Metal cooled reactors have several advantages over pressurized water reactors. For one, their power density is greater, additionally, the coolant is unpressurized, improving safety.
However, there are some downsides. The Soviets' Project 705 class submarines were powered by liquid metal reactors utilizing a lead-bismuth alloy as coolant. This alloy had a freezing temperature of roughly 400K. As a result, the reactors had to be run constantly, even while the submarines were in port (there were facilities to provide superheated steam to the reactors while the subs were docked, but they broke down and were never repaired). This reduces the lifetime of the reactor. Another coolant choice which has been used operationally is NaK (Sodium-Potassium). This alloy is liquid at room temperature, but reacts violently with water or air. I'm not an expert, but this seems like a bad thing.
It seems to me that if appropriate coolants could be found, it seems that liquid metal fast reactors could see more widespread acceptance. To my untrained eye, gallium looks like a good choice. Its melting point is relatively close to room temperature (~303K), and the boiling point is quite high (over 2600K). Also, gallium is less reactive than sodium or other alkali metals. It appears that there has been some research on this topic: http://www.sciencedirect.com/science/article/pii/S0149197000000640(unfortunately, the article is behind a paywall), and it looks quite promising.
Anybody have any opinions on this, or suggestions for alternative coolants?
During the Cold War, many neutral states made efforts to develop nuclear weapons. Very few of these resulted in a working bomb. One of these failures was Switzerland; http://nuclearweaponarchive.org/Library/Swissdoc.html
Interestingly, the Swiss also managed to cock up their nuclear power program; http://en.wikipedia.org/wiki/Lucens_reactor
The Manhattan Project gets all the glory(it deserves it), but the Soviets quickly developed their own atomic weapons. They had some help through espionage, but I think it might be another piece of McCarthyism to dismiss Soviet atomic scientists.
Here is a post on the Nuclear Secrecy Blog on the early program. Good insight, but not the end-all-be-all of information on the subject.
A Model of the First Lightning/Joe 1 bomb?