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A contribution towards a standard leach test for immobilized low and medium level waste
NUCLEAR AND CHEMICAL WASTE MANAGEMENT, Vol. 3, pp. 125-129.1982 Printed in the USA.
0191-815X/82/020125-05$03.00/0 1982 Pergamon Press Ltd.
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST FOR IMMOBILIZED LOW AND MEDIUM LEVEL WASTE R. A. J. Sambell United Kingdom Atomic Energy Authority, Materials Development Division, AERE Harwell, Didcot, Oxfordshire OX11 ORA, United Kingdom
C. Smitton Central Electricity Generating Board, Scientific Services Division, Wythenshawe, Manchester, United Kingdom
A. Elsden British Nuclear Fuels Ltd., Risley, Cheshire, United Kingdom
ABSTRACT. The evaluation of the safety, acceptability and environmental
impact of an immobilized waste requires the identification and characterization of its relevant properties. One of the more important properties is its chemical stability in aqueous environments. This has led to the development of a wide variety of so-called “leach tests.” There is now internationally a strong body of opinion that there should be a standardized approach to this particular test. This paper discusses the categories of application of test results as they apply to the management of immobilized low and medium level waste products. It then describes a procedure for carrying out a leach test that has been adopted as an interim national standard within the United Kingdom (UK). It concludes that there is now a sufficient concensus of expertise to allow the establishment of an international standard.
dionuclides, or an inactive analogue, to an aqueous medium as a function of time. There is a wide range of matrices into which radionuclides have been incorporated: glasses, ceramics, cements, metals, polymers, and bitumens. The choice of matrix depends on several considerations including the following: type of waste (low-, medium-, and high-level waste; alpha-bearing waste, plutonium contaminated material); - nature of the waste (solid, liquid, gaseous, massive, and particulate); - chemical compatibility between waste and matrix; - physical compatibility between waste and matrix; - compatibility between waste and processing plant. Thus, a wide variety of test methods have been developed and adapted by workers in this field in response to their local evaluation requirements. In the absence of international or in most cases national agreement on methodology it is often difficult, if not impossible, to compare the results obtained by different groups of workers even for similar waste/ matrix combinations. The choice of methods of determination is gener-
INTRODUCTION
The evaluation of the safety, acceptability and environmental impact of an immobilized waste requires the identification and characterization of its relevant properties and the parts of the waste management cycle in which these properties are important. Prop erties which may be considered include mechanical strength, thermal stability, chemical stability, flammability, radiation stability, and biodegradability. One of the more important potential initiating events for the return of radioactivity from immobilized wastes to man is exposure to water: surface or rain water in the case of transportation and storage, groundwater in the case of geologic disposal, and seawater in the case of seabed disposal. For this reason the chemical stability of a waste in aqueous environments is invariably assessed. Routinely, this is done by carrying out a leach test. This involves the direct or indirect measurement of the release of raRECEIVED317182; ACCEPTED3/15/82. Acknowledgements-The authors wish to thank those colleagues who contributed to the proposal, in particular, A. Haighton, CEGB, and T. M. Valentine, UKAEA.
125
126
R. A. J. SAMBELL,C. SMFI”lrON, AND A. ELSDEN
ally governed by the use to which the test results are to be put. The major applications for test results are listed below: - radiological assessments of waste disposal; - demonstration of compliance with regulations in waste processing, storage, transport, and disposal; - comparison of various immobilization matrices; - provision of quality assurance in solidification plant operations; - an understanding of the mechanism(s) of leaching; - assessments by regulatory and advisory bodies, that will lead to sound decisions regarding the acceptability of an immobilized waste product.
relate to the release of radioactivity from immobilized waste. However, a specific example in which a standard test procedure could be applied concerns the transport of an immobilized waste from say a nuclear power station site to an engineered store or repository. If it is desired to transport this waste under the low level solids (LLS) category of the IAEA regulations (l), it is necessary to demonstrate that the loss of activity from the immobilized waste cannot exceed 0.1 A2 Ci in one week when the outer packaging is lost and the immobilized waste is totally immersed in water, where A2 is the activity limits of radioactive material, for all forms other than special when transported in a Type A package. The designated values for some radionuclides are shown in Table 1.
