The influence of tritium build-up in Cernavoda NPP systems on gaseous and liquid emission after four years of operation

Most of the tritium in a CANDU reactor is formed by thermal-neutron-capture reactions, 2H(n,γ)3H, which occurs both in the moderator and heat transport system. Very small amounts of tritiated heavy water may escape from moderator and heat-transport systems of CANDU reactors during maintenance and normal operation. Tritium emissions of Cernavoda NPP were continuously monitored since 1996, when the plant became operational. HTO is continuously sampled on molecular sieves and measured twice a week. Statistical methods were applied to find the emission trend and to develop criteria for identifying abnormal emissions, and to predict the further emissions. During routine operation of a CANDU reactor, various gaseous, liquid, and solid radioactive wastes are generated. The design of its systems ensures that these are minimized but small quantities of gaseous and liquid wastes are continuously discharged at very low concentrations. CANDU reactors are both moderated and cooled by heavy water (D2O). Tritium is produced in CANDU reactors by neutron reactions with deuterium boron and lithium and by ternary fission. Activation of deuterium is by far the most important mechanism which is responsible for the production of about 89 TBq of tritium per MW(e) per year compared to only 0.7 TBq of tritium per MW (e) per year produced by ternary fission. Most of the tritium present in CANDU reactors is in the form of tritiated heavy water - DTO. The sources of tritiated heavy water vapor in reactor building, air are the leaks from the main reactor systems or their auxiliaries. Special dryers were designed and are used to remove moisture from different ventilation systems of a CANDU reactor in order to maintain the gaseous tritium emissions below the limits established by the national authorities. Tritium emissions are monitored both at the stack and in liquid discharges to demonstrate the compliance with the approved Derived Emission Limits. A specialized laboratory using Liquid Scintillation Spectrometry methods currently determines tritium activities in effluent samples. All information about the radioactivity of liquid and gaseous effluent is stored into a dedicated data base and used to make periodical reports. Based on four years data the present paper made an analysis of tritium emissions in gaseous effluents in order to highlight the trends and to provide a method to identify abnormal tritium emission. (authors)