The migration and accumulation of radionuclides in the Ravenglass saltmarsh, Cumbria, UK

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University ofSouthampton
THE MIGRATION AND ACCUMULATION OF
RADIONUCLIDES IN THE RAVENGLASS
SALTMARSH, CUMBRIA, UK
December 1999
A thesis submitted for the degree ofDoctor ofPhilosophy
Jung-Suk Oh (BSc, MSc)
Faculty ofScience
School ofOcean and Earth Science
Southampton Oceanography Centre
ABSTRACT
Profiles of 241Am, 137Cs, 238Pu, 239240pu & 24lPu have been studied in four
cores
collected from the Ravenglass saltmarsh, Cumbria and one core collected from the Irish Sea,
near the effluent pipeline of the Sellafield nuclear reprocessing plant. Major and trace
elements have also been measured to look at the effects of sediment composition and any
redox controls on element redistribution. In addition, almost one hundred surface scrape
samples were collected from the Ravenglass saltmarsh and the same radionuclides were
measured to study the lateral distribution of radionuclides. Three ofthe Ravenglass
cores
have well preserved the history ofSellafield discharge since the plantbecame operational in
1952. There are no significantpost depositional processes and redoxchanges for allthe
cores
studied. However, one of the saltmarsh cores collected from the front ofthe marsh shows
that surface erosion has occurred. Surface scrape sample results also show that erosion is a
significant process whichresults inthe redistribution ofsedimentswithin the marsh.
The core collected from the Irish Sea adjacentto the effluentpipeline shows sediment
mixing has operated and these mixed sediments are the main source of material to the
saltmarsh. Usingthe Sellafield discharge records from 1952 onwards, sediment mixing rates
have been estimated using a simple mathematical model and they are inferred to be between
80 % and 90 %. Isotopic ratios ofPu isotopes have been compared to those estimated ratios
corresponding to particular sediment mixing rates and show good agreement up to 30 cm
belowthe surface. There is less correlation between Sellafield discharge ratios and measured
ratios in cores below 30 cm andthis may be due to unreliable discharge data or more likely
the advective redistribution ofmaterial in the saltmarsh. A contribution of241Am originating
from the decay of 24IPu in cores was determined. The ingrown 241Am contributes
significantly to the total 241Am measured. Based on the Sellafield discharge record, it is
estimated that approximately 46 % of total 24IAm in the system has originated from the
decay of241Pu discharged from the plant since 1952. Spatial distribution ofradionuclides in
the Ravenglass saltmarsh is controlled by sediment accumulation associated with tidal
inundation frequency and the clay content in the sediments. Surface erosion also plays a
major role in redistributing saltmarsh sediment labelled with older Sellafield discharge
within the marshtogetherwith the sediments originated fromthe RiverEsk.
ACKNOWLEDGMENTS
I would like to acknowledge the guidance and help of Dr. Ian Croudace, Dr. Phillip
Warwick and Dr. Arthur Sanchez during this project. They are not only my supervisors but
also good friends. I also thank Mrs. Sonia Bryant (formerly of the Geosciences Advisory
Unit) and Mrs. Wink Janes in every aspect while I was in the Geology Department (Sadly, it
does not exist anymore). Dr. Rex Taylor, Dr. Andy Milton, Dr. Andy Cundy, Mr. Frank
Wigley and Mr. Thorsten Warneke have provided me with every help I needed while I was
carrying out this project. There are some Koreans I would like to thank for their wonderful
friendship. Dr. Ki-Hong Shin, Mr. Young-SeogKim, Mr. Kyu-Yong Choi & Mr. Jae-Kyoon
Kang are those Koreans always willing to drink a couple of pints with me whenever I was
happy or depressed. I am also grateful to my parents for their financial assistance, and
grandmother and two lovely sisters for their encouragement. In addition, the support from
my parents-in-law and my wife's two sisters and a brother is also very appreciated. All my
friends in Korea working hard in their areas have given me motivation to pursue the PhD.
