With a global push to reach net zero, the need to consider small modular reactors (SMRs) to reduce
costs and accelerate delivery of nuclear power has become clear. SMRs would benefit greatly from
simplifications in design which could also reduce engineering, operation and running costs. One
particular simplification under consideration is a boron-free coolant chemistry whereby reactivity
management could be maintained through control rod operations. Issues associated with boric acid
within the coolant chemistry could be eliminated, which can increase efficiency, particularly at the
end of cycle.
To elevate the pH of the coolant water, combatting the radiation hydrolysis effect, lithium is added
to the coolant chemistry as lithium hydroxide. In the absence of boron, lithium has been found to
accelerate the corrosion of the zirconium alloy fuel cladding in some circumstances. The mechanisms
that underpin this phenomenon have not been identified and formed the basis for this thesis.
This work was divided into two areas of investigation where, first, a simulation route was
undertaken to explore potential predictions which might provide an indication of the mechanisms
that could cause the accelerated corrosion of the zirconium alloys. These simulations were focused
on the likely location that Li would reside within the bulk or grain boundary ZrO2 structures and how
the Lithium might affect the oxide layer. From this, the impact of Li on defect chemistry could be
assessed. Once the simulations provided sufficient data and focus, experimentation could then take
place with the aim of validating the simulated results.
Literature provided a number of hypotheses:
1. Lithium forms solution into the bulk zirconium oxide reducing the crystal volume, promoting pore
growth which might allow oxygen to pass through the oxide layer and reach the metal surface [1,2].
The work carried out within this document used a combination of density functional theory,
Brouwer diagram and solution energy calculations to produce a number of predictions. The
solution energy of lithium into the bulk oxide structures, however, was high indicating that the
solution of lithium would be unlikely. This result led to the second hypothesis where lithium
might form solution along the grain boundaries.
2. Previous works have identified that grain boundaries can be complex in structure [3,4].
Investigations were carried out to compare solubilities of Li in these amorphous grain boundaries
and compare these values to the bulk solubilities.
In this work, it was found that lithium formed solution into the amorphous structure and the
Brouwer diagrams suggested that an increase of lithium within the grain boundaries would
also increase the oxygen vacancy concentrations, particularly at the water oxide interface.
This provided an indication that oxygen could be transported through the oxide layer via
oxygen vacancy sites along the grain boundaries.
From the results of the simulations, amorphous ZrO2 and lithium doped amorphous ZrO2 were
created and characterised. Through thermal analysis, it was found that lithium produced
amorphous phase stability where an increase in lithium would increase the temperature
required to crystallise the sample. It was also found that lithium segregates outside of the
resulting crystallised bulk oxide and that the lithium phase was highly soluble in water. This
did provide verification for the simulations that showed low solubility of lithium into the bulk
material and good solubility into the amorphous structure.
The work within this thesis outlines the above hypotheses and highlights the route by which Li can
accelerate corrosion of Zr-alloys by preferentially attacking grain boundaries in the protective oxide
layer.