Re: On 8/1945 A-Bombs

Merlin Dorfman <dorfman@xxxxxxxxxxxxxxx> wrote:

> As long as people are willing to answer, I'm willing to ask :-)
>
>> Correct. "Depleted" uranium, i.e. pure U-238, is
>> useless for reactors and bombs. That's why it's
>> quite casually shot off in AP ammunition. It's not
>> even significantly radioactive.
>
> This is because U-238 does not spontaneously fission?

Well, it does - but at an extremely slow rate. U-238's
half-life is 4.5 billion years, U-235's is 71 million
years, over 60 times faster. More importantly, U-238
does not sustain a chain reaction. Whack U-235 or Pu-239
with a neutron - it fissions, producing energy, two
smaller nuclei, and _two_ free neutrons. The two neutrons
whack can whack two nuclei, producing four neutrons, and
away we go! Actually, some of the free neutrons are lost,
but as long as the average result of one fission is more
than one free neutron whacking other nuclei, the reaction
grows. This ratio is the 'k' factor. k >= 1 is a sustained
chain reaction. Note that the interval from one nucleus
fissioning to its daughter neutrons fissioning other nuclei
is on the order of a billionth of a second. If k = 1.5,
the energy release would grow by 10 to the 176th power in
just one millionth of a second. Boom!

But U-238 doesn't fission - it absorbs the neutron, becoming
U-239. (In a breeder situation, U-239 later beta decays
(neutron into proton + electron) becoming Pu-239. This takes
a few minutes - the half-life of U-239 is 23 minutes - which
is _forever_ on the time scale of a nuclear detonation.) The
Something similar can happen with Pu-239; it depends, I think,
on the energy of the free neutrons involved, whether the Pu-239
fissions or captures the neutron.

> And if pure U-238 were put in a reactor, could plutonium be
>"bred?" (Assuming that natural uranium or some other material
>that does spontaneously fission were used to "drive" the reactor.)

Sure. But why bother? Natural uranium is 99.3% U-238 already.

>>>> There's one other factor. Plutonium reacts faster than
>>>> U-235.
>
>>>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 ...
>
>> Pu-240 causes problems which defeat gun-type use of
>> plutonium, but even pure Pu-239 would fizzle. It has
>> a relatively high rate of decay by spontaneous fission.
>> SF releases neutrons, starting up the chain reaction
>> too early.
>
>Would fizzle if the implosion were not fast enough?

Yes. Or if the implosion was assymetrical or something.
A successful implosion bomb is a non-trivial engineering
problem.

>> Pu-240 does make this problem worse, having an even
>> higher rate of SF. When plutonium is made in a breeder
>> reactor, the longer the 'breeding' mass is left in
>> the reactor, the higher the proportion of Pu-240
>> that is produced. 'Weapons-grade' plutonium is pulled
>> early, when it is 97% Pu-239. 'Reactor-grade'
>> plutonium is left in longer, and is only 70% Pu-239.
>
> I want to be sure that I'm reading this right. Not
>all the uranium is converted to plutonium, right?

Correct. For one thing, after a while some of the Pu-239
'burns' too, i.e. is fissioned and releases energy.

>The numbers you quote are the percent of Pu-239 compared to
>Pu-240, not to uranium.

Yes.

>If that's correct, how much uranium is left when the Pu
>is 97% 239, and when it is 70% 239?

No good idea - probably most of it.

> Interesting that weapons-grade is pulled earlier...
> And, has anybody ever tried pulling the breeding mass
>much earlier--when only a small fraction of the uranium
>has been converted to plutonium, so the fraction of Pu-240
>should be very small, and using simple chemical means to
>separate out the Pu; thus enabling a plutonium weapon that
>could use a gun-type detonation?

No - as mentioned above, even pure Pu-239 would fizzle
in a gun-type bomb. Its rate of spontaneous fission and
neutron emission is too great. The k factor goes above
1 before the mass is fully assembled, so the explosion
starts early blowing the mass apart. Remember - the
assembly process in a gun-type bomb takes milliseconds,
but k-intervals are in billionths of a second.

>What is the advantage of going to the higher fraction
>of Pu-239 in weapons?

Fewer neutrons from spontaneous fission.

As with chemical explosives, the idea is to have something
that is stable and non-explosive, _unless_ deliberately
set off. One wants a substantial difference between the
explosive and non-explosive states. The problem of premature
detonation and dispersal of part of the explosive mass
before it all detonates is an old one.

Pu-240 goes critical too easily. I suspect that if one
wanted to build a very small bomb, it might be done
most easily with Pu-240, though.
--
| The shocking lack of a fleet of modern luxury |
| dirigibles is only one of a great many things that |
| are seriously wrong with this here world. |
| -- blogger "Coop" at Positive Ape Index |
--

.

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