$3\mathrm{.}4$ A sphere containing spent nuclear material and of diameter $0\mathrm{.}6$ m is buried in soil at a depth of $10$ m $.$ The soil thermal conductivity is $0\mathrm{.}6\frac{W}{m}K.$ If the heat generated in the sphere is ${10}^6\frac{W}{m}3,$ determine the surface temperature of the sphere. The soil surface temperature is $10C.$