Degradation mechanism of UN and UN- U3Si2 …...Degradation mechanism of UN and UN-U 3 Si 2 pellets...
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Degradation mechanism of UN and UN- U 3 Si 2 pellets of varying microstructure by comparative steam oxidation experiments Selim Uygur – Ongoing MSc. Thesis Kyle Johnson, Denise Adorno Lopes, PhD, KTH Reactor Physics –Nuclear Fuel Laboratory
Transcript of Degradation mechanism of UN and UN- U3Si2 …...Degradation mechanism of UN and UN-U 3 Si 2 pellets...
Degradation mechanism of UN and UN-U3Si2 pellets of varying microstructure by
comparative steam oxidation experiments
Selim Uygur – Ongoing MSc. ThesisKyle Johnson, Denise Adorno Lopes, PhD,
KTH Reactor Physics –Nuclear Fuel Laboratory
UN: An Accident Tolerant Fuel candidate
High thermal conductivity:
Less thermal inertia after reactor shut down
Much lower peak centerline temperature
Higher U-density enable different cladding options
Performance in contact with water/steam
Little literature on high quality (SP-Sintered) pellets
Ongoing R&D on behavior in high pressure steam
Steam exposure experiments at KTH
1. High pressure batch autoclave @300 °C ,9MPa
Air+Steam
2. Digerator @ 400°C ,1atm
Ar+Steam
N2+Steam
Basic chemistry of UN-Hydrolysis:
𝑈𝑁 + 2𝐻2𝑂∆ 𝑈𝑂2 + 𝑁𝐻3 + ½𝐻2
2𝑈𝑁 + 2𝐻2𝑂∆ 𝑈𝑂2 𝑁 + 𝑈𝑁𝑥 + 𝐻2 with 1.68 < 𝑥 < 1.74 ≡ 𝑈2𝑁3
𝑈2𝑁3 + 4𝐻2𝑂∆ 2𝑈𝑂2 + 8/3𝑁𝐻3 + 1/6𝐻2
Influence of NH3 and N2-partial pressure
Higher NH3 / N2 causes higher inter-granular
U2N3 deposition
EDS /SEM: Precipitation of U2N3, UO2 phases in a
“sandwich” structure at GB
Where does the Nitrogen go to?
Influence of density and grain size
Higher density higher hydrolysis tolerance
Smaller grains higher hydrolysis tolerance
TD: 94-96%, grain size:a)23,8µm (x50) b)6,7µm (x500)
Very high TD(99.88%), large grain sample
As fabricated: a) x500 After 90min 300°C: b) x100, c) x200, d)x1000
Influence of grain size – Findings
UN-hydrolysis rate is controlled by specific
surface available to exposure
Large grain samples crack more easily
Cracking increases the specific inter-granular
surface exposed to hydrolysis
Inter-granular cracking is the main driver of
UN-pellet collapse
Hydrolysis of a UN-10U3Si2 sample
UN-10U3Si2 |TD:98.01, GS:10.8µm As fabricated a)x500, After 30min 300°C: b)x1000, c)x1000
The degradation in the UN-
10U3Si2 sample is almost
exclusively intra-granular
Behavior UN-10U3Si2 vs. pure UN pellets
The degradation mechanism in the UN-U3Si2sample is almost exclusively intra-granular
Current results show also lower degradation
than most UN samples
The role of the U3Si2 phase has yet to be
investigated.
