This paper considers and classifies, as far as possible, the various types of 'unknown' faced during the production of a PCSR. Given that the scope of the unknowns is far greater than that of the knowns at the start of the project, the paper considers some of the techniques used to proceed with a safety justification. The last word goes to Rumsfeld (from his autobiography): "Those who made the decisions with imperfect knowledge will be judged in hindsight by those with considerably more information at their disposal and time for reflection."
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