Good point; compare to the first 5 US reactors, the Chicago Pile and the plutonium breeding Oak Ridge X-10, and Hanford B, D and F reactors. All graphite moderated (very pure graphite, BTW, something the Nazi's couldn't get), all using natural, as in unenriched uranium (at least initially). That was obviously handy for bootstrapping prior to enough Highly Enriched Uranium (HUE, mostly U-235) being available from Oak Ridge's separation plants, which I gather the first major use of was for the Little Boy gun assembly bomb.