As I mentioned before, the major risk to any MSR reactor is not a meltdown (its already melted down silly!) or a LOCA (there are some scenarios that could lead to major core damage but they are unlikely to release radiation into the environment) but either a fire effecting its graphite core (which for a LF reactors running at low vapour pressure is a greater risk than with any other graphite cored reactor) or more likely a burst pipe.
In the chemical industry pipes burst all the time. The consequences can vary from minor “oh! Fiddlesticks we appear to have spilt a load of sulphuric acid on the floor, quick! flush it down the drains before the EPA shows up” to utterly catastrophic, as seen in the Bhopal disaster. Thus the major danger with a LF plant is that somewhere in the lengthy network of pipes that it and its CPP consist of, something breaks. You may recall me expressing distaste for the mass of pipes that is a CANDU reactor; you may recall me going so far as to describe it as “a plumber’s nightmare”. Well in fairness, at least a CANDU is made out of easily welded materials! A LF reactor would likely be made out of materials that are either difficult to weld (nickel alloys) or (for ceramics and refrectory’s) down right impossible to weld! Again, putting such a large network of pipes together and getting it all signed off by the safety inspectors will be a difficult, lengthy and an expensive undertaking. Pipe bursts have already played a role in plaguing the nuclear reprocessing industry with problems, most famously the pipe burst in Thorp that led to 160 kg’s of Plutonium leaking into the basement, triggering a level 3 nuclear alert.
All this pipe work raises a whole host of design issues. As already pointed out we’d have to put much of the chemical works within the reactor containment dome, which would have to be hermetically sealed so that any gases can’t escaped (plus a gas treatment plant and a few emergency gas storage tanks on site). Also we would need to include a high-spec containment area in the basement of the reactor building to catch any escaping radioactive material, or chemical pollutants. But the individual components of the chemical plant/reactor would also need very careful design.
LFTR reactor and dump tank
Take for example the pipe at the base of the reactor that allows us to dump the core to the emergency dump tanks. This is a safety critical component – i.e it HAS to work in certain worst case scenarios. But suppose for example that it bursts during a dump scenario? Obviously we need a containment vessel around the pipe to catch any leaks. Also simply relying on gravity would be inadequate in certain scenarios, a pump on a separate stem (or a tank of inert high pressure gas connected up to the pressure vessel to “encourage” the fuel to drain away), would be necessary. But what if the trigger for the accident is a clogging of fuel channels (as discussed earlier) by solidified fuel? If we dump in that scenario we might cause the dump pipe to clog also, likely leading to a criticality incident or its failure and a breach. So we would need a thermal regulation system around the pipe to ensure it can be heated or cooled as necessary.
Also I don’t like the idea behind this “freeze plug”. I realise the passive safety benefits it brings, but it’s just going to be too slow to act in a real emergency and there’s too much that can go wrong with it. If I were an engineer at such a plant I’d want a big shiny red “dump core now!” panic button on my control panel. So we’d need to put a bypass valve in around the freeze plug, which could be activated in an emergency. In any event, as we’ll periodically have to inspect this pipe, a set of isolation valves would need to be installed around the freeze plug (or any pump) as a matter of routine anyway. Also relying on just one pipe, even an over designed one, doesn’t sound sensible. We have multiple dump tanks anyway, so why not have multiple feed pipes into them?
If you follow through the last paragraph you’ll see how in the process of getting one short section of pipe back to within a reasonable safety margin the result has been for it to balloon into a massively complex system in the space of 5 minutes. Obviously if we were to turn a similar critical gaze on many other parts of the LF reactor design (which of course we can’t as many of them haven’t been designed yet!), I suspect the same thing would happen again. Another common misconception of the supporters of the LF reactor is that it would be quicker to get a LFTR certified than a “solid fuelled” reactor, as the latter require being run through several fuel loads before going into commercial operation (which takes months or years).
However, the benefits of a Molten-Salt fuel system are outweighted by the lengthy inspection process of all that pipe work. All of those joints, valves, flanges and connectors would need to properly inspected (X-ray, ultra-sonic testing, etc.), something that would likely take months – and government inspectors aren’t exactly noted for their speed and efficiency! This is precisely why the ESBWR goes the other way and tries to eliminate complex pipe work. It would also be necessary to run the processing plant (under supervision) for a few months after inspection before the safety authorities signed off on it as ready for commercial operation.
There is also an issue regarding the fact that the Fluoride salts naturally produce hydrofluoric acid (when in contact with moisture) as well as decomposing into Flourine gas over time when cold. Both of these can lead to the release of toxic fumes, bearing in mind that any MSR reactor would inevitably have to have large stockpiles of salt stored on site or nearby, as well as some spent fuel. The MSRE already suffered a release of such gases from stored fuel tank and nearly suffered a criticality incident as a result. Radioactive or not, Fluorine gas is extremely toxic (several times more deadly than chlorine which was used as a chemical weapon in World War I) and anything that risks releases of that must be limited. Another misconception of the LFTR fans is that LFTR’s will not require the same large exclusion zones as other reactors. Actually, once the merest hint of this fluorine gas risk is raised, bearing in mind the consequences of incidents such as Bhopal, it would likely make getting a LFTR build anywhere near a large populated area, even harder than it would be for a conventional reactor.
A LFTR is essentially just a glorified chemical plant and getting chemical plants approved anywhere near a populated area is always difficult, doubly so I suspect if the locals learn it will be processing radioactive materials!