# What if we had commercial fusion power?

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#### phyzguy

There have been several recent threads on the feasibility of fusion power. There is reason to be hopeful. This study from MIT claims, due to the breakthroughs in high-temperature superconductors, that an economically feasible Tokamak can be built in the foreseeable future.

For the purpose of this thread, let's not debate that. Let's assume that the technical problems were solved and we had a working design for a fusion reactor with similar economics to existing fission reactors. Then my question is, is there any reason to believe that there would be greater public acceptance of this technology than there is of fission technology?

It's true that the radioactive waste produced by a fusion reactor will be less and less long-lived than that produced by a fission reactor. However, the radioactive inventory and waste stream from a fusion reactor will still be enormous. Public opposition to fission power is not based on quantitative arguments, but is more based on an "all radiation is bad," mindset. I worry that even if we had a working fusion reactor, the public response would be, "Wait a minute, I thought you told us this was clean technology? Now we find out that it still generates large amounts of radioactive waste."

This article highlights my concern. In many ways fusion reactors will have the same problems that lead to the public opposition to fission power. Comments?

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#### gleem

Well there you go. Same issues as fission. Still unsolved problems and who know what we will find when we actually start running them. So I see a significant cry of "not in my back yard". Is the devil we know better than the one that we don't?

Well all that said I think we should have a demonstration unit running for a significant period of time to fully assess the problems that must be addressed before going to the general public.

#### russ_watters

Mentor
There have been several recent threads on the feasibility of fusion power. There is reason to be hopeful. This study from MIT claims, due to the breakthroughs in high-temperature superconductors, that an economically feasible Tokamak can be built in the foreseeable future. For the purpose of this thread, let's not debate that.
Fair enough, but I'm still curious about it -- I'm not seeing the economic feasibility discussion in that link. Could you be more specific about where it is?
....is there any reason to believe that there would be greater public acceptance of this technology than there is of fission technology?
I think fusion is and will be viewed as the panacea solar is. I don't think acceptance is something to be concerned about.
Public opposition to fission power is not based on quantitative arguments, but is more based on an "all radiation is bad," mindset. I worry that even if we had a working fusion reactor, the public response would be, "Wait a minute, I thought you told us this was clean technology? Now we find out that it still generates large amounts of radioactive waste."
If 60 years and the prospect of imminent doom are not enough to turn the public's opinion on fission, I don't expect fusion's can be turned easily either.
It's true that the radioactive waste produced by a fusion reactor will be less and less long-lived than that produced by a fission reactor. However, the radioactive inventory and waste stream from a fusion reactor will still be enormous.

This article highlights my concern. In many ways fusion reactors will have the same problems that lead to the public opposition to fission power. Comments?
Blasphemy! Of course, fusion is the power of the sun and like solar is perfectly safe, clean, plentiful, scaleable and above all, completely free!

#### Tiran

One item missing from the list is the relative possibility of a meltdown. Given that fusion isn't based on a radioactive pile, I would imagine this is a rather unlikely possibility.

The unlikelihood of a Chernobyl or Fukashima alone may greatly affect public opinion in the positive.

Personally, I think that orbital (or regular) solar collectors would be a better way of powering the energy grid, but fusion would be an excellent replacement for powerplants on submarines and ships - or spacecraft.

#### anorlunda

Mentor
Practical fusion power plants are decades away (No, really. Real soon now.) But everything associated with electric power is undergoing dramatic change.

Thirty years from now significant changes are likely. Central power plants may be illegal. Power utilities as a business model may disappear. The public's thirst for energy may be quenched. The grid as we know it may not exist. blah blah.

All that makes it completely pointless to speculate on future public opinion on a particular power plant design.

By analogy, it would be like a public poll at the time of the Wright Brothers about airport noise 80 years into the future.

#### phyzguy

Practical fusion power plants are decades away (No, really. Real soon now.) But everything associated with electric power is undergoing dramatic change.

Thirty years from now significant changes are likely. Central power plants may be illegal. Power utilities as a business model may disappear. The public's thirst for energy may be quenched. The grid as we know it may not exist. blah blah.

All that makes it completely pointless to speculate on future public opinion on a particular power plant design.

