^^^ the russian help was mainly in minaturizing the HEU mobile nuclear reactor.
India earlier focussed on a plutonium based mobile nuclear reactor... since India has had more experience with plutonium, than HEU.
Though, finally, India had to settle for HEU based mobile nuclear reactor... that is the prime reason for setting up the Uranium enrichment plant at Rattehali.
The mobile nuclear reactor is only the second HEU based plant in India, apart from the Tarapur nuclear reactors.
That said, there is very little between Charlie, Akula and the Arihant.
Another issue, I read somewhere, with china's mobile nuclear plants is high radiation leakage, which is hazardous for their crew.