A friend asked me what is the difference between a “Slow” and a “Fast” Nuclear Power Plant, a curiosity probably stirred by the recent announcement of the Beloyarsk 4 NPP going online.
First of all, 789 MWe Beloyarsk 4 is not such a big deal, capacity-wise. It is part of the Beloyarsk complex, designed to deliver around 2.3 GW of power when fully operational, which is big but not in the league of the true giants like the 8 GW Kashiwazaki plant in Japan or the 6.2 GW Bruce plant in Canada. Russia itself has several 3.8 GW plants like the one powering St.Petersburg (still called the Leningrad Plant).
Beloyarsk 4 is however the world’s largest FAST reactor, about 1.5 times larger than the second-largest, its sibling Beloyarsk 3.
Fast reactors use an “inefficient” moderator like liquid sodium: neutrons are not slowed up as much and when they hit U238 nuclei (the non-fissile vast majority of natural uranium), transform them into Pu239 which then decays into U239 which is even more fissile than traditionally used U235. In short, fast reactors produce some of the fuel they consume and, in certain conditions, all of it and even more. This last kind is called “breeder” and it was popular in the late Seventies as it also bred Plutonium, used in you-know-what.
Excluding this last species, for which nobody has much use nowadays (and instead is faced with the decommissioning headache) fast reactors are more fuel-efficient than traditional light-water reactors: Beloyarsk 4 should achieve 80 GW/ton/day (compared with about 50-60 for a light water reactor) and an upcoming follow-up unit is expected to achieve 120 GW/t/d.
Of course, the engineering of a liquid sodium moderator is a little more complex (for one thing, it is prone to exploding), but the design has been around for decades and is therefore rather stable and well-understood.
The question for me is why, given Russia’s richness in uranium (4th largest reserves in the world after Australia, Kazakhstan and Canada) they are investing in more fuel-efficient plants; for more discussion of NPPs see also these other posts.