The authors have studied the dynamical features of limit cycle oscillations in a nonlinear reactor system equation with feedback time delay in both control system and temperature effect by taking account of the experience in the pulsed reactor NSRR before its recent modification.The features of competition between the two delayed feedback effects are analyzed. A theoretical method for predicting analytically the mean amplitude of limit cycle oscillation is also developed by combining the method of Krylov-Bogoliubov and that of nonlinear transformation. Further, the effects of colored external noise on the profiles of power oscillations is investigated from the viewpoints of "global stability" and "non-linear response".Their new findings are summarized as follows : (1) Without an external noise, chaotic oscillations can arise when the product of the applied reactivity and the delay-time is very large. However, it does not appear when this product term takes a small value. (2) On the other hand, near the first Hopf bifurcation under the influence of external noise, the limit cycle oscillation becomes chaotic one as a superposition of quasi-periodic oscillations, which can be identified by the stochastic Van der Pol equation. The sinusoidal limit cycle oscillation of reactor power observed in the NSRR has such a stochastic nature rather than kinematic one.