Database of radiological incidents and related events--Johnston's Archive

Chelyabinsk-70 criticality accident, 1968

compiled by Wm. Robert Johnston
last modified 14 September 2005

Date: 5 April 1968

Location: VNIITF, Chelyabinsk-70, Chelyabinsk, Russia, USSR

Type of event: criticality accident with uranium assembly


A criticality accident occurred with an assembly comprised of a highly enriched uranium sphere (90% U-235) with a polyethylene-filled cavity in the center, all to be surrounded by a natural uranium reflector. Two experimenters were conducting an experiment comparable to one conducted earlier in the day with no polyethylene in the core cavity, erroneously expecting little difference in the system performance. While lowering the upper hemisphere of the outer reflector, the system became critical and produced high power readings, prompting the emergency system to lower the lower hemisphere of the outer reflector, ending the excursion. The experimenters called for help and completed dose estimates. The individual closest to the assembly received a dose of 2000 to 4000 rem and died 3 days later; the other received a 500-1000 rem dose and died 54 days later.

Consequences: 2 fatalities.


© 2004, 2005 by Wm. Robert Johnston.
Last modified 14 September 2005.
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