For both sodium cooled and lead cooled fast reactors, smaller unit capacity could facilitate:

— Effective use of auxiliary passive decay heat removal systems with environmental air in natural draught acting as an ultimate heat sink;

— Achievement of a relatively high heat capacity of the primary (or primary and adjacent intermediate) coolant system at its reasonable size, resulting in a slower progression of transients.

Specifically for sodium cooled fast reactors, smaller reactor capacity could facilitate achieving a negative whole core void coefficient of reactivity to prevent the progression of design basis accidents into severe incidents, otherwise possible at a start of sodium boiling.

Specifically for lead cooled fast reactors, smaller reactor capacity could facilitate simplified seismic protection and improved seismic response [2].