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ORNL-2106 - the Molten Salt Energy Technologies Web Site

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u <strong>the</strong> salt-metal interface, and <strong>the</strong> activity of chro-<br />

mium at <strong>the</strong> surface if maintained at some level<br />

appreciably less than 0.16. In <strong>the</strong> colder zones<br />

<strong>the</strong> diffusion gradient is away from <strong>the</strong> salt-metal<br />

interface, and <strong>the</strong> chromium activity is maintained<br />

at some activity slightly higher than 0.16. Since<br />

diffusion at <strong>the</strong> lower temperature is slower than<br />

that at <strong>the</strong> higher temperature, <strong>the</strong> rate-control ling<br />

step is presumably <strong>the</strong> low-temperature diffusion<br />

process. It may be that <strong>the</strong> intermediate-temperature<br />

regions, in which <strong>the</strong> chemical driving force is less<br />

but <strong>the</strong> diffusion rate is faster, are <strong>the</strong> most im-<br />

portant regions; <strong>the</strong>re is no a priori way of telling.<br />

If <strong>the</strong> dynamic-corrosion and mass-transfer phe-<br />

nomena are examined in this light, it is obvious why<br />

nei<strong>the</strong>r flow rate nor uranium concentration of <strong>the</strong><br />

fuel mixture is an important factor affecting corro-<br />

sion attack and mass transfer in lnconel systems.<br />

If it is assumed that most of <strong>the</strong> corrosion observed<br />

involves mass transfer to a dilute alloy, <strong>the</strong> lack of<br />

an effect of <strong>the</strong> surface-to-volume ratio becomes<br />

comprehensible. If <strong>the</strong> relative effect of diffusion<br />

rate vs driving force is noted, <strong>the</strong> reason for <strong>the</strong><br />

poor correlation of corrosion with temperature drop<br />

can be understood; with a ZrF4-bearing fuel, corro-<br />

sion may be worse with a top temperature of 1 500OF<br />

and a 2WoF drop than with a top temperature of<br />

1500OF and a 400OF drop.<br />

The argument points up <strong>the</strong> fact that, if <strong>the</strong> low<br />

temperature were made sufficiently low, deposition<br />

of CrO might be possible in ZrF4-bearing systems;<br />

<strong>the</strong> temperature coefficient of <strong>the</strong> reaction is not<br />

known below 600OC. It is possible that <strong>the</strong> light<br />

frosting of chromium often observed in <strong>the</strong> cdd<br />

lcgi<br />

as <strong>the</strong> solvent was initiaGd. The equipment and<br />

experimental techniqu:s for this study were very<br />

similar to those described previously for <strong>the</strong> study<br />

PERIOD ENDING JUNE 10, 1956<br />

of <strong>the</strong> reduction of FcF by H, in NaF-tF4.9'11<br />

Equipment changes included <strong>the</strong> substitution of<br />

nickel as <strong>the</strong> container material in place of <strong>the</strong><br />

mild steel used in <strong>the</strong> previous study. Operational<br />

changes consisted in <strong>the</strong> use of hydrogenhelium<br />

mixtures of known composition instead of pure<br />

hydrogen in <strong>the</strong> preparation of <strong>the</strong> equilibrating<br />

gaseous streams. This was found to be necessary<br />

after <strong>the</strong> results of preliminary experiments indi-<br />

cated <strong>the</strong> necessity for using small partial pressures<br />

of hydrogen and high partial pressures of HF in<br />

order to maintain measurable quantities of NiF, in<br />

solution.<br />

The initial experiments were made in order to<br />

ascertain <strong>the</strong> solubility of NiF, in <strong>the</strong> solvent<br />

mixture NaF-ZrF4 (53-47 mole %). The values<br />

obtained by filtration of <strong>the</strong> saturated solution and<br />

chemical analysis of <strong>the</strong> filtrate when a total of<br />

0.17 wt 56 Ni was added as hiF, are summarized<br />

in <strong>the</strong> following tabulation:<br />

Temperature Solubility of NiF,<br />

(OC) (PPd<br />

550<br />

575<br />

600<br />

625<br />

400<br />

600<br />

980<br />

1ASO<br />

Solubility values were also obtained by filtration<br />

by Redman (see following section on "Solubility<br />

andstability of Structural Metal Fluorides in <strong>Molten</strong><br />

NaF-Zrf,"), who showed that <strong>the</strong> solubility is a<br />

function of <strong>the</strong> quantity of NiF, added. T0po1'~<br />

also obtained solubility values by measuring <strong>the</strong><br />

electromotive forces of electrolytic cells. The<br />

for <strong>the</strong> solubility at 6WoC hat were ob<br />

tained in <strong>the</strong> different investigations are shown in<br />

Fig. 2.2.2. There appears to be satisfactory corre-<br />

lation of <strong>the</strong>se solubility values with those ob-<br />

tained by filtration methods. The change in solw<br />

bility at 600OC with total quantity of NiF added<br />

may be ascribed to <strong>the</strong> saturating phase Ling a<br />

complex compound ra<strong>the</strong>r than NiF,. The complex<br />

9C M. Blood and G. M. Watson, AIiP Qua. Pmg. Rep.<br />

Sepr. 10, 2954, <strong>ORNL</strong>-1771, P 66.<br />

l0C M. Blood, ANP Quar. Bog. Rep. Dec. 10, 1935,<br />

<strong>ORNL</strong>-201% p 85.<br />

"C M. Blood and G. M. Watson, ANP Quare Pmg.<br />

Rep. Mdrcb 10, 1936, <strong>ORNL</strong>-2061, p 84<br />

12L. E. Topol, ANP Qwr. Prog. Rep. March 10, 1956,<br />

<strong>ORNL</strong>-2061, p 89.<br />

99

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