All SMRs will be designed to meet the same top level set of regulatory requirements. However, the inherent characteristics of each coolant significantly influence the means by which such requirements are achieved.

Neutronic-based coolant density coefficients of reactivity are of sufficient magnitude as to affect the design of all liquid-cooled reactors. For PWRs and BWRs the moderator coefficient is designed to remain negative for accident conditions, but for sodium — and lead/lead-bismuth eutectic-cooled fast reactors the reactivity coefficients which would be present in significant coolant voiding events are unavoidably positive and protected by other design features. Helium-cooled reactors of either the pebble or prismatic type have the unique feature of low power density coupled with a high heat capacity core and reflector that yields a design such that the reactor, upon an increase in temperature, neutronically reduces the power to a very low level, i. e., well below 1% of full power.

The inherent means to cope with severe accident conditions and design basis events are key features of the variously cooled SMRs, namely,

• bounding the potential energy release.

• mitigation of release of fission products by scrubbing in primary coolants.

• response to the loss of coolant accident (LOCA) and provision for ultimate removal of decay heat.