I How to understand Plutonium enrichment in the graph

[Neutroniclad]

http://ncsp.llnl.gov/ARH-600/files/pdfs/III.A.4-2.pdf
For a certain fixed density (let's say 1g/cm3) different enrichment corresponds with different cylinder diameter
20% wt 12.0in
15% wt 11.2in
10% wt 10.4in
5% wt 9.6in
0% wt 8.4in
Why the decrease of enrichment result in a smaller critical diameter?

 

[DrDu]

I think Pu-240 isn't fissionable or not as easily while it may absorb neutrons. Hence the less Pu-240, the less the critical diameter

 

[Neutroniclad]

I think Pu-240 isn't fissionable or not as easily while it may absorb neutrons. Hence the less Pu-240, the less the critical diameter

Thank you for your opinion.
So you mean the number here refers to the enrichment of Pu-240? i.e. 5%wt means 5% Pu-240 with 95% Pu-239.
If so, does 0%wt refers to 100% Pu-239? Or is there something else in the plutonium?

 

[DrDu]

So you mean the number here refers to the enrichment of Pu-240?

Yes, it says so. But I can't judge from only the graph that the rest is Pu-239. However, I would suppose so.