Thursday, July 14, 2011

Nuclear Technology Basics: Part 8 Liquid Metal Cooled Reactors

Introduction

Part 1

Part 2

Part 3

Part 4


Part 5

Part 6

Part 7

Liquid metal-cooled reactors are both moderated and cooled by a liquid metal solution. These reactors are typically very compact and could also potentially be used for naval propulsion. While there are a few currently existing liquid metal-cooled reactors that are being used for electricity generation, most examples are prototypes that have been built around the world as experimental reactors.

1. Sodium-Cooled Fast Reactors (SFRs)



Sodium-cooled fast reactors use a sodium-potassium alloy that remains liquid at room temperature. While the compound reacts violently on contact with air or water, its effects on steel are minimal. This makes NaK a possible coolant and moderator choice for a fast neutron reactor such as this one. Using water as a neutron moderator would effectively reduce the speed of the neutrons to those of thermal neutrons unless it was under massive amounts of pressure, while the NaK coolant and moderator does not need to be pressurized.

Fast reactors have advantages over reactors of other types as they achieve a high fuel "burn-up" ratio and greatly reduce the long-lived actinides that are present in the spent fuel when compared to other reactor types. This has made SFRs an attractive energy option for many countries around the world. Unfortunately, the Integral Fast Reactor project (IFR) was cancelled in the US because during the Clinton administration because of political reasons despite only being thee years away from completion. As of yet, a standard SFR design has not emerged from one of the many prototypes that have been built.

The NaK compound is pumped through the bottom of the reactor where it is heated up by the core. Hot coolant in the primary coolant loop is pushed into the heat exchanger, which is used to heat up coolant within the secondary coolant loop and the heated secondary coolant is used to turn a turbine connected to the generator. After leaving the turbine, the coolant flows into a condenser connected to a heat sink to help absorb some of the excess heat energy. The cold secondary coolant is pumped into the top of the heat exchanger while the primary coolant enters to bottom of the reactor again, completing the cycle. The control rods are inserted into the top of the reactor vessel.

Moderator Type: Liquid metal

Technology: Generation IV

Existing Examples: Three, but many more are being planned and built.

Advantages

-This reactor design has a high fuel burn-up ratio.

-NaK does not corrode steel.

-The reactor design is very compact yet has a high power output for its size.

-Liquid metal-cooled reactors are not pressurized leading to simpler piping systems.

-The liquid metal coolant cannot turn to steam unlike water during a meltdown, making a steam explosion impossible.

-Some variants of this design can be used as breeder reactors.

Disadvantages:

-The high temperature of the reactor could also pose design challenges.

-Sodium reacts violently with water and air.

Variants: BN-350, BN-600, Clinch River, Dounreay Prototype, Fermi 1, Experimental Breeder Reactor 1, Experimental Breeder Reactor 2, Fast Breeder Test Reactor, Integral Fast Reactor, Jōyō, Monju, Phénix, Prototype Fast Breeder, S1G, S2G, SNR-300, Sodium Reactor Experiment, Superphénix, Rapsodie

2. Lead-Cooled Fast Reactors (LFRs)



Fast reactors can also be used with a lead-bismuth coolant and moderator. Lead has a low degree of neutron absorption and does an excellent job of reflecting neutrons. In addition, lead is a very effective radiation shield and its extremely high boiling point makes lead-bismuth eutectic (LBE) an effective coolant even at higher temperatures. While LBE is somewhat corrosive to steel unlike NaK, it is unlikely that this poses a major problem as this issue can be overcome with proper engineering.

The design of the lead-cooled fast reactor is is slightly different from that of the sodium-cooled fast reactor. the LBE coolant on the outside of the coolant module flows downward where it is drawn into a pair of chambers in the center. The core of the reactor is contained with in a block in the center as well which heats the coolant as it is pushed upwards towards the top of the coolant module. This design has a pair of heat exchangers that are inserted into the top of the coolant module with the control rods in the center. Primary coolant transfers its heat energy to the coolant in the secondary coolant loop through heat exchangers where it is sent to a pair of turbines. After circulating through a secondary heat-exchange system to cool the LBE down, the returning secondary coolant is pumped through the top of the reactor to be drawn into the reactor core to complete the circuit.

Moderator Type: Liquid metal

Technology: Generation IV

Existing Examples: None that have been completed, but several LFRs are in the construction phase around the world.

Advantages

-This reactor design has a high fuel burn-up ratio.

-The entire reactor core can be removed and replaced during refueling procedures.

-The reactor design is very compact yet has a high power output for its size.

-Liquid metal-cooled reactors are not pressurized leading to simpler piping systems.

-The liquid metal coolant cannot turn to steam unlike water during a meltdown, making a steam explosion impossible.

-Some variants of this design can be used as breeder reactors.

Disadvantages:

-Lead-bismuth eutectic is slightly corrosive to steel.

-The lead-bismuth eutectic coolant can cause problems if it is allowed to solidify within the coolant circuits.

-The high temperature of the reactor could also pose design challenges.

Variants: OK-550, BM-40A, SVBR-100, Hyperion Power Module, MYRRHA

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