- Email: [email protected]
Use of reference biospheres for proving the long-term safety of radioactive waste repositories
Journal of Environmental Radioactivity 100 (2009) 435–437
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Letter to the editor
Use of reference biospheres for proving the long-term safety of radioactive waste repositoriesq
The evolution of human societies, their living conditions and eating habits and their interchange with and impacts upon evolving ecosystems in the far future is highly uncertain. Assumptions required for assessing long-term radiological impacts therefore need to build upon stylised biospheres as recommended for long-term impact assessments of nuclear waste repositories (ICRP, 1998). A methodology for constructing stylised biospheres and associated exposure pathways, called the Reference Biospheres Methodology, has been developed via an international collaboration, under the auspices of the International Atomic Energy Agency (van Dorp et al., 1999; IAEA, 2003; Crossland et al., 2005). According to the German Draft Safety Requirements Governing the Final Disposal of Heat-Generating Radioactive Waste (2008), long-term radiological prognoses shall be performed on the basis of reference biosphere models. The aim of the workshop, therefore, was to get an actual overview of the experience gained with the Reference Biospheres Methodology and to identify priorities for future research and development. In this two-day workshop invited biosphere experts from various European countries (including Belgium, France, Germany, Sweden and the United Kingdom) participated, representing a wide range of approaches to biosphere modelling within long-term safety assessments for geologic nuclear waste repositories.
1. Agenda The workshop agenda covered three major topics: 1. Climate impact on biosphere modelling, 2. Radionuclide transition processes as considered within biosphere models and/or affecting radionuclide transfer from the geosphere to the biosphere, 3. Radiation protection criteria (regulatory prescriptions on reference biosphere models, dose/risk constraints, size of critical groups etc.). In each session, participants gave short presentations summarising their experience and view points. The topics were then openly and extensively discussed. A set of questions detailing the q
Workshop of the German Federal Office for Radiation Protection (BfS), 26–27 August 2008. 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.01.007
topics listed above had been circulated by BfS prior to the workshop and served as a guideline for presentations and discussions. Session 1 (Climate impact on biosphere modelling) specifically addressed the following questions and subjects: Is it necessary to simulate complete climate cycles including transition phases using a dynamic approach each time a reference biosphere is developed, or is it sufficient to consider different climatic stages as stationary and to model them independently? Do present climate models, for example, as presented in BIOCLIM (2004), include features to reproduce complete climate cycles? Is there a specific climate cycle which generally causes the highest exposures and which, therefore, should be selected for impact assessments? Can biosphere modelling be performed without considering climatic impact on the radionuclide transport in the geosphere or is an integral concept generally required? Are tools available to simulate the impact on processes in the geosphere caused by climatic conditions? Is this issue primarily relevant for cold climate states or for other climatic conditions, too? From the discussions the following conclusions and general consensus were derived. Occasional dissent should not automatically negate the value of a general consensus; rather it helps to identify issues, such as specific aspects of regulatory requirements or details of particular wastes and site conditions, which may result in country- or site-specific differences in approach. The ability to recognise and be able to explain when and why things need to be considered differently is as valuable as knowing where consensus is appropriate. 1. Post-closure safety assessment should take into account any major changes in environmental conditions which affect potential radiation exposure, not just climate change. Such changes may need to be included explicitly in the assessment calculations. 2. There is no specific climate that establishes a ‘worst case’ with regard to human exposure, but there is a trend that a warmer and dryer climate leads to higher doses compared to temperate regions, mainly because the need for irrigation increases. 3. In biosphere modelling it is generally assumed that ecosystems are in equilibrium in response to the modelled stationary climate
436
Letter to the editor / Journal of Environmental Radioactivity 100 (2009) 435–437
