LIQUID METAL FAST BREEDER REACTOR

M Lakshman Kavish

2011CS10229

Group – 2

Introduction: Liquid Metal Fast Breeder Reactor also called as LMFBR shortly is nothing but a Fast

Breeder Reactor with liquid metal coolant. A Fast Breeder Reactor is a nuclear reactor which

produces more fissile product than it takes in. Liquid metal is used as a coolant in fast breeder

reactors mainly to increase the efficiency of the heat transfer system of the reactor. The most

commonly used metal to accomplish heat transfer is sodium lithium is also used to some

extent.

LMFBR overview

Block diagram of LMFBR

Main Working of LMFBR: The plutonium-239 breeder reactor is commonly called a fast breeder reactor, and the

cooling and heat transfer is done by a liquid metal. The metals used commonly for this are

sodium and lithium, with sodium being the most abundant and most commonly used. The fuel

used in a fast breeder reactor needs to be enriched more than that of a light water reactor,

typically 15 to 30%. The reactor fuel is surrounded by a blanket of non-fissionable U-238. No

moderator is used in the breeder reactor since fast neutrons are more efficient in converting U-

238 to Pu-239. At this concentration of U-235, the cross-section for fission with fast neutrons is

sufficient to sustain the chain-reaction. Using water as coolant would slow down the neutrons,

but the use of liquid sodium avoids that moderation and provides a very efficient heat transfer

medium.

The coolant runs through the main core in the same fashion as in the case of water

cooled reactors, picks up heat and runs through heat exchangers releasing the energy to be

used for power generation thereon. The heat from the liquid Sodium is transferred to water in

the second heat exchanger and the steam produced is used to rotate the turbines.

Principle behind using Sodium: Liquid Sodium if the most used coolant for the fast breeder reactors. Sodium is a solid at

room temperature but melts at 98°C. It has a wide working temperature since the boiling point

of Sodium is 892°C. The temperature at which the reactor works is within this range so no

pressurization is required as in the case of light water coolant. It has a large specific heat so that

it is an efficient heat-transfer fluid. Liquid metal is mainly used for fast breeder and light water

for normal reactors because the water when used as coolant also acts as a moderator but not

liquid metal which is useful in the case of fast breeder reactors.

Advantages of using Sodium as a coolant: As discussed above there are lot of advantages using Sodium as a coolant.

Large gap between melting and boiling point which also includes the temperature of

working of the reactor so efficient heat transfer is possible without pressurizing.

Sodium coolants have high heat capacity which provides thermal inertia against

overheating.

In fast breeder reactors no moderator is used but water when used also acts as a

moderator so Sodium is better to be used because Sodium atoms are much heavier than

Hydrogen and Oxygen so neutrons loose less energy when collided with.

Disadvantages of using Sodium as a coolant: The main disadvantage of using Sodium as a coolant is its chemical reactivity. It reacts

with water and air explosively so precautions need to be taken very carefully. This was

the case at the Monju Nuclear Power Plant in a 1995 accident.

Neutrons cause Sodium to become radioactive. However, activated sodium has a half-life of only

15 hours.

Liquid coolants are opaque so it is tough to repair the setup if found malfunctioning.

Other Coolants Used and their Properties in brief: Mercury was the very first liquid metal used for cooling and also thought of as a best

choice as it is liquid at room temperature but its disadvantages include high toxicity,

high vapor pressure even at room temperature, low boiling point, producing noxious

fumes when heated, relatively low thermal conductivity, and a high neutron cross

section, it has fallen out of favor.

Lead has excellent neutron properties (reflection, low absorption) and is a very potent

radiation shield against gamma rays. Lead has very high boiling point so it can cool the

reactor which is working at a very high temperature. However, because lead has a high

melting point and a high vapor pressure, it is difficult to refuel a lead cooled reactor and

also maintain. The melting point can be lowered by alloying the lead with bismuth, but

lead-bismuth eutectic is highly corrosive to most metals used for structural materials.

Tin Although tin until today is not used as a coolant for working reactors as it builds a

crust, it can be a useful additional or replacement coolant at nuclear disasters or loss of

coolant accidents.

Other uses: Liquid metal cooled reactors are used for propulsion purposes in submarines and

nuclear aircrafts for example The Soviet November class submarine K-27 and all seven Alfa-

class submarines used reactors cooled by a lead-bismuth alloy. The only submarine to use

Sodium cooled nuclear reactor is USS Seawolf