Corrosion in Nuclear Power Plant (Cladding Corrosion (Zr + 2H2O → ZrO2 +…
Corrosion in Nuclear Power Plant
Flow Accelerated Corrosion (FAC)
• FAC involve dissolution of normally poorly soluble oxide by combined electrochemical, water chemistry and mass-transfer phenomena.
• The underlying metal corrode to recreate the oxide
• Corrosion mechanism which a protective oxide layer on a metal surface dissolve in a fast flowing water.
Mechanism of FAC
a) Corrosion Process - Production of soluble Fe2+ and their accumulation of the oxide water interface.
b) Mass transfer process - Flowing water remove the soluble ferrous ions by a convective mass transfer mechanism.
Zr + 2H2O → ZrO2 + 4H
a) Effect of corrosion on cladding integrity i) formation of ZrO2 layer → cladding thinning ii) absorption of hydrogen by the zirconium alloy embrittles the cladding
b) Formation of hrdrides and hydrogen embrittlement i) Hydrogen diffuse into the zirconium alloy cladding forming zirconium hydride ii) Hydrogen production process → mechanically weakens the rods cladding thus blister and cracks from upon hydrogen accumulation
c) Rate of zirconium alloy corrosion is controlled by temp. of the metal
d) Cladding transfer to the coolant, temp (↑)
e) Oxide grow thicker, temp (↑), corrosion rate (↑)
Fuel crud deposition
Crud deposition on fuel surface: affect fuel performance
Activated crud is deposited on out-of-core surface: high radiation and high doses
Effect of Li on zirconium corrosion
i) LiOH concentration, corrosion rate (↑)
ii) Dose reduction, have led to the use of higher lithium level due to longer cycle lengths