PUBLIC DOSES ESTIMATION BASED ON EFFLUENTS DATA AND DIRECT MEASUREMENTS OF TRITIUM IN ENVIRONMENTAL SAMPLES AT CERNAVODA E. Bobric, I. Popescu, V. Simionov Health Physics Department, CNE Cernavoda, Romania EMRAS meeting, Bucharest, 2007
Introduction • The layout of the CANDU reactors and the design of its systems ensure that the radioactive waste quantities are minimized, but small amounts of radioisotopes are continuously discharged at very low concentrations through gaseous and liquid effluents. • As national regulations require CNE Cernavoda established the facilities necessary to carry out continuous monitoring of the level of radioactivity in the air, water and soil and to ensure compliance with the basic radiation protection standards.
More than 10 years of experience in CANDU operation at Cernavoda NPP have shown that tritium is mainly released as tritiated water in gaseous and liquid effluents and has the most important contribution to public and professional exposure.
The purpose of this paper is: • To present estimated tritium doses for a member of the critical group near Cernavoda NPP, and • to make a comparison between doses estimated from the results of effluent monitoring and from environmental monitoring results.
Effluent monitoring program • Continuous monitoring of liquid and gaseous effluents (LEM and GEM) • GEM performs continuous noble gases, radioiodine and particulate measurement as well as particulate, radioiodine, water vapours and carbon dioxide sampling.
Effluent monitoring program • The station radiation protection laboratory measures the samples and the results are stored in a database. • Doses to critical group members are evaluated and periodic reports are issued for authorities and public.
Effluent monitoring program • 3H is the most significant radionuclide released in gaseous and liquid effluents, (mostly as HTO) • 3H emissions represents more than 50% of the total radioactivity released.
Critical Group dose estimation using emission data Individual doses for a critical group member have being evaluated using the compartment models recommended by: “CAN/CSA–N288.4–M87, Guidelines for Calculating Derived Release Limits for Radioactive Material in Airbone and Liquid Effluents for Normal Operation of Nuclear Facilities”
Critical Group dose estimation using emission data CAN/CSA–N288.4–M87 uses Specific Activity Model for HTO. OBT is not considered in this standard.
Critical Group dose estimation using emission data Critical group: People from Cernavoda resident at 2 km far from plant. They are self sufficient, producing all their own fruits and vegetables in farms and backyard gardens, as well as most of their animal products. Tritium DEL for Cernavoda airborne emissions is 6.45E+05 GBq/week
Air monitoring indicator stations TLDs locations Environmental monitoring program • Pre-Operational Program (1984 -1996) • Operational program starting in 1996
Environmental monitoring program Sample types: Air: particulate, iodine, water vapors • Soil • Vegetation • Atmospheric deposition • Food samples (milk, fish, pork, beef, poultry, vegetables and fruits).
Environmental monitoring program • Tritium is the only radionuclide arising from Cernavoda NPP which can be measured in environmental samples. • An indicator location for air monitoring is placed in Cernavoda at about 2.5 km distance far from the plant.
Environmental monitoring program To increase tritium measuring sensitivity a ultra-low level scintillator Quantulus 1220 is used since 2001 for environmental measurements. Tritiated water in milk, meat, vegetables and fruits is extracted by azeotropic distillation in toluene and tritium is measured by liquid scintillation spectrometry.
Environmental monitoring program Airborne water vapors are collected by drawing air through molecular sieve. Sampling is continuous during one month, (4 - 6 m3 of air are passed through the collector). The absorbed water is removed from the sieve by heating at 350ºC and the tritium content is measured by liquid scintillation counting
Environmental monitoring program Tritium concentration in air Cernavoda Station
Environmental monitoring program Tritium in food samples - mean values * these values represent the detection limit of the method ** Mean values between Danube water average values from two locations, upstream and downstream the release point and Danube – Black Sea Canal
Environmental monitoring program Tritium doses for Cernavoda people estimated using measured tritium activity in environmental samples
Conclusions • Tritium doses for critical group due to Cernavoda plant airborne emissions between 1996 and 2006 were less than 10 mSv/year. • Tritium doses for Cernavoda people estimated using measured tritium activity in environmental samples are one order of magnitude lower than dose values for critical group members. • An improvement of tritium models and measurement methods is necessary. Considering OBT could improve tritium dose estimations.