Liquid Metal Fast Breeder Reactor

Ren Ruan Shi

Department of Nuclear Engineering, University of California, Berkeley, CA 94720-1730

NE-161 Project

Key words: LMFBR, Sodium, fuel

Abstract

The first reactor in the world which produced electricity was the experimental breeder reactor EBR I in the US in 1951. In the following 30 years, the technology of breeder reactor was developed very fast, especially for the liquid metal fast breeder reactor(LMFBR). This document provides an overview of Liquid Metal Fast Breeder Reactor (LMFBR), including the principle, basic structure, and fuel of LMFBR.

Contents

Introduction

Nuclear energy, though presently being debated and publicly discussed, contributes significantly to the electricity production in industrialized countries, often to more than 30%. Because the world's reserves of U(235) are not adequate to support indefinitely the needs of a growing nuclear power industry based only on burner or converter reactors, the breeder reactors become more and more popular today. With the introduction of breeder reactors, the fuel base switches from U(235) to U(238) or thorium, both of which are considerably more plentiful than U(235). Furthermore, all of the depleted uranium--that is, the residual uranium, mostly U(238), remaining after the isotope enrichment process--can be utilized as breeder fuel. Breeder Reactors are capable of satisfying the electrical energy needs of the world for thousands of years.

Background

The fundamental principle behind the fast breeder reactor concept were discovered before the end of World War II, and the potential impact of breeder reactors on future energy supplies was immediately recognized. The first experimental breeder reactor was a small plutonium-fueled, mercury-cooled device, operating at a power level of 25 kW. A breeder, cooled with a mixture of sodium and potassium, was placed in operation in 1951 at the Argonne National Laboratory in Idaho. This reactor, the Experimental Breeder Reactor-I(EBR-I), produced 200 kW of electricity, is the world's first nuclear-generated electricity--and it came from an LMFBR! Since these early experiments, a consider amount of LMFBRs have been constructed around the world. Right now, LMFBR is the only breeder which has reached a stage of practical commercialiation anywhere in the world.

Basic Principle

The LMFBR operates on the uranium-plutonium fuel cycle or thorium-U(233) fuel cycle. The reactor is fueled with bred isotopes of plutonium in the core, and the blanket is natural or depleted uranium. From the thoery, the number of fission neutrons emitted per neutron absorbed by Pu(239), increases monotonically with increasing neutron energy for energies above about 100 keV. That means that the breeding ratio and breeding gain increase with the average energy of the neutrons inducing fission in the system. Therefore every effort must be made to prevent the fission neutrons in a fast reactor from slowing down. This means the lightweight nuclei must largely be excluded from the core. There is no moderator, of course, so the core and blanket contain only fuel rods and coolant.

Right now, sodium has been universally chosen as the coolant for the modern LMFBR. With an atomic weight of 23, sodium does not appreciablly show down neutrons by elastic scattering. Since sodium is an excellent heat transfer material, an LMFBR can be operated at high power density. This, in turn, means that the LMFBR core can be comparatively small. Furthermore, because sodium has very high boiling point, reactor coolant loops can be operated at high temperature and at essentially atmospheric pressure without boiling, and no heavy pressure vessel is required. The high coolant temperature also leads too high-temperature, high-pressure steam, and to high plant efficiency. Finally, sodium, unlike water, is not corrosive to many structural materials. Reactor components immersed in liquid sodium for years appear like new after the excess sodium has been washed off.

However, sodium also has some undesirable properties. Its melting point much higher than room temperature, so the entire coolant system must be kept heated at all times to prevent the sodium from solidifying. This is accomplished by winding a spiral of insulated heating wire along coolant piping, valves, and so forth. Sodium is also highly chemical reactive. Hot sodium reacts violently with water and catches fire when it comes in contact with air, emitting dense clouds of white sodium peroxide smoke. For these reasons, LMFFBRs are inherently very tight systems and emit far less radiation to the environment than comparable LWRs.

