Environmental Radiochemical Analysis III is an authoritative, up to date review of research contributions presented at the 10th International Symposium on Environmental Radiochemical Analysis. Representing the work of leading scientists across the globe this edition provides information on:
– new methods of radioanalyses
– waste steams during decommissioning
– radioactivity measurements in the environment
– hazard assessment in decommissioning
– improvements in measurement instrumentation
– application of software to measurements
– current IAEA activities for the ALMERA network
– pro ciency testing and research and development in the NDA. This exceptional work o ers an insight into topical areas of research and is a key point of reference for graduates and professionals alike who work across elds involving analytical chemistry, environmental science and technology, and hazards and waste research and disposal.

The Editor Peter Warwick, BA, MSc, PhD, FRSC, CCHem is Professor of Environmental Radiochemistry, Director of the Centre for Environmental Studies and Head of the Department of Chemistry at Loughborough University. Professor Warwick is a part Chairman of the Radiochemical Methods Group, Analytical Division, the Royal Society of Chemistry and was awarded the Becquerel Medal by them in 2002 for “outstanding contributions to radiochemistry”. His major research interests are in the chemistry of nuclear active wastes disposal and in developing new methods of analyses for decommissioning wastes.

Environmental Radiochemical Analysis III

By Peter Warwick

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-263-0

Contents

Radionuclide Accumulation at a Hydroelectric Power Dam E. Holm, Y. Ranebo, M. Eriksson, P. Rons and M. Peterson, 1,
Determination of the Transfer of Tritium to Crops Fertilised with Contaminated Sewage Sludge G J Ham, B T Wilkins and D Wilding, 10,
Technetium-99 (99Tc) in Marine Food Webs in Norwegian Seas-Results from the Norwegian Radnor Project H. E. Heldal, K. Sjøtun and J. P. Gwynn, 19,
Measuring Thoron (220Rn) in Natural Waters W.C. Burnett, N. Dimova, H. Dulaiova, D. Lane-Smith, B. Parsa and Z. Szabo, 24,
The determination of Gross Alpha and Gross Beta Activity in Solids, Filters and Water – Validation of Dutch Pre-Norms P.J.M. Kwakman, E. van der Graaf and P. de Jong, 38,
Environmental Measurements of Radioxenon T.W. Bowyer, J.C. Hayes and J.I. McIntyre, 44,
Uptake of Uranium by Spinach Grown in Andosols Accumulating Trace Amounts of Fertiliser-Derived Uranium N. Yamaguchi, Y. Watanabe, A. Kawasaki and C. Inoue, 52,
Mineralogical and Particle Size Controls on 137Cs Abundances in Dounreay Offshore and Foreshore Sands Ian W. Croudace, Phillip E. Warwick and Joe Toole, 60,
Assessment of Possible Sources of Artificial Long-Lived Radionuclides in Environmental Samples by Measurement of Isotopic Composition Z. Varga, G. Surányi, N. Vajda and Z. Stefánka, 68,
A Rapid Method for the Preconcentration of Non-Refractory Am and Pu from 100g Soil Samples E. Philip Horwitz, Anil H. Thakkar and Daniel R. McAlister, 77,
Improvements in Underground Gamma-Ray Spectrometry and the Application of Measuring Radioactivity in Agricultural Samples P. Lindahl, M. Hult. F. Cordeiro, J. Gasparro, A. Maquet, G. Marissens and P. Kockerols, 86,
Responses of U and Pu to Microbially Driven Nitrate Reduction in Sediments M. Al-Bokari. C. Boothman, G. Lear, J.R. Lloyd and F.R. Livens, 95,
An Efficient and Optimised Total Combustion Method for Total H-3 and C-14 in Environmental and Decommissioning Samples J-S Oh, I Croudace, P Warwick and D J Kim, 101,
The Analytical Impact on Tritium Data from Storing Nuclear Decommissioning Samples under Different Conditions Dae Ji Kim, Ian W. Croudace and Phillip E. Warwick, 108,
Radionuclide Recording Levels and Prioritisation of Chemical / Radiochemical Analyses of Magnox Wastes for Nirex Compliance C. Kirby, D.J. Hebditch and R.E, Streatfield, 116,
Application of the Radiological Hazard Potential (RHP) to Radionuelides in Magnox Reactor Decommissioning R.E. Streatfield, D.J. Hebditch and W.H.R. Hudd, 126,
Determination of Tritium Radionuclide and Lithium Precursor in Magnox Reactor Steels W.A. Westall, R.E. Streatfield and D.J. Hebditch, 137,
Sequential Determination of Ca-41/45 and Sr-90 in an Activated Concrete Core F. Rowlands, P. Warwick and I. Croudace, 147,
The Chemistry of Ultra-Radiopure Materials H.S. Miley, C.E. Aalseth, A.R. Day. O.T. Farmer. J.E. Fast, E.W. Hoppe. T.W. Hossbach, K.E. Litke, J.I. McIntyre. E.A. Miller, A. Seifert and G.A. Warren, 154,
Independent Radiological Monitoring; Results of a Recent Intercomparison Exercise K.S. Leonard, S. Shaw. N. Wood, J.E. Rowe, S.M. Runacres D. McCubbin and S.M. Cogan, 162,
Routine Application of CN2003 Software to Laboratory Liquid Scintillation Calibration P.E. Warwick, I. W. Croudace and N.G. Holland, 169,
Easy Method of Concentration of Strontium Isotopes from Radioactive Aqueous Wastes for the determination of 90Sr by Liquid Scintillation Counting. Application of Strontium Empore Rad Disks E. Minne, F. Heynen and S. Hallez, 176,
Performance of a Portable, Electromechanically-Cooled HPGe Detector For Site Characterization R.M. Keyser and R.C. Hagenauer, 186,
Nuclear Decommissioning Authority Research and Development Needs, Risks and Opportunities Neil Smart, Andrew Jeapes and Ainsley Francis, 193,
The Performance of UK and Overseas Laboratories in Proficiency Tests for the Measurement of 241Am A.V. Harms, J.C.J. Dean, C.R.D. Gilligan and S.M. Jerome, 200,
Current IAEA Activities and Future Plans for the Almera Network Chang-Kyu Kim, Paul Martin and Gyutla Kis-Benedek, 207,
Isotope Index, 217,
Subject Index, 219,

