Re: On 8/1945 A-Bombs

Rich Rostrom <rrostrom.21stcentury@xxxxxxx> wrote:
> Merlin Dorfman <dorfman@xxxxxxxxxxxxxxx> wrote:

My understanding is improving! Thanks to Rich, Cary, Mike,
etc.

....

>>- The natural uranium tamper: can I infer that while natural
>>uranium will not sutain a chain reaction, it will fission if
>>bombarded by neutrons?

> "Natural" uranium _will_ sustain a chain reaction; the
> Stagg Field reactor was built from natural uranium...

....

> However, the density of U-235 in natural uranium is far
> too low to sustain an _explosive_ chain reaction.

> The primary component of natural uranium (> 99%) is U-238.
> U-238 cannot sustain a chain reaction at all. But when
> bombarded with neutrons, U-238 will fission and release
> energy.

So the Stagg Field reactor depended on the small fraction
of U-235 in order to function? And if a reactor were built
from depleted uranium, it would not work? (How low would the
U-235 content have to go before a chain reaction would be
impossible?

....


>>the bomb [is] reassembled...
>>If so, was this done manually at close range? ... but this
>>could not have been very healthy for the person doing it.

> Why? The plutonium "pit" was coated with nickel to prevent
> chemical contamination and contain the alpha particle
> emission. Alpha emissions are low energy, low penetration -
> a sheet of tissue paper stops them. Alpha emitters are
> dangerous only when inhaled or ingested.

Isn't there spontaneous fission in the pit? (Producing
neutrons that would penetrate the coating.) Not enough to be
self-sustaining of course.

>>- Am I correct that the pit alone was sub-critical, but the
>>compression of it caused by the implosion made it supercrtical,
>>i.e., less mass is required for criticality if it is
>>compressed?

> Exactly. The chain reaction is when neutrons released by
> fission of one atom bang into other atoms. The closer
> together the atoms are, the greater the probability that
> a neutron hits another atom instead of escaping out the
> side of the reacting mass.

> There's one other factor. Plutonium reacts faster than
> U-235. When a critical mass is assembled 'mechanically',
> in a gun type device, the mass becomes more and more
> critical as the pieces come together, and in fact become
> critical before assembly is complete. But U-235 reacts
> slowly enough that the assembly is completed, so the
> chain reaction propagates all through the mass before
> the energy release blows it apart.

Is it the speed of reaction of U-235 vs. Pu-239? I
had understood that the "fizzle" behavior was due to
unavoidable contamination of Pu-239 by Pu-240 which is
formed when Pu-239 remains exposed to the same radiation
(neutrons) that catalyzes the conversion of U-238 to Np and
then to Pu. I.e., if the Pu-240 could be removed (analogous
to separating U-235 from U-238), the fizzle behavior would
not occur and a "gun" type mechanism could be used for a
plutonium bomb.
--

.

Relevant Pages

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