Finally, veiy special thanks to my wife, Ok-Kyung, who always encouraged and supported
me in all aspects. She devoted herselfto look after three kids (?), me and our lovely two
children, Sang-Hoon & Seung-Yeon. Without her support, I would not have completed this
project.
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CONTENTS
1. Introduction
1
1.1. Introductiontothe study
1
1.2. Aims ofthe study
4
1.3. Background informationto the study
5
1.3.1. Radionuclides ofinterests in this study
5
1.3.1.1.241Am
5
1.3.1.2.137Cs
7
1.3.1.3. Plutonium isotopes
8
1.3.2. Distribution coefficients (Kj)
10
1.3.3. Behaviour of137Cs in saline and freshwaterenvironment 11
1.3.4. Behaviour ofredox sensitive elements
12
1.3.5. Establishing achronologyforthe marshes 14
1.3.5.1.210Pb
method
15
1.3.5.2. The use ofartificial radionuclides indating 16
1.3.6. Estuaries
17
1.3.7. Saltmarshes
18
1.3.8. Transportationand interaction ofSellafieldradionuclides 19
1.3.9. Bioturbation
20
2. Areas under investigation
22
2.1. Overview ofstudy area 22
2.1.1. Sellafield site
22
2.1.2. Nuclearfuelreprocessingat Sellafield 25
2.1.3. Fate ofradionuclides followingdischargeto the Irish Sea 28
2.2. Ravenglass saltmarsh
31
3. Sampling locations and methods used for the research 32
3.1. Sampling locations
32
3.2. Gamma spectrometry 35
3.3. XRFanalysis
36
3.3.1. Beadpreparation
36
3.3.2. Pellet preparation
36
3.4. Separation ofplutonium
37
3.4.1. Introduction
37
3.4.2. Experimental
38
3.5. Determination of241Pu in sediments
41
3.5.1. Introduction
41
3.5.2. Experimental
44
4. Results
47
4.1. Core samples
47
4.1.1. Majorandtrace elementresults in cores 47
4.1.2. Radionuclideresults in cores 49
4.2. Surface scrape samples
57
4.2.1.241Am and I37Cs activities in surface scrape samples (1980 and 1997) 57
4.2.2.238Pu, 239'240Pu and 241Pu activities in surface scrape samples (1997) 57
5. Evaluation ofsediment mixing
61
5.1. Introductionto sediment mixing 61
5.2. Determination ofthe sedimentmixingusing coreresults 65
5.3. Conclusions
70
6. Irish Sea sediments
71
6.1. Majorandtrace element data
71
6.2. Vertical distribution ofradionuclides
71
6.3. Pu isotopicratios inthe core 73
6.4. Ingrown241Am from the decayof241Pu
77
6.5. Conclusions
79
7. Verticalvariation ofradionuclides in the Ravenglass saltmarsh 80
7.1. Major andtrace elementdata
80
7.2. Sediment accumulation rates inthe saltmarsh 83
7.3.210Pb datingforR-96-007
core 85
7.4. Erosion ofthe saltmarsh
86
7.5. Vertical distribution ofradionuclides in saltmarsh cores 87
7.6. Variations inPu isotopic ratios in saltmarshcores 92
7.7. Implication of241Pu activities in saltmarsh cores 94
7.8. Conclusions
96
8. Spatial variations across the Ravenglass saltmarsh 97
8.1. Spatial distribution ofradionuclides in the saltmarsh in 1980 and 1997 97
8.2. Variations inPu isotopic ratios in 1997 surface scrape samples 103
8.3. Erosion ofthe saltmarsh
106
8.4. Loss on ignition data
107
8.5. Conclusions
109
9. Overall conclusions
110
10. References
112
Appendices
128
A.1. Details ofcores collected 128
A.2. Analytical methods
128
A.2.1. Sampling and storage
128
A.2.2. Sample preparation
128
A.2.3.210Pb determination
129
A.2.4. Electrodeposition ofPu &U
130
A.2.5. Detection limits inroutineXRF analysisusingaRhanode X-raytube 131
A.3. Geochemical data, 1-96-002 (subtidal) core 132
