# Compact Tokamaks



## Foxbat (Jun 8, 2016)

Could the stalled fusion progress be about to get a kickstart?
Compact tokamaks: the approach to bring fusion energy within reach


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## Ray McCarthy (Jun 8, 2016)

Foxbat said:


> Could the stalled fusion progress


I don't think it is stalled.


> Traditional designs have moved to larger dimensions, culminating in the ITER experiment currently under construction in the south of France. This will be over 30m tall and weigh about 23,000 tonnes. The demonstration reactor that follows, dubbed DEMO, will likely be slightly bigger again. When ITER was being designed in the 1990s, it was believed that the only feasible way to increase fusion power was to increase machine size. But the size and complexity of ITER has led to very slow progress in the fusion program, with first fusion set for the mid 2020s. Tired of waiting so long and recognising the inherent difficulties of such a big project, some have been questioning the possibility of a smaller way to fusion.


The apparent slow progress is because it's not a priority. Economics driven.

The current round of USA VC funding is nothing to do with developments in science. There is no evidence any of it other than General Dynamics (which is looking for funding but not getting it. It may work as a smaller solution because it's not a tokamak). Small tokamaks are a nice idea, but there is no science to back it up.
So why are these people not investing in the small General Dynamics "bottle" design? Because in the real world such development to make it commercial isn't $100M but $10 Billion. That's why they are looking for a development partner and investment.

Really the linked article is "aspirational" and has no substance or science.
The Lockheed Martin / General Dynamics Compact Fusion Reactor is a real breakthrough.
LOCKHEED: We Made A Huge Breakthrough In Nuclear Fusion
Lockheed Martin Progresses on Nuclear Fusion Power Plant

A tokamak needs to be huge to have a net output. It's physics. There is a way to cheat and get net output today. Co-site at nuclear power stations. A Fusion reactor produces a lot of neutrons, which don't contribute to power generation. If nuclear power station waste is packed around the tokamak, it gets hot and is more quickly converted to low level waste, safe to dispose of. The heat makes steam for electricity turbine. This has been tested and gives a net output.


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## Foxbat (Jun 8, 2016)

Just my opinion: Not sure how effective these would be in reality.
Hybrids use high energy neutrons to create fission  in U-238. It gets hot because it's starting a new fission process in material not normally used. UK AGRs use slow neutrons to create the fission process. The use of a moderator in reactors may be problematic when it comes to a hybrid because its job is to slow energetic neutrons to increase the precentage chance of collision with a Uranium 235 nucleus(precisely- it seems- what is not required to create fission in U-238). It's not the easiest thing to remove in a reactor that has gone critical (although may be possible using techinques similar to graphite core sampling). 
PWRs  could be worse because the water is both moderator and coolant.


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## Ray McCarthy (Jun 8, 2016)

Foxbat said:


> Not sure how effective these would be in reality.


It's been tested. The fusion part is fail-safe, anything breaks and there is no fusion, thus no neutrons.
It's waste that's used, not the unused fuel, so a runaway fission reaction isn't possible. That's my understanding. You need fission fuel above a certain level of purity to get "meltdown", by definition, the idea is to use exhausted fuel rods.


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## Foxbat (Jun 8, 2016)

Ray McCarthy said:


> It's been tested. The fusion part is fail-safe, anything breaks and there is no fusion, thus no neutrons.
> It's waste that's used, not the unused fuel, so a runaway fission reaction isn't possible. That's my understanding. You need fission fuel above a certain level of purity to get "meltdown", by definition, the idea is to use exhausted fuel rods.


Nuclear fuel is primarily U-238 with anywhere between 1.5% to 4.5% U-235 enrichment (for UK AGRs). So, essentially, the unused U-238 and a small amount of transuranic elements _is _the waste.


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## Ray McCarthy (Jun 8, 2016)

Foxbat said:


> So, essentially, the unused U-238 and a small amount of transuranic elements _is _the waste.


I don't know what reprocessing there is between removing "used" rods and using "waste" around the tokamak, except that it allegedly makes the waste almost safe enough to bury in your garden and supposed to be an inherently safe way to get more power output, a net output from experimental fusion reactors today.
The problem is that regular nuclear power is very much out of fashion.


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