ROLE OF PASSIVE SAFETY DESIGN FEATURES IN DEFENCE IN DEPTH

Some major highlights of passive safety design features in the 4S-LMR, structured in accordance with the various levels of defence in depth [VIII-2, VIII-3], are described below.

Level 1: Prevention of abnormal operation and failure

(A) Prevention of transient over-power:

• Elimination of feedback control of the movable reflectors;

—A pre-programmed reflector-drive system, which drives the reflector without feedback signals;

—The moving speed of the reflector is approximately lmm/week;

• The limitation of high speed reactivity insertion by adopting electromagnetic impulsive force (EMI) as a reflector driving system;

• The limitation of reactivity insertion at the startup of reactor operation;

• Negative whole core sodium void worth;

• Power control via pump flow rate in the power circuit (no control rods in the core).

(B) Prevention of loss of coolant:

• Double boundaries for primary and secondary sodium in SG tubes and continuously operating leak detection systems.

(C) Prevention of loss of flow:

• Primary EM pumps are arranged in two units connected in a series in which each single unit takes on one half of the pump head;

• A combined system of EM pumps and synchronous motor systems (SM) ensures sufficient flow coastdown characteristics.

(D) Prevention of loss of heat sink:

• Redundant and diverse passive auxiliary cooling systems (RVACS and IRACS or PRACS) with natural draught of environmental air acting as a heat sink.

(E) Prevention of sodium-water reaction:

• A leak detection system in the heat transfer tubes of the SG using wire meshes and helium gas, capable of detecting both:

—An inner tube failure (water/system side of the boundary);

—An outer tube failure (secondary sodium side of the boundary).

Level 2: Control of abnormal operation and detection of failure

The inherent and passive features contributing to such control are:

• All negative temperature reactivity feedback coefficient;

• Negative whole core sodium void worth;

• Effective radial expansion of core (negative feedback);

• Large thermal inertia of the coolant and the shielding structure;

• Two redundant power monitoring systems, the primary and the secondary; balance of plant temperature monitoring system; EM pump performance monitoring system, cover gas radioactivity monitoring system, etc.

Level 3: Control of accidents within the design basis

The inherent and passive features contributing to such control are:

• Metallic fuel (high thermal conductivity, low temperature);

• Low liner heat rate of fuel;

• Negative whole core sodium void worth;

• All negative temperature reactivity feedback coefficient;

• Low pressure loss in core region;

• Effective radial expansion of core (negative feedback);

• Redundant and diverse passive auxiliary cooling systems (RVACS and IRACS or PRACS) with natural draught of environmental air acting as a heat sink;

• Increased reliability of reactor shutdown systems achieved by the use of two independent systems, with each having enough reactivity for a shutdown, including:

—The drop of several sectors of the reflector;

— Gravity driven insertion of the ultimate shutdown rod.

• Increased reliability of the sodium leakage prevention systems achieved by the use of double wall SG tubes with detection systems for both inner and outer tubes.

Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of consequences of severe accidents

The inherent and passive features contributing to such control are:

• Redundant and diverse passive auxiliary cooling systems (RVACS and IRACS or PRACS) with natural draught of environmental air acting as a heat sink;

• Inherent safety features of a metal fuelled core, such as excellent thermal conductivity and low accumulated enthalpy;

• Low linear heat rate of fuel;

• Negative whole core sodium void worth;

• Large inventory of primary sodium to meet the requirements for increased grace periods;

• The rapid system of sodium drain from the SG to the dump tank as a mitigation system for sodium-water reaction.

Level 5: Mitigation of radiological consequences of significant release of radioactive materials

The inherent and passive safety features of the 4S are capable of eliminating an occurrence of fuel melting in any accident without scram (AWS) or anticipated transient without scram (ATWS), see Annex XIV and Annex XV in [VIII-1].

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