APPLICATION OF TEST RESULTS Radiological Assessment of Immobilized Waste Product The source terms for radiological assessment of waste disposal are the individual rates of release of each radionuclide from the immobilized waste product, in units of bequerels or curies per second. For unit mass of immobilized waste, the rate of release of each nuclide may be expressed as dQ = BFLBqs-‘(Cis-I), dt where B is the activity concentration of nuclide in the waste, Bqg-’ (Ci g-l), F is the surface area, m’, and L is the leach rate, g m-' s-‘. Most leaching experiments can be designed to measure the product of these three terms for a representative selection of nuclides. Experiments on the leaching of vitrified high-level waste have shown that chemical differences between nuclides may lead to entirely different mechanisms and rates of release, so it is necessary to include enough different nuclides in the leach tests that the release rates of all others present in the waste may be reliably inferred by chemical analogy. However, in reporting the results of leach tests it is common practice to separate the three terms in the above equation. This can lead to difficulties in reassembling the information for the purposes of a radiological assessment. Clearly, data for a radiological assessment must be derived from a leach test that simulates the environment under consideration. Nevertheless, results from a standard test may be the only comparative data available. Even so, it is only going to be of value if there is access to “unprocessed” data on which to carry out the applicable analysis. Demonsfrafion of Compliance with Regulations in Waste Processing, Storage, Transport, and Disposal At the present time there are few regulations that
Comparison of Different Immobilization Matrices The selection of an immobilization matrix is a complicated procedure involving the scrutiny of many parameters. One of the major inputs is likely to be the chemical properties of the immobilized waste which is generally dictated by the immobilization medium or matrix. Again, the release rate of radioactivity to an aqueous medium will be a property of interest. The extent to which leach data from one product to another can be compared will depend on an adequate description of the product and the similarity of the test procedure. Quality Assurance During Immobilization Plant Operation In order to ensure that satisfactory operation of a waste immobilization process is taking place, the use of quality assurance testing on the product in the plant may be required. The reasons for which such testing is carried out vary. In some cases the checks are used to ensure that in the absence of full and detailed characterization of the physical and chemical nature of the wastes to be processed, the immobilized waste product remains satisfactory or that this aim is achieved by process adjustments based on the checks that are carried out. One such check is a standard leach test. In this case, the test will need to be of the shortest practicable duration. The time interval involved will depend entirely on obtaining a sufficient
TABLE 1 Nuclide Co-60 cs-137 S-90 Y-90 Am-241 Mixed fission products Pu-239 Ru-106
AZ, Ci
7 20 0.4 10 0.008 0.4 0.002 7
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST
number of decay events in unit time (counts per second) to allow an accurate determination of the activity release to the water sample. An Understanding of the Mechanism(s) of Leaching The release of radioactivity from an immobilized waste to an aqueous environment is acknowledged to be due to one or more mechanisms, some of which are diffusion, dissolution, corrosion (adherent and breakaway), ion exchange. Clearly, the mechanism may depend as much on the chemistry of the water (including pH and conductivity) and the temperature and pressure, as on the nature of the waste product. In this sense, it is not possible to have an arbitrary standard method. However, it is still highly desirable that meaningful comparisons be made between the data from different laboratories. This will only be possible if certain basic requirements are standard practise: for example, a prescribed description of the product and the aqueous medium, a simulated seawater standard, size and shape of the sample, ratio of surface area of sample to volume of aqueous medium, clearance between sample, container and surface of liquid, method of support, temperature (or range of temperatures), pressure (or range of pressures), frequency of sampling, and presentation of original, unprocessed data. Assessments by Regulatory and Advisory Bodies Such assessments will be made against a background of requirements such as safety considerations, transport regulations and radiological assessment of the hazard to the biosphere during storage or disposal. Again, one of the many inputs will be leach rate and if intercomparisons are to be possible then standardization of the leach test and data presentation is essential. THE PROPOSED TESTING
PROCEDURE
FOR LEACH
The establishment of a standard leach test will be as a result of an evolutionary procedure based on experience and a fuller realization of the complexity of the process of leaching in most waste/matrix systems. In the UK, the three agencies dealing with nuclear waste [viz., United Kingdom Atomic Energy Authority (UKAEA), Central Electricity Generating Board (CEGB), and British Nuclear Fuels, Ltd. (BNFL)] have reached agreement on the procedure for a static leach test. The proposal is but one of several (2-4) such attempts to establish a standard and due acknowledgement is made to the work of Hespe (2) in laying the foundations for a standard test. However, in common with the remainder, the UK proposal differs in some details. Where differences occur, an attempt is made to justify our
