This manuscript presents the outcomes from the quantitative risk assessment (QRA) of the integration of a generic hydrogen cogeneration facility into the protected area of a reference nuclear power plant (NPP). The latter is Rivne NPP in Ukraine, and the hydrogen production plant (HPP) is assumed to use electricity from the NPP to operate in base-load mode. The presented study applies HAZID technique to identify those postulated initiating events (PIEs) that lead to the worst-case scenario in the HPP and therefore major external hazard to the NPP, which is regarded as an hydrogen explosion. The study quantifies likelihood of occurrence of the two identified PIEs and potential accident scenarios. It evaluates as well the consequences of the worst-case scenario by means of empirical and analytical expressions in the process safety literature, together with conservative conditions. The major external hazard is the storage facility due to the large hydrogen inventory. Nevertheless, storage is assumed outside of the NPP perimeter, and safety distances can be adjusted accordingly. In the electrolyser facility, a detonation of a hydrogen vapor cloud in the gas processing unit would only violate the 10 kPa criterion over the nearby standby diesel generators, under upper-bound conditions. An explosion of the buffer tank would have more severe consequences, requiring mitigation measures such as blast walls to ensure an adequate safety level under such conservative accidental conditions.