Thermal hydraulic and mechanical modelling of the full-scale in situ test (FISST)

Abstract

In 2018, Posiva Oy started the Full-Scale in Situ System Test (FISST) within the ONKALO® underground facility. FISST is a full-scale application of the KBS-3 V reference disposal concept that involves the back section (approximately 50 m) of Demonstration Tunnel 2 (DT2) and its two experimental deposition holes. Two key objectives were identified in designing the test. The first one involved the demonstration that the KBS-3 V concept can be implemented safely and according to plan, whereas the second aimed at the provision of a full-scale simulation of the emplacement of high-heat-generating waste. This work focuses on the simulation of hydraulic, thermal and mechanical (THM) evolution of the FISST experiment. It describes the development of a holistic numerical methodology that integrates the hydrogeological description of the geosphere achieved through discrete fracture network (DFN) modelling, into a continuum THM modelling framework. Further, it provides an evaluation of the predictive capability of the models against available sensor data that capture the early phase of the test.