127
choice of procedure. The test currently accepted as standard in the UK is as follows. Description of the Product An accurate description of the product is essential if meaningful comparisons are to be made. Confusion has arisen in the past in describing the composition of the product. In our view the least ambiguous unit to use is that of volume and the volume of waste should be reported as a fraction or percentage of the whole. The Container Any container is regarded as suitable provided it does not react with the leachant nor absorb significant quantities of activity and is sufficiently resistant to the radiation dose received during the test. Polypropylene, PTFE, and glass are used in the various UK laboratories. The Leachant Specification The leachant should be demineralized water with a maximum conductivity before use of 150 ~LS m-l (1.5 pmho cm-‘). This degree of demineralization is readily obtained and insofar as Hespe proposed this arbitrary value it has been adhered to in the UK. Likewise, the temperature of 25 f 5 “C is retained. The volume of leachant should be a function of surface area of the sample. The ratio, volume of leachant
= 25 f 5 mm,
exposed surface area of sample has been chosen since it allows the condition relating to depth of leachant around the specimen to be met without undue dilution of the radioactivity released to the leachant. Sample Specification The sample should be representative in terms of manufacturing process, homogeneity, and microstructure (i.e., grain or particle size distribution and porosity). In the case of cement, bitumen, and polymer matrices, intrinsic exotherms can cause significant temperature rises in full-scale products. Such temperature rises should be simulated in laboratory samples by external heating. Surface roughness should be removed and the sample carefully cleaned of dust particles by air blasting. The specimen should be in the form of a right cylinder of dimensions 50 i 10 mm by 50 f 10 mm. Edge effects and size effects are not well understood and may be more or less significant depending on the waste/matrix system. For these reasons, any generalization of the shape and size could be prejudicial to comparative assessment, particularly in the short term. The choice of a cylindrical shape is not arbitrary. As more programs move towards
128
full-scale production and full-scale size of end product, the trend will be towards on-line sampling and core sampling. In the latter case, a cylindrical sample is obtained and it is logical that the on-line sample should be the same shape and size. The whole of the specimen should be exposed to the leachant. This obviates the difficulties of sample preparation if only one plane surface is exposed (2). It also gives more reliable data in the early stages of the test by helping to maximize the release rate of radioactivity and hence the number of counts per second. The sample should be supported in such a way that the proportion of surface area masked by the support is negligible. The sample should be positioned in the leachant container so as to give at least 10 mm of leachant surrounding the specimen in all directions. Leachant Renewal and Sampling The leachant should be decanted off rather than the specimen moved to a new container. This simplifies the task of monitoring the container for residual activity when the test is finally completed, including any particulate matter which may have arisen. The presence of particulate matter should be recorded. The frequency of leachant renewal is really one of convenience at the start of a test and once every 24 h ensures that sampling falls naturally within the working day. Renewal should continue once a day for the first seven days, thus providing the raw data for the transport regulation (release of O.l& Ci in one week). Then once a week for the next 8 weeks, once per month during the following four months and then once every 6 months for as long as is considered necessary. During the test the pH and conductivity of the leachant should also be recorded at the same frequency. Analysis of the Leachate Leachate aliquots shall be representative of the used leachant. If solids are present in the leachate these should be analyzed with respect to radioactivity and chemical composition. Analysis of the leachate should be carried out for the isotopes of interest and of radiological significance. Presentation of Results The volume of waste in the solidified product and the specific /3, y, and o activity should be quoted. The breakdown of activity should show the significant isotopes in terms of both activity and radiological effect. All the original “unprocessed” data should be tabulated and appended to the presentation. The results of the leach testing should be expressed as the incremental leach rate, R,, as a function of time, t set of leaching, where
R. A. J. SAMBELL,C. SMiTI’ON,AND A. ELSDEN R. =
(2) (s)
(-l-)ms+,
where a. = radioactivity leached during the leachant renewal period n; Ao = radioactivity initially present in the specimen; F= exposed surface area of specimen (m’); v= volume of specimen (m”); and t, = duration of leachant renewal period (s). Values of R. from the equation above are averaged over the time interval, t. and should be plotted against t. - (tn - t.-1)/2. In those cases, where R. becomes virtually constant with time, the mean value of R, should be quoted together with the accuracy. The results may also be expressed as cumulative fraction of radioactivity leached as a function of time, and a leaching factor derived if appropriate. In some cases workers would wish to use this test for investigating leach rate of transuranic alpha emitters while being aware of the difficulties as discussed in Appendix I of the International Standards Organization (ISO) standard (3).
