Estimating doses from criticality accidents

["Minnema, Douglas" <Douglas.Minnema@ns.doe.gov>]

RADSAFErs,

With the recent experience in Japan and questions raised on this listserver,
I dusted off some old material I had worked up for a class I did about 6
months ago on criticality safety.  Thought you might find it interesting.

While each criticality accident tends to be very unique, and expert advice
should always be sought, one can estimate the doses that might be received
from such an accident based on first principles.  To me it helps in
visualizing the levels of concern from such an accident. 

The key to evaluating the doses is to know the number of fissions involved
in the event.  This is probably where the greatest difficulty lies in
analyzing a hypothetical accident, although after the fact it is somewhat
easier since it can be determined from fission product yield measurements.
Therefore, I will assume for this discussion that the yield is 1E18
fissions.  Note that the results will scale directly with the number of
fissions.

Neutron Dose:

At a distance from the source, neutron flux generally follows 1/r^2.  Assume
that there are 2.5 neutrons per fission, and 50% leakage (leakage is
typically very high in such systems,  this is a guesstimate, but probably
not too far off).  Also assume that the energy of the leaking neutrons is
500 KeV (again, a guesstimate, but not too bad since many such accidents are
either unmoderated or undermoderated, and thermal neutrons get captured
close to the system), which results in a dose conversion factor of about
2.34E-9 rad/(n/cm^2).  The neutron dose would be:

		D(neutrons) = 2.34E-9 x (0.5 x 2.5 x 1E18)/(4 x Pi x r^2)
[rads]

At 10 meters (r = 1000 cm) this would be 232 rads.

Prompt Fission Gammas: