RE: Fisson Bomb Geometries...

[Paul lavely <lavelyp@UCLINK4.BERKELEY.EDU>]

Title: RE: Fisson Bomb Geometries...

> Close, but off in a few details.  If
>
> you read the same story I did in
>
> school, at a junior high grade level, it was not very accurate. 
>
> Whether
>
> this was for security or storyline, I can't say.

>

The below is copied from an excellent review - "A Review of Criticality Accidents 2000 Revision by Thomas P. McLaughlin

LA-13638  Issued: May 2000  LANL  Los Alamos, New Mexico 87545

 

B. BARE AND REFLECTED METAL ASSEMBLIES
1. Los Alamos Scientific Laboratory, 21 August 194537,44
Plutonium core reflected with tungsten carbide; single excursion; one fatality, one significant exposure.
2. Los Alamos Scientific Laboratory, 21 May 194637,44
Plutonium core reflected with beryllium; one fatality, seven significant exposures.

Two accidental excursions occurred with the same
core and were, in several respects, quite similar. The core
consisted of two hemispheres of *-phase plutonium
coated with 5 mils of nickel. The total core mass was
6.2 kg; the density was about 15.7 g/cm3.

In the first accident, a critical assembly was being
created by hand stacking 4.4 kg tungsten carbide bricks
around the plutonium core.  The lone experimenter was
moving the final brick over the assembly for a total
reflector of 236 kg when he noticed from the nearby
neutron counters that the addition of this brick would
make the assembly supercritical. As he withdrew his
hand, the brick slipped and fell onto the center of the
assembly, adding sufficient reflection to make the
system superprompt critical. A power excursion occurred.
He quickly pushed off the final brick and
proceeded to unstack the assembly. His dose was

estimated as 510 rem from a yield of E16 fissions. He

died 28 days later.

An Army guard assigned to the building, but not
helping with the experiment, received a radiation dose of
approximately 50 rem. The nickel canning on the
plutonium core did not rupture.

In the second accident, the techniques involved in
creating a metal critical assembly were being demonstrated
to several people. The system consisted of the
same plutonium sphere reflected, in this case, by
beryllium. The top and final hemispherical beryllium
shell was being slowly lowered into place; one edge was
touching the lower beryllium hemisphere while the edge
180° away was resting on the tip of a screwdriver
(Figure 42). The person conducting the demonstration
was holding the top shell with his left thumb placed in an
opening at the polar point.

The yield of this excursion was 3E15 fissions;

again, there was no rupture of the nickel canning. The
eight people in the room received doses of about 2100,
360, 250, 160, 110, 65, 47, and 37 rem. The man who
performed the experiment died nine days later.

The results of fission rate calculations

> The configuration was a subcritical Pu
>
> sphere with hemispherical Be
>
> reflector shells. The assembly was subcritical at all but full
>
> assembly, and
>
> the reflectors should have been separated on spacer shims at a
>
> subcritical
>
> point.  Instead, they were merely held apart by a hand-held
>
> screwdriver, to
>
> allow the geometry to be varied.  The researcher was also holding
>
> the upper
>
> unit by means of a hole in the polar point. The screwdriver slipped
>
> and the
>
> sphere was fully assembled.  Around 3E15 fissions later, he
>
> separated the
>
> assembly.  This was half a second or less in time.
>
>
> He received about 2100 rem, and died nine days later.  The seven
>
> observers

> got from 37 to 360 rem.

 

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