[Symmetry]

It has the advantage that the energy it gives off can be be converted directly to electrical energy rather than driving an external heat engine, so despite the greater difficulty of ignition its not obviously a worse choice.

[fusionadvocate]

That is incorrect. Recent advances using attosecond lasers enable new tricks and fusion conditions to be realized tabletop. Search also for plasmonics. Using nano antennas and intense lasers to accelerate protons and electrons in a tabletop device (previously required large machines).

[ben_w]

Fusors already enabled desktop fusion reactors, literally high school science fair projects even a couple of decades back.

What stops Fusors and Polywells from having already given us this decades ago with P-B11 etc. is that the cross section for fusing is so much lower than the cross section for elastic scattering, and that elastic scattering loses so much energy to EM via bremsstrahlung.

[rnhmjoj]

Unfortunately is pretty far from "less than 10 years", which I guess you got from the half life of tritium. Tritrium radiocativity, in the form of tritium retained in the plasma facing materials, does contribute in that order of years if done properly, but neutron activation dominates and it's unavoidable. The actual numbers are in the order of hundres of years, still a lot less than fission high level waste, but let's not make unreasonable expectations around fusion, please.

You can find here a good comparison in terms of radiotoxicity vs years after plant shutdown for a few designs in this article [1].

[1]: https://doi.org/10.1016/j.fusengdes.2018.05.049

[elashri]

> we know how to build and operate them at 95%+ efficiency. Fission can provide all the power we need.

I am not sure what do you mean by 95%+ efficency here. But if you are talking about the entire process of getting the energy/power from the nuclear reactor this is not possible. You are still limited by carnot cycle. Even the most advanced reactors like HTGRs [1] operate with efficiency about 45%.

If you have some other definition of efficiency than the standard then it would be good if you define that.

[1] https://en.wikipedia.org/wiki/High-temperature_gas-cooled_re...

[atomic128]

See discussion of "capacity factor" here:

https://news.ycombinator.com/item?id=41858892

It's the same as when we talk about the efficiency of a GEMM kernel on a particular piece of hardware. As efficiency approaches 100% the kernel is saturating the hardware's capacity to perform multiply/add.

[howenterprisey]

The term "capacity factor" should generally always be used for that concept, because "efficiency" has its own, different, meaning for power plants.

[_neil]

You posted this same comment[0] nearly word for word a month ago. Why is that? Not sure why, but “many on hacker news fantasize about fusion (not fission)” stuck in my head.

[0] https://news.ycombinator.com/item?id=41677250

[Borg3]

Thats least of your problem imo. Neutron corrosion is bigger problem. There is trick to use Lithium shielding, with create Tritium needed for Fussion. But not sure how effective it is, especially for long term reactor lifetime. Those reactors are very expensive, not sure if its worth to shut it down every year and replace entire Li shielding...

[throwup238]

I think beryllium is a better candidate. It can be grown as a single crystal and there’s lots of research into using it for shielding in nuclear lightbulb reactors.

[perihelions]

You're overlooking the other requirement of the blanket—that it captures fusion neutrons to breed tritium, and provides a self-sustaining, closed fuel cycle. Lithium is mandatory for a D+T reactor blanket, because of these reactions:

https://en.wikipedia.org/wiki/Tritium#Lithium

(Do you have a link about that beryllium nuclear lightbulb rocket? It sounds interesting).

[rnhmjoj]

Beryllium is a good plasma facing material (low Z, low retention, low activation) and acts as a neutron multiplier, but it's highly toxic: only a few months ago ITER announced they scrapped the design of the first wall because working with beryllium was causing too many complications and slowing the project even more.

It's also so rare to be completely unsuitable for a power plant: a single DEMO-like reactor with a ceramic blanket (HCCB design) would require 70% of the world beryllium output to build and then burn through 200kg/year. Essentially you could only build a couple of these.

[LargoLasskhyfv]

Who is researching nuclear lightbulb reactors?

