PROVISIONS FOR SAFETY UNDER EXTERNAL EVENTS

The safety design features of the AHWR intended to cope with external events and external/internal event combinations are described in detail in [VI-5].

The reactor is provided with an inner pre-stressed concrete containment designed to provide leaktightness in the case of a large break LOCA, and an outer secondary containment that protects the inner containment from external events including aircraft impacts.

Location at a high elevation counters the effects of flood related events as well as probable maximum precipitation, maximum possible sea level etc. in extreme environmental conditions.

AHWR structures, systems and equipment are being designed for high level and low probability seismic events such as an operating basis earthquake (OBE) and safe shutdown earthquake (SSE). These are also called S1 and S2 level earthquakes respectively. Seismic instrumentation is also planned in accordance with national and international standards.

Safety related buildings are protected from turbine generated low trajectory missiles.

Fire protection measures comprise physical separation, barriers, and the use of fire resistant materials at potential systems, and also minimize the inventory of combustible material.

Closing dampers in the ventilation systems provide for detection of poisonous gases and minimize their ingress into structures and air intakes. Air bottles with a capacity of 30 minutes are provided to supply fresh air to operating personnel.

Important nuclear auxiliary systems are located inside the reactor building and in the basement, to the extent possible.

As outlined in previous sections, the AHWR incorporates many inherent safety features (e. g., negative void coefficient of reactivity, and passive systems that require no external power and no operator actions to accomplish certain safety functions. The design provides for several heat sinks that remain available with loss of external coolant supply, such as the gravity driven water pool (GDWP) with 6000 m3 of storage capacity, ensuring a three day grace period for decay heat removal; fire water storage, providing cooling of the important auxiliary systems for eight hours; the moderator, which in AHWR acts as an ultimate heat sink; and the emergency water reservoir. All of these features/systems are intended to secure plant safety in the case of both internal and external events and their combinations.

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