Steve Howe and the Center for Space Nuclear Research (CSNR) suggests using a Nerva like nuclear rocket stage to go from orbit to the moon It would enable 40% more cargo to be delivered on each trip. Previous work, including a NASA study, have suggested that it would cost only $2.5 to $3 billion to develop the nuclear rocket technology. This would be recovered by saving two trips to the moon.
Improving nuclear thermal performance (pdf) with oxygen afterburner or lasers for higher temperatures Although no performance gain is detailed it sounds like a 50% boost in ISP and thrust could be achieved.
Mars mission studies that also considered nuclear thermal The old tested performance of nuclear thermal was ISPs of about 800. Most new studies suggest ISP of 925 would be very achievable. Russian NTR fuel elements would allow ISP of 960+.
A 2005 NASA presentation on nuclear thermal Systems with ISPs of 1010 considered.
Wikipedia on different kinds of nuclear thermal rockets The solid core reactor is the one that was experimentally developed and which is the type being discussed. There are higher performing liquid and gas core versions.
The closed cycle (gas core) nuclear thermal engine actually has much more in common with the solid-core design, but this time limited by the critical temperature of quartz instead of the fuel stack. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a rather respectable specific impulse of about 1500-2000 seconds (15–20 kN·s/kg).
Calculations show that this rocket would have both very high thrust (1.3 x 107 N, for one design) and a very high specific impulse (66 kN·s/kg for the same design), a rare combination of traits in the rocket world.