In the tests of containment sump strainer, the solid debris such as fiber and particulate is put into the test system. And then the liquid-solid two-phase flow comes into being. Three main tests are carried out to measure pressure drop of the debris bed, which are clean screen test, thin bed test and full fiber/particulate load test. If there is vortex on the water surface, air may be inhaled into the pump, and then cavitation erosion would appear. So the vortex formation is carefully observed in these tests.

Clean screen test is performed without debris, which is shown in Fig. 15.

The thin bed test is conducted for the purpose of determining the amount of fiber fines which are necessary to completely cover the strainer. When the full coverage of the strainer screen is visually observed, the strainer screen should be photographed. The post-test photograph of thin bed test is shown in Fig. 16. It can be seen that the strainer is completely covered by fibrous debris and the thin bed is formed.

Full fiber/particulate load test is performed to determine the head loss associated with the maximum fibrous and particulate debris load. In this test, full debris load is put into the test tank and the strainer is covered by more debris than that in the thin bed test. And the post­test photograph of full fiber/particulate load test is shown in Fig. 17.

As the safety nuclear apparatus, the containment sump strainers filter the debris out of the recycling water and provide the filtered water for the emergency core cooling system (ECCS) and the containment spray system (CSS). In order to keep the normal operation of ECSS pump and CSS pump, the containment sump strainers must guarantee sufficient NPSH (net positive suction head). Then the pressure loss due to the accident-generated debris accumulated on the sump screens should be one of the most important parameters of he containment sump strainers. And liquid-solid two-phase flow will appear when the accident-generated debris is flushed into the recycling water. NPSH of ECCS pump and CSS pump will be directly affected by this two-phase flow, of which the characteristics are important and significant for researchers to investigate.

In the tests of containment sump strainer, the solid debris such as fiber and particulate is put into the liquid. And the liquid-solid two-phase flow comes into being, which is different from the flow patterns in tube-bundle channels. The liquid-solid two-phase flow and the debris bed covering on the containment sump strainer are carefully observed and recorded in the tests.

In the recycling course, the ECCS and CSS systems would adopt the water in the containment as pump source when the water in PTR001BA was used up. The debris generated by LOCA or HELB would be transported to the containment ground floor with the elevation of -3.5m. And a fraction of debris would accumulate on the sump screen which could induce pressure loss and might lead to the pump failure of ECCS and CSS systems. The debris transportation fraction to the sump strainers is analyzed by numerical simulation.

The authors take Daya Bay PWR for example to establish a 3-D computational model, with the purpose of studying the debris types and contents transported to the sump strainers. According to the actual dimension of the containment sump strainer in Daya Bay nuclear power station, a 3-D CAD model is established, as shown in Fig. 18. The altitude of the CAD model is ranging from -3.5m to 0m.

Then this CAD model is imported into Gambit, and the computational grid can be plotted. The cooper mode is adopted in defining the computational grid. In the complex locality and the key position, the computational grid is refined to resolve the important features. For the main part of the model, 5*5*5cm mesh spacing is induced in x-y-z directions. And the total cell amount in the model is 7,166,332, which is shown in Fig. 19.

In CFD model, the water temperature in the containment is set as 120°C and the pressure is 1.99bar. In this circumstance, the water is sub-cooled and the water density is 943kg/m3, and the viscosity is 2.32*10-4 Pa s.

According to the mass conservation principle and momentum conservation principle, continuity equation and momentum equation are established.

Fig. 18. 3-D CAD model of containment

Where, p is the liquid density, kg/m3. u is the flow velocity, m/s. t is time, s. ц is the dynamic viscosity Pa s. S is the source term.

In the CFD calculation, the general transportation equations are established with the use of k-£ technique.

Where, pt is the turbulent viscosity coefficient. Gk is the turbulent energy generated by time — average velocity gradient. ok and o£ are turbulent Prandtl number of k equation and £ equation.

The velocity field of water flow in the containment is shown in Fig. 20. The water would tumble the sunken debris along the ground floor or lift debris over a curb in the area where the water velocity is high enough.

The experimental and numerical results gained above can provide necessary basis for the analysis of the properties of containment sump strainer and for the design of new-type containment sump strainers.