From the very beginning there were questions about how to remove heat produced in a fast-neutron reactor’s compact core. Sodium was chosen as the best coolant based on theoretical research and experiments.

A serious drawback of sodium is that it burns in water. A number of experiments at IPPE and experience gained with the BR-5, BN-350, BOR-60 and BN-600 suggested that this problem is not a major issue for fast-neutron-reactor safety. Even the 1973 sodium fire at the BN-350 did not affect reactor safety. Problems with steam — generator design were corrected step-by-step.

Fortunately, before the startup of construction of the BN-350, it was discovered that steel swelling under high neutron fluence was problematic. At the last moment, the fuel assembly design was modified to take into account the swelling and the planned burn-up of the fuel was limited (though subsequently increased on the basis of further experience). Irradiation tests on different types of steel were made in the BOR-60, BN-350 and BN-600 reactors for consideration in future projects.19

Mercury was initially considered as a coolant but is highly corrosive to most reactor materials. Although sodium-potassium alloy is a good coolant (with a low melting temperature of approximately 20 oC so that a heating system is not required for liquefaction), the alloy is more flammable then pure sodium.

Lead and bismuth and their alloys are more promising as fast-reactor coolants. Neutrons do not lose a significant amount of energy when they collide with the heavy nuclei of these elements. They are not flammable and do not react with water. At the same time, however, they are significantly more corrosive to steel than sodium with corrosion properties that are dependent on the oxygen content in the alloy. Research on lead-bismuth alloys was initiated in 1951.20 They are effective coolants for compact nuclear reactors, which is why they were used for submarines.21 During the past decade, the alloy was considered as a coolant for a new type of fast-neutron reactor, the BREST project. Rosatom22 has decided to build an experimental 75 MWt reactor with lead-bismuth coolant (SVBR-75/100) before developing a commercial prototype.23

Thus far, most of the fuel in the BN-350 and BN-600 reactors has been uranium dioxide. Some experience with mixed-oxide (MOX) uranium-plutonium fuel was acquired in the BOR-60 reactor and in a few experimental fuel assemblies in BN — 350 and BN-600. Limited experience in carbide and nitride fuel was gained with the BR-5/10 but not enough to deploy these fuels in a future fast-neutron reactor.

Recently, the director general of the Research Institute of Atomic Reactors, Alexander V. Bychkov, declared that most of the problems of vibro-packed fuel — fabrication technology have been solved and that it is ready for commercial implementation.24 However, full-scale experiments with closed fuel cycles have not been conducted.

Seven tons of BOR-60 fuel have been reprocessed, 4 tons of which were MOX and some of the separated plutonium was recycled. A number of questions are unresolved. How will a transition between bench scale and commercial scale technology influence the quality of the fuel pins and assemblies? If pyro-chemical processing is used, the degree of separation of the fission products will have to be determined.