# Little Boy (bomb) fission question

1. Oct 22, 2009

### pf_001

Hi, can someonse explain me some things about fission?

Supposedly, the Little Boy bomb consisted of a single bullet of uranium 235 that was to be shot at a bigger mass of uranium 235 to achieve critical mass, then an initiator would introduce a burst of neutrons so fission could occur.

Two things.

1. Why is a mass needed to be shot at another mass to achieve critical mass? Is it because it gets compressed?

2. I keep seeing that splitting an atom of uranium 235 releases 200 MeV according to e=mc^2. Can someone show this to me?

Thanks

2. Oct 22, 2009

### mgb_phys

You need to reach the critical condition quickly so that the neutrons generated will all hit other atoms of fissile material. If you do it slowly there will be some heat generated which might melt or partly destroy the mass before you have a full criticality. This is called a fizzle although it can still be one heck of a bang!

It's an experimental result, you obtain it by weighing the mass of the U235 before the reaction and the sum of the masses of the particles created. Actually you do it the other way around - you measure the 200Mev of energy and infer the loss of mass.

3. Oct 22, 2009

### CRGreathouse

Right. Although compression can work -- that's the (wasteful but reliable) design of Fat Man.

4. Oct 22, 2009

### Staff: Mentor

Why wasteful. The implosion device requires a smaller mass. The gun type is less efficient.

Nuclear weapons systems are prompt supercritical. The critical or supercritical mass must be assembled rapidly before it starts to heat, which changes the density, which allows neutrons to leak out of the assembly, which would reduce the yield and go subcritical quickly.

The prompt neutron lifetime is on the order 10-7 s, and the objective is to get several orders of magnitude of generations of neutrons, in order to get large amounts of energy in a few microseconds.

The fission process of U235 yields about 205 MeV of energy of which ~180-185 is manifest in the kinetic energy of two fission nuclei and the other 20-25 MeV is distributed in gamma rays, delayed neutrons, and beta particles.

5. Oct 23, 2009

### mgb_phys

Do you have these the right way around?
The little boy was U235 and a gun type, it used twice as much U235 than needed for criticality . You generally need to use a gun for U because it's self fissile and there are enough neutrons flying around that it's hard to get a critical mass together fast enough. But every design of a gun type (by a whole range of countries) seems to have worked first time.

Fat Man was more efficient, it used much less Pu for the same bang. But is much more complicated to detonate. Pu is easier to handle in a criticality, you even have to inject neutrons from an initiator to get a bang.

6. Oct 23, 2009

### PrincePhoenix

Where do the neutrons that initiate fusion come from?

7. Oct 23, 2009

### mathman

Pu bombs need implosion. Gun type detonation would lead only to fissile.

8. Oct 23, 2009

### Staff: Mentor

One would use an (α,n)Be source, e.g. RaBe, PoBe, PuBe source, or an isotope that undergoes sufficient spontaneous fission.

9. Oct 23, 2009

### Bob S

10. Oct 24, 2009

### willem2

You have these the wrong way around in another way.

You can't use a gun for Pu, because Pu has too much spontaneous fissions (because of contamination with Pu240). The gun works slower than an implosion, and the chance that a
spontaneous fission will set it off to soon is too big.
Guns types are indeed less efficient, but much simpler to make.

Initiators have to be used with implosion bombs, to get the reaction to start at just the right
time because the bomb will fly apart in a very short time even without a nuclear explosion.

11. Oct 24, 2009

### A/4

You actually can use a gun-assembly design with Pu-239, just not very effectively. The barrel would have to be enormously long because the assembly speed must be significantly higher than for U-235 (in order that the core doesn't fizzle). Originally, the Manhattan Project was studying the feasibility of this type of design, but abandoned it for the implosion mechanism. The device was called "Thin Man".