Patent GB-1101353-A (GB)
Title: Manufacture of nuclear reactor fuel elements
Inventor: SMITH HARRY
IPC: G21C21/12
Filed: 1964-04-07
Abstract: 1,101,353. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 25 March, 1965 [7 April, 1964 (2)], Nos. 14223/64 and 14225/64. Heading G6C. A clamp for applying longitudinal compressive loading on the sheaths of a number of fuel elements whilst the fuel elements are subjected to a process of external pressurization at an elevated temperature, comprises end clamping members 13, 14 which engage with the ends of the sheaths 3 of the fuel elements 1 and which are mounted in a common restraint structure, the overall lengthwise thermal expansion of which structure is such that the degree of thermal expansion of the sheaths 3 is constrained to be substantially equal to the degree of longitudinal thermal expansion of the fuel members 2 contained within the sheaths, when heated to the temperature at which pressurization is to be applied. The restraint structure comprises a tie-bar 10 coupling upper and lower end support plates 11, 12 on which the end clamping members 13, 14 are mounted. The tie-bar 10 is of composite form comprising two co-joined sections 15, 16 of equal length joined by a spigot joint 17 and a butt weld 18. The section 15 is of mild steel while the section 16 is of stainless steel. The tie-bar 10 is of greater  length than the fuel elements 1 and has upper and lower threaded portions 19 and 20, respectively. The end support plates 11, 12 are mounted on the tie-bar 10 by means of spigot nuts 21 and lock-nuts 22 screwed on the threaded portions 19, 20. Internal end faces 23 on the end clamping members 13, 14 bear against the ends of helical fins 4 on the sheaths 3. Each of the end clamping members 13, 14 is fitted with four anti-twist blades 24 welded in longitudinal slots 25 and projecting radially inside the end clamping member 13, 14 to engage with longitudinal grooves 6 in the fins 4 of the sheaths 3, which grooves in the finished fuel element serve to locate longitudinal splitter fins. The upper end support plate 11 is fitted with a number of clamping screws 33 which have end spigots 34 engaging with spigot holes 35 in the upper end clamping members 13. Clamping pressure is applied to the sheaths 3 through the upper end clamping members 13, the clamping screws 33 being screwed down by means of a torque wrench to the required degree of tightness. Pressurization may be carried out in oil, by pre-heating in the oil at 250‹ C. and 100 p.s.i. pressure for 15 minutes, the pressure then being increased to 12,000 p.s.i. and maintained for 30 minutes. Alternatively, the fuel elements may be subjected to pressurization in gas at a temperature of 500‹ C. In the latter case, the length of the tie-rod 10 between the end support plates 11, 12 should be 50% mild steel and 50% stainless steel while in the former case, 75% of this length should be mild steel and 25% stainless steel. The desired relative proportions of mild steel and stainless steel are obtained by altering the positions of the end support plates 11, 12 along the tie-rod 10 by adjustment of the spigot nuts 21 and the lock nuts 22. In general, the higher the temperature at which pressurization is applied the larger the proportion of stainless steel required. In a similar manner, the clamp is adjustable for use with fuel elements of a variety of lengths. In a further embodiment, the fuel elements are mounted end to end in pairs, the outer ends of each of the fuel elements being fitted with end clamping members as described above and the adjoining ends connected by pairs of end coupling members. An important feature of these clamps is that they provide for the simultaneous application of a longitudinal compressive loading on a number of fuel elements mounted side by side in close proximity to one another, the transverse dimensions of the assembly of fuel elements and clamp being less than the overall transverse dimensions of a similar assembly of fuel elements each mounted in an individual clamp. Hence more fuel elements may be accommodated in a pressurization chamber of a given size.