In an intact circuit accident, the heat sink is no longer available, e. g. to the steam generator secondary side or to the turbine.

In the AP1000/600 designs, under accident conditions, heat is transferred to the in­containment refuelling water storage tank (IRWST) via a passive residual heat removal heat exchanger (PRHR HX). This is connected to the reactor cooling system forming a full pressure, closed, natural circulation cooling loop (Hochreiter, 1992).

In, for example, a loss of normal feedwater scenario, the PRHR can remove sufficient heat to prevent operation of the pressurizer safety valves. The PRHR HX is activated following reactor trip and loss of power. If the pumps are operating, the flow through the passive RHR heat exchanger will be forced convection from the higher pressure cold leg to the hot leg. However, if the pumps are not operating, the flow direction will be reversed and by natural circulation from the hot leg to the top of the PRHR heat exchanger to the cold leg.

The EP 1000 incorporates a similar system (Yadigaroglu et al., 1998).

Other designs are summarised in Yadigaroglu et al. (1998).

In the SWR 1000 design, there are Emergency Condensers connected to the RPV without valves and immersed in the core flooding pool.

In all the above cases, a further step is required to transfer heat from the pools to the ambient. These are described later in the containment section.

Other designs, e. g. KNGR Chang et al. (1997), some CANDU systems and some Siemens systems utilise cooling of the secondary side via a condenser.

The VVER-1000 and AC-600 systems make use of condensers outside the containment via a natural circulation air-cooled system.

Finally, the ESBWR and the Indian heavy water moderated light boiling water cooled AHWR utilise isolation condensers condensing steam from the RPV.

The passive cooling of the moderator in CANDU reactors employs a similar approach.