Issues affecting the assessment of impacts of disposal of radioactive and hazardous wastes

James Wilson (Quintessa Ltd.) presented.

The work presented was undertaken within a BIOPROTA project in 2017. BIOPROTA is an

international forum that aims to resolve key issues in biosphere assessments for radioactive waste disposal. The project was therefore driven by radioactive waste disposal assessment needs, but there are many parallels for legacy sites.

The project was supported both technically and financially by a wide range of organizations from Norway, Finland, Japan, Canada, Spain, Sweden and the UK. A project report has been completed

and is to be made available at www.bioprota.org and issued as an NRPA report (NRPA, 2018 in prep.).

A large body of scientific and regulatory guidance materials were reviewed in the project. Key findings from the project are summarized below.

 It is clear that for many waste management organizations, non-radioactive substances present in radioactive wastes can pose a significant challenge, but good inventory data are often lacking for these substances.

 The release and transport of radionuclides and chemical contaminants should be modelled taking account of the same features, events and processes (as far as is possible).

 It is appropriate to assess exposures of humans to ionising radiations in terms of effective dose, and to assess exposures to chemical pollutants in terms of intake rates by ingestion or air concentrations. However, it is important to recognise that these are intermediate measures of impact and they both need to be related to potential health effects in order to achieve a balanced decision.

 There are a wide range of adverse health effects associated with different chemotoxic substances that occur via a range of mechanisms. Therefore, simple index quantities (such as weighted total exposures) cannot be recommended for application across wide ranges of chemicals or between chemicals and ionising radiations. However, there are contexts in which index quantities can be useful, notably for chemicals such as dioxins and dioxin-like compounds, where Toxic Equivalency Factors (TEFs) may be applied.

 It is difficult to assess the effects arising as a result of exposure to a variety of toxic agents, such as synergistic effects, although some qualitative work has been done on this topic by Radioactive Waste Management Ltd in the UK.

 Whilst standards for protection are available for ionizing radiations and chemical pollutants that are generally based on precautionary approaches, they are not applied consistently, which makes comparison of the effects of different stressors or the overall impact of multiple stressors difficult.

A number of potential ‘priority’ chemicals have been identified that could be the focus of future work, as detailed in under Table 4-2. For substances such as chromium, it is necessary to consider the form since some forms are more toxic than others (Cr (VI), for example, is of particular concern due to its carcinogenicity). Uranium compounds are both chemotoxic and radiotoxic.

TABLE 4-2.POTENTIAL ‘PRIORITY’ CHEMICALS.

Substance Key Toxicological Properties (chronic effects)

Arsenic (As) Dermal effects, vascular effects (‘black foot’ disease) and

carcinogenic properties are generally of concern for environmental exposures.

Beryllium (Be) Inhalation associated with chronic beryllium disease, lung cancer.

Cadmium (Cd) Nephrotoxin, may cause bone disease

Chromium (Cr) Hexavalent chromium (Cr{VI}) is of greater concern than trivalent Cr(III). Importance of inhalation pathway

Mercury (Hg) Neurotoxin, nephrotoxin, developmental effects

Lead (Pb) Neurotoxin; exposure may also lead to cardiovascular effects; also

‘probably carcinogenic’

Uranium (U) Nephrotoxin and radioactive

Polycyclic Aromatic Hydrocarbons (PAH) Group of carcinogenic compounds, Benzo(a)pyrene has been studied extensively. Carcinogen, mutagen and reproductive toxin

Asbestos Mesothelioma (+lung cancer)

Key aspects that need to be considered in risk assessments were reviewed. For chemicals, the main exposure pathways are oral ingestion or inhalation. Dermal exposure may also be important, but this can be harder to assess quantitatively. For radiological assessments, these same pathways are important, but external irradiation is also considered.

The bioavailability and bio-accessibility of chemicals are important considerations governing uptake, but risk assessments tend to rely on total intakes, although it is becoming more commonplace to undertake biokinetic modelling.

A key difference between chemicals and radiation is that chemotoxic substances may have dose-response curves that exhibit thresholds at environmental exposure levels. In contrast, for radionuclides it is non-threshold effects that are of primary interest in assessing and mitigating environmental exposures. For threshold effects, a level of exposure can be determined, above which there may be the potential for adverse physiological effects. This allows tolerable daily intake (TDI) values to be determined for chemotoxic substances, often on the basis of no observable adverse effect level (NOAEL) or lowest observable adverse effect level (LOEL) data.

Such data are often only available from animal studies and therefore require the application of uncertainty factors to account for inter-species variation. Uncertainty factors are also applied to account for intra-species variation and deficiencies in source data. Uncertainty factors may also need to be applied to data from human populations, but the overall uncertainty in setting a TDI may be lower than that associated with using data from animal studies. It should be noted that intakes of a chemical above its TDI do not necessarily guarantee that appreciable health effects will be observed within a human population.

Non-threshold effects are largely determined through cancer risk models based on animal data and uncertainties can be significant due to high to low dose extrapolation and interspecies variation.

Typical intakes used for setting standards and guidelines have an associated excess lifetime cancer risk of ~10^-4 to ~10^-6. For substances with non-threshold effects (for a given exposure pathway), the objective is to minimize exposures, and the principle of ALARP applies (i.e. efforts should be made to ensure exposures are “as low as reasonably practicable”). In recent years, there has been a move towards using benchmark dose modelling for setting limits on intakes of chemotoxic substances.

There have been a number of recent case studies on assessing risks posed by chemotoxic substances associated with radioactive materials. An example is the illustrative assessment of human health risks arising from the potential release of chemotoxic substances from a generic geological disposal facility for radioactive waste reported by Wilson et al (2011).

The key conclusions from the BIOPROTA project are that there is a continuing need to move towards some common measure of hazard that supports identification of risk management priorities for mixed hazardous waste and, while the overall picture, including the different regulatory contexts, remains complex, the non-radiologically hazardous components of many radioactive wastes appear to relate to relatively few elements and materials that are already reasonably well understood (such as U, Pb, Cd, and asbestos). Progress would, therefore, be most effective if it focusses on a limited set of hazardous components, especially for the relatively large volumes of LLW and very LLW arising in the decommissioning of nuclear facilities and remediation of legacy sites.