conditions. Consideration of transition phases between stationary climates, however, could be of importance, as they might result in higher releases, e.g. due to an accumulation and then acute release of radionuclides below an ice shield during a glaciation event. The methodology needed for such sequential climate modelling has been developed (BIOCLIM, 2004), but the phenomenological understanding and model parameterization are still limited. 4. Geosphere/biosphere interface processes are considered important for estimating potential release rates into the biosphere. The influence of climate on the hydrological cycle is a key aspect. As a consequence, the traditional approach of performing geosphere and biosphere simulations separately with an interface, at which radionuclide concentrations resulting from the geosphere model serve as input for the biosphere simulations (typically groundwater or well), should be justified on a case by case basis. 5. The approaches and simulation tools developed within the BIOCLIM project may be a valuable methodology for estimating the impact of climate variations on biosphere processes, but spatial downscaling needs much effort. 6. Site-specific scenarios differ drastically depending on whether glaciation has to be assumed. Session 2 (Radionuclide transport processes within biosphere models and/or relating to transfer from the geosphere to the biosphere) specifically addressed (i) whether knowledge and models exist with regard to radionuclide transition processes between single temporally and/or spatially separated biosphere models and, (ii) which aspects have to be taken into consideration for modelling radionuclide transport from the geosphere into the biosphere. The following conclusions evolved from the discussions: 1. Processes at the biosphere and geosphere transition zone (e.g. groundwater recharge rates) are of major importance since they determine the degree of dilution which affects radionuclide concentrations in deeper groundwaters. Cooperation of experts is needed. As stated above, assumptions on biosphere and climate may have major feedback on geosphere processes. Understanding and simulation of such interactions may be essential for modelling radionuclide mobility during climate transition phases. 2. Site-specific assessments are essential to model the transport processes. 3. Future focus should be directed toward more detailed assessment of the potential influence of environmental changes on groundwater flow and radionuclide transport; emphasis should be on the dynamics of changes and related accumulation processes. 4. There should be a clear understanding of the groundwater flow system so as to define surface outlets of radionuclides. In Session 3 (Radiation protection criteria), the focus was (i) on the experience gained and on open problems with regard to specific reference biosphere models, and (ii) on the specification of the critical groups to be considered. The results can be summarised as follows: 1. Since wells show high variability with regard to water extraction capacity and other properties, it is not sensible to define a single standard well and assume that this is adequate for testing safety. However, the dilution of radionuclides eventually present in groundwater, which is assumed to occur in a well, considerably affects the calculated exposures to members of the critical group. This combination of factors could limit the applicability of commonly used reference biosphere well scenarios (Santucci et al., 2005). In addition, well water extraction may not in all cases be regarded as the most conservative scenario (IAEA, 2003).
2. Information on the influence of climate change on mobility and biotic accumulation of radionuclides in various ecosystems is for some radionuclides, such as Se-79, partially available from the literature (e.g. Smith, 2008), but may be inadequate for others (e.g. Tc-99). 3. Biosphere models should be based on a site-specific prediction of potential future developments. For each of these, the specific biosphere models should balance the generic nature of any longterm prediction with the complexity needed for including all major pathways to man. 4. Selection of the parameters for the hypothetical critical groups (or reference persons) should take into account the distribution of behaviours, consistent with viable existence (including at least at some level of sustainability) in the biosphere system assumed. 5. Simulations should be performed for various age groups. Following the reasoning given by ICRP (1998), it is common practice to estimate the annual dose or risk for adult individuals. For communication with the public, however, it is important to document that other age groups are also considered and adequately protected, as members of the public generally are well aware of the fact that children show higher radiation sensitivity than adults. 6. Probabilistic simulations of potential long-term consequences of nuclear waste repositories have become routine. They usually mix probabilistic and deterministic components within the simulations. For example, dose coefficients are generally assumed to have single values for each chemical form and age group. Excluding parameters, such as dose coefficients, from uncertainty analysis may lead to an underestimation of overall uncertainties and over concern about less impacting parameters. Probabilistic approaches may therefore provide benefits in terms of demonstrating compliance with dose or risk constraints and they may also help to elucidate the degree of pessimism that can be inadvertently included within a purely deterministic approach (Smith et al., 2008). Based on these conclusions, the following priorities for future research are suggested: Radionuclide accumulation and release processes during climate transition phases should be studied systematically and compared to those at stationary climate conditions. Processes in the geosphere/biosphere transition zone should be modelled in more detail with special regard to long-term erosion of soil and rock and to climate induced changes to hydrology. Consideration should be given to understanding the circumstances in which well scenarios are not conservative for estimating doses to humans. Uncertainties in dose coefficients should be taken into account when developing an overall understanding of uncertainty in the post-closure safety assessment.
