1 _____________________________________________________ INVESTIGATING THE EFFECT OF LOSS-OF-COOLANT ON THE STABILITY OF WATER-COOLED REACTOR DESIGN MODELS A.I. Oludare 1 , M.N. Agu 2 , P.O. Akusu 3 , O. E. Omolara 4 , A.M. Umar 5, L.N. Okafor 6 and S.O. Adedayo 7 1 Nigerian Defence Academy, Department of Physics, Kaduna 2 Nigeria Atomic Energy Commission, Abuja 3 Nigeria Atomic Energy Commission, Abuja 4 Ahmadu Bello University, Department of Mathematics, Zaria, Nigeria 5 Energy Commission of Nigeria, Department of Nuclear Science & Technology, Abuja 6 Nigerian Defence Academy, Department of Mathematics and Computer Science, Kaduna 7 National Open University of Nigeria, Department of Information and Technology, Abuja _________________________________________________________________ ABSTRACT Loss-of-coolant in the operating reactor usually causes instability in the reactor. To investigate the cooling problem of the reactor, two tests were conducted on safety margin. The first was applied to design temperature and design coolant flow rates while, the second was applied to design temperature and design volume of the fuel in the reactor core. The tests were carried out on some typical Water-Cooled Reactor Design (WCRD) models using Linear Regression Analysis Techniques. The results of the statistical analyses on these types of nuclear reactor models reveal that the WCRD models promises stability under application of small size of uranium (fuel) at 9g and below than large size of uranium (fuel) at 12g and above. Meanwhile, at 9g of fuel element the reactor seems to be most stable and safer as the regression plot was optimized. The safety margin prediction of up to 1.25% 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. The implication of this research effort to Nigeria‟s nuclear power project development. Keywords: Linear Regression Analysis, Water-Cooled Reactor Design Model with Normal Pressure, Water-Cooled Reactor Design Model with Abnormal Pressure, Safety Factor, Ỳ, Optimization, Stability Margin in Nuclear Power Reactor Designs Corresponding author: email: isaac_abiodun@yahoo.com INTRODUCTION The unsteady state of the coolant can affect the reactor stability since the rate of coolant flow is either low or high nor abnormal, the reactor may be overheated-up and hydrogen could built-up within reactor core of nuclear power plant while the steady state of the coolant will provide stability in the reactor that will minimize heat generation in the reactor core and also disallow fuel melting that may produce decay heat in the core assemblies which may degenerated to hydrogen built-up that can make reactor to melt or fail. As identified in the case of Fukushima Daiichi Nuclear accident March 2011, Reuters reported that Fukushima Daiichi Unit 3 has lost cooling capability and may be experiencing melting of the core, eventually, reactor 1- 4 was written-off. The overheating of the reactor core could lead to temperature rise and gradual pressure built-up in the system. Removal of residual heat could not be assured in the case of Fukushima Daiichi nuclear plant accident in Japan, as reported to Reuter on 11 th March 2011 by Mark Hibbs, a nuclear expert at the Carnegie endowment for international peace. The cooling of reactor core could not be assured in the case of Fukushima Daiichi nuclear plant accident. Seawater was being pumped into the reactor in an attempt to cool down the radioactive core. A catastrophic built-up of hydrogen gas inside the building that housed the reactor sparked an explosion, destroying the structure and throwing radioactive debris into the air. Also, this phenomenon of malfunctioning of cooling system has led to the Three Mile Island Nuclear Power Plant accident near Harrisburg, Pennsylvania in USA, in which TMI-2 reactor was destroyed in 1979. Chernobyl disaster in 1986 rated worst nuclear accidents in history.