If a nuclear reactor is abruptly shut down, extremely large amounts of poison accumulate, which makes it impossible to restart the reactor for about two days or more after shut-down. Hence it is desired that the duration in which the reactor cannot restart is as short as possible. For this purpose, the problem of optimally controlling Xenon poison in the reactor for a given control duration, is considered. This problem is approached by a direct numerical technique, avoiding the conventional difficulties of nonlinear two point boundary value problem. The computational results are presented and compared with the previous ones.