Next generation pool-type power plants will require advanced control techniques to meet operational and safety goals. This is the conclusion after conducting control experiments in the Experimental Breeder Reactor II (EBR-II) to demonstrate a supervisory so-called passive control scheme. The proportional-integral-derivative controller in EBR-II did not adequately compensate for disturbances to inlet temperature that occurred during normal power changes. The key to better control is to take into account the stratification and energy interchange mechanisms in the primary pool through which these disturbances feed. A model-based control approach for solving this problem is described. A lumped parameter model of the EBR-II primary pool is developed and validated using experimental data. A disturbance rejection method is then used to design a controller that minimizes the effect of those disturbances that cause poor setpoint tracking. The implementation of the controller and results are given in a companion paper.