Fast reactors

Liquid metal fast breeder reactor.

As indicated by the name, the LMFBR uses a liquid metallic coolant,a suitable choice where good heat-transfer characteristics are required.The basic nuclear configuration for all fast breeders is similar. The coreis a compact arrangement of fuel assemblies that are similar to those

used in light water reactors except that the fuel material is more highlyenriched, the fuel pins typically have a smaller diameter, and the claddingis stainless steel rather than zircaloy.

Instead of using 235U or 233U as the fissile material, the present choice is toload the core with 239Pu; this is the inevitable product of the current generationof light water reactors, and because of its high value of eta (neutrons producedper neutron) in a fast spectrum, It is much more profitably used in thebreeder than in thermal reactors. (235U, on the other hand probably does notyield enough neutrons for a practical breeder; this is not true of 233U, but thelatter nuclide is not presently being produced in any large quantity). The fertilenuclide now chosen is 238U, at least in the LMFBR. This is expected toproduce more than enough 239U to replace that which is consumed in the core.The “core” itself would be a mixture of oxides of plutonium and uranium.Surrounding this core would be a blanket of uranium. In both cases, the uraniumwould be almost entirely 238U. Fission would occur primarily in the coreregion and conversion would occur in both regions. Regular reprocessingwould be needed to recover the bred fissile material.

The blanket assemblies would have rods with the same compositionthroughout. Core assemblies, on the other hand, would have fissile loadsalong the portion of the rods that constitutes the core, but only fertilematerial at the top and bottom of the rods. In this way, the entire core issurrounded with blanket regions.

Power generation in the core will be quite intense, compared withthermal reactors. As a result, the coolant must have good heat transferproperties. In the present case, the LMFBR, this has led to the choice ofsodium as the coolant. A metal that is liquid over a large range of temperature,sodium can successfully cool the very compact core. Additionaladvantages of great importance are that it can be used at essentiallyatmospheric pressure thereby making design easier, and that it canoperate at high enough temperature to permit a higher plant efficiency(see Appendix C) than water-cooled reactors can achieve. Sodium,however, is highly reactive chemically. At reactor operating temperatures,it burns if exposed to air; moreover, it reacts violently with water.Stringent efforts must be made to prevent any breaks or corrosion that

lead to sodium leakage.

In all LMFBR designs, the sodium that cools the core is not used to raisethe steam that drives the turbogenerators. Instead, there is an intermediatesodium loop which avoids the possibility of releasing radioactive sodium duringany steam generator problem. This requires use of an intermediate heatexchanger as an intermediate heat exchanger as an interface between the primaryand secondary sodium loops. It has the effect of more effectively isolatingthe primary sodium-filled reactor from any water. It does not, however,eliminate the difficulty of designing steam generators which effectively separatesodium and water.

Two basic types of LMBR are being considered. The form most favoredby other countries is the “pot” (pool) type, in which not only thecore, but also a number of other components are contained in the reactorvessel. (An example is the French Phenix reactor). The vessel is filledwith sodium at roughly atmospheric pressure, in which are immersed thecore, refueling machines, the primary coolant pump, and the intermediateheat exchanger, so that the entire primary sodium loop is contained in thesame vessel. This assembly markedly reduces the amount of external piping.The alternative scheme, called the “loop” arrangement, developed is

more similar to conventional LWR systems in that individual componentsof the heat transfer system are connected by pipes, and the reactor vesselonly contains the core and associated equipment. Such a system is employedin the Clinch River breeder reactor design. In either arrangementof the primary system, the vessels containing primary components aresurrounded by guard vessels, so that any rupture of the primary systemdoes not lead to a large loss of sodium. Finally, in either, the secondarysodium proceeds from the intermediate heat exchanger to a steam generator,which produces steam for driving the turbines

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Вариант 9

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