The Advanced Test Reactor (ATR) is a 250-MW irradiation facility used to test reactor fuels and other materials, and also to produce radioisotopes. The ATR core is divided into five regions, or lobes, that normally operate at different power levels. To support future irradiation programs, it is desired that the maximum lobe power be increased 10% (from 60 to 66 MW). A modification to ATR's emergency core cooling system is proposed to ensure that adequate safety margins would be maintained during a loss-of-coolant accident (LOCA). The modification being considered is the addition of an accumulator injection system. The RELAP5 thermal-hydraulic code and the SINDA thermal analyzer were used to simulate the two most challenging design-basis LOCAs identified in the ATR Safety Analysis Report. Calculations were performed both with and without accumulator injection. The results indicate that a 10% increase in maximum lobe power is achievable. Minimum thermal margins increased more than 40% when accumulator injection was simulated.