The Sodium Coolant

The concept of a fast neutron reactor (FNR) requires the use of a non-neutron moderating coolant, which de facto explains the elimination of water: hydrogen is an excellent moderator, widely used for these properties in pressurized water power plants. The other criteria also required for a heat transfer fluid of this type are :

• a low neutron capture cross-section;

• ability to extract high power density;

• good radiation resistance and reduced corrosion on circuits ;

• reduced or, if possible, zero harmfulness;

• high industrial availability and low acquisition costs.

Sodium is one of the best candidates for meeting all these criteria. This explains why it has become so popular in the fast breeder reactor sector for the construction of experimental reactors worldwide, and why it is the exclusive choice for the construction of demonstration and prototype reactors.

Chemically speaking, sodium is an extremely powerful reducing agent. In fact, it is used as a catalyst in the chemical industry because of this particular property. This reducing effect implies a high reactivity with air and a very lively reaction with water. The development of today's fast breeder reactors is therefore closely linked to the specific technology of sodium. The specific features and technological concepts of this type of reactor are often a direct consequence of the physico-chemical properties of sodium and its use as a coolant.