Wireless systems are gaining momentum within nuclear power plants (NPPs) due to their ease of deployment and higher mobility over their wired counterparts. Particularly, industrial wireless sensor networks (IWSNs) are now being used to improve plant efficiencies by replacing periodic surveillance and making data more readily available. IWSNs however have yet to reach their full potential due to several concerns that include IWSN system reliability and availability. Application constraints, such as resource limitations and deployment restrictions, can negatively impact the reliability and availability of a system. To evaluate the reliability of a IWSN system for a NPP monitoring application, it is important to understand how different IWSN device types and network communication protocols affect the overall performance of the system. This paper provides the background necessary to describe an IWSN system reliability and availability, in terms of IWSN device types, and network communication characteristics. Different approaches for modelling IWSN reliability and availability are also discussed.