Protection of the reactor cavity against overpressurization is an important part of the safety system of RBMK-1500. A simplified schematic of this system is shown in Figure 3.4. Cavity pressure exceeding 0.31 MPa has been described as having the possibility to lift the upper-head shield assembly breaking the reactor seal, the pressure tubes, and affecting the operation of other safety functions. The requirement for overpressure protection system is to have the capacity for coping with DBA involving one pressure tube rupture. The new overpressure protection system that has the capacity to withstand multiple rupture of up to 9 pressure tubes under conservative assumption of simultaneous ruptures was installed at INPP in l996. Pressure relief is provided by pressure relief tubes which connect the reactor cavity to the ACS. The reactor cavity venting system vents the steam-gas mixture from the cavity to the pressure suppression pools where steam is condensated while the gas is retained in the leaktight spaces.
Fig. 3.4 Simplified schematic of the reactor cavity overpressure protection system:
1 - reactor; 2 - the fifth ACS suppression pool; 3 - suppression pools 1-4; 4 - steam distribution devices; 5 - membrane safety devices