This paper examined the control rod drop-failure in nuclear power plants. Safety margin test was conducted on some typical water-cooled reactor design (WCRD) models at an accident situation, secondly safety margin test was carried out on the thermal efficiency and thermal power output of the reactor when power supply failed and thirdly, safety margin test was perform on the reactor in relation to the high temperature within reactor core and the fuel temperature. The results of the statistical analysis on these types of nuclear reactor models reveals that the typical water-cooled reactor design (WCRD) models promises most stability under thermal efficiency of 45% and above. Meanwhile, at anything below 45% thermal efficiency the fuel element seems to be unstable in the reactor as the regression plot could not find it optimal. At this point the fuel temperature seems at maximum, the reactor agrees to be stable as the regression plot was at the best fit, that is the least squares method finds its optimum when the sum, S, of squared residuals became minimal. Safety margin prediction of 4.42% was validated for a typical WCRD model as an advantage over the current 5.1% challenging problem for plant engineers to predict the safety margin limit.