By analogy, it would be like a public poll at the time of the Wright Brothers about airport noise 80 years into the future.
Well, in that case, maybe there is no point in continuing this thread. No objection from me if you close it.

#### gmax137

Well, in that case, maybe there is no point in continuing this thread.
Hey, not so fast! On a technical note, can someone compare the decay heat from a fission reactor to the corresponding residual heat from a fusion reactor? I just don't know anything about fusion, is there any appreciable decay heat? In a fission plant, safety is all about removing decay heat once the reactor is shutdown.
Is that even an issue with a fusion machine?

#### Klystron

Gold Member
[snip]
Thirty years from now significant changes are likely. Central power plants may be illegal.
[...snip...]
By analogy, it would be like a public poll at the time of the Wright Brothers about airport noise 80 years into the future.
@anorlunda Please elaborate on the possible illegality of central power plants unless you mean this an example of specious argument?

Your 'Wright bro's' analogy has merit as for instance newspaper and popular science publications from that era (circa 1905+) often discuss the drawbacks of automobiles frightening livestock; not envisioning that automobile traffic would rapidly replace horse-drawn vehicles even on farms. The concepts of horse and horsepower achieved near-mystical standing among the public; a mystique that quickly accrued to automobiles. Modern industries such as solar try to accrue similar mystique to sell their products.

Consider also that the typical pre-20th landowner likely owned horses, understood equine basics (e.g.; feeding hay vs. oats), and measured useful energy in horse units. A 21st homeowner can purchase solar technology, install panels on their house, even feed excess electricity back to the public power grid. Similar to recycling, adopting solar panels provides intrinsic benefits and status to the homeowner analogous to pre-20th horse ownership.

If these analogies have merit, public acceptance of fusion power sources might improve as the technology scales to more compact units such as in public transport and personal vehicles. Meanwhile the 'fission dirty; fusion clean', 'fission dangerous; fusion safe' rubrics may have to do within the end user community.

#### PeterDonis

Mentor
One item missing from the list is the relative possibility of a meltdown. Given that fusion isn't based on a radioactive pile, I would imagine this is a rather unlikely possibility.
It's an impossibility. There is no significant stored energy inside the reactor once it stops.

#### Tiran

It's an impossibility. There is no significant stored energy inside the reactor once it stops.
That doesn't mean that fusion reactor can't have some non-fission accident that would release considerable energy and atomize radioactive structure sending contaminates into the air.

#### phyzguy

That doesn't mean that fusion reactor can't have some non-fission accident that would release considerable energy and atomize radioactive structure sending contaminates into the air.
I think the most worrisome accident from a fusion reactor would be a leak that released a large amount of tritium. A fusion reactor would likely have on the order of 10 kg of tritium in inventory. If I calculate correctly, this is ~ 100X the radiation released at Fukushima, so an accident that released even 1% of the tritium inventory would be an equal amount to Fukushima. Also, since tritium is easily taken up by the body, it can have significant biological effects.

The metal structure of a fusion reactor becomes radioactive through neutron activation, but it's hard to imagine an accident that would disperse the solid structure of the reactor into the environment.

#### PeterDonis

Mentor
I think the most worrisome accident from a fusion reactor would be a leak that released a large amount of tritium.
This is the type of incident that secondary containment structures prevent.

#### phyzguy

This is the type of incident that secondary containment structures prevent.
If the secondary containment were always successful, we wouldn't have had a radiation release at Fukushima.

#### PeterDonis

Mentor
If the secondary containment were always successful, we wouldn't have had a radiation release at Fukushima.
The reason the secondary containment failed at Fukushima was loss of decay heat removal, due to poor siting of the backup switchgear (and also to the secondary containment not being designed to handle that amount of stress--since the design assumed decay heat removal would be present). A fusion reactor has no decay heat, so no decay heat removal.

#### Klystron

Gold Member
There have been several recent threads on the feasibility of fusion power. There is reason to be hopeful. This study from MIT claims, due to the breakthroughs in high-temperature superconductors, that an economically feasible Tokamak can be built in the foreseeable future.

The picture above the super-conductor article says much about this thread. The blue-shadow human figure gives a rough size factor of current technology. The cut-away view of the torus reminds me of the dee's in early cyclotron diagrams. Here's 2 2019!

