Molten salt nuclear reactor that eats radioactive waste gets funded

a reply to: truttseeker

There are parts to using liquid graphite

Molten Salt Reactors use a molten salt, not a liquid graphite. The graphite is still solid and is used as the moderator. Therefore, most Molten Salt Reactors are thermal reactors (i.e. not fast reactors). Molten Salt also has a very high boiling temperature, very high heat capacity. These factors make them very stable and easy to control.

liquid sodium that make the core inherently more unstable

With fast reactors using liquid sodium it depends on the specifics. Core size, core geometry, fuel type (oxide or metal) are all factors. Smaller liquid sodium fast reactors with metal fuel are very stable under many conditions, like when cooling is lost. On the other hand, larger liquid sodium Fast Reactors with MOX fuel, especially those optimized for high breeding ratios, tend to be less stable, sodium boiling can occur much more easily. Sodium boiling will (in absence of other negative feedback mechanisms) melt the fuel and possibly change core geometry. Fast Reactors tend to react much more quickly than thermal reactors.

Of course, accidents that change the core geometry of a fast reactor can be disastrous, leading to what is known as a HCDA.

Pwr plants are generally more well self sustaining

They are easy to control yes, you're going to have a hard time convincing people they're easier to control than some kinds of Molten Salt Reactor.

event of an emergency h2 buildup should not be an issue

It was an issue at Three Mile Island and Fukushima.

pressure can most certainly be relieved if it gets too high

I tihnk my initial understanding wasn't correct. Here is a corrected version:

Fundamentally PWRs operate at pressure. This prevents the water from boiling. If cooling is lost then the temperature will rise and some water will turn to steam, raising the pressure. The pressure will then continue to rise until cooling is reestablished or it is vented into containment, which causes loss of coolant. So basically, if cooling is loss then meltdown occurs. With a coolant with an extremely high boiling point, that probably won't happen.

Some newer PWRs like the AP1000 are designed so that as long the containment is cooled via passive safety, the core will remain covered. That can last several days. The EPR is designed to catch the core and prevent it from melting through the basemat (I'm not sure what the off-site release will be in this case). Older PWRs simply are not protected against eventual loss of containment after prolonged loss of cooling. There are probably things that can be retrofitted to older reactors - such as hydrogen recombiners to prevent hydrogen explosions and filters so that the containment can be vented in a known way. But if you leave them long enough without cooling fundamentally, then a meltdown will occur.

If a meltdown occurs I don't know how likely is it that it will melt through the concrete basemat.

They just took poor care of their plant and that was the main cause of their catastrophe.

At Fukushima the measures of dealing with hydrogen did not work since there was no electricity (iirc, newer ones as supposed to be passive?). The containment was also fairly poor for that design of reactor. People being stupid and not planning properly isn't an excuse, since people will always occasionally do this. Better design the safety into the plant to be inherit and there from the start - not reliant on tsunami walls, or diesel generators.

originally posted by: C0bzz
a reply to: mbkennel


1. What water is in the system?

When there's an accident.

2. In what situation is water required?

It's on fire.

4. The idea is that continuous reprocessing removes waste products from the fuel, rather than let it accumulate. In addition, the fissile inventory is fairly low.

This means that every reactor has to be a high-level liquid waste reprocessing plant!! That is really a nasty job and dangerous, at present, only Savannah River did any of that dirty work. I wouldn't trust a utility to run this sort of job.

Spill = reactor offline for a decade.

5. The final form of the waste is probably not going to be a liquid.

6. The melting point of the fuel is over 450 C.

7. A melt-down in a conventional reactor is dangerous for completely different reasons than for this. In a PWR, if the core looses cooling for an extended period of time, hydrogen is produced when fuel cladding reacts with water in an exothermic reaction. This makes the core hotter and creates hydrogen gas. Hydrogen gas can explode and destroy the containment (newer plants are designed to mitigate this). If cooling is re-established with containment breached, the cooling water will leak everywhere and spread contaminants to the environment. And (regardless of a hydrogen explosion) if cooling still isn't provided then pressure can build till containment fails, at which point the reduced pressure allows the water to boil (and escape into the environment taking the fission products with it) then the core will melt onto or through the floor.

In a Molten Salt Reactor none of those can happen. The boiling point of the fuel/coolant is extremely high, it's not likely going to become a gas. As far as I know no there are no explosive materials. In an emergency, all that is needed is to drain the core onto a plate with reasonably large surface area and leave it there.

In other words, melt down onto the floor which was the terminal and really bad outcome of a PWR is now a 'safety feature' in a molten salt reactor?

The accident scenario is when # happens, earthquake + rain + flood.

Of course there are failure modes of a PWR which are avoided, but there is a big cost.

Personally I think smaller reactors with a larger surface/volume ratio (so 100% passive cooling) with SOLID fuel is what I'd prefer.

Something other than PWR needing active cooling and explosive failure modes? Sure!

Dissolving large amounts of fuel + hence waste products in a hot water soluble thing with a requirement to process it at industrial scale (note that no humans could possibly go into this room for years, what do you do when there's a leak? And there's always a leak). Do not want!

Even with reprocessing fuel fresh from the regular reactors to make Pu at Savannah River, they could take it out and let it decay for a few months. Not with this---it has to be processed with maximum radioactivity having been very recently fissioned.

Molten Salt reactor = Molten high-level radioactive waste in hot water soluble carrier which has to go through many pipes and filters reactor. The salt isn't the problem.