Two references I have in my nuke library at home that discuss the shortage of Uranium in the Soviet Union in the early 1960s. However these two references to not reference the RDS-220 specifically.
"Cost of Uranium Concentrates to the Soviet Union" CIA 1973 (declassified in 1994)
"Enriched Uranium vs. Plutonium: Proliferant preferences in the choice of fissile material" by Joel Ullom 1994
Most of my nuclear references are in my office which I will check tomorrow when I get to work.
Just a quickie.
Uranium for the Soviet Union Nuclear program was supplied primarily from Romania though the SOVROM-CUARTT enterprise. This supply was stopped in 1961 due to political issues.
http://www10.antenna.nl/wise/index.h.../chapter1.html
This lead to the Soviets starting their Acid
In Situ Leaching industry, in 1961 as a way of increasing their domestic production of Uranium. I imagine that this switching from imported Uranium from Romania to developing a new procedure accounted for the shortage I remember reading about (and am still looking for the citation)