The present work analyzes the thermal-hydraulic behavior of the CANDU supercritical water reactor (SCWR) using a 1-D numerical model. The possibility of a static instability, the Ledinegg excursion, is investigated, which reveals it can occur only in a hypothetical condition, far from the proposed operating regime of the CANDU SCWR. The investigation demonstrates the possibility of density wave oscillations (DWOs), a dynamic instability, in the operating regime of the CANDU SCWR and its marginal stability boundary (MSB) is obtained. The phenomenon of the deterioration in heat transfer is observed, and the related investigation shows that the strong buoyancy effect is responsible for its appearance inside the heating section of the channel of the CANDU SCWR core. The MSB is found to be inadequate in determining the safe operating zone of the reactor because the wall temperature can exceed the allowable limit from metallurgical consideration. The investigations also determine the safe as well as stable zone where the CANDU SCWR should operate in order to avoid the maximum temperature limit and DWOs.