Environmental Radiochemical Analysis V (Special Publications, Volume 351) - Hardcover

Hardcover

ISBN 10: 1782621555 ISBN 13: 9781782621553

Publisher: Royal Society of Chemistry, 2015

Synopsis

Incidents in the past have made scientists aware of the need for accurate methods of radionuclide analyses in order to estimate the risk to the public from released radioactivity. This book is an authoritative, up-to-date collection of research contributions presented at the 12th International Symposium on Environmental Radiochemical Analysis. Representing the work of leading scientists from across the globe it presents information on radiochemical analysis, measurement of radioactivity, naturally occurring radioactive materials, radioactively contaminated land, fate of radionuclides in natural and engineered environments and behaviour and analysis of radionuclides in radioactive wastes. This essential work will be a key reference for graduates and professionals who work across fields involving analytical chemistry, environmental science and technology, and waste disposal.

About this title

About the Author

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.

From the Back Cover

Incidents in the past have made scientists aware of the need for accurate methods of radionuclide analyses in order to estimate the risk to the public from released radioactivity. This book is an authoritative, up-to-date collection of research contributions presented at the 12th International Symposium on Environmental Radiochemical Analysis. Representing the work of leading scientists from across the globe it presents information on radiochemical analysis, measurement of radioactivity, naturally occurring radioactive materials, radioactively contaminated land, fate of radionuclides in natural and engineered environments and behaviour and analysis of radionuclides in radioactive wastes. This essential work will be a key reference for graduates and professionals who work across fields involving analytical chemistry, environmental science and technology, and waste disposal.

Excerpt. © Reprinted by permission. All rights reserved.

