A Review of Criticality Accidents A Review of Criticality Accidents

5. Oak Ridge National Laboratory, 30 January 1968 41
233 U solution assembly; reactivity added by air bubble movement; single excursion; insignificant exposures.
Routine critical experiments were underway to
determine the critical concentration of an aqueous
solution of uranyl nitrate in a thin aluminum sphere
(5.84 l volume) with a thick water reflector. The
uranium contained 97.6% 233 U at a concentration of
167 g/ l. The solution density was 1.23 kg/ l.
The solution height in the sphere could be adjusted
through the vertical motion of an external 55 mm
diameter cylindrical tank. This adjustment tank was
connected to the sphere by a 13 mm diameter flexible
line. The system had achieved criticality, and measurements
were being taken to determine incremental
reactivity values. Lowering of the adjustment tank did
not provide the expected reduction in reactivity. An air
bubble was visually observed in the line connecting the
adjustment tank to the sphere. In an attempt to remove
the bubble, enough solution was drained to the supply
reservoir to achieve subcriticality. The adjustment tank
was then moved up and down in an effort to dislodge
the bubble. The motion was repeated at least twice. At
a time when no adjustments were knowingly being
made, the reactivity increased rapidly, all shutdown
devices functioned, and the radiation alarm sounded.
It is assumed that motion of the air bubble caused
the addition of enough solution to the sphere to change
the system from subcritical to essentially prompt
critical. The yield of the excursion was determined to
have been 1.1 × 10 16 fissions. Approximately 90 m l of
solution was expelled from the tank into the water
reflector and onto the nearby floor and equipment. The
modest cleanup required was accomplished promptly.
Simple modification of the experimental configuration
precluded future introductions of air bubbles.
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