Is there any document about the difference between U235 and U238 properties?
Chemically, isotopes are essentially indistinguishable - they have the name Z and same number of electrons, and the difference in mass of the nucleus doesn't cause a substantial difference in chemical properties, but there is just enough difference to separate the isotopes by laser isotopic separation.
The nuclear properties are quite different and these are published in many textbooks or available from IAEA/NEA and other sources.
For example - http://www.nndc.bnl.gov/sigma/index.jsp Just click on the element.
My guess is he is interested in fast fission cross sections...for pease! [sic]
Separate names with a comma.