Environmental Radiochemical Analysis V

By Peter Warwick

The Royal Society of Chemistry

Copyright © 2015 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-155-3

Contents

CHARACTERISATION OF RADIOACTIVELY CONTAMINATED LAND AT DOUNREAY, AN AREA WITH A HIGH NATURAL BACKGROUND J.A. Heathcote, 1, 
THE USE OF IN SITU GAMMA MEASUREMENTS IN LAND SURVEYS P.D Rostron, J.A. Heathcote and M.H. Ramsey, 15, 
TESTING OF A COMPUTER PROGRAMME FOR CALCULATING THE Sr-85 YIELD AS WELL AS FALLOUT Sr-89 AND Sr-90 IN A SINGLE LSC MEASUREMENT WITH DETECTION LIMITS CONFORMING TO ISO 11929 D. Tait, T. Vagt, G. Kanisch and N. Roos, 30, 
CHARTING A PATH TO THE FUTURE J. Martin, 39, 
FALLOUT OF Pu-238 OVER MADAGASCAR FOLLOWING THE SNAP 9A SATELLITE FAILURE E. Holm, C. Rääf, N. Rabesiranana, R. Garcia-Tenorio and E. Chamizo, 44, 
TRANSPORT AND ACCUMULATION OF STABLE METALS AND RADIONUCLIDES IN DULAS BAY, NORTH WALES H. Al-Qasmi, G.T.W. Law, N.D. Bryan and F.R. Livens, 50, 
EXPERIMENTAL STUDIES ON THE UPTAKE OF TECHNETIUM-99 TO TERRESTRIAL CROPS L.W. Ewers and J. Brown, 61, 
A PILOT STUDY OF THE LUNG SOLUBILITY OF RADIOACTIVE MATERIALS USED AT AWE T.J. Miller, 67, 
AN IMPROVED METHOD FOR THE DETERMINATION OF IODINE-129 IN ENVIRONMENTAL AND NUCLEAR DECOMMISSIONING SAMPLES L. Townsend, M. Chimes, J. Gregory, C. Talbot-Eeckelaers and M. Wharton, 75, 
ANALYTICAL CHALLENGES POSED BY NORM CONTAMINATED SITES D. Read, 81, 
CHARACTERISATION OF HIGHLY TRITIATED WASTE STREAMS RESULTANT FROM THE OPERATION OF THE CHAPELCROSS POWER STATION PROCESS PLANT (CXPP) FOR THE PURIFICATION OF TRITIUM D. Bridgewater, 92, 
OXALOHYDROXAMIC ACID AS A REDUCTOMETRIC TITRANT FOR THE DETERMINATION OF OXYGEN TO METAL RATIO IN NUCLEAR FUEL F. Khan and S. Agrawal, 95, 
THE DETERMINATION OF TECHNETIUM-99 IN SOIL, SILT, SAND AND SEDIMENT BY ICP-MS USING ISOTOPICALLY ENRICHED RHENIUM AS A YIELD MONITOR M. Bell, A.M.J. Lees and N. Scott, 100, 
AUTOMATED SYSTEM FOR NEUTRON ACTIVATION ANALYSIS S.S. Ismail, 111, 
ALGAL BIOSORPTION – IS IT A VIABLE ALTERNATIVE IN TREATMENT OF RADIOACTIVELY CONTAMINATED EFFLUENTS? A. Kutner and V. Nesbitt, 122, 
LONG TERM TRENDS IN FAR-FIELD EFFECTS OF MARINE RADIOACTIVITY MEASURED AROUND NORTHERN IRELAND V.E. Ly, S.M. Cogan, W.C. Camplin, L. Peake and K.S. Leonard, 134, 
A SEARCH FOR DARMSTADTIUM IN NATURE J.W. Mietelski, P. Gaca and E. Tomankiewicz, 144, 
A COMPARISON OF ANALYTICAL METHODS FOR SCREENING GROSS ALPHA AND BETA RADIOACTIVITY IN WATER BY LIQUID SCINTILLATION COUNTING AND GAS FLOW PROPORTIONAL COUNTING M.B. Nisti, A.O. Ferreira, M. P. Campos, C.H.R. Saueia and B.P. Mazzilli, 148, 
SPINACH POWDER IS A SUITABLE MEDIUM FOR INTERLABORATORY COMPARISONS (ILC) OF ENVIRONMENTALLY RELEVANT NUCLIDES N. Roos and D. Tait, 152, 
DETERMINATION OF RADIOACTIVITY LEVELS AND 222Rn EXHALATION RATES IN SOIL AROUND IPEN FACILITIES M.B. Nisti, C.H.R. Saueia and B.P. Mazzilli, 159, 
UPTAKE OF NATURAL URANIUM BY NATIVE PLANTS GROWN IN A HIGH BACKGROUND RADIATION AREA E.I. Shabana, M. M.T. Qutub and A.A. Kinsara, 164, 
DETERMINATION OF POLONIUM-210 AND LEAD-210 IN IRON- AND STEEL-MAKING MATERIALS F. Dal-Molin, D.R. Anderson and D. Read, 175, 
WASTE MANAGEMENT CHARACTERISATION STRATEGY : FROM RAW WASTE TO WASTE CONTAINERS FOR LONG TERM DISPOSAL C. Lamouroux, G. Piot, P. Strock and F. Cochin, 185, 
BEHAVIOUR OF RADIONUCLIDES AND ENVIRONMENTALLY SIGNIFICANT ELEMENTS IN NUCLEAR FUEL ELEMENT DEBRIS (FED) C.G. Holmes, J.A. Caborn, C.R. Harvey, N.A. Hodge, D.F. Lee, B. Kralj, D. Wickenden, A.Patel and C. Saunders, 195, 
FUKUSHIMA ACCIDENT FALLOUT IN FOOD AND FODDER IN GERMANY D. Tait and N. Roos, 207, 
UNCERTAINTIES IN MEASUREMENTS USING LSC QUENCH-CURVES G.J. Knetsch and P.J.M. Kwakman, 214, 
Subject Index, 222, 
Isotope Index, 225, 

