The design philosophy of the 4S-LMR is to emphasize simple, passive and inherent safety features as a major part of the defence in depth strategy. The ultimate objective in the 4S-LMR safety design is to eliminate the requirement of population evacuation as an emergency response measure.

The inherent safety features of the 4S-LMR are:

• Low power density in the core;

• Good thermal characteristics of the metallic fuel bonded by sodium;

• Negative reactivity coefficients of temperature;

• Negative sodium void reactivity coefficients;

• Large coolant inventory;

• Elimination of active or feedback control systems operating inside the reactor vessel;

• Elimination of components consisting of rotating parts (application of static devices such as EM pumps);

• Limitation of the radioactivity confinement area (no on-site refuelling and no systems for fuel loading/ unloading and shuffling, no fuel storage facilities in the reactor or on-site);

• Multiple barriers against fission product release, including:

—The fuel cladding;

—The reactor vessel, the upper plug and the IHX tubes;

—The top dome and the guard vessel as containment;

• Relatively small radioactive inventory of a small power reactor;

• Prevention of a sodium leakage and mitigation of its impact or influence if it occurs through double boundaries for sodium with a detection system for small leakage occurring in the event of one boundary failure:

—The reactor vessel and guard vessel for primary sodium;

— Double piping, tubes and vessels for secondary sodium, including heat transfer tubes of the SG.

The passive safety systems of the 4S-LMR are the following:

• An automatic sodium drain system from the SG to the dump tank — if a sodium-water reaction occurs, an increase in cover gas pressure in the SG causes secondary sodium to drain rapidly to the dump tank located beneath the SG (without rupture disks);

• Two diverse and redundant passive shutdown (residual) heat removal systems operating on natural convection of the coolant and natural air draft (PRACS or IRACS and RVACS).

For shutdown (residual) heat removal, two independent passive systems are provided; RVACS and IRACS (or PRACS, see Section VIII-1). The reactor vessel auxiliary cooling system (RVACS) is completely passive and removes shutdown heat from the surface of the guard vessel using natural draught of air. There are no valves, vanes or dampers in the flow path of the air; thus RVACS is always working, even in normal (rated) operation. Two stacks are provided to obtain sufficient draft.

The IRACS removes shutdown heat via the secondary sodium. In normal shutdown, heat is removed by forced circulation of air with a blower driven by normal electric power; IRACS can also remove the required amount of heat solely through natural circulation of both air and sodium in the case of postulated initiating events.

The 4S-LMR incorporates no active safety systems. However, there are several active systems providing normal operation of the reactor at rated (or derated) power. In normal operation heat is removed from the core by forced convection of sodium driven by EM pumps. The compensation of burnup reactivity swing is performed by very slow upward movement of the reflector. An advanced driving mechanism for such movement is being considered [VIII-1].

No information was provided on whether certain systems of the 4S-LMR are safety grade.