CONCLUDING REMARKS The role of leaching during transport, storage, disposal and accident situations is an important factor in the waste management cycle. The development of test methods to characterize the behavior of real full-size immobilized packages is likely to be crucial in these areas of the waste cycle. The testing method will have to allow for theoretical scaling factors and achievement of a basic understanding of the physical/ chemical processes involved if full-scale-type testing is to be avoided. This, together with the areas for application of test results discussed in this paper, make it essential that an internationally agreed method is established as soon as possible. The IAEA’s recognition of this need is seen in the form of Hespe’s draft recommendations. Most work in this field is now based on the Hespe method, but many workers apply their own modifications, particularly in the light of experience of evaluation work on low-and mediumlevel waste streams. At the present time, the International Standards Organization is examining this problem and the American Nuclear Society and United Kingdom reactions to these earlier drafts provide important inputs into a final version of the recommendations. The need to establish a data base for radiological and regulatory assessments requires that environmental testing is carried out. Nevertheless, the
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST
establishment of a standard routine is still highly desirable. For example, the replacement of demineralized water by a standard seawater simulant in the “standard test” straightforwardly allows the generation of another set of data for comparison and intercomparison evaluations. It is suggested that the various draft proposals provide a sufficient concensus of experience on a wide enough international basis for an “international standard leach test” to be designed and published, and that this should be done as quickly as possible.
129
REFERENCES 1. International Atomic Energy Agency. Regulations for the safe transport of radioactive materials. IAEA Safety Series No. 6 (1979). 2. International Atomic Energy Agency. Leach testing of immobilized radioactive waste solids, a proposal for a standard method, E. D. Hespe, ed. AI. Energy Rev. 9: 195-207 (1971). 3. International Standards Organization (ISO). Long-term leach testing of radioactive waste solidification products. ISO/TC 85/SC 5/WG 38, Draft IS0 Standard (1979). 4. American Nuclear Society Standards Committee, Proposed standard, measurement of the leachability of solidified low-level radioactive wastes, in preparation by ANS-16.1 Working Group.
0191-815X/82/020125-05$03.00/0 1982 Pergamon Press Ltd.
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST FOR IMMOBILIZED LOW AND MEDIUM LEVEL WASTE R. A. J. Sambell United Kingdom Atomic Energy Authority, Materials Development Division, AERE Harwell, Didcot, Oxfordshire OX11 ORA, United Kingdom
C. Smitton Central Electricity Generating Board, Scientific Services Division, Wythenshawe, Manchester, United Kingdom
A. Elsden British Nuclear Fuels Ltd., Risley, Cheshire, United Kingdom
ABSTRACT. The evaluation of the safety, acceptability and environmental
impact of an immobilized waste requires the identification and characterization of its relevant properties. One of the more important properties is its chemical stability in aqueous environments. This has led to the development of a wide variety of so-called “leach tests.” There is now internationally a strong body of opinion that there should be a standardized approach to this particular test. This paper discusses the categories of application of test results as they apply to the management of immobilized low and medium level waste products. It then describes a procedure for carrying out a leach test that has been adopted as an interim national standard within the United Kingdom (UK). It concludes that there is now a sufficient concensus of expertise to allow the establishment of an international standard.
dionuclides, or an inactive analogue, to an aqueous medium as a function of time. There is a wide range of matrices into which radionuclides have been incorporated: glasses, ceramics, cements, metals, polymers, and bitumens. The choice of matrix depends on several considerations including the following: type of waste (low-, medium-, and high-level waste; alpha-bearing waste, plutonium contaminated material); - nature of the waste (solid, liquid, gaseous, massive, and particulate); - chemical compatibility between waste and matrix; - physical compatibility between waste and matrix; - compatibility between waste and processing plant. Thus, a wide variety of test methods have been developed and adapted by workers in this field in response to their local evaluation requirements. In the absence of international or in most cases national agreement on methodology it is often difficult, if not impossible, to compare the results obtained by different groups of workers even for similar waste/ matrix combinations. The choice of methods of determination is gener-
INTRODUCTION
The evaluation of the safety, acceptability and environmental impact of an immobilized waste requires the identification and characterization of its relevant properties and the parts of the waste management cycle in which these properties are important. Prop erties which may be considered include mechanical strength, thermal stability, chemical stability, flammability, radiation stability, and biodegradability. One of the more important potential initiating events for the return of radioactivity from immobilized wastes to man is exposure to water: surface or rain water in the case of transportation and storage, groundwater in the case of geologic disposal, and seawater in the case of seabed disposal. For this reason the chemical stability of a waste in aqueous environments is invariably assessed. Routinely, this is done by carrying out a leach test. This involves the direct or indirect measurement of the release of raRECEIVED317182; ACCEPTED3/15/82. Acknowledgements-The authors wish to thank those colleagues who contributed to the proposal, in particular, A. Haighton, CEGB, and T. M. Valentine, UKAEA.