CHAPTER 1

RADIONUCLIDE ACCUMULATION AT A HYDROELECTRIC POWER DAM

E. Holm, Y. Ranebo, M. Eriksson, P. Roos, M. Peterson

1 INTRODUCTION

There are about 1200 hydroelectric power plants in Sweden and several thousands of other water reservoirs for other purposes. World wide there are about 45 000 large dams in the world, the vast majority of which were constructed after 1950 and in total there are several millions of smaller ponds. These dams produce several benefits including supply of irrigation water, hydropower generation, flood control, recreation, fishing and others.

One hundred and ninety of the hydro electric plants in Sweden have depths larger than 13 m and up to 125 m. The history of these plants is well known with respect to physical parameters, construction year, etc. Generally the time of construction is different for different plants along a river. The constructed dams, which regulate the water flow, might act as flocculation basins. Very high sedimentation rates have been reported, up to 20 cm per year. High sedimentation rates will limit the time of operation in shallow dams and increase the potential hazard if the sediments are reintroduced to the environment. World wide there has been a large number of dam failures and in Sweden there have been 8 cases of serious floods and 2 cases of dam failures. Remedial actions for dams have been studied.

The retention of dissolved silicated (DSi) artificial dams in Sweden and Finland has been studied which show a reduction in the delivery to the coastal zone.

It is well known that anthroogenic radionuclides such as radiocaesium and plutonium together with natural 210Pb are accumulated in sediments and can be used for the dating/growth rate determination of the sediments. The flux of these radionuclides depends on factors, such as physical and chemical properties, biological factors etc,. Water dams along rivers will stop the water flow and might act as effective traps by sedimentation processes and accumulate material that otherwise would be transported to the sea.

We have studied the accumulation and vertical distribution of 210Pb, 137Cs and 239+240Pu in sediment cores from a dam (Granö hydroelectric power dam) situated in the Mörrum River at SE Sweden (Figure 1, Table 1). This dam was constructed in 1958, about the time when the first large scale world wide fallout from nuclear test occurred.

There are in principle two sources for anthropogenic radioactivity in rivers in Sweden,1954-58 (20%) and 1961-62 (80%), and the Chernobyl accident, 1986. These events are quite distinct in time and by using isotopic ratios and radionuclide ratios the two sources can be distinguished from each other. China and France also conducted nuclear tests during 1960-1980.

The dam at Granö was also selected for logistic reasons and the activity levels were expected to be high enough to allow meaningful analysis of the experimental data. Some basic parameters are given in the table below. The river receives water from a large upper lake, (area 150 km2 , 139 m over the sea level) and the catchment area of the lake is 3150 km2. The lake is shallow (mean depth 3 m) and the residence time of the water is rather short. The water flow is variable but the annual average is 26 m3 s-1. The mean residence time of the water is then only 6.6 months.

