What is the difference between a uranium bomb and a plutonium bomb?

Answer 1

The difference simply comes down to the fissile material (i.e. uranium or plutonium) used as fuel in the core (aka pit) of the bomb. There is also a third type that is less well known among the general public, the composite bomb which uses both uranium and plutonium together as fuel in the core (aka pit) of the bomb. In the composite bomb the uranium and plutonium could potentially be alloyed together, but are more likely (for metallurgical reasons) prepared as separately cast and machined nested shells that must be assembled to create the core (aka pit) of the bomb.

The secondary difference is the choice of rapid assembly mechanism used in the bomb to assemble a subcritical mass of fissile material into a supercritical mass of fissile material rapidly enough to avoid having stray neutrons initiate a chain reaction prematurely causing a fizzle. A fizzle may do no more than simply melt the bomb or may do as much as producing a yield of a few tons of TNT equivalent, but is unpredictable in effect and thus undesirable. There are several potential sources of stray neutrons ranging from cosmic rays to the materials of the bomb itself. If it were possible to obtain fissile materials that are isotopically pure (e.g. uranium-235, plutonium-239) the choice of rapid assembly mechanism would be a nonissue, but isotopic purity is simply an impossibility on the industrial scale that these materials must be produced. The issues with each fissile material are as follows:

• uranium - must be enriched from natural uranium and thus will always contain some uranium-238 which is not fissionable and captures the high speed neutrons needed to sustain the chain reaction. The usual form of weapons grade uranium used in US bombs is called oralloy and is 93.5% uranium-235 and 6.5% uranium-238. Neither of these isotopes has a significant probability of spontaneous fission and as the remaining uranium-238 actually poisons the chain reaction some, a uranium bomb can use either the gun or implosion rapid assembly mechanism with no risk of a fizzle happening.
• plutonium - must be produced in a reactor from uranium-238. Typical weapons grade plutonium as produced in reactors is about 93% plutonium-239, 6% plutonium-240, and 1% plutonium-241. Unfortunately the plutonium-240 isotope has a high enough probability of spontaneous fission that with this much of it in the fuel a bomb using the gun rapid assembly mechanism would certainly fizzle, leaving the implosion rapid assembly mechanism as the only viable choice for either a plutonium bomb or a composite bomb.

In summary the three types of atomic fission bombs differ as follows:

1. uranium bomb - uranium fueled, may use either gun or implosion rapid assembly mechanism
2. plutonium bomb - plutonium fueled, must use implosion rapid assembly mechanism
3. composite bomb - uranium & plutonium fueled, must use implosion rapid assembly mechanism

Answer 2

Its not the material the determines the yield and amount of damage.

However the highest yield pure fission bomb was the MK-18 tested in the Ivy King shot on November 16, 1952 at 500 kilotons yield, used Uranium. The spontaneous fission rate of Plutonium is too high to permit building pure fission bombs of that high yield (they would fizzle, causing them to melt without producing significant yield).

However as the critical mass of Plutonium (6.4 kilograms inside a depleted Uranium tamper/reflector) is significantly less than the critical mass of Uranium (18.4 kilograms inside a depleted Uranium tamper/reflector), bombs built using Plutonium usually have higher material efficiency (higher yield per given mass of material used).

Answer 3

The two basically mean the same thing. Some people use the term "atomic" to refer to a pure fission bomb, and "nuclear" to refer to te more powerful fusion bomb.

Also there are modern compact fusion bombs with lower yield than many older pure fission bombs that used to be stockpiled. In this case the fission bomb could cause more damage, but damage magnitude was not the goal, reduced cost and small size for a given amount of damage was the goal.