Noble Gases Monitoring In Gaseous Effluents at Cernavoda NPP

The main objectives of effluents monitoring programs, are to limit releases of radioactivity to the environment so that the exposures are not likely to exceed dose limits established by the competent authority and to provide reasonably assurance that the levels of radioactivity added to environment are well below levels that could result in adverse effects on the ecology of the environment. The Gaseous Monitoring System installed and commissioned in Unit 2 of Cernavoda NPP is a modern and reliable equipment that allows an accurate monitoring of the atmospheric emissions for assuring the control of releases and for comparison with the limits established by the authorities. Also, having the benefit of using measurement features like on-line monitoring (through Radiation Monitoring System), on-line -spectrometry with NaI detection system and supplementary γ - spectrometry with HpGe detector, the system allows to identify the activities of the plant that can lead to significant radiological impact regarding the levels of emissions and their magnitude (e.g.: Annulus Gas System purges, PHT Storage Tank purges, defective fuel removal, maintenance work on active systems). One of the most significant applications of the Noble Gases Monitoring Channel at Unit 2 is represented by the monitoring of the gaseous emissions during and after the removal of the fuel suspected to be defective. On-line surveillance of the noble gases emissions was performed to prevent the attaint of the alarm set - point and to reduce, as much is possible the levels of radioactive releases. By taking in account the total activity previously released, the real - time emissions rate and trend evolution, it was established if the criteria for the manual isolation of the Containment System (Reactor Building Boxed-Up) are fulfilled or not. The γ - spectrometry with NaI and HpGe detection systems allowed to identify the components of noble gases nuclides mixture present in the discharges and inside of the Reactor Building, in order to estimate the time period of the Reactor Building isolation required to reduce nuclides concentration by radioactive decay.(author)