FLiBe Capsule Testing of 316H

Abstract

This milestone was originally envisioned for completion in FY20 and was delayed due to the COVID-19 response and lack of FLiBe salt. In March 2021, FLiBe salt was received from Kairos Power. The nine capsule experiments with type 316H stainless steel specimens and 316 capsules were completed at 550° and 650°C for 1000 h to bracket the conditions expected in the subsequent thermal convection loop (TCL) experiment. In three capsules, Be metal was added for redox control. The specimen mass losses were low, similar to those observed previously for FLiNaK salt and the addition of Be reduced the average mass loss. Initial characterization suggested that some reaction with Be may have occurred. Therefore, Be was not included in the TCL experiment.

Corrosion of 316H Stainless Steel Specimens in Two FLiBe (LiF-BeF₂) Salt Batches

“…A dramatic increase in the Cr and Fe contents in the salt were observed as well as minor increases in Mn and Ni. These results were presented in a previous report [16]. Similar salt measurements for this salt have not been performed.…”

“…Metallographic cross-section images of the batch #1 specimens were shown in a previous report [16]. Figure 3 shows example cross-sections for the batch #2 specimens.…”

Measuring the Dissolution of Cr and Fe at 550°-750°C in FLiNaK and FLiBe

“…Isothermal capsule experiments were conducted in 4340 steel, or low carbon arc cast Mo capsules (25 mm outer diameter x 100 mm tall x 1.2mm wall) [2][3][4][5]. For these dissolution experiments, larger coupons (~10 x 20 x 1.5 mm) of unalloyed Fe and Cr (nominally 99.99% purity) were used and had a 600 grit polished finish.…”

“…[1] Current developers are interested in 316H stainless steel due to its ready availability and ASME code qualification. Previous studies of 316H stainless steel compatibility in molten fluoride salts have shown general agreement that Cr is selectively removed from the alloy during the exposures in molten salts [2][3][4][5]. However, the extent of Cr dissolution depends on the initial purity of the salt and Fe also readily dissolves in the salt.…”

Corrosion of 316H Stainless Steel in Flowing FLiBe (LiF-BeF₂)

“…Purity of the salt is reliant on purification methods which typically use the hydrofluorination process [8][9][10] developed during the 1950's and 1960's. Moving beyond static, isothermal compatibility experiments [5][6][7][11][12][13], flowing salt experiments in a thermal gradient are needed to study mass transfer, which is the major concern with potentially corrosive working fluids [13][14][15]. Historically, thermal convection loops (TCLs) [13][14][15][16][17][18][19][20] have been used as an initial assessment of compatibility before investing in a forced convection loop.…”