I think that this is an informative graphic. It comes from a presentation about risk based ALARP (As Low As Reasonable Foreseeable) by David Forsythe, Head of Nuclear Safety, Decommissioning from Sellafield at the 2016 SRP Conference.
He discussed the dilemma posed by legacy facilities that are below modern standards of safety, where several facilities are interdependent and for which decommissioning poses additional risks. Do you “keep the plane on the ground” where you believe risks are lower or do you decommission? (You can gain some impression of the difficulties posed by the Sellafield site from this NDA Document.)
The blue line in the diagram above (taking from the presentation with permission) shows the development of risk with respect to time for a well designed decommissioning project. It accepts that risk may rise as the work gets underway but will then drop below the initial value as the work progresses. The art is to minimise the area under the curve and steer clear of yellow, orange or red regions. Prevarication will lead to risks not being reduced and probably increasing with time as the systems degrade. This leads to, at best, an unacceptable time at risk (Region C) or, at worst, an eventual failure of the system or reaching a position from which the overall programme cannot be delivered (Region E).
David explained that the processes that manage delivery and the processes that manage the upper safety boundary need to balance to deliver ALARP and secondly that the ALARP “landing plan” dictates what “safe enough” looks like for the components of the plan.