A.4. Geochemical data, R-96-000 (saltmarsh) core 133
A.5. Geochemical data, R-96-007 (saltmarsh)
core 135
A.5.1. X-radiograph ofcore R-96-007 137
A.6. Geochemical data, R-96-008 (saltmarsh) core 138
A.7. Geochemical data,R-97-003 (saltmarsh) core 140
A.8.1. Cumulative activities of241Amwithvariousmixingrates 142
A.8.2. Cumulative activities of241Amwith various mixingrates* 143
A.9. Cumulative activities of137Cs with various mixingrates 144
A.10. Cumulative activities of238Pu withvarious mixing rates 145
A.ll. Cumulative activities of239>240pu with various mixingrates 146
A.12. Cumulative activities of24IPu with various mixingrates 147
A.13. Ratios of238Pu/239'240Pu in discharges with various mixing rates 148
A.14. Ratios of241Pu/239'240Pu indischarges with various mixingrates 149
A.15. Ratios of241Pu/238Pu in discharges with variousmixingrates 150
A.16.238Pu/239'240Pu ratios in cores 151
A.17. 24IPu/239'240Pu ratios in cores 152
A.18.241Pu/238Pu ratios in cores 153
A.19. Ratios of238Pu/239>240Pu in 1997 surface scrape samples 154
A.20. Ratios of24IPu/239>240Pu in 1997 surface scrape samples 155
A.21. Ratios of24IPu/238Pu in 1997 surface scrape samples 156
A.22. Loss on ignitiondatafor surface scrape samples (%) 157
A.23. Publications
158
LIST OF FIGURES
Figure 1-1. Location ofresearch site (Ravenglass saltmarsh) 3
Figure 1-2. Productionofradionuclides in a light waterpowerreactor 6
Figure 1-3. Schematic representation ofthe zonation ofmarine sediments with
respectto Mndiagenesis
13
Figure 1-4. Decay schemeof238U leadingtothe generation of2I0Pb 16
Figure 1-5. Simplified diagram ofradionuclidepathways and possible
processes
duringthe transportation
20
Figure 2-1. Locations ofnuclearfacilities inUnited Kingdom 23
Figure 2-2. Annual discharges ofradionuclides from Sellafield 24a
<
Figure 2-3. Schematic representation ofthe reprocessingofused fuel at Sellafield 27
Figure 2-4. Surface and bottom mean residual currents in the Irish Sea 29
Figure 2-5. Composition ofIrish Seabed sediments
30
Figure 3-1. Irish Sea andthe sampling location
32
Figure 3-2. Sampling locations
33
Figure 3-3. The views ofthe Ravenglass Saltmarsh
34
Figure 3-4. An efficiencycurve forthe gamma spectrometer 35
Figure 3-5. Schematic diagram ofthe separation
40
Figure 4-1. Geochemistryandradionuclide results for1-96-002 core 52
Figure 4-2. Geochemistry andradionuclideresults forR-96-000 core 53
Figure 4-3. Geochemistryandradionuclideresults forR-96-007 core 54
Figure 4-4. Geochemistryandradionuclideresults forR-96-008 core 55
Figure 4-5. Geochemistryand radionuclideresults forR-97-003 core 56
Figure 4-6. Surface scrape dataforboth 1980 and 1997 241Am 58
Figure4-7. Surface scrape dataforboth 1980 and 1997 137Cs 59
Figure 4-8. Surface scrape datafor 238Pu in 1997 samples 59
Figure 4-9. Surface scrape datafor239-240Pu in 1997 samples 60
Figure 4-10. Surface scrape datafor241Pu in 1997 samples 60
Figure 5-1. Isotopic ratios ofPu isotopes forthe Sellafield dsicharges 62
Figure 5-2.238Pu/239>240Pu ratios in cores 63
Figure 5-3.241Pu/239>240Pu ratios in cores 63
Figure 5-4.241Pu/238Pu ratios in cores 63
Figure 5-5. Schematic diagram ofmixingprocess 65
Figure 5-6. Schematic diagram ofsediment mixing model 66
Figure 5-7. Profiles ofradionuclides from Sellafield with various degrees ofmixing 67