A study on timeframe definition and reference evolution of the geological system for safety assessment. Case study on the Horonobe URL site

Describing system evolution is an important task in safety assessment and site investigation strategies. This paper presents the method developed to describe system evolution using a hybrid approach incorporating safety functions and FEPs and its application in a case study of the geological evolution of the Horonobe URL site. Based on a site descriptive model and newly developed Horonobe-specific FEPs, important events and processes were placed in a timeline along with the main safety functions of the engineered and natural barriers. Based on the events and processes, we defined a thermal and re-saturation phase, a climatic phase and a geological evolution phase. In the climatic phase, the safety function of retardation in the deeper part of the Wakkanai Formation is important, therefore, advection, dispersion and sorption in the domain should be assessed based on a nuclide migration scenario. In the geological evolution phase, on the other hand, uplift and erosion are important events. Thus, uplift, erosion and the consequent thermal, hydraulic, mechanical and chemical processes are involved in the reference evolution of the geological system. Through the application and associated discussion, the hybrid method, comprising the FEPs and the timeframe of safety functions, has been found to be applicable to other sites, albeit, with minor modifications. (author)