Containment of Indian PHWR is an ultimate barrier, which is designed to envelope whole reactor systems, to prevent the spread of active air-borne fission products in accident condition as well as normal operation. Containment had different compartment, one such compartment is Fuelling Machine Vault (FMV) which encloses Fuelling machine, Primary Heat Transport System (PHTS) such as Header & Feeder to core. The FMV environment is closely monitored and kept at relatively low pressure with respect to other compartment of the containment. In order to control the environment inside FMV the opening area of FMV with other area is critical and should be kept minimum. FMV is also connected with Pressure Relief Chamber (PRC) and Steam Generator Vault (SGV) to relief the pressure during Loss of Coolant Accident (LOCA). Further during design extension conditions, hydrogen may be generated due to metal-water reaction and released to FMV, which will further rise and move to other area of the containment. In-order to protect the containment from threat of hydrogen, large opening area is required from FMV to SGV or PRC for pressure equalisation and dispersion of hydrogen. Another design parameter for the civil structure is the differential pressure across various walls and floors during accident conditions, to keep the differentials pressure to lower value large area is required for the initial flow from FMV to other containment volumes. As large opening area requirement for low Differential pressure, hydrogen distribution during accident and small area requirement during normal operation are two opposite requirement, hence it is important to optimise these opening for the fulfilment of both the conditions during normal operation as well as accident conditions. Studies had been done for the optimisation of the opening area in between FMV and other connecting area that is PRC and SGV which is above the FMV. These calculations had been done by System thermal hydraulic computer code PACSR-SI-2.0.