CHAPTER 1

CHARACTERISATION OF RADIOACTIVELY CONTAMINATED LAND AT DOUNREAY, AN AREA WITH A HIGH NATURAL BACKGROUND

J.A. Heathcote

D2003/Z10, Dounreay Site Restoration Ltd, Thurso KW14 7TZ, UK

1 PURPOSE OF PAPER

The Dounreay experimental reactor site is being decommissioned, with the expectation that some time before 2333, the land will be suitable for 'any reasonably foreseeable use', To achieve this, unacceptably contaminated materials must be identified and characterised for disposal as waste, and there must be a demonstration that residual materials meet relevant standards. Appropriate radio-analytical methods are essential to support these activities. Potential complications are the range of nuclear materials used at the Dounreay experimental site and the presence of a relatively high natural background. Radio-analytical techniques used must also be consistent with the commercial reality of the UK's decommissioning framework – there is no net financial benefit to the UK from decommissioning Dounreay since the land itself has little value, and therefore analytical methods must be cost-effective.

This paper reviews experience gained from using the full range of commercial analytical techniques, to identify how contamination can be distinguished from background, using γ-spectrometry and gross α+β measurement.

2 BACKGROUND TO DOUNREAY

Construction of the Dounreay experimental facility commenced in 1955 on the site of a World War II military airfield, to investigate the commercial feasibility of fast breeder reactor technology. Three reactors were built and operated

• DMTR U-fuelled thermal reactor, 1958 – 1969;

• 12R, U-fuelled fast breeder reactor, 1959 – 1977;

• PFR, MOX-fuelled fast breeder reactor, 1974 – 1994.

These were supported by fuel re-processing and fabrication plant, investigation laboratories, engineering workshops and ancillary facilities. The properties of uranium and plutonium fuels were investigated and a small amount of work was done with thorium fuels. Thorium is also present in refractory thoria in crucibles etc., where its high melting point is relevant, rather than its nuclear properties.

Reactor operations ceased in 1944 fuel re-processing ceased in 1997 and fuel manufacture ceased in 2004. The sole focus now is on decommissioning, with the intention that interventions will be complete by c. 2030 enabling the site to achieve Interim End State and enter a period of passive institutional control. During this period of institutional control the only processes operating are envisaged to be radioactive decay and passive chemical transformation, and the flow of water under gravity. Monitoring will be in place to confirm this. No later than 2333, it is expected that the site will reach a Final End State, when it is suitable for un-restricted use. It is envisaged that such use might be small-scale self-sufficient farming, which was the use of the land prior to construction of the airfield.

3 REQUIREMENTS OF SITE RESTORATION

At Final End State it is expected that the site will be removed from the nuclear licensing regime. The relevant test is "A demonstration that any residual activity, above background radioactivity, which remains on the site, which may or may not have arisen from licensable activities, will lead to a risk of death to an individual using the site for any reasonably foreseeable purpose, of no greater than 1 in a million per year".

A performance assessment has been made to calculate the incremental activity concentrations equivalent to the risk threshold specified in the de-licensing criterion. It is also necessary to be able to determine the 'background'.

Selected incremental activity concentrations appropriate for the large areas (~1000 m2) average for soils within one metre of ground surface are given in Table 1, for different periods of possible institutional control. These comprise the main nuclides of concern at Dounreay. The values for longer periods of control are expected to fall to the values for zero years control during the control period, as a result of radioactive decay and leaching by rainfall. For comparison, current waste clearance standards are also provided. Values given are for single nuclides; multiple nuclides are combined using the sum of quotients rule. The treatment of thorium and uranium currently differs between Scottish and European legislation.

The background comprises radiation from natural geological materials, the consequences of international fall-out, and the expected consequences of authorised discharges to the environment from Dounreay's operations. These are excluded from the definition of radioactive waste in UK law. Rocks in the Dounreay area locally contain relatively high levels of uranium and thorium, to the degree that mining has been considered in the vicinity. The uranium in bedrock at Dounreay comprises a general background value of 2-6 ppm with local accumulations of up to 33 000 ppm, especially in organic-rich material including fossil fish, estimated by chemical means. The Environment Agency reports typical average activities for Th-232 and U-238 in English soil as 0.021-0.037 Bq/g and 0.013-0.032 Bq/g respectively (1 ppm U = 0.0122 Bq/g).

