The kick-off of commercial power production by the 800 MWe BN-800 reactor at Beloyarsk Nuclear Power Station in Russia last year could be a defining moment in the history of the nuclear power industry. “A step into the future” was the phrase used by Alexander Kharichev of Rosatom, Russia’s state atomic energy corporation, to describe the occasion. Whilst this may sound somewhat grandiose, on 27 June 2014 when the Rosatom engineers brought the first commercial fast breeder reactor to criticality – meaning that the nuclear chain reaction was self-sustaining – an important page was surely written. Fast breeder reactors ensure that a single fuel charge can last more than a century, and that the uranium is used in a way which is 60% more efficient than in any other standard reactor. The Beloyarsk unit therefore represents a big step towards a new generation of reactors with enhanced safety and minimal waste production. The name fast breeder reactor comes from two of their most important features: the use of “fast” neutrons and the ability to “breed” fresh fuel with each nuclear reaction.
FAST NEUTRONS. In a typical nuclear reactor, the neutrons produced by the reaction need to be slowed down, or “thermalized”, in order to reach the right energy level to produce a nuclear reaction by fissioning an atom of uranium-235. Although this form of uranium represents only 0.7% of naturally occurring uranium, it is currently the only viable nuclear fuel. The abundant uranium-238 isotope which makes up the remainder is a so-called fissionable atom, but is not fissile like the 235 isotope. A fissionable element needs to be hit by a fast neutron to undergo a fission reaction, so it is very important to preserve the energy level of neutrons produced by the reaction rather than slowing them. For this reason the reactor has been designed to avoid any thermalization, and water cannot be used as coolant as it would slow neutrons – the BN-800 reactor instead uses liquid sodium for this purpose.
THE CONVERSION RATIO. The other key trait in a fast breeder reactor is a conversion ratio (or breeding ratio) which is greater than one, meaning that for every nuclear fission, more than one fissile atom is created. There is no creation of any new material, just the mutation of some elements, such as the conversion of uranium-238 to fissile plutonium-239. In principle, this kind of reactor can convert fissionable atoms into fissile ones and use either as fuel. This versatility was the primary goal of the design peculiar to the Beloyarsk reactor.
UNIQUE DESIGN. The reactor is formed of a core section containing a mix of uranium-238 and plutonium-249 (known as mixed oxide fuel) and a so-called “blanket”of uranium-238, which is bred by the neutrons coming from the core. The new nuclear fuel produced in the blanket is periodically removed to be used in the core or in other fast reactors. This distinctive feature is the main asset of this kind of reactor.
COMPETITIVE EDGE. There are other important advantages too. When fully operational, fast breeder reactors burn mainly uranium-238, extending the life of uranium reserves by more than a thousand years compared to traditional reactors using uranium-239. In addition, they can burn actinides, which are the most dangerous elements present in nuclear waste. Although the waste will still take a thousand years to return to its natural level of radioactivity, this still represents a huge improvement on the typical timespan of millions of years.
NO FAIRYTALE. There are some downsides too. First of all the sodium used as coolant is corrosive and very reactive if it comes into contact with water. To prevent this, two heat exchangers are needed to avoid any contact between the sodium in the reactor and the steam sent to the turbine. Another drawback is a positive void coefficient of reactivity, which means that the reactivity (and consequently the power) tends to increase when voids such as steam bubbles form in the coolant – making the system unstable in case of overheating. The Russian engineers’ unique knowledge of fast breeder reactors derives from considerable experience gained with the smaller BN-600 reactor, operated at Beloyarsk since 1980. The implementation of the BN-800 reactor in the commercial grid is without doubt a great challenge, but should be regarded as a real turning point in the history of power generation. It now remains for fast breeder technology to prove its potential as a virtually CO2-free source of energy which combines the benefit of an abundant natural fuel source with the capability to dispose of nuclear waste and plutonium from nuclear warheads.