4. METHOD OF SOLUTION
THETA1B solves the two-dimensional diffusion
equation for a single fuel rod and the one-dimensional
conservation of energy equation for the associated fluid channel.
The two solutions are linked at the fuel rod-fluid interface by
the boundary conditions of surface temperature and heat flux. The
energy equations in the fuel rod and fluid channel are solved by
several numerical differencing techniques. Normally the fuel rod
equation is solved explicitly and the fluid equation is solved by
the method of characteristics. The heat transfer model considers
forced convection in subcooled liquid and superheated vapor,
nucleate boiling, forced convection vaporization, transition
boiling, and stable film boiling.