Since the AGR fuel element contains both enriched uranium as well as an amount of integral moderator, in theory it is just possible that an uncontrolled mix of fuel and moderator could result in accidental criticality. Because of this, special fuel reactivity cal­culations are performed in which the potential for criticality in all the various circumstances which could prevail during transport and storage of AGR fuel is assessed. These studies, known as ‘Criticality Safety Assessments’, are deliberately pessimised to remove any doubts about the adequacy of the assumptions made, by assuming, for example, artificially high fuel enrichments together with the worst possible configu­rations of elements in storage, and further allowances are made for unusal conditions such as flooding of the fuel stores. Each part of the fuel route is sub­jected to a Criticality Safety Assessment so that suit­able administrative guidelines and controls can be provided for the safe Spelling and storage of the fuel. Safety of the fuel in transport is also appraised in a similar manner and the end-product of all such assessments is the setting of safe working standards. Good housekeeping is of the utmost importance since extraneous moderating materials could be placed ad­jacent to the fuel store. Graphite, in the form of broken sleeves, as well as protective polythene bags and new fuel box packing materials, which are known to contain hydrogen (an excellent moderator), are the most obvious examples. It has been demonstrated that the small quantities of polythene bags and packing materials present are insufficient to cause a criticality hazard, but it is nevertheless considered essential that such materials should not be allowed to accumulate unnecessarily.