An exact solution based on the explicit formulation of the optimal time-displacement operator by using the confluent form of the Sylvester theorem is presented for synthesizing suboptimal control of nuclear reactors with spatially distributed parameters. The inverse of the neutron velocity is regarded as a small singular parameter, and the model adopted for simplicity is a cylindrically symmetrical reactor. The Helmholtz mode expansion is used for the application of the optimal theory for lumped parameter systems to the spatially distributed parameter systems. A numerical example is given showing the expedience of the present method.