The flow diagram of EFWS system is shown in Figure 3.2. When activated, the EFWS provides makeup water to the main circulation circuit. This system can act as part of the ECCS long-term subsystem, as described in Section 3.2., or it may act independent of the ECCS. In the former mode of operation, the emergency coolant is injected into the GDH. For some upset conditions, the ECCS is not activated, but the makeup is still required, e.g. low feedwater flow. For such conditions, the EFWS provides additional coolant to the steam separators.
The largest and most important system protecting the plant personnel and the environment from radiation in the event of an accident, is the accident confinement system. The ACS structure is shown schematically in Figure 3.3. The system used at the Ignalina NPP belongs to the pressure suppression category of containment’s. The ACS must ensure the protection of personnel and environment from radioactive contamination above authorized limits following any DBA, including the ultimate DBA. This is achieved in the following way:
The release of clean air and the use of sprinklers allows the ACS pressure to be reduced to the relative atmospheric pressure, which decreases the possibility of release of radioactive materials to the environment. The ACS also serves as a water reservoir. This water is used for emergency core cooling, as well as to condensate any steam discharged by the safety/relief valves.
The defence-in-depth concept is not completely applied to Ignalina NPP with respect to confining all parts of the reactor coolant circuit, e.g. upper sections of pressure tubes, steam-water communication lines between steam separators. If a break occurs in these lines, steam is discharged directly to the atmosphere.
The free volume of the ACS is relatively small (less than 2800 m3) with a high degree of compartmentalization. It is obvious that high H2 concentration of 1% cladding oxidation can locally occur in some of the compart-ments. Analyses have shown that flammable concentrations are avoided for DBA, but no evidence was provided that the hydrogen removal capacity is sufficient to cope with the maximum credible hydrogen production during beyond DBA.
Fig. 3.3 Simplified schematic of the accident confinement system [4]:
1 - fuel channel; 2 - main circulation pump; 3 - suction header; 4 - pressure header; 5 - group distribution header; 6 - ECSS header; 7 - suppression pools; 8 - ACS heat exchanger; 9 - air discharge pipe section; 10 - steam pipe from MSVs and SDV-A; 11 - air-gas mixture from the reactor cavity