MODCONV is a FORTRAN Computer code using the primitive form of the conservation equations for natural convection in porous medium to calculate the velocity and temperature distribution in the moderator of a typical CANDU reactor under normal and shut down operations, as well as postulated LOCA for which the rupture in the system is external to the core. Appropriate velocities are imposed at the boundary walls so that the free convection equations set can be used to model a mixed convection problem. Purely explicit, donor cell finite differencing scheme is used. A heat exchanger model using NTU method is also included to give a realistic inflow temperature for the moderator. The heat load distribution is calculated according to a channel power map, but can be redefined through input. / Thesis / Master of Engineering (MEngr)