Why does U235 fission to U236 18% of the time

[Daniel Bolstad]

First of all, let me say I'm not a physicist, an undergrad, study physics or have any particular education surrounding physics. I'm actually simply a photographer with a special interest in science - so if possible, keep any answers simple.

Right, so I was reading about the fission of U235, and it said (the source being Wikipedia) that around 18% of the time it will not fission when hit by a neutron, but rather become U236 and emit the extra energy as gamma radiation. Why is this? Is there a quantum-mechanical reason for this? If so, why is the result not truly random?

Secondly, it says that U236 is long-lived and unwanted. From what I've understood though, U236 is very unstable (which is why it is used for creating nuclear chain reactions). How can both claims be true? What separates the 82% of U236 that is so unstable that it fissions from the 18% of U236 that is apparently stable and long-lived?

Any answers (hopefully as simple as possible) are appreciated! :)

 

[mfb]

Why is this?

Both processes are possible. It would be odd if one would not occur. Either you get immediate fission or you get a U-236 nucleus.

f so, why is the result not truly random?

The result is truly random. It is impossible to predict what a specific nucleus will do, all you can do is give a probability.

U-236 is not unstable (on relevant timescales for reactors), and not used in chain reactions.

 

[Daniel Bolstad]

Excellent, thanks for the answer!
So that means the U235 fissions at the moment of impact with the neutron, and never gets a chance to turn into U236? (Except for 18% of the time?)
Because from what I'd understood from this it first turns into a U236 atom, then almost instantly fissions into lighter elements (or becomes stable).

 

[mfb]

So that means the U235 fissions at the moment of impact with the neutron, and never gets a chance to turn into U236?

At least not U-236 in its ground state. For very short-living intermediate states naming can be a bit arbitrary.

 

[David Dilmore]

Sorry to seem stupid here..In a reactor fueled with U235. An atom of U235 on absorbing a thermal neutron turns into U236. Now the just created U236 atom is unstable and 82% of the time the nucleus will split creating two new atoms. Why for 18% of the time, does the nucleus not split.. it is unstable. Does it become stable by another decay route.
I am doing a MOOCs course on Futurelearn.. The Science of Nuclear Energy and am confused about why the nucleus doesn't always split. Lots of papers and articles blithely talk about the products of the fission of the nucleus... but none explain why it only happens 82% of the time. Help.. I spent all last night online trying to find the answer... maybe I'm just too thick to understand...sigh

 

[mfb]

The created U236 is an excited state*. It either decays to a ground-state U236 or fissions. Two different possible decay modes.

*or something that is hard to call a state at all.