Fuel assembly for gas-cooled nuclear reactors

In a fuel assembly for gas-cooled nuclear reactors, having a plurality of fuel rods fastened to a mounting plate formed with through bores serving as passageways for a coolant, the fuel rods having respective perforated end caps by which the fuel rods are connected to a fission-gas exhaust system, the end caps being formed with a respective external thread and being threadedly secured thereby in the mounting plate and simultaneously connected to the fission-gas exhaust system through channels formed in the mounting plate, the mounting plate and the end caps having surfaces formed thereon having a residual roughness of from 4 to 16 microns and being sealingly pressable against one another by the threaded connection between the end caps and the mounting plate, includes a threaded sleeve serving as means for threadedly securing the respective end caps to the mounting plate, the threaded sleeve being provided with means for gripping the sleeve by a tool and being formed with an internal thread in threaded engagement with the external thread of the respective end caps and an external thread in threaded engagement with a corresponding internal thread formed in the mounting plate, the external thread of the threaded sleeve having a greater pitch than the internal thread thereof.

The invention relates to a fuel assembly for gas-cooled nuclear reactors of 
the type known from U.S. Pat. No. 4,077,839, assigned to the same assignee 
as that of the instant application and, more particularly, to such a fuel 
assembly having a plurality of fuel rods fastened to a mounting plate 
formed with throughbores serving as passageways for a coolant, the fuel 
rods having respective perforated end caps by which the fuel rods are 
connected to a fission-gas exhaust system, the end caps being formed with 
a respective external thread and being threadedly secured thereby in the 
mounting plate and simultaneously connected to the fission-gas exhaust 
system through channels formed in the mounting plate, the mounting plate 
and the end caps having surfaces formed thereon having a residual 
roughness of from 4 to 16 microns and being sealingly pressable against 
one another by the threaded connection between the end caps and the 
mounting plate. This type of fastening of ventilated fuel rods to a rod 
support or mounting plate of a fuel assembly has proven to be especially 
good for fuel rods with casing tubes having no spacer elements, the 
insertion of such fuel rods requiring no specific rotary-angle 
orientation. It has been found, in the interim, however, that it is 
advantageous to provide posts or helical ribs on the casing tubes for the 
mutual spacing of the fuel rods, the function of the posts or helical ribs 
being assured only if the fuel rods are inserted and secured with 
corresponding mutual rotary-angle orientation in the composite fuel 
assembly. 
The form of fastening disclosed in the aforementioned U.S. Pat. No. 
4,077,839 was only conditionally useful for fuel rods of the foregoing 
type. 
It is accordingly an object of the invention to provide a fuel assembly for 
gas-cooled nuclear reactors which has an improved means for fastening the 
fuel rods to the mounting plate which, on the one hand, ensures the 
desired surface sealing of the fuel rods with respect to the rod mounting 
plate and, on the other hand, also affords adjustment of the desired 
rotary-angle orientation of the individual fuel rods without difficulty as 
they are being sealed with the mounting plate. 
With the foregoing and other objects in view, there is provided a fuel 
assembly for gas-cooled nuclear reactors of the type mentioned 
hereinbefore and disclosed in U.S. Pat. No. 4,077,839 wherein means for 
threadedly securing the respective end caps to the mounting plate include 
a threaded sleeve that is provided with means for gripping the sleeve by a 
tool, the threaded sleeve being formed with an internal thread in threaded 
engagement with the external thread of the respective end caps and an 
external thread in threaded engagement with a corresponding internal 
thread formed in the mounting plate, the external thread of the threaded 
sleeve having a greater pitch than the internal thread thereof. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in 
fuel assembly for gas-cooled nuclear reactors, it is nevertheless not 
intended to be limited to the details shown, since various modification 
and structural changes may be made therein without departing from the 
spirit of the invention and within the scope and range of equivalents of 
the claims.

Referring now to FIG. 1 of the drawing, there is shown a fuel rod 12 
fastened in a rod holding plate 13. The fuel rod 12 is provided with an 
end cap 14 having an outer hexagonal surface 145 as a tool gripping device 
and being formed centrally with a bore 143 for removal or discharge of 
fission gas. This bore 143 is aligned with a bore 134 formed in the rod 
holding plate 13 as a connection or inlet to a fission-gas removal or 
discharge system 131. The fuel rod 12 or the fuel-rod end cap 14 is sealed 
with the rod holding plate 13 by means of an end face 142. To fasten the 
end cap 14 in the rod holding plate 13, the latter is formed with a 
threaded sleeve 147 formed with an internal thread which threadedly 
engages an external thread 151 formed on a rod-shaped elongation or 
extension 150 of the end cap 14. The external thread 148 of the sleeve 147 
is screwed into the threaded bore 132 of the rod holding plate 13. 
Since the external thread 148 has a greater pitch than the internal thread 
149, the suitable unit pressure of the end face 142 is attained by 
twisting or turning the threaded sleeve 147 through the fuel rod 12 which 
is secured by means of a torque wrench or spanner formed with the 
aforementioned hexagonal surface 145. Reversed turning or twisting of the 
threaded sleeve 147 loosenes or releases the connection between the fuel 
rod and the rod holding plate 13. 
The threaded sleeve 147 and the components engaging or mating therewith 
can, however, also be so constructed that the external and internal thread 
have an opposite sense of direction of rotation. This type of construction 
has the advantage that the fuel rod to be fastened can be inserted end 
first into the rotationally oriented position thereof and can remain in 
that position. This is of importance especially for fuel rods with helical 
spacing fins. 
For the protection or security of the sealed condition of the fuel rod 
mounting, the threaded sleeve 147 is fixed to the end cap 14 or to the 
elongation or extension 150 thereof by a spot weld 152 or equivalent 
fastening means. 
As a protection against rotation and, simultaneously, to fix or establish 
the required rotary-angle position of the individual fuel rods 12 in the 
composite fuel assembly, the end caps 14 are formed with a respective bore 
154, as shown in FIG. 2, through which a common pin 155 is insertable and 
thereby ensures continued maintenance of the given rotary orientation of 
the fuel rods. These bores 154 are obviously set or formed in a manner 
that they assure the required positioning of non-illustrated parts of 
spacing ribs for non-illustrated spacers between the individual fuel rods 
of which the end caps 14 are shown in FIG. 2. 
Finally, it should again be emphasized that the pitch of the external 
thread on the sleeve 147 must be greater than that of the internal thread, 
the transmission ratio for the adjustment of the sealing pressure being 
predeterminable through the value of the phase difference of the threads.