The Overview Report describes the philosophy behind QPAC, including the novel ‘model as input’ approach which allows users to define the exact processes to be included, and gives examples of how QPAC has been applied by Quintessa to complex problems in the fields of energy and the environment.  For example, QPAC has been used to model gas flow through graphite bricks in nuclear reactor cores; the sedimentation, compaction and geochemical transformation of clays in natural environments; the impact on ecosystems of naturally-occurring fluxes of CO₂ from deep sources to the surface; and the thermal, hydrological, mechanical and chemical processes involved in resaturation of underground disposal facilities for nuclear waste.  All of these applications and more are described in the QPAC Overview Report.