ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

58
of the reactor. The rate of transfer of chromium is limited by the rate
at which the transferred ~~~QIII~LKII an diffuse into the alloy in the low-
temperature regi~n~. " 9 20$ 56
The results of two decades of sophisticated
corrosion testing have dem~nstrated the validity of this mechanism and
have shsm that such C O ~ ~ O S ~ Owill - ~ " p r ~ to ~ e be only a trifling problem
for MSBW. Appreciable chromlium depletion WQU$~ be expected to a depth of
less than 0.13 mm/year (0.5 mil/year) in metal at TO~~C.''~~'
The initial attack, which is not serisus if proper purification of
the salt and cleaning of the system have occurred, can be mitigated by
the presence of a small quantity of UFq along with UF4 in the salt. Csn-
trol of the oxidation states of plutonium and protactinium and of certain
fission products, along with control of the oxidative effects of the fis-
sion process, furnish more cogent reasons for maintaining %iF3 in the fuel
mixture. Slight continuing corrosion is affected very little (if at all)
by the presence of small quantities of UF3.
The unclad moderator graphite is not wetted by or chemically reactive
to the Standard MSRE QF HSBR fuel COmpQSit%QnS, and these facts appear Un-
19% 203 57,58
changed by intense irradiation and the eo~se~uences of fissi~~~.
Estimates22 are that the MSRE graphite moderator stack (3700 kg) acquired
less than 2 g of uranium during ~perati~~l of the reactor. ObviousPy, no
appreciable interaction of graphite with the fuel (whose UF3/&YFb, ratio was
never above 0.02) occurred in that reactor. Mowever, g i ~ a e sufficiently
~
high UF3/UP4 ratio, formation of uranium carbides must be expected, and
this should be avoided, TQth a d Gilpatrfck, Who used SpectHOphQtOmetrg
in a graphite cell with diamond windows to assay equilibrium UF3 and UF4
concentrations, have carefully studied uranim carbide fomati~n using
Li2BeF4 (Refs. 59 and 60), other LiF-BeF2 mixtures,6Q ,61 and LiF-BeP2-TkP4
(72-16-12 mole X) (Ref. 41) as solvents. A surprising finding of these
studies is that, contrary to generally accepted themodynamic data," U C ~
is the stable carbide phase over the temperature interval 550 to 700°C.
Figure I12 S~QWS the results6' of equilibration experiments in MSBR fuel
solvent. Apparently, at the lo~est temperature (565°C) within a BNSR,
"~srrosion in the presence of fission and fission products is more
complex. See Sects, 3.3.2 and 3.4 for additional details.