|
Regarding the dispersion of Ir-192 in an explosion,
there was a real case of this occurring at ORNL some years ago. A couple of us
gave talks on the radiation protection aspects at the next annual Health Physics
Society meeting.
If memory serves, what happened was that at DOE's
INEEL site (Idaho), the precursors were prepared in disk form
and were placed into very long thin cylinders. These were then sealed by
welding. Since INEEL's reactor was down, the cylinders were sent to ORNL's High
Flux Isotope Reactor (HFIR) to be neutron-irradiated. By this process, the
precursor material was activated to form Ir-192.
The rods were then removed to a cask
underwater to cool for a time. This cask was then lifted out of the
water and set upright (long axis vertical) at the side of the pool to await
transport to an ORNL hot cell. It sat there for hours. Then, by a procedure that
had been used various times before, the cask was lifted by a crane and rotated
90 degrees in the air so that it could be moved over to a waiting flatbed
truck to be transported on its side (long axis horizontal).
However, this time, as the cask was rotated, the
chirpers, the area dose rate monitors, the air monitors, and
the evacuation alarm all went off. The health physics technicians
had the cask lowered back onto the ground on its side and evacuated
all workers in the reactor bay. A short time later, the cask was rotated
back and set down upright on the floor by a reactor supervisor, operating
the crane, since the dose rate was thought to be emanating from the bottom
of the cask (or at least this seemed to be the conservative course of
action).
Characterization of the radioactivity showed that
there were bits of Ir-192 on the reactor bay floor. These were collected in
various ways, such as by sticky tape mounted at the end of a pole. Eventually
the cask was sealed off on the bottom and moved to the hot cell for
examination.
What happened? Well, up to this time the welds were
not inspected radiographically at INEEL. A faulty weld on one rod thus
went undetected. A small hole in the weld allowed water to enter during the
cooling time underwater. When the cask was set out by the side of the pool, the
heat of decay caused the water to flash rapidly to steam. The hole in the weld
was big enough to allow the initial slow ingress of water, but not big enough to
allow the egress of the steam burst. Thus the rod was blown open near the top
(the rod material actually ruptured) and bits of Ir-192 were blown out the hold.
This was easily seen when the rod was examined in the hot cell. The bits lay
under the cask or fell out around the shielding ball as the cask was rotated,
and that is why they were not detected until the cask was lifted and
rotated.
Although the heat of the explosion was not enough
to melt or vaporize the Ir-192, the force was clearly enough to fragment some of
the disks. Most of the bits were of larger than respirable size, but some Ir-192
was found on the air monitor filters later on and some was presumably
dustlike.
SO.....with respect to the question under
discussion about the dispersal of Ir-192, my guess is, from this real example,
that if a detonation could be arranged so as to produce maximum fragmentation,
you could get some significant dispersion of small particles that could be
carried by air, including some of respirable size. It doesn't seem as though
this process would be efficient (i.e., most of the Ir-192 would be in largish
fast-settling bits), but somebody might be able to optimize it.
Janet Westbrook
|