Tube loading sleeve for pilger mill

To minimize the incurrence of defects in the inner surface of a nuclear fuel cladding tube produced in a pilger mill from a Zircaloy tube, a loading sleeve is inserted onto the pilgering mandrel assembly in close fitting relation, and the Zircaloy tube is then inserted onto the sleeve in close fitting relation. The sleeve, formed of ABS plastic, is then withdrawn, leaving the Zircaloy tube loaded on the mandrel assembly preparatory to reducing the Zircaloy tube down to cladding tube dimensions in the pilger mill.

The present invention relates to the manufacture of nuclear fuel rods and 
particularly to the production of the Zircaloy cladding or tubing utilized 
in nuclear fuel rods. 
BACKGROUND OF THE INVENTION 
In the production of Zircaloy cladding tubes for nuclear fuel rods, 
Zircaloy tube stock having an outer diameter (OD) of, for example, 2.5 
inches, is cold worked by numerous passes in a pilger mill down to an OD 
on the order of 0.5 inches with a wall thickness of, for example, 0.025 
inches. Of course, these dimensions will vary depending on the particular 
fuel rod design in production. The finished tube must meet strict 
dimensional specifications and be essentially defect-free. Of particular 
concern are defects on the inner surface of a finished tube due to the 
presence of an extremely thin pure zirconium liner. If this previously 
applied uniform liner is disrupted during the pilgering process, any 
buildups of the soft zirconium are worked into the tube wall cross 
section, resulting in rejectable defects which are discovered during 
ultrasonic inspection. These defective tubes must then be scrapped, 
resulting in lost labor, material and throughput. 
SUMMARY OF THE INVENTION 
It is accordingly an objective of the present invention to minimize the 
instances of defects on the inner surface of Zircaloy tubing incurred 
during pilger milling of a tube down to nuclear fuel cladding dimensions. 
In achieving this objective, Applicant has discovered that the inner 
surface defects are largely created during the tube loading stage when the 
Zircaloy tube stock is inserted on the mandrel rod assembly preparatory to 
the final pilgering pass. Any metal burrs on the surface of the mandrel 
rod assembly will inflict scratches in the soft zirconium liner as the 
tube inner surface slides over the mandrel assembly during loading. The 
burrs disrupt the liner uniformity and cause a buildup of zirconium at the 
tail ends of scratches inflicted by the burrs. The zirconium buildups are 
then worked into the tube wall during the pilgering process and result in 
defects destroying the efficacy of the finished tube as nuclear fuel 
cladding. 
To solve this defect problem in accordance with the present invention, a 
sleeve is inserted over the mandrel rod assembly prior to loading the 
Zircaloy tube on the mandrel. The sleeve, formed of a suitable material 
such as acrylonitrile butadiene styrene (ABS) plastic, serves as a 
protective barrier between the mandrel surface and the inner surface of 
the Zircaloy tube. The smooth outer surface of the sleeve preserves the 
zirconium liner integrity as the Zircaloy tube slides over the sleeve 
incident to loading, and as the sleeve is subsequently removed preparatory 
to pilgering, with the result that inner surface defects in the finished 
cladding tubes are dramatically reduced.

DETAILED DESCRIPTION 
Referring to the drawing, a pair of counter rotating forming rolls or dies 
10 of a pilger mill are mounted by conventional means (not shown) for 
oscillatory axial motion relative to a tapered mandrel 12 of a mandrel 
assembly. The mandrel is affixed to one end of a mandrel rod 14, whose 
other end is terminated by a tail piece 16. The mandrel assembly, which 
may be in excess of thirty feet in length, is cantilever mounted by a 
conventional chuck (not shown), which grips the tail piece. A Zircaloy 
tube stock 18 is loaded onto the mandrel assembly and is gripped by a 
conventional holder (not shown), which acts to pass the Zircaloy tube 
through the dies in increments of axial and rotational motion. 
In accordance with the present invention, to protect the delicate zirconium 
liner on the inner surface of the Zircaloy tube during the loading step, 
an elongated loading sleeve 20 is first inserted onto the mandrel assembly 
in close fitting relation. Preferably, one end 20a of the sleeve is 
cone-shaped to conform with the conical termination of tail piece 16 to 
establish a rightward-most position of the sleeve on the mandrel assembly. 
The Zircaloy tube is then inserted onto the mandrel assembly over the 
sleeve in close fitting relation. The inner surface of the tube thus 
slides over the outer surface of the sleeve as the tube is inserted from 
left to right into a loaded position. Sleeve 20 is then slid out leftward 
from between the tube and the mandrel assembly. Once the sleeve is 
removed, the tail piece of the mandrel assembly is gripped by the mandrel 
chuck, and the Zircaloy tube is gripped by its holder to prepare the 
pilger mill for operation to reduce the Zircaloy tube down to nuclear fuel 
cladding tube dimensions. 
Sleeve 20 must of course possess a smooth outer surface to avoid scratching 
the zirconium inner liner during relative sliding motion incident to 
loading the tube and removal of the sleeve. To accommodate handling, the 
sleeve should also have sufficient strength to remain rigid despite its 
considerable length, e.g., fifteen feet, and thin wall thickness, e.g., on 
the order of 0.020 inches. Acrylonitrile butadiene styrene (ABS) plastic 
has been found to be an eminently suitable sleeve material. However, other 
high strength plastics having the requisite structural attributes are also 
applicable as sleeve materials. The inner and outer diameters of the 
sleeve are of course determined by the cladding tube size to be produced. 
However, to facilitate application and removal, the sleeve should have an 
inner diameter in the range of 5-10 mils greater than the outer diameter 
of the mandrel rod and an outer diameter in the same 5-10 mil range less 
than the inner diameter of the Zircaloy tube.