A cylindrical tube has an inner diameter of 2 cm and a wall thickness of 1 cm. The tube is evacuated. In the center, there is a radiation source along its axis that sends 1e5 W/m2 to the inner surface of the tube. K for the tube wall is 2 W/m K. If the outer surface of the tube is cooled with water at 298 K with a convective heat transfer coefficient of 100 W/m2 K, determine the minimum temperature rating for the tube material for safe operation.