#### Tiran

Without using the word "meltdown", can you describe what risk of a fusion reactor you are referring to in the quote below, that you think is missing from the list?
To clarify, I meant that a fusion reactor will be perceived to not have a likelihood of having the equivalent of Chernobyl or Fukushima type radiation containment failure, and that is why that item is rightfully missing from the list. And since it is missing from the list, a fusion reactor might garner support that a fission reactor lacks.

But I said that I "imagine it is a rather unlikely possibility" because the inside of a fusion reactor is the temperature of the inside of a star, and I have no idea what sort of possible fusion reactor failure modes could lead to a containment failure serious enough to unleash the energy necessary to destroy the reactor structure. It just seems likely that the answer is not 'zero'. Whether that failure is a full on nuclear detonation or merely enough instantaneous heat to slag a building, both would produce results of great public concern because radioactive material would be released.

It is a similar conundrum to battery powered cars. They obviously are more efficient and should pollute less than fuel burning cars, but that thinking contains the assumption that battery production and replacement pollution is negligible. Again, I don't have that information, but it is also unlikely to be zero. The battery car bandwagon will grow, but if some unexpected level of environmental impact is brought to light, public opinion could quickly turn.

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#### Lord Jestocost

Gold Member
2018 Award
1. A fusion reactor is much less likely to have an event like a "meltdown". You said it is "impossible", but phyzguy agreed with me in post 12.
On the hompage of the ITER organization (https://www.iter.org/sci/Fusion) one reads:

"No risk of meltdown: A Fukushima-type nuclear accident is not possible in a tokamak fusion device. It is difficult enough to reach and maintain the precise conditions necessary for fusion—if any disturbance occurs, the plasma cools within seconds and the reaction stops. The quantity of fuel present in the vessel at any one time is enough for a few seconds only and there is no risk of a chain reaction."

#### PeterDonis

Mentor
the inside of a fusion reactor is the temperature of the inside of a star
Yes, but temperature is not the same as heat content. The inside of a fusion reactor is a plasma with a very low density, and even at fusion temperature, there simply isn't that much energy there--many orders of magnitude less energy than is stored inside a fission reactor.

Whether that failure is a full on nuclear detonation or merely enough instantaneous heat to slag a building
Consider this: a power plant producing a typical amount of electrical power (1000 MW or so) is producing roughly a ton of TNT worth of heat every second. This is true regardless of the type of fuel it is burning; it's just simple numbers--a factor of 3 to 4 loss in the conversion from thermal to electrical power means 3000 to 4000 MW thermal, which is about 4 billion Joules every second, and a ton of TNT is $4.184 \times 10^9$ Joules.

For any power plant that injects fuel and burns it right away, which includes oil, natural gas, and fusion, the only potential heat source inside it at any given time is the fuel needed to produce a few seconds or so worth of heat when burned (the quote that @Lord Jestocost gave from the ITER page indicates a few seconds' worth). So at most there is a few tons of TNT's worth of heat in the fuel inside the reactor, if it is burned.

A nuclear bomb has the explosive energy of tens to hundreds of thousands to millions of tons of TNT, all released in a tiny fraction of a second. So clearly a fusion reactor has many orders of magnitude too little fuel inside it to produce a nuclear detonation of this kind.

A bomb containing roughly a ton of TNT (or the equivalent) can collapse an ordinary building if exploded. But it doesn't do that by melting the building; it does it by converting the energy in the TNT into a high pressure shock wave in the air that causes the building's structure to fail. Combustion of fuel inside a fusion reactor doesn't do that: there is no air, and the plasma inside the reactor has much too low a density to form a shock wave. Also, the energy produced by fusion is mostly carried by neutrons, which don't interact with the plasma anyway, they just hit the first wall, which is designed for that. So nothing like a conventional explosion that could collapse a building can happen inside a fusion reactor even while it is running.

And, of course, if the reactor is shut down, fuel stops flowing and there is no more heat produced.

Here's another way to look at it: I said above that a power plant producing a typical amount of electrical power is producing roughly a ton of TNT worth of heat every second, regardless of the type of fuel it is burning. That means that conventional oil or natural gas power plants are doing that too. (They produce the heat at a lower temperature than a fusion reactor--a few thousand degrees vs. roughly ten million--but the density inside them is correspondingly higher, so the heat content is the same.) So if those plants don't have a failure mode where the entire building gets collapsed because something goes wrong with the reaction, a fusion reactor won't either.

