The IMSR uses a liquid fuel, a fundamental departure from a conventional nuclear reactor that use solid fuel. In turn this implies a fundamentally different safety profile. Three risks define the safety profile of a conventional reactor:
- Loss of coolant and decay heat removal;
- Hydrogen production;
- Extremely high operating pressures.
The principal challenge for all reactor systems is the management of decay heat. In this critical area of decay heat management the IMSR excels. The IMSR’s liquid salt mixture serves as both the coolant and the fuel. Molten salts are excellent coolants and have high heat capacities, which explains their use in solar power arrays. An IMSR cannot lose coolant for the simple reason that the coolant and the fuel are one and the same. An IMSR cannot melt down, because the fuel is already molten, and naturally operates at high temperatures. High temperatures permit simple and effective cooling methods.
Conventional Nuclear commonly uses water as a coolant. If decay heat is not managed, the water creates hydrogen, which is explosive and in some accident scenarios has exploded. The IMSR operates in an environment completely devoid of water and steam. Decay heat in an IMSR cannot create a pathway to explosive chemicals. This is a key safety feature of an IMSR.
Finally, the IMSR operates at atmospheric pressure, while a conventional nuclear reactor operates at 160 atmospheres of pressure due to the need to maintain water for fuel cooling. It is a substantial engineering challenge to contain the contents of the core, including the coolant, at such pressures in all conceivable scenarios. This results in the requirement for multiple containment domes and other heavily engineered solutions. Operating at atmospheric pressure the IMSR has no such need. This greatly reduces the IMSR’s capital costs and physical footprint. The IMSR excels in these three critical safety areas, and does so to such a degree that the IMSR is walk-away safe. The loss of power or secondary coolant are not the accident scenarios for an IMSR. With this safety profile, the IMSR represents an entirely new paradigm in civilian nuclear safety.