For the safe disposal of high-level nuclear waste (HLW), it is required to understand the mechanism of Thermal-Hydrological-Mechanical-Chemical (T-H-M-C) coupling in the area nearby the deposition holes with or without buffer material.

Due to the long term thermal inputs generated by decay heat from the spent fuel, the development and implementation of coupled mechanisms in HLW project may not be neglected since the failure mechanisms of the waste package are all interrelated in terms of T, H, M, and C.

The total systems approach based on N2 diagram (or Rock Engineering System, Hudson, 1992) may be considered to identify the coupled interactions to be incorporated in the modeling of near field environment of the emplaced HLW canister. The N2 diagram or interaction matrix has been frequently used in the field of systems engineering to help identify the interactions or interfaces between major factors from systems perspective.

The N x N interaction matrix consists of N leading diagonal components and N²-N off-diagonal components. The leading diagonal components, A_ii, represent the parameter of the system to be investigated. The off-diagonal component, A_ij, represents the interface (or impact) of the parameter A_ii on A_ij and sometimes called as the one-way interaction component. The interaction matrix is not usually symmetric since interaction Aij is not always same as A_ji under the normal situation. Once the diagonal parameters, A_ii, are set, then the N2 diagram forces us to consider each interaction component, A_ij, to complete the matrix. Therefore, the interaction matrix can be used as a thinking tool to construct a system using parameters and interactions between those parameters.