Fuel cost accounting for AGRs is performed within the ADOS suite by a specially provided computer pro­gram which conducts its calculations on a channel — by-channel basis. At regular intervals and for each channel, the cost of the heat generated is computed from a knowledge of the ‘fuel worth’ at the time (initially set to the replacement fuel cost at the time of loading to the reactor), the irradiation accrued within the period and the proportion of stringer life remaining before discharge. The total reactor heat cost is obtained by summating the individual channel fig­ures. In this way the program enables precise records of reactor and individual channel heat costs to be maintained within the ADOS suite.