is there one functional unit per segment?
You can find the chassis ("segment") layout on page 2-3 of the Cray 1 Hardware Reference.
As you can see, the middle sections contain the functional units, and the left and right sections contain storage.
but what is a module?
Module types are listed in Appendix B of the above reference manual. With names like "address adder", "program branch control", "floating add coefficient add (front half)" you can get some idea of the complexity of the functions that are implemented by a module.
how does the functional completeness of NAND correspond to modules?
I have no idea what you mean by this question. Edit:
If you are talking about the design choice to use mostly one type of logic circuit to implement everything: The reference manual says on page 2-5
With minor exceptions, one type of logic gate is used for the central processing unit. This is an ECL circuit with either four or five inputs and with both normal and inverted outputs available to drive loads. One four-input gate and one five-input gate are packaged in a 16-pin flat pack. All latches, adders, subtracters, etc., are made of this basic gate.
Wikipedia indicates that these are NOR (or rather, NOR+OR) gates.
So "the completeness of NAND" doesn't "correspond" to modules at all.
The fact that you can implement all logic either with NAND or NOR allows the design choice to use a single component in the majority of places, leveraging benefits of scale (easier to produce, easier to swap out). But this still has no connection to "modules"; you can build up everything out of such components, no matter if it's subdivided in "modules" or not.
where is the memory contained?