Design Features

Advantages

Disadvantages

Neutron spectrum

Fast

· large fission to absorption ratio

· minimize radioactive waste formation

· burnup extension

· sustainability (breeding)

· potential for disruptive power excursions

· low margin to prompt criticality (Pu-239 compared to U-235)

Thermal

· robust reactivity against fluctuations in physical parameters

· safer margin to prompt criticality

· long operational experience

· inefficient fuel exploitation

· larger radioactive waste formation

Coolant

Sodium

· superior thermal hydraulic properties/heat transfer characteristics

· excellent neutronic properties and economy

· good compatibility with structural materials

· significant reactivity insertion issues (sodium boiling, large coolant temperature coefficient)

· high chemical activity with water, steam, and air (explosion risk)

· optical opacity

Lead

· good natural circulation and heat transfer properties

· superior neutronic characteristics and performance

· chemically inactive

· low cost (lead is abundant)

· high melting point (freezing potential)

· erosion and corrosion potentials (need for coating)

· Polonium-210 activity build up

· optical opacity

Molten Salt (fluorides or chlorides)

· high density-specific heat product (large grace period)

· high boiling temperatures (no void reactivity insertions)

· chemically inactive

· optical transparency

· high melting point (freezing potential)

· neutronically challenging

· small thermal conductivity limiting the power density

· pumping constraints (large viscosity)

Inert Gas (e.g. Helium) [26]

· high breeding ratio

· small void reactivity coefficient

· chemically inactive (inert)

· no corrosion/material challenges

· optical transparency

· potential for direct Brayton cycle (lower capital costs)

· high neutron leakage

· relatively poor heat transfer characteristics

· water ingress concerns (positive reactivity insertions)