NaK pool-boiler solar receiver durability bench test. Volume 2, metallurgical analysis

We report on the analysis of the principal materials used in the construction of a NaKbased pool-boiler. The device was operated for 7500 hours and accumulated = 1000 thermal cycles to a peak temperature of 750°C. Haynes 230, a Ni-Cr-W alloy used to fabricate the pool-boiler vessel was found to

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perform satisfactorily. Air-side corrosion of the pool-boiler vessel was insignificant. The internal surface of the alloy exhibited evidence of some NaK-induced elemental dissolution. This dissolution was somewhat more extensive where the alloy was exposed to the liquid metal compared to regions exposed only to NaK vapor. However, the corresponding metal loss in all regions was inconsequential, never exceeding more than a few microns. Autogenous seam welds of the alloy responded in a similar fashion, exhibiting only minimal metal loss over the course of the experiment. While mechanical property measurements revealed a 50% loss in ductility of the alloy compared to baseline measurements of as-received material, there remained adequate ductility for the anticipated operating environment. An enhanced boiling nucleation surface comprised of stainless steel powder brazed to the vessel ID showed no change in its structure. It remained intact and showed no evidence of cracking after the repeated thermal cycling. Other materials used in the experiment showed more extensive degradation after exposure to the NaK. IN 600, a Ni-Cr-Fe alloy used to fabricate thermowells, exhibited extensive surface and intergranular dissolution. Grain boundary dissolution was sufficiently severe in one of the thermowells to cause an air leak, resulting in the termination of the experiment. BNi-3, a brazing alloy used to join the pool-boiler vessel, endcaps and thermowells, showed evidence of some dissolution where it was exposed to the NaK as well as thermal aging effects unrelated to the liquid metal environment. However, all brazes remained structurally sound throughout the experiment. A nickel metal ribbon exhibited evidence of catastrophic dissolution, resulting in the formation of deep (> 30 pm) pits and cavities. A zirconium metal foil used to getter oxygen from the NaK became extremely brittle, the result of the formation of either Zr-oxides or Zr-Ni intermetallic compounds.

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