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Laser spot welding of appendages to Zircaloy-4 tube for PHWR reactor fuel

  • Posted on: 7th May, 2024

Zirconium alloys are widely used in thermal nuclear reactors as fuel sheath and other structural components due to their low neutron absorption cross section, adequate strength at reactor operating temperature and excellent corrosion resistance in water.

A PHWR fuel bundle consists of a cluster of fuel rods whose ends are welded by endplate to form a bundle. The fuel rod consists of UO2 pellets loaded inside a 0.40 mm thick Zircaloy-4 alloy tube encapsulated at both ends by end plugs. In order to prevent damage to fuel rod during fuel bundle loading / unloading in the reactor, three bearing pads are welded to the outer fuel rods.

These bearing pads provide bearing surfaces while charging and discharging the bundle in reactor. It also facilitates proper coolant flow.

Bearing pads are currently joined with the thin fuel sheath either by induction brazing with beryllium as filler material or by Resistance Projection Welding (RPW). Induction brazing process has been adopted in Canada, whereas, Resistance welding process is used at NFC, Hyderabad, India.

Induction brazing process requires heating the components to a temperature to above 1000 deg C, resulting in generation of larger HAZ. It may also develop chronic beryllium disease to the operators due to toxicity of beryllium.

For RPW, no NDT is available to check the quality of the welds. Weld shear strength is evaluated by destructive method on sample basis. Therefore, a new welding technique without use of toxic beryllium and amenable to NDT may benefit production activities and in reactor fuel performance.

Laser welding is considered to be an attractive joining technique with low heat input and is extensively used in many industries for joining different metals and alloys.

The major advantages of laser welding are precise heat control, repeatability, flexibility, lower distortion, smaller HAZ and higher welding speed.

In this work we report laser spot welding of bearing pads with Zircaloy-4 fuel tube. Welds were made with different heat inputs using optical pulses of different temporal width. Welds were evaluated by metallography, radiography and tensile shear testing.

This article is shared by Aniruddha Kumar, D. B. Sathe and R. B. Bhatt of Fuel fabrication, Nuclear Recycle Board, BARC, Tarapur, Maharashtra

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