At the time of construction of the dam there will have been a decomposition of radionuclides (and other pollutants) trapped in vegetation, especially mosses and peat. Depending on depth, ventilation, etc. anoxic areas of the dam might develop. Granö hydroelectric power plant at Mörrumsan was constructed in I 958, i.e. just at the beginning of the nuclear area. This means that at that time only small amounts of radioactivity were trapped in biota in the flooded area.

2. METHODS AND RESULTS

2.1 Sampling and measurements

Sediment cores were taken in the accumulation area of the dams during the winter of 2004-2005 and the summer of 2005 . A small boat was be used and during the summer and during winter time the ice was used as a sampling platform. The sediments were cores with a diameter of 31.16 or 55.39 cm. and were sliced in 1 cm. The maximum possible core depth was generally around 35 cm corresponding to the time of construction.

Radiocaesium (137Cs, T1/2 = 30 years) in the samples was measured using gamma spectrometry by solid state well-type HpGe gamma spectrometry.

Thereafter an aliquot of each slice is analysed for lutonium isotopes and 210Pb. Plutonium was separated by anion exchange using 242Pu as radiochemical yield determinant and measured by alpha spectrometry using solid state ion implanted Si detectors.

Pb-210 (T1/2 = 21 years) was analysed after radiochemical sefgaration of its daughter product 210Po (T1/2 = 163 days) and alpha spectrometry. 209Po is used as radiochemical yield determinant.

2.2 Deposition and accumulation of 210Pb.

Pb-210 was analysed in all cores. Typical distributions with depth are shown in Figures 2-7. The maximum possible coring depth was about 25-35 cm which should represent the time of construction of the dam. This corresponds to a sedimentation rate of 5-6 mm per year. The distribution in all cores is quite similar. It is evident that the sedimentation rate has not been constant. We have an abrupt discontinuity in the sediment record and dating by 120Pb is not easy. One can distinguish two periods. We see an early phase after the construction when the sedimentation rate was low, around 1 mm per year about 25-35 cm depth and then the rate increased to 20-30 mm per year. This rather sudden change can have several explanations. 40-50 years ago there was a significant ditching of the catchment area, especially peat bogs, to obtain more useable land for agriculture or foresting. Forest management techniques have changed such that clearance of larger areas is now commonplace, rather than harvesting of individual trees and both processes cause erosion, loss of vegetation and soil to the lake.

The area has been forested especially with pine and spruce and there is much less of agricultural land today. The humic material from the peat bogs will cause increased humic concentrations in the lake. Radionuclides can be associated with humic colloids and even radionuclides such as plutonium may be transported long distances.

The accumulated activity of 210Pb from 0 to 35 cm varied from 11 000 – 19 000 Bq m-2 with a mean of 14 200 +/- 3 400 Bq m -2. We can not assume that 210Pb has reached a state in the cores where decay equals the input to the sediments. This would require a core corresponding to 200 years. Anyway the average annual deposition has been larger than 360 – 600 Bq m-2 year-1 which is significantly higher than the expected annual flux for the area, about 70 Bq m-2 per year, while disturbed lakes in Sweden show fluxes 2-3 times higher than atmospheric ones. The annual deposition today is around 200 Bq m-2 year-1 but has been over 1000 Bq m-2 year-1 right after the change in sedimentation rate.

2.4 Accumulation of 137Cs

The distributions of 137Cs as a function of depth in the sediment cores are displayed in Figures 7-12. As can be seen a maximal concentration is observed at a depth of about 20 -24 cm with one exception where the maximum occurs at 10 cm (core 2857). In this core the maximal coring depth was also shorter-about 25 cm. This depth should for this core represent the year 1958 while for the other cores this year refers to 30-35 cm depth. If the depth for the maximal concentration represents 1963 then the sedimentation rate during 1958 to 1963 should have been about 3 cm per year which is in disagreement with the data for 210Pb. The peaks are however broad. It is expected that there is some delay from initial deposition to maxima in the sediments due to the time needed for sedimentation processes. The input from the catchment area is in the early phase of fallout important especially during the Spring thaw. In our case we also have the change of the catchment area described under the accumulation of 210Pb. These factors certainly would broaden the peak. We have then not even considered migration and diffusion in the sediments. If we look at the total area content (Bq m-2) the maximal accumulation occurred at 30-32 cm except for core 2857 when it occurred at 21 cm. This is explained by the fact that the density of the sediments have a very low density in the upper part and much higher density deeper down. The accumulation actually started soon after the construction of the dam and using the integrated deposition as a basis for extrapolation we get a completely different sedimentation rate of about 2 mm per year which is in better agreement with the 210Pb data. We must also consider the nuclear tests 1957-58, when 25% of the fallout took place.

