Studying the removal of the residual heat of the reactor in a final stage of an accident with loss of coolant is very important for proving that a nuclear power plant is safe. In this stage of the accident with WWER-640 reactor, the tanks of the emergency core cooling system are empty and the emergency pool is filled with water. At this time of emergency cooling of the reactor, a rather complex heat transfer mechanism based on natural circulation of the coolant is realized. Key elements and systems of the reactor installation are involved in this process. The natural circulation circuit is a system of three vessels with a free liquid surface. These vessels are the reactor, the fuel pool, and the emergency pool. Several pipes connect them to each other, in accordance with a plan, by which the residual heat of the reactor is to be removed by using the emergency pool. The steam generated in the core during natural circulation of the coolant through the reactor enters the pressurized containment space. In this way, the steam bubbles through the liquid layer in the pools. The heat from the pressurized volume is removed by cooling the outer surface of the metallic protective shell of the containment. This is done by using a system of passive heat removal from the pressurized

shell (SPHR PS), which also utilizes the principle of natural circulation of the corresponding coolant. The condensate formed on the cooled internal surface of the protective shell is returned to the emergency pool. Thus, the necessary amount of water is maintained in the emergency pool. Therefore, during the stage of pool cooling of the reactor, the thermal- hydraulic processes that take place in the reactor, the containment, and the emergency system, are interrelated. So, to substantiate measures taken in the design for ensuring safety of the nuclear power unit, we will have to comprehensively simulate the processes mentioned above.

In order to carry out the proposed experiments connected with the WWER-640 reactor, the PACTEL was reconstructed (Fig. 2). The main parts of the PACTEL have been kept in the experiments; such as the instrumented pressure vessel, the downcomer, the lower plenum, the core, and upper plenum. The remaining components and systems were excluded.

Two water tanks installed into the PACTEL, simulating the emergency cooling and fuel storage pools. These vessels are open to atmosphere at the top. The larger diameter of the two water tanks models the emergency cooling pool, the smaller one models the fuel storage pool. Geometrical characteristics are presented in the Table I. Each tank has a hydraulic link to the downcomer of the PACTEL with a single horizontal pipe. Similarly, each tank is linked to the upper plenum of the PACTEL with hot-leg connections. The level balancing line (LBL) interconnects the two water tanks (Fig. 3). All of the added pipelines provided with isolation valves to form a circulation loop for each scenario of the performed test series.