Multibarrier system

Geological disposal of high-level radioactive waste (HLW) in Japan is based on a multibarrier system composed of engineered and natural barriers. The engineered barriers are composed of vitrified waste encapsulated in a canister, metal overpack and buffer material. Highly compacted bentonite is considered as the candidate buffer material, mainly because of its low hydraulic conductivity and high sorption capacity of radionuclides. 

In Sweden, a repository design of KBS-3 system has been develop (SKB, 1999). The KBS-3 is a multibarrier system to isolate the spent nuclear fuel. The spent nuclear fuel is placed in corrosion-resistant 5-m long copper canisters. Each of the canisters is surrounded by an engineered barrier system (EBS) of bentonite clay in separate deposition holes excavated along tunnels in... 

Figure 7.7 shows the plots of m (x) vs x for the resonant state with n = 17 of the above multibarrier system. Figure 7.7a refers to the symmetric... 

Figure 7.7 Plot of Un(x)p in arbitrary units, for n - 17, along the internal interaction region of the multibarrier system considered in Figure 7.6. Figure (a) refers to the periodic case, whereas Figure (b) corresponds to the disordered system, exhibiting quasi-localization of the resonant state. See text.

Most theoretical treatments of decay consider the time evolution of an initial state tunneling out of single well [81-85]. However, the present-day possibility of designing the potential parameters of artificial quantum systems [61, 86], opens the way to study the issue of decay in more complex potential profiles as exemplified by semiconductor multibarrier systems of finite length which are formed by a succession of alternating barriers and wells [57] and other artificial multibarrier structures as ultracold atomic gases in optical lattices [86]. 

G. Garcia-Calderon, R. Romo, J. ViUavicencio, Internal dynamics of multibarrier systems for pulsed quantum decay, Phys. Rev. A 79 (2009) 052121. 

A. R. Boccaccini, J. Janczak, D. M. R. Taplin and M. Kopf, The Multibarriers-System as a Materials Science Approach for Industrial Waste Disposal and Recycling Application of Gradient and Multilayered Microstructures, Environmental Technology 17, 1193-1203 (1996). 

Physically, (138) is obvious. That is a uniform multibarrier system attains the steady state at t - oo. By substituting the solution (137c) into (la) it is easily checked that (137c) is the desired solution. A somewhat simpler expression of (137c) is possible the use of (116) leads to... 

The principle of defence in depth is implemented primarily by means of a series of barriers which should in principle never be jeopardised, and which must therefore be violated in turn before harm can occur to people or the environment. These barriers are physical, confining the radioactive material at successive locations. The barriers may serve operational and safety purposes, or may serve safety purposes only. Power generation is only allowed if this multibarrier system is not jeopardised and is capable of functioning as designed. 

The multibarrier system protects humans and the environment in a wide range of abnormal conditions. Planned countermeasures are provided, as a further component of defence in depth, against the possibility that radioactive material might still be released from the plant. 

The isolation and safety functions of HLNW deep geological repositories are based upon the multibarrier concept, where a number of containment and isolation barriers are put in place. A schematic view of the multibarrier HLNW concept is given in Fig. 1. The main barriers of the system are the waste matrix itself a metallic container (either corrosion resistant like Cu or Ti, or based upon stainless steel) a buffer material (normally bentonite) and finally the host rock itself (essentially granite or clay, although salt domes are also being considered). 

In order to exemplify the above results we calculate the transmission coefficient vs energy in multibarrier resonant tunneling structures which may be modeled by a one-dimensional system formed by alternating N-1-1 rectangular barriers with N rectangular wells. We assume that the electrons possess the same effective mass through the system and that the tunneling process is coherent i.e., elastic [61]. 

The long term safety of high level radioactive waste disposal shall be based on the multibarrier concept, and shall be assessed on the basis of the performance of the disposal system as a whole. 

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