Sodium Cooled Fast Breeder Reactor Power Plants

Fast breeder reactors, known as liquid metal fast breeder reactors (LMFBRs), use 239Pu as the fissile nuclide, 238U as the fertile material, and fast rather than thermal neutrons. Reactors of this type have been built and operated in France, Japan, the former Soviet Union, the United Kingdom, and the United States to demonstrate their feasibility and to gain experience in their operation. The largest LMFBR power plant built up to mid-1998 is the 1240-MWe SuperPhenix located at Creys-Malville, France. These reactors are called breeders because they are designed to produce more 239Pu than they "burn" by fission. They can breed with 239Pu as the fissile material because 239Pu has a larger cross section than 235U for fission with fast neutrons and because the neutron absorption cross sections for fission products and for structural and other materials in the core usually decrease with increasing neutron energy, making a larger fraction of the neutrons released in fission available for converting 238U to 239Pu.

0ne fuel that has been used in fast reactors is M0X containing about 20% Pu02 and about 80% U02. Stainless steel is a suitable cladding material for the fuel rods. Depleted uranium as U02 also is used as a blanket around the core so that 239Pu is produced in both the core and the blanket.

Water cannot be used to cool the core of a fast reactor because it would function as a neutron moderator as well as the coolant. Sodium is not a good neutron moderator, and its physical properties make it a suitable coolant.35

35Lead has been proposed as a substitute for sodium.

It has a melting point of 98.7°C and a boiling point of 883°C. As liquid sodium passes through the core, 23Na captures neutrons to form 24Na (14.95 h) and 22Na (2.604 y) by (n, 7) and (n, 2n) reactions, respectively. The coolant is, therefore, highly radioactive quite apart from any contamination by fission product leakage from fuel elements. Sodium is spontaneously flammable in air and must be handled in an inert atmosphere. Furthermore, it reacts violently with water to liberate hydrogen, which generally ignites. Although the technology for using molten sodium as a coolant is not new, sodium leakage problems have plagued operation of the few sodium-cooled power reactors that have been built.36 Most, including the SuperPhenix and one in Japan following a sodium fire have been shut down at the time of writing.

In one type of fast reactor (pool type), the core is immersed in a pool of molten sodium, which is circulated through the core by pumps located in the pool of sodium. Hot sodium (about 500°C) also is pumped from the pool through a primary loop and an intermediate heat exchanger, where heat from the pool is transferred to nonradioactive sodium in a secondary loop. Specially designed heat exchangers having double-walled tubes to prevent accidental contact of the sodium with the water are then used to transfer heat from the nonradioactive, liquid sodium to water and generate steam.

In the second type of fast reactor (loop type), molten sodium is pumped through the core as for a PWR. The heat is transferred to a second loop of sodium and then to a steam generator as for the pool type reactor. The pool type design has been more popular than the loop type.

Breeding 239Pu enables operation of nuclear power plants by utilizing 238U, the most abundant isotope of uranium. Breeder reactors are seen as a means to energy independence by nations that must import their fossil and nuclear fuel. They are also seen as a potential source of material that could be used for nuclear weapons by terrorists and rogue nations.

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