Corrosion Product Deposition and Release at Nuclear Fuel Sheaths in Boiling Water^★

Deposition and release mechanisms for corrosion products at nuclear fuel element surfaces in boiling water have been considered in detail. Particles deposit from turbulent eddies, as in non-boiling water. Also, water which evaporates at nucleation sites causes a convective flow normal to the heating surface. Particles in the convective flow are filtered as the water percolates through deposits to reach nucleation sites. If the water pH is low (<9.3 at 25 C), magnetite crystals will grow from solution on the fuel sheaths in both ammoniated and lithiated water. As well, the higher the dissolved hydrogen concentration, the larger the quantity of magnetite precipitated. The release mechanisms depend strongly on water chemistry. If there is no dissolution mechanism for release, then deposits grow until spalling limits their thickness. Results from an in-reactor experiment are used to postulate a spalling mechanism which depends on surface heat flux. If deposits can dissolve, then deposits are thin (<200 mg/m²), and deposition is only by particles from turbulent eddies.