Here's the link, and no mention of nuclear engines just the RD-180 for the Atlas rocket. One would think that if you are going to Mars, that you would consider Nuclear.
It is an irony that Russian engines are being used for US space boosters--but the efficiency and cost make them very competative with US engines.
The RD180 is a high efficiency, high pressure staged combustion rocket engine utilizing liquid oxygen and kerosene (well, actually RP-1 which is a specially formulated version of jet fuel with additives for turbopump lubrication.)
The RD180 are used in the Russian Zenit booster (which is the booster used by Sea Launch too.) The RD180 is 1/2 of the bigger RD-170 engine which has a larger turbopump that feeds four-thrust chambers instead of two.