References BIOCLIM, 2004. Modelling sequential biosphere systems under climate change for radioactive waste disposal. EU-Contract FIKW-CT-2000-00024, information available at: http://www.andra.fr/bioclim. Crossland, I.G., Pinedo, P., Kessler, J.H., Torres-Vidal, C., Walters, B., 2005. ‘‘Reference Biospheres’’ for solid radioactive waste disposal: the BIOMASS methodology. J. Environ. Radioact. 84, 135–149. van Dorp, F., Egan, M., Kessler, J.H., Nilsson, S., Pinedo, P., Smith, G., Torres, C., 1999. Biosphere modelling for the assessment of radioactive waste repositories; the development of a common basis by the BIOMOVS II reference biospheres working group. J. Environ. Radioact. 42, 225–236. IAEA, 2003. ‘‘Reference Biospheres’’ for Solid Radioactive Waste Disposal. Report of BIOMASS Theme 1 of the BIOsphere Modelling and ASSessment (BIOMASS) Programme IAEA-BIOMASS-6. International Atomic Energy Agency Safety, Vienna.
Letter to the editor / Journal of Environmental Radioactivity 100 (2009) 435–437 International Commission on Radiological Protection, 1998. Radiation protection recommendations as applied to the disposal of long-lived radioactive waste. Publication 81. Ann. ICRP 28 (4). Santucci, P., Kontic, B., Coughtrey, P., McKenney, C., Smith, G., 2005. Use of contaminated well water, example reference biospheres 1 and 2A. J. Environ. Radioact. 84, 151–170. Smith, G.M., Merino, J.,, Kozak, M., 2008. Uncertainty and variability in biosphere dose conversion factors for the groundwater release scenario for Yucca Mountain. In: Proc 2008 International High-Level Radioactive Waste Management Conference, Las Vegas, 7–11 September. American Nuclear Society. Smith, K. (Ed.), 2008. Report of an International Forum on Se-79 in the Biosphere Hosted by the Nationale Genossenschaft fu¨r die Lagerung
437
radioaktiver Abfa¨lle. Wettingen, May 2008. Report available at: http:// www.bioprota.com.
Gerald Kirchner Federal Office for Radiation Protection, Germany E-mail address: [email protected] 27 January 2009 Available online 25 February 2009
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Letter to the editor
Use of reference biospheres for proving the long-term safety of radioactive waste repositoriesq
The evolution of human societies, their living conditions and eating habits and their interchange with and impacts upon evolving ecosystems in the far future is highly uncertain. Assumptions required for assessing long-term radiological impacts therefore need to build upon stylised biospheres as recommended for long-term impact assessments of nuclear waste repositories (ICRP, 1998). A methodology for constructing stylised biospheres and associated exposure pathways, called the Reference Biospheres Methodology, has been developed via an international collaboration, under the auspices of the International Atomic Energy Agency (van Dorp et al., 1999; IAEA, 2003; Crossland et al., 2005). According to the German Draft Safety Requirements Governing the Final Disposal of Heat-Generating Radioactive Waste (2008), long-term radiological prognoses shall be performed on the basis of reference biosphere models. The aim of the workshop, therefore, was to get an actual overview of the experience gained with the Reference Biospheres Methodology and to identify priorities for future research and development. In this two-day workshop invited biosphere experts from various European countries (including Belgium, France, Germany, Sweden and the United Kingdom) participated, representing a wide range of approaches to biosphere modelling within long-term safety assessments for geologic nuclear waste repositories.
