Fast-neutron Reactor - Disadvantages

Disadvantages

• The reactor's criticality responds within the flight time of the neutrons across the core. Design of a fast reactor is therefore more demanding, because there is no moderator whose thermal or mechanical behavior can adjust the reactor, and neutron lifetime is lower than in a thermal reactor, since neutrons diffuse without slowing down. Fast reactors cannot be reliably stabilized with control rods, which are too slow and have much smaller capture cross sections at fast energies than thermal energies. Most designs are stabilized either by doppler broadening or by thermal expansion of the fuel, a neutron poison or a neutron reflector.
• Due to the low cross sections of most materials at high neutron energies, critical mass in a fast reactor is much higher than a thermal reactor. In practice, this means significantly higher enrichment: >20% enrichment in a fast reactor compared to <5% enrichment in typical thermal reactors. Since enrichment is the most expensive step in the fuel cycle, this significantly increases the initial costs of a fast reactor. It also opens the door to Nuclear proliferation issues.
• Sodium is often used as a coolant in fast reactors, because it does not moderate neutron speeds much and has a high heat capacity. However, it burns in air, and is very corrosive. It has caused difficulties in reactors (e.g. USS Seawolf (SSN-575), Monju). Although some sodium-cooled fast reactors have operated safely (notably the Superphénix), sodium problems can be prevented by using lead or molten chloride salts as a coolant.
• Since liquid metals have low moderating power and ratio and no other moderator is present, the primary interaction of neutrons with liquid metal coolants is the (n,gamma) reaction, which induces radioactivity in the coolant. Boiling in the coolant, e.g. in an accident, would reduce coolant density and thus the absorption rate, such that the reactor has a positive void coefficient, which is dangerous and undesirable from a safety and accident standpoint. This can be avoided with a gas cooled reactor, since voids do not form in such a reactor during an accident; however, activation in the coolant remains a problem. A helium-cooled reactor would avoid this, since the elastic scattering and total cross sections are approximately equal, i.e. there are very few (n,gamma) reactions in the coolant and the low density of helium at typical operating conditions means that the amount neutrons have few interactions with coolant.