The reason fission reactors have this unique sort of failure mode, then, is evidently that there is a lot more stuff inside the reactor than just a few seconds' worth of fuel at any given time. There is a few years' worth (in a typical pressurized water reactor) of fuel inside, plus all of the fission products. So there is a huge source of heat that is still there even after the reactor is shut down. (Newer designs like pebble bed reactors are an attempt to address this problem.)

#### anorlunda

Mentor
The reason fission reactors have this unique sort of failure mode, then, is evidently that there is a lot more stuff inside the reactor than just a few seconds' worth of fuel at any given time. There is a few years' worth (in a typical pressurized water reactor) of fuel inside, plus all of the fission products. So there is a huge source of heat that is still there even after the reactor is shut down. (Newer designs like pebble bed reactors are an attempt to address this problem.)
Very good explanations Peter. But that last paragraph is unclear.

The primary safety problem with LWRs is not the quantity of fuel in the core but rather the quantity of fission products. Fission products produce decay heat, thus requiring continuous cooling. The large mass of unburned fuel is not a heat source after shutdown.

If the containment is breached and everything inside were to leak out (not realistic), it is the fission products, not the unburned fuel that creates hazards. U235 and U238 are not a radiation hazard, even if leaked.

Pebble bed reactors have the advantage of storing most unburned fuel and most fission products external to the reactor vessel. They also operate at high temperatures, so that even if not cooled following shutdown, they won't melt.

However, like all designs, pebble beds have their own safety problems.
1. Rainer Moormann (2008). "A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts". Forschungszentrum Jülich, Zentralbibliothek, Verlag. Berichte des Forschungszentrums Jülich JUEL-4275.
2. ^ Rainer Moormann (1 April 2009). "PBR safety revisited". Nuclear Engineering International. Archived from the original on 30 May 2012.

#### PeterDonis

Mentor
The primary safety problem with LWRs is not the quantity of fuel in the core but rather the quantity of fission products. Fission products produce decay heat, thus requiring continuous cooling. The large mass of unburned fuel is not a heat source after shutdown.
Yes, this is correct. But there is still a connection with the quantity of fuel in the core: the quantity of fission products is only an issue because the whole fuel assembly stays in the reactor for a few years--i.e., because the reactor is only fueled once every few years. If only a few seconds' worth of fuel was in the reactor at any given time, the fission products would be removed along with the burnt fuel every few seconds. (As you note, designs like the pebble bed reactor are basically trying to achieve something like this.) In such a design, decay heat after shutdown would not be an issue.

#### rbelli1

Gold Member
A fusion reactor would likely have on the order of 10 kg of tritium in inventory. If I calculate correctly, this is ~ 100X the radiation released at Fukushima, so an accident that released even 1% of the tritium inventory would be an equal amount to Fukushima. Also, since tritium is easily taken up by the body, it can have significant biological effects.
Only a small portion of that will be in the reactor at any given time. I would think that there would be some mechanism put in place to limit the spillage in the event of a catastrophic failure. Also when gaseous tritium is released it has the tendency to go straight up and rapidly escapes the atmosphere. Not that releases of it are healthy. Storage as oxide would prove rather more dangerous in the event of a spill.

https://www.iter.org/sci/FusionFuels

The intent is to produce the fuel as an ongoing part of operation. Only a small quantity will be needed on site at any given time. I would hope that the operators of fusion plants (if we ever get that far) will keep the inventory of expensive and dangerous fuel to a minimum.

BoB

#### phyzguy

The intent is to produce the fuel as an ongoing part of operation. Only a small quantity will be needed on site at any given time. I would hope that the operators of fusion plants (if we ever get that far) will keep the inventory of expensive and dangerous fuel to a minimum.
BoB
Do you have a source to back this up? The reactor requires a "blanket" capable of breeding tritium. This consists of lithium, tritium, and some sort of neutron multiplier, like lead or beryllium (see the source you cited). I suspect that the blanket will be highly radioactive, and represent a large inventory of radioactive material. The source you cited also says that a typical reactor will need ~150 grams of tritium per day. Given this, a total on-site tritium inventory of 10 kg, between fuel ready to be injected and tritium resident in the blanket, seems reasonable to me. Also, the fact that the tritium is stored somewhere else instead of at the reactor site doesn't lessen the chance of a release.

