An alternative approach is to take more advantage of inherent forces such as gravity in the design of safety systems.

An additional advantage is that this approach results in simplified systems since it can eliminate the need for some redundancy, e. g. in emergency power supply systems. Some of the important reactor types in this category are listed in Table 11.2. Details of the various passive features of these plants were given in the previous chapter.

In evolutionary passive designs there are three typical components to protect against faults and accident conditions:

— cooling of the core via natural circulation, e. g. in intact circuit faults;

— gravity-driven cooling systems to mitigate LOCAs, and;

— passive containment cooling systems (PCCS).

In passive systems, the active components, e. g. pumps, diesels, fans, etc. are dispensed with and so there is no need for the redundant active safety grade systems associated

Table 11.2. Evolutionary water reactors incorporating passive systems

Reactor Description

AP600, 1000 Innovative decay heat removal via passive systems

EPP

SWR 1000 Utilisation of natural forces and phenomena,

e. g. gravity, natural circulation, passive injection

ESBWR

VVER-640 Passive containment cooling

CANDU 6(E)

Table 11.3. Passive plant reduction in components

50% fewer valves 35% fewer pumps

80% less pipe (no safety grade pumps)

80% fewer heating, ventilating and cooling units (safety grade) 35% less seismic building volume 70% less cable

AP1000: Set to Compete (2002).

with active systems. This leads to simplification and hence potential scope for capital cost reduction. Similarly, the main ultimate heat sink is often the ambient air and hence there is no need for active service water systems.

Passive safety systems do not require the framework of safety support systems that are needed for current generation plant, including AC power, HVAC, cooling water systems and associated seismic buildings containing these components. Table 11.3 illustrates the consequential reductions in a number of key components compared with current generation plant (AP1000: Set to Compete, 2002).

Examples of evolutionary passive plants also include both light and heavy water reactors: PWRs — AP1000, AP600, EPP; BWRs — SWR 1000, ESBWR; and HWR — CANDU 6(E).