125
126
R. A. J. SAMBELL,C. SMFI”lrON, AND A. ELSDEN
ally governed by the use to which the test results are to be put. The major applications for test results are listed below: - radiological assessments of waste disposal; - demonstration of compliance with regulations in waste processing, storage, transport, and disposal; - comparison of various immobilization matrices; - provision of quality assurance in solidification plant operations; - an understanding of the mechanism(s) of leaching; - assessments by regulatory and advisory bodies, that will lead to sound decisions regarding the acceptability of an immobilized waste product.
relate to the release of radioactivity from immobilized waste. However, a specific example in which a standard test procedure could be applied concerns the transport of an immobilized waste from say a nuclear power station site to an engineered store or repository. If it is desired to transport this waste under the low level solids (LLS) category of the IAEA regulations (l), it is necessary to demonstrate that the loss of activity from the immobilized waste cannot exceed 0.1 A2 Ci in one week when the outer packaging is lost and the immobilized waste is totally immersed in water, where A2 is the activity limits of radioactive material, for all forms other than special when transported in a Type A package. The designated values for some radionuclides are shown in Table 1.
APPLICATION OF TEST RESULTS Radiological Assessment of Immobilized Waste Product The source terms for radiological assessment of waste disposal are the individual rates of release of each radionuclide from the immobilized waste product, in units of bequerels or curies per second. For unit mass of immobilized waste, the rate of release of each nuclide may be expressed as dQ = BFLBqs-‘(Cis-I), dt where B is the activity concentration of nuclide in the waste, Bqg-’ (Ci g-l), F is the surface area, m’, and L is the leach rate, g m-' s-‘. Most leaching experiments can be designed to measure the product of these three terms for a representative selection of nuclides. Experiments on the leaching of vitrified high-level waste have shown that chemical differences between nuclides may lead to entirely different mechanisms and rates of release, so it is necessary to include enough different nuclides in the leach tests that the release rates of all others present in the waste may be reliably inferred by chemical analogy. However, in reporting the results of leach tests it is common practice to separate the three terms in the above equation. This can lead to difficulties in reassembling the information for the purposes of a radiological assessment. Clearly, data for a radiological assessment must be derived from a leach test that simulates the environment under consideration. Nevertheless, results from a standard test may be the only comparative data available. Even so, it is only going to be of value if there is access to “unprocessed” data on which to carry out the applicable analysis. Demonsfrafion of Compliance with Regulations in Waste Processing, Storage, Transport, and Disposal At the present time there are few regulations that
Comparison of Different Immobilization Matrices The selection of an immobilization matrix is a complicated procedure involving the scrutiny of many parameters. One of the major inputs is likely to be the chemical properties of the immobilized waste which is generally dictated by the immobilization medium or matrix. Again, the release rate of radioactivity to an aqueous medium will be a property of interest. The extent to which leach data from one product to another can be compared will depend on an adequate description of the product and the similarity of the test procedure. Quality Assurance During Immobilization Plant Operation In order to ensure that satisfactory operation of a waste immobilization process is taking place, the use of quality assurance testing on the product in the plant may be required. The reasons for which such testing is carried out vary. In some cases the checks are used to ensure that in the absence of full and detailed characterization of the physical and chemical nature of the wastes to be processed, the immobilized waste product remains satisfactory or that this aim is achieved by process adjustments based on the checks that are carried out. One such check is a standard leach test. In this case, the test will need to be of the shortest practicable duration. The time interval involved will depend entirely on obtaining a sufficient
TABLE 1 Nuclide Co-60 cs-137 S-90 Y-90 Am-241 Mixed fission products Pu-239 Ru-106
AZ, Ci
7 20 0.4 10 0.008 0.4 0.002 7
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST
