Abstract: Fuel rod 2 has a fuel cladding tube 3, and upper end plug 5 and a lower end plug 6 welded to the ends of fuel cladding tube 3. He gas 1 and U pellets 4 are filled in the fuel cladding tube 3. The He gas pressure 1 is about 8 atm, and the average crystal grain size of the U pellet 4 is in the range of 30-60 .mu.m. The fuel rod can be manufactured at a lower manufacturing cost, while maintaining the fuel integrity the same as in a conventional fuel rod.

Abstract: With an arrangement that the axial distance from the upper end of a first opening area to the upper end of the second opening area is equal to the axial distance from the upper end of a first coupling member to the upper end of a second coupling member, when a spring member is moved upwardly relative to round cells, the upper end of the second coupling member comes into contact with the upper end of the second opening area, and the spring members receives upwardly acting forces from fuel elements. The spring member is thereby prevented from inclining with respect to flow of a coolant. Simultaneously, the upper end of the first coupling member contacts the upper end of the first opening area, and the spring member is stably held in a condition where it is aligned with the direction of the coolant flow.