Hydrogen uptake into Zr-based fuel claddings
A new method for direct electrical conductivity measurements at nanoscale
In this study the conductivity/resistivity of oxide formed on Zircaloy-2 nuclear fuel cladding is measured.
The same cladding grade (LK3/L) is used to compare the oxide formed after short and long residence times, respectively 3 and 9 cycles, in a Boiling Water Reactor (BWR). The oxide in the vicinity of the metal-oxide interface is studied. The measurements were carried out using a novel technique developed at PSI, using micromanipulators (micromanipulator) inside a FIB/SEM to probe different thicknesses of oxide and hence directly measure the resistance and calculate the resistivity of that thickness of oxide.
The results from the 3 and 9 cycle oxides are compared. It is demonstrated that the resistivity of the oxide in the vicinity of the interface of the 9-cycle cladding increases with distance from the interface, much faster than that of 3-cycle cladding to a maximum level. The study is correlated to previous work, performed at PSI, characterizing these 2 materials by EPMA and other techniques.
Oxide conductivity measurement on spent fuel
- Hydrogen deteriorates the mechanical properties of fuel rod cladding
(possible problem for handling and transport of spent fuel after long dry storage)
- Low resistivity of the protective oxide reduces the H-uptake
- Direct measurement of the conductivity would deliver critical info for the validation of models
Experimental new findings:
The resistivity of the oxide at the metal interface increases with distance from the interface
- the structure of the oxide layer near the interface is critical for H-pickup
Nb-containing alloys show lower resistivity in the oxide close to the metal interface
- better performance with respect to hydrogen pick-up (I.e. lower HPUF)
The resistivity in the oxide close to the metal interface increases with reactor time
- higher hydrogen uptake late in life
Relevance: the model is better understood, new influences on cladding development.