number of decay events in unit time (counts per second) to allow an accurate determination of the activity release to the water sample. An Understanding of the Mechanism(s) of Leaching The release of radioactivity from an immobilized waste to an aqueous environment is acknowledged to be due to one or more mechanisms, some of which are diffusion, dissolution, corrosion (adherent and breakaway), ion exchange. Clearly, the mechanism may depend as much on the chemistry of the water (including pH and conductivity) and the temperature and pressure, as on the nature of the waste product. In this sense, it is not possible to have an arbitrary standard method. However, it is still highly desirable that meaningful comparisons be made between the data from different laboratories. This will only be possible if certain basic requirements are standard practise: for example, a prescribed description of the product and the aqueous medium, a simulated seawater standard, size and shape of the sample, ratio of surface area of sample to volume of aqueous medium, clearance between sample, container and surface of liquid, method of support, temperature (or range of temperatures), pressure (or range of pressures), frequency of sampling, and presentation of original, unprocessed data. Assessments by Regulatory and Advisory Bodies Such assessments will be made against a background of requirements such as safety considerations, transport regulations and radiological assessment of the hazard to the biosphere during storage or disposal. Again, one of the many inputs will be leach rate and if intercomparisons are to be possible then standardization of the leach test and data presentation is essential. THE PROPOSED TESTING
PROCEDURE
FOR LEACH
The establishment of a standard leach test will be as a result of an evolutionary procedure based on experience and a fuller realization of the complexity of the process of leaching in most waste/matrix systems. In the UK, the three agencies dealing with nuclear waste [viz., United Kingdom Atomic Energy Authority (UKAEA), Central Electricity Generating Board (CEGB), and British Nuclear Fuels, Ltd. (BNFL)] have reached agreement on the procedure for a static leach test. The proposal is but one of several (2-4) such attempts to establish a standard and due acknowledgement is made to the work of Hespe (2) in laying the foundations for a standard test. However, in common with the remainder, the UK proposal differs in some details. Where differences occur, an attempt is made to justify our
127
choice of procedure. The test currently accepted as standard in the UK is as follows. Description of the Product An accurate description of the product is essential if meaningful comparisons are to be made. Confusion has arisen in the past in describing the composition of the product. In our view the least ambiguous unit to use is that of volume and the volume of waste should be reported as a fraction or percentage of the whole. The Container Any container is regarded as suitable provided it does not react with the leachant nor absorb significant quantities of activity and is sufficiently resistant to the radiation dose received during the test. Polypropylene, PTFE, and glass are used in the various UK laboratories. The Leachant Specification The leachant should be demineralized water with a maximum conductivity before use of 150 ~LS m-l (1.5 pmho cm-‘). This degree of demineralization is readily obtained and insofar as Hespe proposed this arbitrary value it has been adhered to in the UK. Likewise, the temperature of 25 f 5 “C is retained. The volume of leachant should be a function of surface area of the sample. The ratio, volume of leachant
= 25 f 5 mm,
exposed surface area of sample has been chosen since it allows the condition relating to depth of leachant around the specimen to be met without undue dilution of the radioactivity released to the leachant. Sample Specification The sample should be representative in terms of manufacturing process, homogeneity, and microstructure (i.e., grain or particle size distribution and porosity). In the case of cement, bitumen, and polymer matrices, intrinsic exotherms can cause significant temperature rises in full-scale products. Such temperature rises should be simulated in laboratory samples by external heating. Surface roughness should be removed and the sample carefully cleaned of dust particles by air blasting. The specimen should be in the form of a right cylinder of dimensions 50 i 10 mm by 50 f 10 mm. Edge effects and size effects are not well understood and may be more or less significant depending on the waste/matrix system. For these reasons, any generalization of the shape and size could be prejudicial to comparative assessment, particularly in the short term. The choice of a cylindrical shape is not arbitrary. As more programs move towards
128
full-scale production and full-scale size of end product, the trend will be towards on-line sampling and core sampling. In the latter case, a cylindrical sample is obtained and it is logical that the on-line sample should be the same shape and size. The whole of the specimen should be exposed to the leachant. This obviates the difficulties of sample preparation if only one plane surface is exposed (2). It also gives more reliable data in the early stages of the test by helping to maximize the release rate of radioactivity and hence the number of counts per second. The sample should be supported in such a way that the proportion of surface area masked by the support is negligible. The sample should be positioned in the leachant container so as to give at least 10 mm of leachant surrounding the specimen in all directions. Leachant Renewal and Sampling The leachant should