Commercially Feasible Fusion Reactor

[etudiant]

At an ITER visit two years ago, a senior specialist from the PPPL showed a chart tracking the figure of merit of fusion devices, a combination of temperature and confinement time, over the past 50 plus years. The improvement trend was pretty linear and about 10 years away from reaching levels sufficient for viable fusion.
So I'm much more hopeful than most here that there will be practical fusion devices by 2030, even though ITER will still only be in its shakedown phase at that point.

 

[girts]

I remember reading some popular science articles about fusion VS fission some time ago and what I noticed is that many of them echo some slogans like fusion is much cleaner etc (to which i know the answers myself) but what especially attracted my attention was the emphasis that fusion would produce much more power than fission, which I think comes from the fact that the fusion of certain light elements release more energy per nucleon than fission of the known fissile elements like Uranium or Plutonium per nucleon, which I assume is true.

But isn't this notion false or at least not objective because even if the per nucleon energies are higher, any practical devices have other constraints like particle density and confinement times etc aka Lawson criteria.
As far as I can see current fusion devices are far less energy dense than current fission reactors so for a given size would produce less thermal energy correct?
Because enriched U235 fuel rods are very dense so even if the per nucleon energy is lower there are many more of them in a given space while in a fusion reactor the per nucleon energy is higher but there are far far less nucleons in a given space not to mention the fact that only a small fraction of those nucleons fuse at any given time, so would it be fair to say that current fusion energy density is much worse than that of fission devices?What is there to do to increase the fusion energy density? Do we have to increase the density of plasma and it's fusion rate?

 

[mfb]

Comparing the power per fuel mass across different energy sources is meaningless if you don't take the different availability, cost and so on of these things into account.
Both deuterium and lithium are easily available, luckily.

The power per volume would be roughly similar, the power per mass would be much higher for fusion, the available energy per volume of the fuel would be much lower for fusion. As corollary, fusion reactors would exchange their fuel much more often (on a timescale of minutes instead of months).

 

[girts]

Ok so basically what you are saying is that if we took 1kg worth of U235 and put next 1kg worth of deuterium and tritium (500g each) the power we could extract from DT using fusion would be much more than the maximum power we could extract from fission using U235, because the fusion process of DT releases something like 17.6Mev per nucleon? while fission of U235 was 7Mew?

but because U235 is a solid heavy metal while Deuterium and Tritium are gaseous under atmospheric temp and conditions that 1kg worth of fuel takes up much more space right so from an energy density per volume viewpoint it being a gas is far worse?

Does this mean that once we arrive at a 1000 MW or more fusion generating plant it would need very large tanks for deuterium and tritium storage so that it is readily available if peak power is demanded from the machine?
I assume one can cool deuterium and tritium to very low temps using cryogenics and then they could be stored as liquids greatly reducing the space but that would require energy, I wonder how much though?

 

[mfb]

Fission of one uranium nucleus releases about 200 MeV, a bit less than 1 MeV per nucleon.
DT fusion releases about 18 MeV, a bit more than 3 MeV per nucleon (~4.5 MeV per nucleon if we don't consider the neutron which is recycled to breed new tritium).

but because U235 is a solid heavy metal while Deuterium and Tritium are gaseous under atmospheric temp and conditions that 1kg worth of fuel takes up much more space right so from an energy density per volume viewpoint it being a gas is far worse?

Sure, but it does not matter, as storage costs for both are negligible. A fusion power plant would work with a few kilograms of deuterium and tritium on site, enough to power it for days. Handling tritium is a bit difficult due to its radioactivity, but the deuterium to run a power plant for years can easily be stored in a small room.

 

[etudiant]

From a power industry perspective, none of the current fusion designs make much sense.
They all involve enormous capital costs, because the devices are enormous to offset our lack of skill in managing plasmas.
Big complex devices are not practical, we can't even run a nuclear fission plant at acceptably low enough failure rates.
So the hope is that mainstream efforts such as ITER will get superseded by some more creative approach.
There is plenty of time, remember that ITER only anticipates structural completion by 2025 and tritium based power generation tests by 2035.
Assuming ITER has shown feasibility, a prototype power generator is to be built around 2050. So other approaches have at least 30 years to show their stuff.

 

[mfb]

Big complex devices are not practical, we can't even run a nuclear fission plant at acceptably low enough failure rates.

We do run fission power plants at acceptable failure rates. Apart from a ridiculous design and a huge operator error, there was just one accident with a problematic release of radioactive material - after one of the largest earthquakes ever recorded. With zero expected deaths from the accident itself (compare this to 20,000 from the tsunami). Anyway, fusion power plants cannot explode, so this point is irrelevant for them.

So other approaches have at least 30 years to show their stuff.

And they had decades already. So far tokamaks and stellarators look the most promising.

 

[girts]

well I would say it like this, both Chernobyl and Fukushima were entirely human errors not mechanical ones, they were just errors on different levels, chernobyl was a unsafe design built because it was easy and fast to build and cheap while very powerful at the same time and served multiple goals simultaneously, also the operators that carried out the experiment went berzerk due to multiple complicated reasons such as incompetence, stress under supervisors and authorities etc etc.

as for Fukushima, I think the planners made some cost cuts that can be labeled as criminal in the long term, they could have put the emergency diesels more uphill away from coasts on higher ground, few hundred meters of electrical cables don't cost that much , a melted down nuke plant and cleanup costs like 1000 times the price of double safety standards and yet corporations still make everything as cheap as they can so that it fits the bill.the only real mechanical fault to my mind was Three Mile Island, where equipment malfunction and wrong readings made the operators unaware and blind to the real and troublesome state of affairs.

just my two cents.

 

[LURCH]

http://iopscience.iop.org/article/10.1088/1741-4326/aa7f41/meta

Reactor walls meaning some outer containment of neutrons and other particles emitted from the reactor core?
Could those walls be constructed in a way that produces useful isotopes when they are worn out so have to be recycled.

I recall reading a long time ago about the possibility of using liquid lithium as the wall of the reaction chamber. The lithium would continually flow, so any contaminants are carried away, damage to the chamber wall is nearly impossible , and the lithium bombarded by neutrons would produce tritium. Found a link to a more recent article, working on how to post it here.