At the end of 2010 there was still no spent fuel repository in operation for power reactor fuel. The only similar project and operational facility that can provide lessons for spent fuel management facility is the waste isolation pilot plant (WIPP) in the US. This is a repository for transuranic waste of military origin operating in New Mexico.10

The main characteristics that influence spent fuel and high-level waste (HLW) disposal are the content of the long-lived radionuclides in the spent fuel and HLW, their heat generation and radiation levels. Heat generation limits the amount of waste that can be disposed in a given volume of rock. High radiation levels require that all radiation waste handling is shielded and uses remote handling systems. The amount of long-lived radionuclides requires the safety of the repository to be considered for tens of thousands of years. The main design objective of any radioactive material repository is to confine waste and isolate it from the environment. Furthermore, the fundamental task of any repository design, operation and performance is to demonstrate compliance with all applicable laws and regulations. Many of these are developed under the principles and guidelines developed by, or under, the auspices of the International Atomic Energy Agency (IAEA). Disposal facilities for spent nuclear fuel and HLW are typically designed for a deep geological formation with adequate rock characteristics. The reason for this is that adequate long-term safety needs to be provided without reliance on active controls and continuing maintenance of the closed facility. Geological repositories are therefore designed to be passively safe and with the idea that no indefinite institutional control is required to ensure safety. During the operating period of the disposal facility, institutional control will have to be maintained to provide assurances on safety, safeguards and security. One of the key problems with the disposal of nuclear fuel is the long-term radio toxicity of its radionuclides, in particular actinides. Figure 15.13 shows the radiotoxicity of radionuclides in spent fuel over the long time periods relevant to spent fuel disposal. The isolation of nuclear fuel to prevent release into the environment is essential.

The other issue for spent fuel disposal that has to be taken into account in choosing the location and geology of the disposal site and the design of engineered barriers is the decay heat over long periods of time. Figure 15.14 shows decay heat and its contributors over the long periods of time relevant to disposal. The third aspect of spent fuel disposal safety that is to be taken into account in the

design of engineered barriers is contact with water and demonstrating that criticality cannot be reached. Applying a burnup credit that uses realistic depleted concentrations of fissile radionuclides in the design of the repository can significantly reduce the cost of the repository and the transportation of spent fuel to the repository.

In any spent fuel disposal project there are universal repository programme constituents10 that need to be prepared:

• legal and regulatory framework

• organizational structure

• siting approach

• design concept

The following are basic technical principles that are applied for all well-developed disposal concepts:

• Encapsulation of spent fuel (or HLW) in a tight canister with a very long expected lifetime.

• Assurances that the conditions in the repository will allow the canister to remain intact and tight for as long as possible (such conditions include, for example, mechanical stability, stable geochemical conditions and very limited ground water movement, which could bring corrosive agents into contact with the canisters).

• Backfilling the repository with appropriate materials and locating it in geological media that, together with the backfill, strongly limit water movement and eventually waste movement when the integrity of the canisters finally breaks down.

An example of a spent fuel repository demonstrating all these principles is shown in Fig. 15.15, which is a Canadian project for spent fuel disposal managed by the Nuclear Waste Management Organization (NWMO) in Canada and which is also very similar to concepts in other countries.