Hesketh, P, Tillotson, S & Watts, R 2023, 'Using uncertainty analysis to unravel cost risks for nuclear and mine site closure', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, https://doi.org/10.36487/ACG_repo/2315_084 (https://papers.acg.uwa.edu.au/p/2315_084_Hesketh/) Abstract: Closure programs in both the mining and nuclear sectors can run into the hundreds of millions to billions of dollars and are prone to large increases as planning develops, which give rise to budgetary and governance concerns. These increases in cost are often the result of not sufficiently understanding the uncertainty surrounding different activities. The uncertainty is often hidden and derives from a lack of knowledge, assumptions based on a weak knowledge base, an optimism bias (particularly relating to long tail items) and/or the enhanced scope of closure. Cost risk analysis can be used to unravel where uncertainty lies, with the majority of the cost range often being concentrated within a disproportionately small number of items. In understanding which activities provide the greatest levels of uncertainty subsequent studies can be focused to narrow the uncertainty (and cost range) providing better outcomes. Keywords: cost, risk, waste management, planning, studies programmes, knowledge base