We adopt and apply a tiered risk assessment approach in which the detail of the assessment increases with the complexity of the problem. This is required in UK regulations, and we have emphasised the idea in guidance written for the UK’s Nuclear Decommissioning Authority. The approach recognises that it is important that the models need to be fit for purpose in their level of detail. In some cases, simple models – such as those developed by Quintessa for regulators – can be appropriate. For more complex problems, Quintessa can draw on substantial expertise in detailed modelling of aspects like environmental change, groundwater transport and chemistry. We have developed complex “whole site” models for clients when required.