Regulatory Requirements

Introduction

The previous sections should have indicated that the deep disposal of radioactive wastes is a complex process, with various issues relating to future human safety that need to be taken into account. In the next article we will examine some of the requirements of the safety cases that need to be developed, to show that consequences to human populations in the future remain at acceptable levels following the construction, operation and closure of a deep repository. To structure the development of the safety cases, a number of regulations have been developed. These set out the legal and other requirements that need to be fulfilled before permission to construct a repository can be given.

In the UK, The various environment agencies are responsible for ensuring that the repository safety case meets the required regulatory standards. These standards are set out in a report known as the Guidance on Requirements for Authorisation (known colloquially as the GRA) [1]. This guidance is currently being revised, and a consultation draft was published for comment and review in May 2008.

Guiding Principles

The GRA runs to about 80 pages in length, and the new draft regulatory guidance runs to over one hundred pages. However, at the heart of the GRA are four basic principles that need to be adhered to when developing a repository concept and the associated safety cases. These guiding principles are set out below.

Principle 1: Independence of safety from controls:

Following the disposal of radioactive waste, the closure of the disposal facility and the withdrawal of controls, the continued isolation of the waste from the accessible environment shall not depend on actions by future generations to maintain integrity of the disposal system.

This principle reflects the ethical position that the generation who obtained the benefit from the nuclear materials is responsible for their safe disposal, for both the present and future generations. In particular, the degree of isolation of the wastes from the accessible environment should be sufficient that future generations do not need to engage in programmes of monitoring, surveillance, preventative or remedial actions or other intervention measures to ensure their continued safety.

Principle 2: Effects in the far future:

Radioactive wastes shall be managed in such a way that predicted impacts on the health of future generations will not be greater than relevant levels of impact that are acceptable today.

A deep geological repository is capable of providing a period of isolation that will last for many thousands of years. However, it is inevitable that natural or anthropogenic processes will result in releases of radioactivity to the accessible environment, at some stage in the future. This principle requires that the radiological detriment to future communities, arising from these releases of radioactivity, should not be greater than levels that are embodied in dose and risk limits prescribed at the present time. Risk limits and targets are covered in Principle 4.

Principle 3: Optimisation (as low as reasonably achievable):

The radiological detriment to members of the public that may result from the disposal of radioactive waste shall be as low as reasonably achievable, economic and social factors being taken into account.

The aim of optimisation studies is to ensure that the design and construction of the repository is such that radiological and other detriments have been reduced to the lowest levels achievable, without incurring excessive costs. The implication is that further reductions can only be achieved by incurring disproportionate additional expenditure. However, due consideration must be given to the dose and risk limits and targets that apply. For example, it is unlikely that a satisfactory safety case can be developed if the repository is shown to be optimised, but the radiological detriments exceed the relevant dose and risk limits.

Principle 4: Radiological protection standards:

After control is withdrawn, the assessed radiological risk [of death or serious health effect] from the facility to a representative member of the potentially exposed group at greatest risk should be consistent with a risk target of 10^-6 per year (i.e. 1 in a million per year).

This principle provides the main qualitative measure against which future human health impacts will be judged, for a particular repository concept. Safety and performance assessments (see next article) are used to make estimates of radiological risk, which can then be compared with this target. An interesting question arises if the safety and performance assessments indicate that risks exceed this target. The GRA makes the following comments:

For risks larger than 10^-6, the developer shall demonstrate that the design is optimised, so that any changes that might reasonably be instituted to enhance the performance would lead to increases in expenditure disproportionate to the reductions in risk.

For risks below the target, the regulator will look for sound science and good engineering practice as being applied to design, construction, operation and closure.

It is not anticipated that risks greater than 10^-5 will be considered acceptable.

The risk value of 10^-5 is a risk limit, and as noted under Principle 3, exceeding this limit is unlikely to be acceptable, whether the repository design is optimised or not.

Additional Guidance

The regulations in the GRA cover many additional aspects, many of which can be safely skipped in a brief survey. However, it does provide useful guidance on two important aspects of deep repository development and safety analysis, namely uncertainty and future human behaviours (which are important in determining how human radiation exposures could occur after the repository is closed).

In this context, the term "uncertainty" refers to our ignorance, or lack of knowledge, about the repository system that we are considering in safety assessments. Because of the timescales involved and the complexity of the processes that operate in a repository system, uncertainty is an unavoidable aspect that needs to be addressed in determining repository safety.

There are four principal types of uncertainty:

1. Uncertainty in the future evolution of the repository system and its environment;

2. Uncertainty in the behaviour of future human populations;

3. Uncertainty in the form of the conceptual and mathematical models of the repository system and its environment, which are used to produce estimates of radiological dose and risk;

4. Uncertainty in the parameters and data that are used in the mathematical models.

In the context of uncertainty in human actions, the problem is that they are largely unpredictable and yet can have a significant impact on the health effects arising from a repository disposal system or region of contaminated land. For example, in the future people may drill or excavate in the region of the repository environment. Human activity may also change the landscape around the repository environment and changes in habits may affect the radiological impact of the repository wastes on future generations.

One means of dealing with uncertainty in human habits and behaviour is to adopt a stylised approach, in which it is assumed that possible future human behaviours are restricted to those that are observed at the present time, or which have been observed in the past. This circumvents the problem of trying to make predictions about future human behaviour, or technological developments that could be beneficial (e.g. finding a cure for cancer). This is the approach advocated in the GRA.

References

[1] Environment Agency, Scottish Environmental Protection Agency and Department of the Environment for Northern Ireland, Disposal facilities on Land for Low and Intermediate Level radioactive wastes: Guidance on Requirements for Authorisation (Radioactive Substances Act 1993), HMSO, London, 1997.

Next: Performance Assessments