Nuclear energy is perceived as a solution for global environmental pollution, and alternative future clean energy production technology free from CO2 emission. The nuclear energy production is based upon two nuclear processes i.e. nuclear fusion and fission process. Deuterium is used as a fuel in nuclear fusion power generation method, but its availability has been the major issue as its natural abundance is very low. Deuterium can be obtained from hydrogen isotope mixtures, and currently, it is being separated by high energy intensive, time consuming and not so efficient processes like cryogenic distillation and hydrogen sulfide exchange process. Recently, the Quantum Sieving (QS) exploited separation of deuterium from hydrogen isotope mixture using nanoporous materials has emerged as an efficient and less energy intensive process. For hydrogen isotope separation, various nanoporous materials like Metal Organic Frameworks (MOF), Covalent Organic Frameworks (COF) and Nanoporous carbon based materials have been used. MIL-53(Al) is a well-known flexible MOF material and its pore structure changes from narrow pore (np) to large pore (lp) under the influence of external stimuli such as temperature, pressure and guest molecules, and this phenomenon is known as breathing effect. The MIL-53 can maximize the QS effect as the pore size will be adjusted by the breathing effect. However, the study of the hydrogen isotopes separation using flexible structure lacks an in-depth separation mechanism and requires analysis from a microscopic point of view. Towards this, quasi-elastic scattering experiment was performed to investigate the kinetic behavior of hydrogen and deuterium on the pore structure of MIL-53 (Al), and Time of flight (TOF) was used for QENS analysis.