With the reactor at full gas flow, final trimming to full power commences. A comprehensive survey of operational parameters is carried out to ensure no abnormalities. Some temperature trimming on regu­lating rods is carried out to achieve a level tempera­ture distribution at the reactor outlet for optimum boiler performance. A detailed assessment of reactor power and temperatures is carried out to determine the margins to operating limits, temperatures can then be raised as permitted by the margins. It will be two days before the xenon concentration reaches its equi­librium level, and throughout this period the bulk rods will be inched out to maintain criticality. Tem­perature is limited to a few degrees below the normal operating limit by the deep bulk rod insertion asso­ciated with low xenon concentration, also it takes some time to achieve optimum operating conditions because of the several inter-related factors to be op­timised, so full power will not be achieved for up to two days.