Originally posted by snoopy
The first nuclear bomb did not use beryllium/polunium.
Oh, sure it did. Fat Man used a fairly stock urchin design.
And since Fat Man was what was toggled off at Trinity, yes, the first nuclear bomb used a BePo urchin initiator.
Gun type bombs like Little Boy don't strictly need one, but in fact that one used an initiator as well, only it was an ABNER - you only use urchin in
spherical compression weapons.
The purpose of the initiator is to provide a source of neutrons.
That part's right.
It's not required for a nuclear reaction/explosion, but the yield without it will be reduced drastically. The neutrons are already present in the
Plutonium. and when compressed they will collide with each other and radiate more neutrons. But the berylium/polonium provides an abundant source of
Neutrons which means that the number of collisions will increase greatly and thus produce a much greater yield.
But this part's not. Ok. Lots of matter 'has neutrons in it', that's not the issue. edit: also the "collide with each other and radiate more
neutrons" part is just incorrect.
You're sort of conflating the boost system and the initiator here, and you can't do that. They are similar but have different intent.
The supercritical assembly serves one purpose - it's a neutron amplifier with positive feedback and a significant gain. The purpose of the initiator
is to provide the initial input at just the right time.
Gun type weapons don't need initiation at all, because the segments will fuse when they hit. An initiator is useful but not a requisite - eventually
the supercritical mass will emit a spontaneous neutron and start the ball rolling. Clean U235 will do that about every 60th of a second.
Plutonium has the other problem - Pu240 contamination will cause it to emit lots and lots of spontaneous neutrons. That's actually a problem. You
can't build a gun-type bomb out of plutonium because of the spontaneous emission - it won't assemble quickly enough that way to prevent a fizzle
yield. The mass has a high probability of emitting a spontaneous neutron and initiating itself while the neutron gain is not much over 1.0, and you
will get a dud as the mass scatters itself and not much more.
That's why you generally only use spherical compression systems with plutonium, it goes from sub-critical to super-critical very quickly (there's
several tricks to that) as it's compressed, so the window of time where you'd get a fizzle yield during compression due to a spontaneous emission is
Now, though, you've got the other side of the coin. With spherical compression, if you DON'T get a spontaneous emission within a very narrow moment
of time, you will reach maximum compression and the pit will begin to disassemble due to Rayleigh and Munroe instabilities and rebound expansion. In
other words, it tries to squirt out like water from your hands.
That window's pretty narrow too. You can't count on the emission of a spontaneous neutron during that time. You HAVE to initiate a spherical
compression device for that reason. It doesn't have anything to do with the yield per se. Well, whether you get one or not, I guess.
The initiator may not and in most cases does not emit more than 2 or 3 neutrons into the supercritical mass during the time it's active. That's all
you need to set it off.
Now, the other concept you're mixing in is the boost system of later designs. In that case, you can inject tritium into the pit just before/during
the start of compression. When the initiator fires, the mass will begin to react and the tritium will start to fuse. The T-T reaction takes more
energy than it releases, but what it releases is LOTS of neutrons. It may be a negative contributor in total energy, but it is a way to get a sackload
of neutrons into the pit while it's still supercritical. So, yes, your concept is a correct one but you're attributing it to the wrong bits in the
weapon. The neutron output of the boost system is many orders of magnitude higher in terms of neutrons per second than the initiator's.
Design has gone all the way around though, and the new "no maintenance" weapon that Sandia won the contract for does not use a traditional tritium
But my only point is that one could make a basic uranium bomb. It just wouldn't be nearly as powerful, but it would be powerful enough to serve the
purpose of terrorism. Just 1% efficiency would be devastating even if tiny compared to a current thermonuclear bomb.
Not with a spherical compression weapon, even when it's Uranium. The A.Q. Khan designs are all spherical compression uranium. They will disassemble
far too quickly to rely on a spontaneous emission. Of all the designs, that one would need the initiation most. You could build a gun type weapon,
though, which would not need an initiator.
There are other bomb topologies which we don't use for various reasons, but if you don't mind a low yield for your fissiles, you could use either
plutonium or uranium. We only designed one weapon that used 'an alternate topology' in the past. It doesn't meet several of the safety
requirements. But if I had to make one using a machine shop, it's probably what I'd do.
[edit on 8-9-2007 by Tom Bedlam]