1. Agenda The workshop agenda covered three major topics: 1. Climate impact on biosphere modelling, 2. Radionuclide transition processes as considered within biosphere models and/or affecting radionuclide transfer from the geosphere to the biosphere, 3. Radiation protection criteria (regulatory prescriptions on reference biosphere models, dose/risk constraints, size of critical groups etc.). In each session, participants gave short presentations summarising their experience and view points. The topics were then openly and extensively discussed. A set of questions detailing the q
Workshop of the German Federal Office for Radiation Protection (BfS), 26–27 August 2008. 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.01.007
topics listed above had been circulated by BfS prior to the workshop and served as a guideline for presentations and discussions. Session 1 (Climate impact on biosphere modelling) specifically addressed the following questions and subjects: Is it necessary to simulate complete climate cycles including transition phases using a dynamic approach each time a reference biosphere is developed, or is it sufficient to consider different climatic stages as stationary and to model them independently? Do present climate models, for example, as presented in BIOCLIM (2004), include features to reproduce complete climate cycles? Is there a specific climate cycle which generally causes the highest exposures and which, therefore, should be selected for impact assessments? Can biosphere modelling be performed without considering climatic impact on the radionuclide transport in the geosphere or is an integral concept generally required? Are tools available to simulate the impact on processes in the geosphere caused by climatic conditions? Is this issue primarily relevant for cold climate states or for other climatic conditions, too? From the discussions the following conclusions and general consensus were derived. Occasional dissent should not automatically negate the value of a general consensus; rather it helps to identify issues, such as specific aspects of regulatory requirements or details of particular wastes and site conditions, which may result in country- or site-specific differences in approach. The ability to recognise and be able to explain when and why things need to be considered differently is as valuable as knowing where consensus is appropriate. 1. Post-closure safety assessment should take into account any major changes in environmental conditions which affect potential radiation exposure, not just climate change. Such changes may need to be included explicitly in the assessment calculations. 2. There is no specific climate that establishes a ‘worst case’ with regard to human exposure, but there is a trend that a warmer and dryer climate leads to higher doses compared to temperate regions, mainly because the need for irrigation increases. 3. In biosphere modelling it is generally assumed that ecosystems are in equilibrium in response to the modelled stationary climate
436
Letter to the editor / Journal of Environmental Radioactivity 100 (2009) 435–437
conditions. Consideration of transition phases between stationary climates, however, could be of importance, as they might result in higher releases, e.g. due to an accumulation and then acute release of radionuclides below an ice shield during a glaciation event. The methodology needed for such sequential climate modelling has been developed (BIOCLIM, 2004), but the phenomenological understanding and model parameterization are still limited. 4. Geosphere/biosphere interface processes are considered important for estimating potential release rates into the biosphere. The influence of climate on the hydrological cycle is a key aspect. As a consequence, the traditional approach of performing geosphere and biosphere simulations separately with an interface, at which radionuclide concentrations resulting from the geosphere model serve as input for the biosphere simulations (typically groundwater or well), should be justified on a case by case basis. 5. The approaches and simulation tools developed within the BIOCLIM project may be a valuable methodology for estimating the impact of climate variations on biosphere processes, but spatial downscaling needs much effort. 6. Site-specific scenarios differ drastically depending on whether glaciation has to be assumed. Session 2 (Radionuclide transport processes within biosphere models and/or relating to transfer from the geosphere to the biosphere) specifically addressed (i) whether knowledge and models exist with regard to radionuclide transition processes between single temporally and/or spatially separated biosphere models and, (ii) which aspects have to be taken into consideration for modelling radionuclide transport from the geosphere into the biosphere. The following conclusions evolved from the discussions: 1. Processes at the biosphere and geosphere transition zone (e.g. groundwater recharge rates) are of major importance since they determine the degree of dilution which affects radionuclide concentrations in deeper groundwaters. Cooperation of experts is needed. As stated above, assumptions on biosphere and climate may have major feedback on geosphere processes. Understanding and simulation of such interactions may be essential for modelling radionuclide mobility during climate transition phases. 2. Site-specific assessments are essential to model the transport processes. 3. Future focus should be directed toward more detailed assessment of the potential influence of environmental changes on groundwater flow and radionuclide transport; emphasis should be on the dynamics of changes and related accumulation processes. 4. There should be a clear understanding of the groundwater flow system so as to define surface outlets of radionuclides. In Session 3 (Radiation protection criteria), the focus was (i) on the experience gained and on open problems with regard to specific reference biosphere models, and (ii) on the specification of the critical groups to be considered. The results can be summarised as follows: 1. Since wells show high variability with regard to water extraction capacity and other properties, it is not sensible to define a single standard well and assume that this is adequate for testing safety. However, the dilution of radionuclides eventually present in groundwater, which is assumed to occur in a well, considerably affects the calculated exposures to members of the critical group. This combination of factors could limit the applicability of commonly used reference biosphere well scenarios (Santucci et al., 2005). In addition, well water extraction may not in all cases be regarded as the most conservative scenario (IAEA, 2003).