Radiochemical analysis supports site restoration in four ways:

• Investigation to scope the need and extent of remedial work;

• During remedial work, to control operations and in particular in deciding when it is possible to stop;

• In sentencing removed material for disposal as radioactive waste;

• In verifying that remedial work has achieved the required standard.

There are particular operational constraints on the second and third of these because plant will be standing and temporary stock-piles of waste need segregation and safe management, while the results of analysis are awaited. Results within seven days would be very good, 24 hours is even better. Commercially available techniques for nuclides of concern for Dounreay site restoration are listed in Table 2. Only gross α+β and γ-spectrometry are potentially available on 24-hour turnaround, requiring no more than drying and grinding of the sample before counting, with no chemical preparation.

Dounreay has had a programme of site characterisation for restoration purposes for a number of years. The current operational framework was established c. 2006 and data from this date have been reviewed for the purposes of this paper. Three different laboratories have been used for most of the work. Most samples are analysed for gross α+β and by γ-spectrometry. A subset of samples were analysed for Sr-90 by chemical separation and β-counting, and for U isotopes, Pu isotopes and Am-241 by chemical separation and α-spectrometry. The samples comprise rock, engineering soils, and concrete from floor-slabs and similar constructions. The work presented here is based on 1623, sets of analyses from Lab 1, 802 from Lab 2, and 79 from Lab 3. The nature of the materials sampled is such that the samples sent to each laboratory are broadly from the same population and therefore the results would be expected to show similar central tendencies.

4.1 Gross Alpha and Beta

Gross α+β is measured on soils using a simple proportional detector, which discriminates counts into two channels. The beta channel effectively has a lower cut-off at around 300 keV and therefore does not respond to all β-emitters. The source is dried and the powdered material is presented as a notionally thin film or as a thick pellet. The sensitivity is determined by calibration and the technique is only semi-quantitative.

The results from the complete dataset are shown in Figure 1. There are three clear features. The bulk of results cluster in the region gross-β 0.1 Bq/g, gross-α 0.1-5 Bq/g. There is a small cluster of results around gross-β 0.1 Bq/g, gross-α 0.1 Bq/g, and there are various results with much higher activities, particularly gross-β. There is evidence of a systematic difference between Lab 1, using the thick pellet technique, and Labs 2 and 3, using the thin film technique and this has been confirmed as statistically significant. The median gross-α activity is 0.37 Bq/g and the median gross-β activity is 1.3 Bq/g (Labs 2 and 3 only, median used since it is less affected by extreme values).

When the activities of the bulk of the results are compared with the clearance levels in Table 1, the difficulties at Dounreay become clear. Sample activities typically exceed clearance levels for α- and β-emitters, even though most of these results are for uncontaminated material.

A small number of samples have anomalously low gross-β. These are from a single analytical batch and, with the benefit of hindsight, seem to represent an analytical difficulty. These gross-β results are inconsistent with the concentration of β-emitters determined by other techniques.

It is clear that some contaminated materials are present, identified by atypically high gross-α or gross-β.

4.2 Gamma Spectrometry

4.2.1 Measurement Method. Ideally, γ-spectrometry is performed on dried and powdered material so that a uniform geometry can be achieved. There is, however, a time saving in counting the material 'as received', at natural moisture content and with less well constrained geometry although loosely packed into a standard container. Most of Lab 1Ks results were obtained in this way.

Gamma spectrometry can quantify the common contaminant Cs-137 via the 662 keV photo-peak of its short-lived daughter Ba-137m. A number of other key potential contaminants can be identified from characteristic photo-peaks, including Co-60 and Nb-94). There are also γ-emitting naturally occurring radionuclides K-40; Ac-228, Ra-224, Pb-212, Bi-212, Tl-208 as daughters of Th-232; U-235; Th-234, Pa-234m, Ra-226, Pb-214, Bi-214, Pb-210 as daughters of U-238.

