[acidburnNSA]

These were among the only Lead-bismith cooled nuclear reactors ever operated. Sodium-potassium eutectic or pure sodium coolant is a better heat transfer fluid but in a water environment it's just a little crazy due to sodium-water chemistry. The US Navy tried sodium coolant in the USS Seawolf [1] and it was smaller, quieter, and more powerful, but they had problems with the steam generators. Not an unsolvable problem but definitely a challenge. The success of the water cooled competitor eventually led to the predominance of water cooled reactors in the commercial fleet.

Passively safe Lead-cooled commercial reactors are still thought about today. Challenges include high pumping power required due to high density, erosion issues at high coolant velocity, and corrosion issues without very stringent oxygen control.

[1] https://en.wikipedia.org/wiki/USS_Seawolf_(SSN-575)

[dragonwriter]

> The success of the water cooled competitor eventually led to the predominance of water cooled reactors in the commercial fleet.

By commercial you mean operational (as in non-experimental Navy vessels.) There's precious few commercial nuclear vessels in history (four, each from a different country and I don't think they were ever operational all at the same time), so talking about the dominance of a particular nuclear technology being dominant in a “commercial fleet” seems misplaced.

[acidburnNSA]

I meant to refer to the commercial fleet of commercial nuclear power plants making electricity on land, almost all of which have designs directly descended from naval designs. I guess "fleet" isn't a great word to use in the context of naval vessels. Whoops!

[brandmeyer]

Its not commonly known, but the initial generation of commercial power reactors in the US is basically the result of technology transfer from the Naval Nuclear Propulsion program.

[jabl]

I wonder if we'll see non-pwr naval reactors anytime soon. A high temp reactor would allow using a supercritical CO2 Brayton cycle which would take up much less space.

On another note, what would happen if a MSR powered ship would sink and the fuel salt would come into contact with sea water? Would the salts dissolve into the water? Much as I like MSR's as a concept, if so this would make them a quite bad idea on a ship...

[MertsA]

The Uranium tetrafluoride itself is basically insoluble in water. But the fuel isn't really the issue, Uranium is a toxic heavy metal but in terms of irradiating the environment outside of a reactor it's not very radioactive at all. People always clamour over Uranium having a half life of billions of years but what that means is that Uranium basically doesn't decay on its own. It's fissile, so hit it with a neutron and you'll induce it to split, but once it leaves a reactor and is subcritical you don't have to worry about the radiation from Uranium.

What you do need to worry about is the radiation from the products of fission as those are the nasty components of nuclear waste. Not only do you have the direct products which have a tiny half life compared to Uranium, those direct products have a decay chain of their own and in most reactor designs out there those products stay in the reactor so not only do you have the decay to contend with, you also have additional fission since it's still being bombarded by neutrons. This is what leads to the toxic soup of radioactive waste, you get tons of isotopes of elements so while the fuel itself wouldn't be too terribly awful for the environment the partially burned fuel is dramatically more radioactive.

The fuel salt itself though if it's anything similar to what was used in the MSRE or FLiBe's new Thorium breeder reactor would actually be insoluble in water and would freeze, hopefully trapping the waste in the frozen salt just like vitrification of nuclear waste. I am not a chemist, and any knowledge I have on the subject is just the result of a hobbyist interest in the subject, but I would think that in the event of a MSR being sunk and breached that the frozen fuel salt would keep most all of the radioactive products dissolved in it safely contained.

[acidburnNSA]

MSRs use either Th/U-FLiBe or U/Pu-NaCl and both of those would easily dissolve in water. But honestly radioactive inventory of these cores is small enough that one or two breached systems, while catastrophic, wouldn't have significant biological consequences as it disperses in the vast oceans. Certainly this wouldn't be acceptable for civilian operations but I wouldn't put it past an ambitious military. Problem with Molten Salt in a submarine is size. The fuel salt is very low density compared to metallic nuclear fuels, by about an order of magnitude. That would require more volume to be critical. Small thermal/epithermal solid fueled reactor cooled with water, liquid metal, or even solid fuel/salt cooled reactors (like the FHR) are appropriate for submarines but probably not full-on liquid-fueled MSRs.

[leeter]

Not in the US Navy. Part of the reason the US Navy standardized on PWR is that while it's not the most efficient it is something you can restart at depth if necessary. The lesson of how much that matters was driven home by the loss of the USS Thresher.

[acidburnNSA]

It should be technically feasible to restart a liquid-metal cooled reactor at depth just as easily as a PWR, especially a Na- or NaK-cooled one. Certainly keeping the coolant liquid after shutdown can be a challenge but as long as the plant has operated for a while there should be significant decay heat to keep it warm, and trace heating in the pipes could be powered by auxiliary power if necessary.

[cocoablazing]

This had more to do with safety system failres and procedures regarding operation of critical valves than poison kinetics.

LM designs were abandoned prior to this accident due to availability and maintainability issues.

[brandmeyer]

The kind of emergency restart procedure employed today would work just fine on a MSR design. We're talking restarts on the order 1-10 minutes after an emergency shutdown. That's not nearly enough time to solidify the core, even when you are drawing some steam for emergency propulsion needs while the reactor is shutdown.

[cocoablazing]

To what end?

The USN has, to a material degree, perfected the compact PWR.

Their experience in fuel design, materials, cooling, support systems, control systems, and processes pose an enormous barrier to new systems other than those that could truly radically simplify the reactor system and yield a huge gain in reduced manpower and increased reliability.

I think the best marginal gains are coming and will come from developments in tactical and ISR systems.