Abstract: The counting system includes a portable gamma ray detector having a detecting portion electrically connected to instrumentation for analyzing the radioactivity of nuclear reactor fluids, a radiation shielded panel including a plurality of openings for receiving the detecting portion of the detector, and sample vessels for receiving a radioactive fluid located behind the panel. Samples of radioactive fluids such as reactor coolant, reactor coolant off-gas and containment air, are selectively introduced into the sample vessels. The reactor coolant is cooled, de-pressurized and de-gassed and the off-gas and the liquid are routed to separate sampling vessels. The radioactivity of the radioactive fluid contained in each sample vessel is determined by simply inserting the detecting portion of the detector into a corresponding panel opening. Radiation shielded plugs are inserted into each of unused panel openings to shield the operator against radiation.