be decanted off rather than the specimen moved to a new container. This simplifies the task of monitoring the container for residual activity when the test is finally completed, including any particulate matter which may have arisen. The presence of particulate matter should be recorded. The frequency of leachant renewal is really one of convenience at the start of a test and once every 24 h ensures that sampling falls naturally within the working day. Renewal should continue once a day for the first seven days, thus providing the raw data for the transport regulation (release of O.l& Ci in one week). Then once a week for the next 8 weeks, once per month during the following four months and then once every 6 months for as long as is considered necessary. During the test the pH and conductivity of the leachant should also be recorded at the same frequency. Analysis of the Leachate Leachate aliquots shall be representative of the used leachant. If solids are present in the leachate these should be analyzed with respect to radioactivity and chemical composition. Analysis of the leachate should be carried out for the isotopes of interest and of radiological significance. Presentation of Results The volume of waste in the solidified product and the specific /3, y, and o activity should be quoted. The breakdown of activity should show the significant isotopes in terms of both activity and radiological effect. All the original “unprocessed” data should be tabulated and appended to the presentation. The results of the leach testing should be expressed as the incremental leach rate, R,, as a function of time, t set of leaching, where
R. A. J. SAMBELL,C. SMiTI’ON,AND A. ELSDEN R. =
(2) (s)
(-l-)ms+,
where a. = radioactivity leached during the leachant renewal period n; Ao = radioactivity initially present in the specimen; F= exposed surface area of specimen (m’); v= volume of specimen (m”); and t, = duration of leachant renewal period (s). Values of R. from the equation above are averaged over the time interval, t. and should be plotted against t. - (tn - t.-1)/2. In those cases, where R. becomes virtually constant with time, the mean value of R, should be quoted together with the accuracy. The results may also be expressed as cumulative fraction of radioactivity leached as a function of time, and a leaching factor derived if appropriate. In some cases workers would wish to use this test for investigating leach rate of transuranic alpha emitters while being aware of the difficulties as discussed in Appendix I of the International Standards Organization (ISO) standard (3).
CONCLUDING REMARKS The role of leaching during transport, storage, disposal and accident situations is an important factor in the waste management cycle. The development of test methods to characterize the behavior of real full-size immobilized packages is likely to be crucial in these areas of the waste cycle. The testing method will have to allow for theoretical scaling factors and achievement of a basic understanding of the physical/ chemical processes involved if full-scale-type testing is to be avoided. This, together with the areas for application of test results discussed in this paper, make it essential that an internationally agreed method is established as soon as possible. The IAEA’s recognition of this need is seen in the form of Hespe’s draft recommendations. Most work in this field is now based on the Hespe method, but many workers apply their own modifications, particularly in the light of experience of evaluation work on low-and mediumlevel waste streams. At the present time, the International Standards Organization is examining this problem and the American Nuclear Society and United Kingdom reactions to these earlier drafts provide important inputs into a final version of the recommendations. The need to establish a data base for radiological and regulatory assessments requires that environmental testing is carried out. Nevertheless, the
A CONTRIBUTION TOWARDS A STANDARD LEACH TEST
establishment of a standard routine is still highly desirable. For example, the replacement of demineralized water by a standard seawater simulant in the “standard test” straightforwardly allows the generation of another set of data for comparison and intercomparison evaluations. It is suggested that the various draft proposals provide a sufficient concensus of experience on a wide enough international basis for an “international standard leach test” to be designed and published, and that this should be done as quickly as possible.
129
REFERENCES 1. International Atomic Energy Agency. Regulations for the safe transport of radioactive materials. IAEA Safety Series No. 6 (1979). 2. International Atomic Energy Agency. Leach testing of immobilized radioactive waste solids, a proposal for a standard method, E. D. Hespe, ed. AI. Energy Rev. 9: 195-207 (1971). 3. International Standards Organization (ISO). Long-term leach testing of radioactive waste solidification products. ISO/TC 85/SC 5/WG 38, Draft IS0 Standard (1979). 4. American Nuclear Society Standards Committee, Proposed standard, measurement of the leachability of solidified low-level radioactive wastes, in preparation by ANS-16.1 Working Group.