An analytical procedure is presented for evaluating and correlating various heat transfer and economic parameters in order to optimize core size and configuration in the design of heterogeneous reactors. Maximum wall temperature calculations are used to determine a large range of reactor operating conditions with core size and shape as parameters. Information gained from crossplotting such information may then be used to estimate the sensitivity of the cost of the reactor system to some operating variable such as coolant flow rate, again with core size as a parameter. This information, coupled with fuel cost data as determined from nuclear calculations, can then be used to select an optimum core size and optimum operating conditions for a given system.