Gamma spectra can be quite complex, as a result of both complicated emission spectra and detector artefacts. Radiochemical data are from The Radiochemical Manual unless indicated otherwise. There is the possibility of peak overlaps as a result of finite detector resolution, two of which are significant to the present discussion:

• Ra-226 186.21 keV, 3.59% probability overlaps with U-235, 185.72 keV, 57.2% probability. When the isotopic abundance in natural uranium and secular equilibrium of Ra-226 are taken into account, these two gamma peaks are of similar height;

• Ra-224 240.987 probability, is between Pb-212, 238.625 keV, 43.4% probability and Pb-214, 241.91 keV, 7.5% probability. The former is certain to be present, as a short-lived daughter of Ra-224.

Consideration of the expected abundances in natural uranium in secular equilibrium, and the emission probabilities, indicates that the peaks for Ra-226 and U-235 are of very similar intensity in such material.

Gamma-emitting daughters provide the potential to determine thorium and uranium activity concentrations without using alpha techniques.

4.2.2. Effect of Fuel Processing on Actinide Decay Chains. Chemical separation of thorium results in material containing Th-232 and Th-228 only. Secular equilibrium of Pb-212 and Bi-212 from Th-228 takes ~40 days governed by the half-life of Ra-224. Secular equilibrium of Ac-228 takes around 50 years, governed by the half-life of Ra-224. Chemical separation of uranium results in material containing U-234, U-235 and U-238 only. Subsequent enrichment for fuel increases the proportions of U-235 and inevitably of U-234. Th-234 and Pa-234m achieve secular equilibrium with U-238 within a year, but other γ-emitters take a very long time to reach equilibrium, governed by half-lives ~104 y, and are therefore not yet present in fuel uranium in any significant quantity.

Both decay chains involve a gaseous isotope. Rn-220 in the Th-232 decay chain is short-lived (<1 min) and is unlikely to diffuse from solids; Rn-222 in the U-238 decay chain is longer-lived (3.8 d) and diffusion before daughters grow in is more credible. Short-lived daughters after 'the radon gap' will exist for analytical purposes if their radon parents are present. It is noted that Pb-210 at the end of the U-238 chain is long-lived on the time scale of sample analysis (22 y), and will remain after sample collection even if Rn-222 is lost subsequently.

4.2.3 Results for Th-232 Decay Chains. Considering Th-232 first, Pb-212 has been chosen as the reference nuclide since it has the largest number of measurements above detection in the dataset. The Pb-212 results from all three labs are closely similar and the means are not significantly different, thus these data are regarded as consistent.

Data for Ac-228 from Labs 2 and 3 show a good correspondence with the 1:1 relationship expected from the decay series, Figure 2. Lab 1 shows a slight low bias to about 80% of the expected value. There are very few outliers. The full dataset has been examined for these points but no clear conclusions have been drawn that explain internal inconsistencies in the data. There is no clear evidence for disequilibrium between Ac-228 and Pb-212 and little would be expected, given the site's history. A plot of Bi-212 against its parent Pb-212 shows good correspondence (Figure 3), but significant variability when compared with measurements of Tl-208, allowing for the decay branch after Bi-212 (36%), Figure 4. Lab 1 also reports Ra-224 and Th-232. Th-232 is not a gamma emitter and the measurement is derived from a weighted mixture of Tl-208, Pb-212 and Ac-228 lines. The key line is the Tl-208 line at 583.14 keV, 30.25%. These data show that Ra-224 is consistently higher than expected, presumably as a result of the peak overlap mentioned above. There is a reasonable correspondence with Th-232, which is not unexpected since this measurement is not fully independent of Pb-212.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2015
• Language: English
• ISBN 10: 1782621555
• ISBN 13: 9781782621553
• Binding: Hardcover
• Edition number: 1
• Number of pages: 226
• Editor: Warwick Peter