[defrost]

Thomas Latham was, in 1991 ... today, not so much.

https://ntrs.nasa.gov/api/citations/19920001892/downloads/19...

https://arc.aiaa.org/doi/10.2514/6.1991-3512

[fragmede]

> Many on Hacker News fantasize about fusion (not fission) reactors. These fusion reactors will be an intense source of fast neutrons. All the hardware in a fusion reactor will become radioactive. Not to mention the gamma rays.

My personal ideology about fusion aside, it should be mentioned there is an easy fix for these radiation problems. What you do is put the fusion reactor in space, and collect the energy with specialized fusion energy collectors on Earth (or in space). They'll have the problem that they aren't able to collect energy if the fusion reaction is below the horizon, so this design is imperfect, but having the fusion reaction take place in space means you don't have to deal with a radioactive casing by not including it in your fusion reaction space station design because you don't need any. Just a bit of hydrogen, a tiny bit of helium, and a some time.

[koe123]

For such an approach I’ve always thought it seemed a risk to launch something like that into the air. E.g. what happens if the rocket explodes while taking off? Or something bad happens when in space? Will it rain nuclear material?

[fragmede]

Obviously you just put the nuclear material inside of the in-flight data recorder so it will survive a rocket failure.

If you're being serious, Cassini had those kinds of questions with its launch about its RTG but that didn't have enough nuclear material for it to be a problem.

If we were to try and use a fusion reaction in space, we'd probably use the existing one.

[UltraSane]

Exactly. We should be working on making nuclear reactors cost $1/watt to construct. I can't see a technological reason why they couldn't be that cheap to build.

[ben_w]

> Hacker News is hostile to fission and defeatist (unable to contemplate innovation in fission technology) but this attitude will gradually change.

Lots of us like fission and think the fears are overestimated.

Nevertheless, the observation is that new developments in fission tend to result in the cost increasing, not decreasing.

And I say that as someone with a similar mindset regarding fusion, though for different reasons: you can pick aneutronic fusion reactions… but look at what weapons can proliferate with transmutation from the neutrons you can also choose, and ask which governments will turn them down.

[vilhelm_s]

The radioactivity generated from neutron activation is low-level, so you don't need to worry about accidents releasing lots of radioactivity, or about how to store waste for a long time. There are a lot of people worrying about those two things for fission reactors.

Also, the fuel for fusion reactors is much more plentiful. If we went all in on fission we might run out of easily minable uranium ore in a century or so, so it would be nice to have fusion reactors ready to take over then.

[adrian_b]

The radioactivity generated from neutron activation is not at all low-level, because the neutron flux is huge, providing most of the energy generated by the fusion reactor.

The intense neutron flux will transmute a very high number of atoms, so when taken out of the reactor all materials are very highly radioactive.

What can be hoped is that there may be choices for the materials used in a fusion reactor that will ensure a short enough lifetime for the radioactive isotopes, so that the radioactivity of the contaminated materials will become low-level soon enough.

The studies that I have seen have the target that the radioactive waste produced by a fusion reactor should become low-level radioactive waste after one hundred years.

To reach this target, many commonly used structural materials, like many types of steel, must be completely avoided, e.g. any steel containing nickel, molybdenum or niobium. Even the carbon from steel is a problem, because the radioactivity of C14 will persist for thousands of years.

A smaller fraction of the materials, particularly from highly activated plasma facing and near plasma components, may fail to meet current low-level waste criteria even after one thousand years.

See e.g. the report:

https://assets.publishing.service.gov.uk/media/61ae4caa8fa8f...

[pas]

How does the waste distribution and amount compare to fission reactors? (Pressure vessels, spent fuel rods, what else?)

If we need to to dump every 50 year a few thousand tons of steel into the old uranium mines that seems like a good deal, no?

[lupusreal]

The only hard part of dealing with nuclear waste is the social aspect. If not for that, you can simply and safely dump it into the ocean. Water is excellent shielding and the amount of uranium/etc already dissolved in sea water is absurd. Put it in a stainless steel vessel first if you want most of it to decay before coming into contact with the water, but that's not even necessary.