Unfortunately, sodium absorbs neutrons, even fast neutrons, leading to the formation of the bata-gamma emitter Na(24), with a half-life of 15 hours. Sodium which passes through the reactor core therefore becomes radioactive. LMFBR plants operate on the steam cycle, that is, the heat from the reactor is ultimately utilized to produce steam in steam generators. However, because of the radioactivity of the sodium and because sodium reacts so violently with water, it is not considered sound engineering practice to carry the sodium coolant directly from the reactor to the steam generators. Leaks have often occurred in steam generators between the sodium on one side and the water on the other, and such leaks could lead to the release of radioactivity.

Structure

All LMFBRs have two sodium loops: the primary reactor loop carrying radioactive sodium, and an intermediate sodium loop containing nonradioactive sodium, which carries the heat from the primary loop via an intermediate heat exchanger to the steam generator.

The detailed manner in which the intermediate sodium loop is arranged divides LMFBRs into two categories: the loop-type LMFBR and the pool-type LMFBR. The loop-type appears based on the simpler concept: except for the presence of the intermediate loop, it is not much different in design from an ordinary PWR. All primary loop components, the reactor, pumps, heat exchangers, and so on, are separate and independent. This makes inspection, maintenance, and repairs much easier than when these components are immersed in hot, radioactive, and opaque sodium, as they are in pool-type systems. However, substantial amounts of shielding are required around all the primary loops in a loop-type plant, which makes these plants resemble large, heavily built fortresses.

By contrast, a pool-type LMFBR has no radioactivity leaves the reactor vessel, so no other component of the plant must be shielded. Furthermore, the usual practice is to locate pool-type reactor vessels at least partially underground, so that only the uppermost portion of the vessel requires heavy shielding. It is possible to walk into the reactor room where a pool-type reactor is operating and even walk across the top of the reactor without receiving a significant radiation dose. Therefore, this type of LMFBR is very tight and compact.

The core of an LMFBR consists of an array of fuel assemblies, which are hexagonal stainless steel cans 10 to 15 cm across and 3 or 4m long that contain the fuel and fertile material in the form of long pins. An assembly for the central region of the reactor contains fuel pins at its center and blanket pins at either end. Assemblies for the outer part of the reactor contain only blanket pins. When these assemblies are placed together, the effect is to create a central cylindrical driver surrounded on all sides by the blanket.

The fuel pins are stainless steel tubes 6 or 7 mm in diameter, containing pellets composed of a mixture of oxides of plutonium and uranium. The equivalent enrichment of the fuel, that is plutonium, range beteen 15 to 35% depending on the reactor in question. The fuel pins are kept apart by spacers or in some cases by wire wound helically along each pin. The pins in the blanket, which contain only uranium dioxide are large in diameter, about 1.5 cm, because they require less cooling than the fuel pins. Both fuel and blanket pins are more tightly packed in an LMFBR than in an LWR or an HWR, because the heat transfer properties of sodium are so much better than those of water. The liquid sodium coolant enters through holes near the bottom of each assembly, passes upward around the pins, removing heat as it goes, and then exits at the top of the core.

Conclusion

Although virtually all present day LMFBRs operate with uranium--plutonium oxide fuel, there is considerable interest in the future use of fuel composed of uranium-plutonium carbide, since large breeding ratio are possible with this kind of fuel. This, in turn, is due to the fact that while there are two atoms of oxygen per atom of uranium in the oxide, there is only one atom of carbon per uranium atom in the carbide. Light atoms such as carbon and oxygen tend to moderate fission neutrons, and since there fractionally fewer of the atoms in the carbide that in the oxide, it follows that the energy distribution of neutrons in a carbide-fueledd LMFBR is shifted to higher energies than in a comparable oxide-fueled LMFBR.

In conclusion, since Liquid Metal Fast Breeder Reactor use U(238), a factor of 60 more energy produced from a given amount of uranium. This can solve the energy and resource conservation problem on a worldwide scale. So the Liquid Metal Fast Breeder Reactor will become more popular in the future.

References