CESIUM AND PLUTONIUM ISOTOPES AS TRACERS IN SEDIMENT RECORDS IN THE BAY OF TOULON ( NORTHWESTERN MEDITERRANEAN SEA ) DUFFA C., ARNAUD M., CHARMASSON S.,THEBAULT H. Sous-titre de la présentation IRSN (Nuclear Safety and Radiological Protection Institute)
Context TOULON FRANCE Inner Bay First French military harbour Open Bay General westward circulation 5 km IAEA Symposium – MONACO, 30 March 2011
Hydrological context Two main wind directions : NW (mistral) and E - SE East wind « Mistral » North wind Surface current vectors and water temperature IAEA Symposium – MONACO, 30 March 2011
Hydrodynamic context Wind Surface N S Bottom South – North profile : current intensities Mid-depth current reversal IAEA Symposium – MONACO, 30 March 2011
Objectives and method ▌ To evaluate Pu and Cs contamination levels and origins in marine sediments ▌ To study sedimentation rate and processes Collection and analysis of 51 surface sediment samples and 5 sediment cores (2001 and 2009) Grain-size measurements Gamma, alpha and ICP-MS measurements giving 137 Cs, 210 Pb xs , 238 Pu, 239 Pu, 240 Pu, 241 Pu, 241 Am IAEA Symposium – MONACO, 30 March 2011
Results in surface sediments 1.5 Bq.kg -1 < 137 Cs < 12 Bq.kg-1 Mean 137 Cs activities : in the Open Bay : 3 Bq.kg -1 dry weight in the Inner Bay : 7 Bq.kg -1 dry weight IAEA Symposium – MONACO, 30 March 2011
Results in surface sediments Higher activity levels in the Inner bay correspond to finner sediments IAEA Symposium – MONACO, 30 March 2011
Results in surface sediments The same observations for Pu and Am isotopes. 239+240 Pu mean activity = 2.1 Bq.kg -1 in the Inner Bay, = 1.5 Bq.kg -1 in the Open Bay Mean activity ratios: 238 Pu/ 239+240 Pu = 0.027 ± 0.003 2.5 239+240 Pu/ 241 Am = 3.1 ± 0.6 2 Mean atom ratio: 239+240Pu 1.5 R2 = 0,7 239 Pu/ 240 Pu = 0.188 ± 0.008 1 global fallout origin 0.5 0 0 5 10 15 137Cs 137 Cs and 239+240 Pu mass activities well correlated. IAEA Symposium – MONACO, 30 March 2011
Results in sediment cores 12 9 23 15 52 Projet CARTOCHIM Contribution IRSN- 10 mars 2011 9/13
210 Pb results and interpretation 210 Pb xs (Bq.kg -1 dry weight) 210 Pb xs (Bq.kg -1 dry weight) 210 Pb xs (Bq.kg -1 dry weight) 0 50 100 150 200 0 100 200 0 100 200 0 0 0 2 0.14 cm/y 0.17 cm/y 2 4 5 0.17 cm/y 6 4 1 cm/y 0.26 cm/y 8 10 6 10 12 8 Depth (cm) Depth (cm) 0.21 cm/y Depth (cm) 15 14 10 16 0.18 cm/y 20 18 12 20 14 25 22 24 16 30 26 St12 2009 18 St9 2009 28 St 15 2009 St12 2001 St9 2001 30 35 20 Apparent sedimentation rates in the Inner Bay estimated from the slope of the linear regression Assumptions: - steady state accumulation of sediments - the excess 210 Pb activity of depositing sediment particles is constant IAEA Symposium – MONACO, 30 March 2011
Radionuclides distribution Pu isotopes station 9 SIAT-2001 (Bq.kg -1 dry weight) 137 Cs (Bq.kg -1 dry weight) 0% 50% 100% 0 1 2 3 0 5 10 15 0 1 2 3 4 5 0 1 0 0 2 2 3 5 4 4 5 6 6 1963 10 7 8 Depth (cm) Depth (cm) Cs, Pu and Am measured until 8 Depth (cm) 9 18 cm deep 10 15 10 higher sedimentation rate 12 11 12 0.23 cm/y 20 St9 - 2001 14 14 13 Radionuclides diffusion in 239Pu 16 St9 - 2009 240Pu 16 the sediment pore water < 4 µm 25 15 63 - 200 µm 238Pu 18 St9 - 2001 4 – 20 µm 137 Cs (Bq.kg-1 dry weight) 18 241Am 200 - 2000 µm 17 20 – 63 µm 30 20 20 St 23 2009 35 IAEA Symposium – MONACO, 30 March 2011
Inventories and stocks (derived from st.9) Inventories: Theoretical Direct deposits 239+240 Pu = 0.4 GBq (<20% of the sediment stock) 239+240 Pu = 279 Bq m -2 137 Cs = 895 Bq m -2 137 Cs = 28 GBq (4 the sediment stock) Stocks in the Inner Bay (7.8 km²) : Inner Bay 239+240 Pu = 2.2 GBq 137 Cs = 7 GBq Why? : Pu mainly associated with particles, Cs mainly in dissolve phase in marine waters 80% of the 239+240 Pu in bottom sediments are deriving from catchment basin indirect inputs IAEA Symposium – MONACO, 30 March 2011
Inventories and stocks A 70 km² catchment area 239+240 Pu direct atmospheric deposits = 3.6 GBq Marine inputs mainly through the Las River integrated since the 1960 ’s = 1.8 GBq (50%) Inner Bay IAEA Symposium – MONACO, 30 March 2011
Conclusion • Cesium and plutonium isotopes appear to have a common origin in the Toulon Bay i.e. the global fallout. • Cs ans Pu surface mass activities higher in the Inner Bay. Inputs from the watershed seem to explain the higher levels compared to the other part of the Bay which is under influence of the general water circulation. Sedimentation rate determined via 210 Pb xs profiles 0.2 cm.y -1 in the Inner Bay. • • Caution! Inventories and stocks evaluated with strong assumptions • However, they underline that sediment represent a high plutonium storage compared to cesium which is in agreement with what is known about their behavior in the marine environment. IAEA Symposium – MONACO, 30 March 2011
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