[perihelions]

That doesn't really work because marine life is good at filtering and concentrating a subset of the elements that are in spent nuclear fuel. There are already ocean fish that are too poisonous too safely eat because of (coal-emitted) mercury pollution—and that's only 100,000 tons of mercury, total, in the history of human industry [0]. If you dig in to the hard numbers surrounding spent fuel, it's a much, much more toxic and difficult problem than mercury—diluting it in the oceans is a complete non-starter.

[0] https://en.wikipedia.org/wiki/Marine_mercury_pollution

[meindnoch]

Mercury from burning coal is an extremely dilute pollutant. There's zero hope for capturing and containing it. Nuclear waste in contrast is literally just barrels/boxes of stuff. You can pick it up with a forklift and put it inside a sealed container for the next thousand years.

[cesarb]

> Nuclear waste in contrast is literally just barrels/boxes of stuff. You can pick it up with a forklift and put it inside a sealed container for the next thousand years.

You can't pick it up with a forklift to put it inside the sealed container. That would make the forklift (and its operator) radioactive. You can only use a forklift after it's already within the sealed container. See for instance this real-life video (shot on a nuclear power station in my country), which shows used nuclear fuel rods being put inside a sealed container for long-term storage: https://www.youtube.com/watch?v=7X5K46ALdD0

[meindnoch]

You park the forklift in the storage container and seal it in. It's not that hard.

Anyway, the forklift example wasn't about literally picking up pieces nuclear fuel with a forklift. You obviously use the forklift to move the shielded container around which contains the nuclear waste. Nuclear fuel in general is always at least in a water bath, which shields the neutrons, so your forklift is going to be fine.

In contrast to nuclar fuel, you cannot use a forklift (or any other equipment) to feasibly pick out the evenly mixed mercury atoms from the ocean or the atmosphere that we put there from burning coal.

[LargoLasskhyfv]

s/ocean/subduction zone/

aka the solution to pollution is magmatic delusion err... dilution.

[avery17]

I really hope you aren't serious. Safe dry-cask storage on site is already a fantastic solution.

[criddell]

How long do we have to store it on site? Does it take any maintenance? Is there any reason to be worried about people stumbling upon it and opening it up in the distant future where nobody can read or understand English anymore?

[bongodongobob]

If its half life is so long that you're afraid people won't be speaking English anymore it means it's not that dangerous.

[bongodongobob]

If it's half life is so long that you're afraid people won't be speaking English anymore that means it's not that dangerous.

[lupusreal]

I'm completely serious. It was done extensively during the 20th century and never became an environmental issue. Nuclear waste is a social problem, not a technical problem.

[lazide]

They were referring to the Sun.

[raverbashing]

Fission is "simple" but it seems every designer in the XX century made it as much complicated as possible for not so great reasons (and don't even get me started on the "let's not use breeding reactors" stuff)

Cooling that requires pumps, as an example, should be a non-starter in new projects.

[rob74]

The designs are complicated, well, because in practice it's not as simple as "put these rocks next to each other and they get hot". When you put the rocks next to each other, they not only get hot, but also emit some nasty radiation that has to be shielded. And if the rocks get a liiiitle bit too close together, they might explode, which leads to huge headaches for everyone involved, so you'd better make sure that doesn't happen...

[meindnoch]

>And if the rocks get a liiiitle bit too close together, they might explode

Impossible. Worst thing that can happen without carefully designed explosive lens is a nuclear fizzle.

[empath75]

There are lots of ways that nuclear plants can explode or fail in otherwise catastrophic ways, it doesn't need to be an atomic explosion.

[meindnoch]

Parent was alluding to a nuclear explosion, not a steam explosion or other type of explosion. Other kinds of explosions have nothing to do with the fissile material ("rocks" in their parlance) being "too close together". Steam explosions in particular are caused by boiling water, due to increased reactor power, or inadequate circulation of coolant. In a nuclear reactor, the fuel cells are held in a fixed matrix, and are not moving an inch closer to each other, whether the reactor is operating normally, or a steam explosion is imminent.

In general, nobody was disputing the possibility of steam explosions, or other type of failures at nuclear power plants, thus your comment is besides the point, and irrelevant to this subthread.

[SoftTalker]

Weapons proliferation concerns is/was the reason fission power is so complicated.