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

surface.
72
Outgassing sf the moderator by pumping at reduces pressure and
elevated temperature is necessary and sufficient to prevent contamination
of the fuel by oxide ion from reactive gases from this source.
Oxide films on the structural metal can also contaminate the fuel by
oxide ion, and, as described previously, the dissolved ~e3+$ ~e2+$ and
Ni2+ can be responsible for subsequent metal corr~sion.
In operation sf
the MSRE, the system was flushed with an LiF-BeF2 mixture for cleaning at
start-up and after each shutdown before introduction of the fuel mixture.
This precaution might be unnecessa%y, but it did suffice to keep oxide
contamination caused by surface oxidation of the metal to a minimum.
A small (-1OO-ppm) concentration of ~r2+ in the fuel as a consequence
Of reaction Of the metal with the fuel Cartnot be aVOid@di. However, in the
absence of extraneous oxidants, the reacti~n is very slight, and the pres-
ence Of Cr2' is CQIIIpbet@%y illlX3CuOuS.
Grow-in of the fission products is also unavoidable, as is the pres-
ence of a relatively small steady-state concentration of 3 ~ ~ .
Atmsspherfc contamination. Reaction of the DMSK fuel mixture with
*
oxygen is relatively slow,
but reaction with water vapor is more rapid,
Further, contaminati~n of the fuel with 40 to 50 ppm (by weight) sf oxide
ion could result in precipitation of a uranium-rich (UTB%)02 solid SO~U-
tion, A large ingress of contaminant air would be re~taired to produce
40 ppm Of 02- in the fuel, and the DMSR would be designed amd operated SO
as to minimize the chances sf such contamination. Operation of PISRE
during much Of a fQuK'-yeaaP period With Illany ShUtdOWTls and several lrnimpor
repair opera%ions sho~ed no evidence sf an increase in oxide contamina-
tion level.22 ~reatment the initial fuel charge with anhydrous HF-B~
mixture during its prepratton reataces the 02- concentration to innocuous
Peve%s, and similar treatment of contaminated fuel would serve to re~~ve
the o%-. such treatment might never be required, but in the DMSR, simple
equipment should be included that is capable of treatment to ~emove oxide
fOn should BnadVePtent CoRtPARinatiOn BCCUre
dr
KQW~V~P, the reactor metal at high temperature can readily react
with oxygenB and the fuel can react with the oxides formed in this mannero
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