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These early tests seemed to show that radiation posed no threat even
at very high power levels, but further studies66-69 were conducted primar-
ily to test the wetting of graphite by LiF-BeP2-ZrFq-ThF4-UPq mixtures
under irradiation.
ent temperatures for many weeks revealed appreciable quantities of CF4
and, in most cases, considerable quantities of fluorine in the cover gas.
Careful e~mination20~70~71 strongly suggested that the p2 generation had
not occurred at the high temperature but had occurred by radiolysis of
the mixture in the solid state.
Examination of these capsules after storage at mbi-
This suggestion was confirmed by irradiation of two arrays sf
Nastelloy-M capsules, all containing graphite and LiF-BeF2-ZrF4-UF4 mix-
tures. Two of the capsules in each array had gas inlet and exit lines to
permit sampling of the cover gas as desired.
Gas samples dram from the
test capsules at operating temperatures and at various power levels up to
80 m/m3 showed H~Q F~ (though an occasional sample from the first array
showed detectable traces of CF,). However, during reactor shutdowns with
the capsules at about 3%"6, pressure rises were observed (usually after
an induction period of a few Bmours), and F2 was evolved. In the second
array, the capsules were kept hot during reactor shutdown as well as Bur-
i~lg operation; BO evidence of F~ or CF, was observed. such -17~ generation
at ambient temperatures was subsequently followed for several months in
OREaE hot cells.
The generation diminished with time in a manner corres-
ponding closely with decay of fission-produet activity; F2 evolution at
35°C corresponded t~ about 0,02 molecule per 188 eV absorbed, eodd be
Completely stopped by heating to I.OO"C or above, and could be resfu~ed
markedly by chililing to -7O"C. The F2 e~slution resumed, usualI~ after
a few hours, when temperature was returned to 35 to 50"Ce
These and subsequent experiences, including operation of the MSRE,
strongly indicate that radfolysfs of the molten fuel at reasonable power
densities is not a problem. It seems unlikely, though it is possible,
that DMSR fuels will evolve F2 on coolit~g, If they do, arrangements must
be made for their storage at elevated temperature until a f~a~tion sf the
decay energy is dissipated.