2. Information on the influence of climate change on mobility and biotic accumulation of radionuclides in various ecosystems is for some radionuclides, such as Se-79, partially available from the literature (e.g. Smith, 2008), but may be inadequate for others (e.g. Tc-99). 3. Biosphere models should be based on a site-specific prediction of potential future developments. For each of these, the specific biosphere models should balance the generic nature of any longterm prediction with the complexity needed for including all major pathways to man. 4. Selection of the parameters for the hypothetical critical groups (or reference persons) should take into account the distribution of behaviours, consistent with viable existence (including at least at some level of sustainability) in the biosphere system assumed. 5. Simulations should be performed for various age groups. Following the reasoning given by ICRP (1998), it is common practice to estimate the annual dose or risk for adult individuals. For communication with the public, however, it is important to document that other age groups are also considered and adequately protected, as members of the public generally are well aware of the fact that children show higher radiation sensitivity than adults. 6. Probabilistic simulations of potential long-term consequences of nuclear waste repositories have become routine. They usually mix probabilistic and deterministic components within the simulations. For example, dose coefficients are generally assumed to have single values for each chemical form and age group. Excluding parameters, such as dose coefficients, from uncertainty analysis may lead to an underestimation of overall uncertainties and over concern about less impacting parameters. Probabilistic approaches may therefore provide benefits in terms of demonstrating compliance with dose or risk constraints and they may also help to elucidate the degree of pessimism that can be inadvertently included within a purely deterministic approach (Smith et al., 2008). Based on these conclusions, the following priorities for future research are suggested: Radionuclide accumulation and release processes during climate transition phases should be studied systematically and compared to those at stationary climate conditions. Processes in the geosphere/biosphere transition zone should be modelled in more detail with special regard to long-term erosion of soil and rock and to climate induced changes to hydrology. Consideration should be given to understanding the circumstances in which well scenarios are not conservative for estimating doses to humans. Uncertainties in dose coefficients should be taken into account when developing an overall understanding of uncertainty in the post-closure safety assessment.
References BIOCLIM, 2004. Modelling sequential biosphere systems under climate change for radioactive waste disposal. EU-Contract FIKW-CT-2000-00024, information available at: http://www.andra.fr/bioclim. Crossland, I.G., Pinedo, P., Kessler, J.H., Torres-Vidal, C., Walters, B., 2005. ‘‘Reference Biospheres’’ for solid radioactive waste disposal: the BIOMASS methodology. J. Environ. Radioact. 84, 135–149. van Dorp, F., Egan, M., Kessler, J.H., Nilsson, S., Pinedo, P., Smith, G., Torres, C., 1999. Biosphere modelling for the assessment of radioactive waste repositories; the development of a common basis by the BIOMOVS II reference biospheres working group. J. Environ. Radioact. 42, 225–236. IAEA, 2003. ‘‘Reference Biospheres’’ for Solid Radioactive Waste Disposal. Report of BIOMASS Theme 1 of the BIOsphere Modelling and ASSessment (BIOMASS) Programme IAEA-BIOMASS-6. International Atomic Energy Agency Safety, Vienna.
Letter to the editor / Journal of Environmental Radioactivity 100 (2009) 435–437 International Commission on Radiological Protection, 1998. Radiation protection recommendations as applied to the disposal of long-lived radioactive waste. Publication 81. Ann. ICRP 28 (4). Santucci, P., Kontic, B., Coughtrey, P., McKenney, C., Smith, G., 2005. Use of contaminated well water, example reference biospheres 1 and 2A. J. Environ. Radioact. 84, 151–170. Smith, G.M., Merino, J.,, Kozak, M., 2008. Uncertainty and variability in biosphere dose conversion factors for the groundwater release scenario for Yucca Mountain. In: Proc 2008 International High-Level Radioactive Waste Management Conference, Las Vegas, 7–11 September. American Nuclear Society. Smith, K. (Ed.), 2008. Report of an International Forum on Se-79 in the Biosphere Hosted by the Nationale Genossenschaft fu¨r die Lagerung
437
radioaktiver Abfa¨lle. Wettingen, May 2008. Report available at: http:// www.bioprota.com.
Gerald Kirchner Federal Office for Radiation Protection, Germany E-mail address: [email protected] 27 January 2009 Available online 25 February 2009