I asked a question in similar vain for the "Fermi pile", however this reactor is different. It's purpose was to produce plutonium...now I understand the advantage of that since plutonium can be separated from the "atomic waste mess" much easier than separating U235 from U238.
But now it was mostly U238 but didn't there have to be some U235 in concentrated form to start the reaction or was it just the case that it was U238 and the carbon moderators.
Atomic Heritage Museum has video on the B reactor construction and I saw both video's but does not say if those were the only two components.....if so then the mystery is solved because apparently all that is needed for plutonium production is U238 and carbon. The rest comes out in the wash so to speak. Interesting video's by the way for the history buff.