RE: skyshine from radiography sources, Tokaimura criticalityacci dent

Actually, it is not that unusual for a criticality accident to continue operating for a while. The fatal accident in Russia involving a metal assembly (1997?) operated for about 3 days or so before disassembled by a simple robot. It is not unusual for the initial pulse to not release enough energy to physically disrupt the system adequately for complete shutdown. Furthermore, solution systems may achieve thermal equilibrium at temperatures below the boiling point of the solution, depending on the heat transfer from the system.

Also, the release of shortlived noble gases needs to be considered, especially for solution systems. In models I developed a while back, in a room with 8 room changes per hour the shortlived nobles contributed over 95% of the plume dose at 100 meters. In a surprise to me, I found that at 100 meters, if one ignores shielding (which is often missing anyway), the doses from plume passage for an accident like Tokaimura were essentially equal to the direct doses from the assembly. In my simulations, they were about 1 - 2 rem each. At Tokaimura the nearest residences were only 80 meters away.

If interested, I'd be happy to trade info with you off the list.

Doug Minnema, PhD, CHP

National Nuclear Security Administration

what few thoughts i have are truly my own ...

The Tokaimura criticality accident was very unique in that the criticality continued for approximately 20 hours before the water blanket around the tank could be drained and the system allowed to reach a subcritical state. Usually a criticality accident is a pulse or burst type prompt criticality event. The energy released during the pulse is usually sufficient to change the geometry (through container deformation) or moderation (through boiling off of the water moderator) of the system sufficient to make further criticality pulses impossible or at least less powerful. Of all known criticality accidents world wide (Ref. LA-13638), this accident's duration far exceeded previous accidents. Because some residents were not evacuated until 3.5 to 4 hours after the reaction started and were within a few hundred meters of the source, the doses received by a handful of the nearest members of the public were due primarily to the direct radiation and not sky shine.

The Criticality Slide Rule (Ref. NUREG/CR-6504, Vol. 2 or ORNL/TM-13322/V2) indicates that for the type of accident at Tokaimura, at 30 meters, the skyshine dose is approximately 1/10 the total dose and at 300 meters the skyshine dose is 1/2 the total dose.

I have two separate reports generated soon after the accident that discuss radiological exposures, measurements taken, emergency response, and the steps taken to halt the ongoing criticality. One report was prepared by Valerie Putnam on behalf of the American Nuclear Society and one prepared by the IAEA. Both reports are in PDF format and easily transmitted. I am not aware if they are available on the internet or not.

I'm not sure if this answers Mr. Franta's question, but I hope it helps.

Nuclear Criticality Safety Manager

Portsmouth Gaseous Diffusion Plant