#### artis

I agree with PeterDonis , a fusion reactor can't have a meltdown, and as of this day it can't even have a self sustaining operation as we haven't got to breakeven point in other words power in= power out or 1:1.
Also the argument about plasma having a low , in fact very low density is very correct, speaking plainly the amount of heat energy (inertia) some material has depends on its temperature and weight, plasma may have an extreme temperature but it has almost no weight, tons of Uranium have much much less temperature in a reactor core but much more weight as was mentioned here before so takes much longer to cool. (Sure the complicated explanation involves decay products and half lives etc)

So apart from tritium fusion is very safe because all the other radioactive materials are solid and can't be simply released into environment I think , right?

But I would like to disagree with PeterDonis on Fukushima. I think Fukushima can be labeled as a core meltdown much like Three Mile Island because that is essentially what happened, the cores at Fukushima lost cooling water due to no cooling pumps working and decay heat evaporating the existing water, which then led to meltdown or partial meltdown of the core as in the physical core inside the core vessel melted. The core primary containment vessel may indeed have stayed intact (not sure and too lazy to search now) but still the core inside melted so I don't think it's wrong in Fukushima case to say that there was a meltdown.
When something physically melts and causes the reactor to be deemed lost accompanied by the release of fission products (yes in gaseous state but still) into the environment then I think it is fair to say it's a meltdown, what do you think?

#### gmax137

A fission power reactor generates about 1.8% decay heat at 30 minutes post shutdown. Then for a typical 1000 MWe plant (say 3400 MWth core) at 30 minutes we need to remove 58 MW or 55,000 Btu/sec.

For tritium, wiki says specific activity is 3.57E14 Bq/gram; via 5.7 keV betas plus antineutrinos. The antineutrinos leave carrying away their energy. So if we have 10,000 g tritium * 3.57E14 Bq/gram * 5.7 keV = 2.035E19 keV/sec = 3260 joule/sec = 0.00326 MW 0r 3.1 Btu/sec. Did I do that correctly?

Then the "decay heat" in the fusion machine is 3 Btu/sec vs. 55,000 Btu/sec in the fission machine. The safety case for these two is going to be completely different.

@artis, the 100 tons of uranium in the fission core don't really matter; what does matter is that the fission products generate heat after the reactor is shutdown (no longer critical). In other words, a fission core doesn't really have an "off switch." This is the central engineering issue with fission reactor safety. Apparently the fusion machines do not have this issue (see above).

#### PeterDonis

Mentor
I think Fukushima can be labeled as a core meltdown much like Three Mile Island because that is essentially what happened
It is true that there was core melting and damage in both cases, yes. But in TMI, the core did not melt because of lack of decay heat removal after shutdown; it melted because of a loss of coolant accident while the reactor was operating, because of a combination of faulty instrumentation, violation of NRC rules, and poor judgment on the part of the operators.

I think it is fair to say it's a meltdown, what do you think?
I think there has been enough argument about the usage of the term "meltdown" already in this thread.

As far as the public is concerned, I think the key things to focus on in any incident are:

(1) How much radiation has been released, and in what form? What precautions can be taken to minimize exposure? There is no way to describe this using a single word, whether it's "meltdown" or anything else.

(2) Can the release of radiation be controlled? If it can, as I mentioned in a previous post, then the costs and benefits of any radiation release can be discussed in advance, the timing can be planned, and the public can be warned and given time to take precautions. If the release of radiation is uncontrolled, none of those things can be done, which makes the whole event much more dangerous. There's no way to describe all this using a single word either.

My personal opinion is that the term "meltdown", in the minds of lay people, suggests, not just an event in which a fission reactor core melts, but such an event in which (1) a lot of radiation is being released outside the plant, and (2) the release is uncontrolled. In other words, a scenario like the one described in movies like The China Syndrome (in that movie, the scenario is narrowly avoided, but it is described as the core melting and sinking down through the containment walls and into the earth until it reaches groundwater, causing widespread release of radioactive water and steam). TMI met neither criterion. Fukushima arguably met criterion #1, but not #2.

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