The emergency condenser system consists of four separate heat-exchanger subsystems, each having a nominal heat transfer capacity of 55 MW at about 71 bar reactor pressure. The emergency condensers are connected to the RPV without isolating elements, and thus actually form part of the RPV. Each emergency condenser consists of a steam line leading from the RPV nozzle to a heat exchanger tube bundle. This tube bundle is located inside the core flooding pool at a low elevational position. The outlet on the heat exchanger primary side is the reflooding line with integrated anti-circulation loop.

The working principle of the emergency condenser design is illustrated in Figure 2. Given the normal water level inside the RPV there prevails a stratified condition inside the emergency condenser. The upper part of the steam line is filled with steam while the lower part is filled with water. The water remains cold (except for a small layer below the RPV water level), as the anti-circulation loop prevents hot water from the RPV from entering the reflooding line from below. No convection occurs and thus thermal losses are negligible as long as the water level in the RPV remains normal. The water level in the steam line of the emergency conden­ser is several meters lower because the density of the water inside the RPV is lower than that of the water in the emergency condenser.

This stratified condition changes to natural circulation if the water level inside the RPV drops by more than 0.7 m. Consequentially, when the water level in the emergency condenser then drops by more than 0.5 m, steam enters the heat exchanger bundle. The steam then condenses inside the heat exchanger tubes and the resultant condensate flows via the reflooding line back into the RPV. If the water level inside the RPV is lower than the inlet nozzle of the reflooding line, the maximum driving pressure differential will be reached at a pressure of about 0.5 bar. This pressure differential is used to overcome the flow resistances in the steam line, heat exchanger tube bundle and reflooding line. The emergency condenser continues to function as long as the water level inside the RPV remains lower than 0.7 m below the normal RPV water level. This has been experimentally tested and verified under the direction of Prof.

E. F. Hicken on the emergency condenser test facility at Germany’s Julich Research Center.

On the secondary side, natural circulation also occurs once the emergency condenser begins to work. At low heat transfer rates there is single-phase flow, while at higher rates two-phase flow occurs due to water evaporation. Normally, the water inventory of the flooding pool below the heat exchanger bundle could not be used as a heat sink due to stratification. To overcome this problem, the heat exchanger is enclosed in a chimney. Water enters the chimney at the bottom of the pool and exits at the top several meters above the heat exchanger bundle.