The LOCA challenges reactor safety by raising peak cladding and fuel temperatures from stored core energy and decay heat generation. The response to this challenge differs among the SMR types as follows — however, for fully integral SMRs no large primary coolant diameter piping exists, thus no classic large LOCAs can occur:

• For the water reactors the primary coolant released flashes in the containment, creating steam expansion which pressurizes the containment and can cause mechanical damage to equipment. While equipment can be secured from this threat and containment can be sized both in volume and wall thickness to survive this threat, the threat of exposure of fuel, even after shutdown of the fission process, requires a means to replenish core coolant inventory. Passive, gravity-driven core reflood systems are the current design vehicle. They must be sized both in delivery head and volume sufficient to rewet the cladding if the fuel is uncovered or simply maintain the cladding wet if the reactor system can be designed to prevent core exposure even during a design basis-LOCA as are all integral PWR-type SMRs. For both situations the sufficiency of core coolant inventory reverses the trend of increasing temperature before the zirconium-based cladding reaches the regulatory limit, now 1204 °C, at which its ductility, and hence its integrity, is threatened. Ultimate removal of decay heat is achieved by means of dedicated decay heat removal loops which transfer heat to the environment or passive conduction heat removal through the reactor containment.

• For gas reactors timely replacement of coolant inventory at pressure is impractical. However, the use of high conductivity graphite as the core moderating material offers a radial conduction path for core energy to an ex-vessel heat sink. The graphite core material provides a significant heat sink which maintains temperatures at allowable levels until passive heat removal capability can match the decay heat level. For cores of modest dimensions the length of this path is short enough and the heat storage capacity of the graphite moderator is large enough to allow steady-state power ratings of hundreds of MWe. These ratings are made possible by the use of coated particle fuel with its high 1600 °C limit for onset of significant fission product diffusion or leakage through disrupted fuel coatings.

• For liquid metal reactors the very low vapor pressure of coolant even at the high operating temperature allows the NSSS to be housed in a pool of coolant within a thin-walled reactor vessel which itself is surrounded by a close fitting thin-walled guard vessel. Even upon loss of integrity of the reactor vessel, the coolant inventory is retained in the guard vessel keeping the cladding covered with coolant and the decay heat is removed by a dedicated in-vessel natural circulation coolant loop and/or radial heat flow through the guard vessel to a dedicated air chimney system, both of which discharge heat outside the containment.

• All three design solutions are satisfactory, although they operate on different principles and have configurations of differing passive safety responses.