Multi-barrier concept

Deep geological disposal concepts, in particular for alpha bearing (low or medium level) waste and heat generating high level waste, were developed mainly based on the multi-barrier concept. In this concept, the protection of humans and the environment should be guaranteed by an optimal combination of a number of barriers consisting of the waste matrix, container, buffer and backfill material, the repository structures and the geological environment. 

The geological barrier plays the main role in the multi-barrier concept, in particular where it concerns the long term. Various geological formations are under consideration such as clay, salt, crystalline rocks, tuff, etc. The enqjhasis put in the various national programmes on >ecific rock formations depends on the presence of one or another formation at the national territories [4]. Therefore, there is no best or second best rock formation but each of them has its own advantages and disadvantages. 

Ciurent deep repository projects are based on a multi-barrier concept aiming at ensuring the containment of radioactive substances ... 

The overall safety of a deep repository system relies on a multi-barrier concept including the geological barrier. 

The Concept of defence-in-depth is applied in designing safety systems to achieve functional diversity, i.e. by providing diversely functioning systems that can perform same safety function (e.g. two shutdown systems), multi barriers to prevent release of radioactivity, multi-defence system, using physical separation of systems and components which serve as back-up (in safety functions) to each other, and procuring components for different systems from different suppliers, to the extent possible. Such an approach leads to a design of safety systems which will be tolerant to a wide range of human errors and equipment failures. 

Proposals for the geological disposal of heat emitting high-level radioactive wastes (HLW) have been put forward by many countries including Japan, Canada, Sweden, Switzeriand, USA, Spain and others. The disposal concepts invariably involve underground multi-barrier schemes where bentonite clay is chosen for a number of desirable attributes including its swelling potential and ability to trap the majority of released radionuclides (JNC 1999 Chapman and McCombie 2003). 

Values of barrier heights vary widely between chemical reactions. They control how rapidly reactions take place and how their rates respond to changes in temperature. (Most reactions involve a number of steps and the activation energy may not correspond to the height of any individual activation energy barrier. It is a concept best applied to the individual elementary steps of a multi-step reaction.)... 

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