CAREM safety systems are based on passive features that don’t require active actions to mitigate accidents for a long period. They are duplicated to fulfil the redundancy criteria. One of them that relies on natural circulation is the residual heat removal system (RHRS), Figure XIV-2. It has been designed to mitigate a Loss of Heat Sink, reducing the pressure of the primary system to values lower than the values at hot shutdown, by removing the decay heat. The RHRS is a simple and reliable system that operates condensing steam from the reactor dome in the emergency condensers. This establishes a stratified two-phase natural circulation loop with the primary system.

FIG. XIV-2. Safety system.

The emergency condensers are heat exchangers consisting of an arrangement of parallel horizontal U tubes between two common headers. The top header is connected to the reactor vessel steam dome, while the lower header is connected to the reactor vessel at a position below the reactor water level. The condensers are located in a pool filled with cold water inside the containment building. The inlet valves in the steam line are always open, while the outlet valves are normally closed, therefore the tube bundles are filled with condensate. When the system is triggered, the outlet valves open automatically. The water drains from the tubes and steam from the primary system enters the tube bundles and condenses on the cold surface of the tubes. The condensate is returned to the reactor vessel forming a natural circulation circuit. In this way, heat is removed from the reactor coolant. During the condensation process the heat is transferred to the pool water by boiling process. This evaporated water is then condensed in the suppression pool of the containment. The pool of the RHRS has a volume sufficient to provide an autonomy greater than the grace period for the prototype (48 h).

The RHRS main characteristics are listed in Table XIV-1.

TABLE XIV-1. RESIDUAL HEAT REMOVAL SYSTEM-EMERGENCY CONDENSER FOR CAREM PROTOTYPE

In case of a very small LOCA (lower than 3/4’) the RHRS is also demanded by the reactor protection system to depressurize the primary system to allow the emergency injection system to act.

XI — 4. Conclusions

In order to assess the CAREM primary circuit numerical modelling a High Pressure Natural Circulation Rig (CAPCN) was built in the decade of the 90s. The CAPCN resembles CAREM in the primary loop and steam generators, while the secondary loop is designed just to produce adequate boundary conditions for the heat exchanger. The CAPCN rig reproduces most of the dynamics phenomena of the RCCS, except for 3-D effects. Several tests were performed covering thermal hydraulics, reactor control and operating techniques around the nominal operational point.

Indicative values of the main variables corresponding to the CAPCN operation at full power are shown in Table XIV-2.

TABLE XIV-2. CAPCN NOMINAL CONDITIONS

Most of the tests performed consisted of an initial self-steady state in which a pulse-wise perturbation induced a transient. In this case the perturbation is a thermal unbalance as severe as possible for operational transients. The following test groups were performed:

• Preliminary tests to characterize components and equipment;

• Thermal balance test, instrumentation calibration and evaluation of their accuracy;

• Dynamic test around the operational nominal point without control: thermo-hydraulic response evaluation;

• Dynamic test with the control loops (power pressure);

• Dynamic test at low pressure and temperature.