A solid nuclear fuel with spherical shape of radius r, and thermal conductivity k, generates a certain heat transfer rate per unit volume q. The fuel is covered to control the dissipated heat. The covering material, of external radius r2, has a conductivity k2 and it is surrounded by an air flow which is characterized by a convection coefficient h. 1) Obtain the expression to calculate the thermal resistance of the covering material as a function of the variables given in the statement. 2) To control the right amount of covering material it is desired to know the critical insulation radius r2. Obtain the expression to calculate such radius.