The total deposition from nuclear test fallout was 2 300 Bq m-2 which today would have decayed to 870 Bq m-2. The deposition from the Chernobyl accident was 1 400 Bq m-2 which today would have decayed to 900 Bq m2. In total we then have about 1 770 Bq -2. In the sediments we find a deposition between 2700 and 3 600 Bq m-2 with a mean of 3 400 +/- 950 Bq m-2. The integrated deposition in the dam is about twice that from general integrated fallout. It is very common that the accumulation in the aquatic environment is higher than that from estimated fallout due to run off. The conclusion is that there is not any strong accumulation of radiocaesium in the dam. The annual accumulation today is in the order of 20-50 Bq m-2 year-1 compared to 700-800 Bq m-2 year-1 during the maximal fallout period. It shows that there is still some continuous transport of contaminated material from the catchment area and the lake downstream. The lake is very shallow and the sediments are redistributed by waves.

2.4 Accumulation of plutonium

The cores analyzed for 239+240Pu show a similar distribution as those for radiocaesium, i.e. maxima between 20-25 cm (Figure 13-14). In the same way by looking at annual deposition over time the accumulation started very soon after the construction of the dam. The total integrated deposition of 239+240 Pu in the area from nuclear test fallout is about 27 Bq m-2. The contribution from Chernobyl fallout was very small 1-2 Bq m -2. In the sediment cores we find 170-330 Bq m-2 which is significantly higher than the general integrated fallout. The conclusion is that plutonium is significantly accumulated in the dam.

The present annual accumulation of plutonium in the dam is 0.6-1 Bq m-2 and was 30-90 Bq m-2 during the maximal fallout period.

If we regard the activity ratio 239+240Pu/137 Cs as displayed in Figure 15-16 it will tell us something about the different behavior in the system between radiocaesium and plutonium. In fallout from nuclear tests the ratio was 0.012 and should today have increased to 0.031. Including the Chernobyl fallout the ratio should today be 0.015. The accumulation of plutonium is substantially higher than that for radiocaesium during the whole period since 1958. This is the case especially during the nuclear test fallout period when this ratio is 0.07-0.08.

Generally plutonium is scavenged from the water column to a larger extent than caesium is. The ratio in open ocean water is 0.001 while in shallow areas, with high suspended load, both can be more or less completely removed to the sediments.

The sediments are very organic and anoxic conditions have been formed. It is known that plutonium forms organic humic complexes. In watersheds from the catchment areas the plutonium concentrations are as high as up to 300-400 μBq 1-1 and 137Cs, 30 mBq 1-1. In the major lake from where the river starts the concentrations are respectively 100 and 10 times lower. Our data suggests that concentrations downstream is due to bunding and transport by organic sediments which are transported by the river and trapped in the dam.

3 CONCLUSION

The use of radio lead (210Pb) for dating of the sediments in a dam constructed for hydroelectric power showed a dynamic pattern. The sedimentation rate based on the 210Pb data was low after the construction of the dam , 1 mm per year and increased then to 20-30 mm per year. The reason for this increase in sedimentation rate is probably due to ditching of the catchment area and that forest management techniques have changed such that clearance of larger areas is now commonplace, rather than harvesting of individual trees and both processes cause erosion, loss of vegetation and soil to the lake.

changes in logging of forest and use of land bringing large amounts of material into the upper lake and further down the river system.

The average annual deposition has been larger than 360 – 600 Bq m-2 year-1 which is much higher than the expected annual flux for the area, about 150 Bq m-2 year-1. The maximal concentrations of the anthropogenic radionuclides 137Cs and 239+240Pu occurred at a depth of about 20 cm which should then correspond to the major nuclear tests around 1962-63. Radiocaesium shows a different accumulation pattern and/or higher mobility in the sediments than plutonium and the 1963-peak is quite broad. The radiocaesium fallout from the Chernobyl accident was small in this area and the peak from this event is either rather small or overlaps the distribution from nuclear test fallout, which also broadens the peak compared to plutonium. Radiocaesium is not to any higher degree accumulated in the dam compared to other fresh water systems. Applying a time resolution, Full Width of Half Maximum , gives 10-12 years for 137CS while for 239+240Pu this time resolution is about 6 years.

(Continues…)Excerpted from Environmental Radiochemical Analysis III by Peter Warwick. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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