NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Poster 4.5: Thermodynamic Considerations in the Potential Use of Ni-Be Alloy to Braze Zircaloy–4

S.J. White and Emily Corcoran
  • Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada
  • E-mail: Sheldon.white@rmc.ca and Emily.Corcoran@rmc.ca

Abstract

The construction of fuel bundle elements for CANDU® reactors involves the attachment of numerous small Zircaloy–4 appendages to the thin walled Zircaloy–4 fuel sheathing (cladding). These appendages provide inter-element spacing and improve coolant circulation thereby enhancing heat transfer. The outermost appendages, or bearing pads, provide the spacing between the fuel bundle and the pressure tube and facilitate the safe movement of the fuel bundles within the fuel channels. These appendages have been attached by a specialized brazing process using beryllium as the filler material.

To better control beryllium exposure during the manufacture of CANDU® fuel bundles, a nickel-beryllium brazing alloy to affix the appendages is being considered. A Ni–2wt%Be shows promise in substantially reducing beryllium volatility while minimally perturbing existing fabrication processes.

This work considers Ni-Be foil as a braze filler material. This choice eliminates the vapour deposition coating step, as well as any handling of pure Be in the fuel assembly plant. Avoiding both operations in a high production throughput workplace is a benefit. Furthermore, the use of foil as a means of applying the braze alloy is not a major modification to long established plant operations. Most important is that the alloying of Be with a high concentration of Ni reduces the Be partial pressure substantially during actual brazing (as distinct from the coating operation) without preventing in principle the chemical displacement of the thin ZrO2 passive layer in spite of the considerable reduction in thermodynamic activity of Be due to nickel dilution.

Guidance on the potential use and benefits of Ni-Be braze foil is to be found in the proposed thermodynamic model of the Zr-Ni-Be ternary alloy system developed by interpolations from adaptations of treatments of the component binary systems. To this end, thermodynamic models of the condensed phases of the binary Zr-Be, Ni-Be and Ni-Zr alloy systems have been developed to provide a basis for the construction of a provisional ternary phase diagram that in particular identifies the domain of the liquid phase. The ternary model not only permits the computation of phase equilibria that bear on such essential matters as melting but also provides thermodynamic properties that can be used in other computations on oxidation and partial pressure of Be.