Assessment of surveillance tests used to estimate backup instrument air capacity

In Nuclear Power Plants, Backup Instrument Air (BUIA) is credited as the safety support system powering the mitigating devices used during a design basis accident to ensure that the barriers against radiological releases are maintained. The BUIA normally stored in cylinders or vessels are designed to have adequate capacity to meet minimum mission time requirements, which is confirmed using a Safety Related Surveillance Test (SRST). Normally simple extrapolation of the SRST is performed to determine the available BUIA capacity. This extrapolation is valid dependent upon the conditions and procedure of the test. The available BUIA capacity can be over or under predicted if the thermodynamic changes occurring during the transient, choked flow conditions at the break, and heat transfer from the environment are not rigorously considered. This paper conducts an assessment of the SRST and the typical processes that should be considered for estimating BUIA capacity. (author)