Dose Assessment for Tritium Releases During Normal Operation of NPP - PowerPoint PPT Presentation

Dose Assessment for Tritium Releases During Normal Operation of NPP Irena Malátová*, Juraj Ďúran ** *NRPI, Praha, Czech Republic, VÚJE, a.s. Trnava, Slovakia.** 4 th Meeting WG7, 6 – 9 September 2010, Aix-en-Provence, France

Main source of tritium in the Czech republic are NPPs Dukovany and Temelín, both PWR Type. Doses to population from tritium and other radionuclides in the discharges from NPPs are very low, much lower than authorized limits However, calculation of doses from tritium has some problems from the point of view of its RBE and therefore, often it is a target of anti-nuclear activists. Aim of this work was to find out how realistic are the estimation of doses to public, and if too conservative, to suggest more realistic approach.

Dose constraint and authorized limits for discharges of radionuclides into environment in the Czech Republic Dose constraint for total discharge of radionuclides – 250 µ Sv per year for a representative individual from public (200 µ Sv for airborne discharges, 50 µ Sv for liquid discharges) On the basis of optimization process, site specific authorized limits are set for NPP NPP Dukovany: 40 µ Sv airborne and 6 µ Sv liquid discharges NPP Temelín: 40 µ Sv airborne and 3 µ Sv liquid discharges

Characteristics of the NPPs in the Czech republic and doses to public from their releases. Nuclear Power Plant Dukovany Temelín Power output 4x440MW 2X1000MW Releases to atmosphere (Sv) 1.6E-08 8.0E-09 Releases of tritium (Sv) 3.20E-10 6.40E-11 Releases to hydrosphere (Sv) 1.80E-06 1.10E-06 Releases of tritium (Sv) 1.80E-06 1.10E-06 Doses from liquid releases during normal operation are 2 to 3 order of magnitude higher than from airborne releases. In liquid effluents and therefore in overall dose to population, dose from tritium is dominant. Tritium is the only one radionuclide measurable in the environment (in the rivers) Doses to public are calculated using models of transport of radionuclides in environment in combination with measured released activity of radionuclides into air and water.

Hřensko,river Elbe 100 Germany Volum e activity H -3 [B q/l] 10 1 0.1 1.1.1994 0:00 1.1.1996 0:00 31.12.1997 31.12.1999 30.12.2001 30.12.2003 29.12.2005 29.12.2007 28.12.2009 0:00 0:00 0:00 0:00 0:00 0:00 0:00 Poland Praha NPP Temelin NPP Dukovany Pohansko, river Dyje 100 Volume activity H-3 [Bq/l] 10 Germany 1 28.10.1995 11.3.1997 24.7.1998 6.12.1999 19.4.2001 1.9.2002 14.1.2004 28.5.2005 10.10.2006 22.2.2008 Austria Slovakia

1.E+14 EDU hydrosphere EDU atmosphere 1.E+13 ETE atmosphere ETE Bq hydrosphere 1.E+12 1.E+11 1.E+10 1985 1990 1995 2000 2005 2010 Annual releases to atmosphere and hydrosphere from the Czech Nuclear Power Plants Dukovany and Temelín

GBq/GWe 100000 10000 Loviisa (Finn.) 1000 Annual releases of tritium, normalized to power output, 100 Belleville (France) 10 1 from NPP PWR to air and water (geom.mean) Cruas (France) Biblis B (Germ.) Emsland (Germ.) Neckarwestheim 2 Ringhals (Sweden) Sizewell B (GB) Krško (Slovenia) Paks (Hungary) EDU (CR] ETE (CR) Water Air

Old model for calculation of doses to the public The old model calculation of population doses took into account releases through ventilation stacks into atmosphere and liquid effluents into hydrosphere only. However, in Dukovany NPP the source of cooling water is water from the river dam with the outfall of liquid effluents from NPP. Significant activity of tritium is escaping into air by the way of the cooling towers, contributing thus to the dose from air releases and diminishing dose from hydrosphere. Activity of radionuclides in cooling water is measured periodically in NPP, amount of water vapours is calculated from flow rate above and below NPP.

0.5* (3.99E+12)Bq Ventilation stacks 0.5* (3.99E+12)Bq (HTO vapor and drops) (HTO,HT) (HTO vapor and drop 5.7E+11Bq Cooling towers To the dam: 8.9E+12Bq , Cooling towers To cooling towers is returning 3.99E+12Bq Net to river: 4.9E+12Bq

Modifications in the new model A computer code PTM_HTO has been developed to assess the dose to general public. It takes into account HTO, HT and water drops (1 – 3 % of the released activity of 3 H). Oxidation of HT to HTO and reemission of HTO from soil to the atmosphere are included too. Organically bound tritium (OBT) in vegetations, milk and beef is taken into account. In the same way as in the old model local consumption is assumed (e.g. that all products consumed are locally produced – very conservative approach)

Validation of PTM_HTO Calculated concentration HT [Bq/m3] Model on the data from field 1.8E+06 1.6E+06 experiment 1.4E+06 HT 1.2E+06 1.0E+06 8.1E+05 Experimental vs. 6.1E+05 Theoretical Data 4.1E+05 2.1E+05 1.0E+04 HTO 1.0E+04 5.1E+05 1.0E+06 1.5E+06 Measured concentration HT [Bq/m3] Calculated concentration of HTO [Bq/m3] 250.0 R.M.Brown,G.L.Ogram 200.0 And F.S.Spencer: Field Studies of HT Oxidation and Dispersion in the 150.0 Experiment II, the 1987 June experiment at Chalk River, 100.0 Canadian Fusion Fuels Techn. Project. CFFTP Report No CFFTP – 50.0 G – 88007, October 1988 0.0 0.0 50.0 100.0 150.0 200.0 250.0 300.0 Measured concentrations of HTO [Bq/m3]

Example for releases in 2008 Ratio of doses to individual from released 3 H calculated by modified transport models to the hydro,i original one hydro,a atm,i 100.00 atm, a ratio of doses 10.00 1.00 0.10 A2/A1 B1/A1 B2/A1 Doses to individual per unit of released 3 H activity A1 – without OBT, without hydrosphere, infant releases through cooling 1.0E-18 Hydrosphere, towers adult atmosphere, A2 – with OBT, without 1.0E-19 infant releases through cooling atmosphere, Sv/Bq towers adult 1.0E-20 B1 – without OBT, with releases through cooling 1.0E-21 towers B2 – with OBT, with releases 1.0E-22 through cooling towers A1 A2 B1 B2 type of model

A1 – without OBT, without releases through cooling towers A2 – with OBT, without releases through cooling towers B1 – without OBT, with releases through cooling towers B2 – with OBT, with releases through cooling towers Change of normalized doses from tritium with modification of transport model ratio of normalized doses 1.80 1.60 1.40 to individual 1.20 A2/A1 1.00 B1/A1 0.80 B2/A1 0.60 0.40 0.20 0.00 hydro, i hydro, a atm,i atm,a exposure patway, representative individual

Conclusions. Doses from tritium to population from the Czech NPP are very low (below 2 µ Sv), When OBT is included, doses from airborne discharges increase by about 30 %. Increase of doses from water discharges, which are dominant, is less than 10% Taking into account recirculation of cooling water, overall doses from tritium from NPP Dukovany decreased about 50%. Such approach would be not practical for limitation purposes as the amount of water evaporated through cooling towers is known ex post only. However, for reporting annual doses, a realistic approach would be appropriate.