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.