By the end of the decade a revolutionary new fleet of fission reactors will be on the market.

The gas-cooled fast reactors look to be the most promising. They are high temperature Brayton cycle engines with better efficiency. They use helium coolant which is not as bad for activation as water. They are fast and so can make use of fertile material and will end dependence on U-235. The best part is that all actinides are recycled on site which minimises HLW and makes for a much more efficient operation.

Lead-cooled fast reactors look cool because rather than using flowing coolant, they use natural convection in molten lead, which means passive safety. They are fast and fully recycle but the recycling is done a reprocessing centres.

Molten salt reactors are quite revolutionary. The fuel is dissolved in the coolant allowing for continual extraction of fission products.

Supercritical water cooled reactors are a bit like PWR except the water is under extremely high pressure. That's the supercritical bit, not the core. They can be thermal or fast.

Very high temperature reactors are helium cooled and use either prism blocks or pebbles. They are high efficiency, high burnup and physically meltdown proof. They are also built in modules making construction quicker and cheaper. However, they are open cycle only.

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What does HLW and PRW mean?

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Sorry.

HLW- High level waste: fission products and transuranics
PWR- pressurised water reactor, the most popular type of reactor at the moment where the core is cooled and moderated by high pressure liquid water.

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Glom says that

Very high temperature reactors are helium cooled and use either prism blocks or pebbles. They are high efficiency, high burnup and physically meltdown proof. They are also built in modules making construction quicker and cheaper. However, they are open cycle only.[/quote]

This system sounds interesting just because it may sell to John Q. Public because of its inherent safety. When you say open cycle, to what are you referring to?

Sorry for my ignorance but this sounds very intriguing.

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That's the beauty of HTRs. It is easier to make it clear to the public that they are benign. You could go on about all the safety features of the CANDU, but John Q. Public's eyes would just glaze over. Reactors like Pebble Bed Modular Reactor (South Africa, UK), Gas Turbine- Modular Helium Reactor (USA, Russia) and HTR-10 (China) are more demonstrably safe to the general public.

It's all to do with the different form of fuel to conventional rods. When the fuel temperature increases, the neutron absorption cross section goes down and so the reaction slows down. It's called negative temperature coefficient. If the reactor gets too hot, it goes subcritical and shuts down, purely because the fuel refuses to cooperate and not because of any engineered mechanism. They did a test on the HTR-10 in China where they voided the coolant. The temperature climbed to about 1600 from 1000 operating temperature but then cooled as the core shutdown. The melting point of the fuel was above 2000 so the temperature didn't get anywhere near close to melting.

The open cycle means that fuel goes once through the reactor (actually in the PBMR at least it goes through ten times, because the fuel balls are circulated around, but that's nitpicking). Once the fuel is spent, it is treated as waste. Usually spent fuel contains a lot of actinides which are either fissile or can be turned into fissile isotopes. In the closed cycle, these actinides are extracted from the fission products and incorporated into fresh fuel. The fact that there is no recycling with HTRs may be a major downside, but there is higher burnup. I got the vibe from the PBMR site that there was total fuel burnup, including the fertile material. They said there was nothing commercially useful left in the balls afterwards, but I'm not sure what they define as commercially useful.

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