I know some will be bored with this, but it seems that a lot of the initial objections to Orion have been negated or greatly reduced. The benefits are just so great...

The original Orion design called for 2000 pulse units, far more than enough to attain Earth escape velocity. "Our motto was 'Mars by 1965, Saturn by 1970'", recalls Dyson. Orion would have been more akin to the rocket ships of science fiction than to the cramped capsules of Gagarin and Glenn. One hundred and fifty people could have lived aboard in relative comfort; the useful payload would have been measured in thousands of tons. Orion would have been built like a battleship, with no need for the excruciating weight-saving measures adopted by chemically-propelled spacecraft.

The launch pad would have been composed of eight towers, each 250 feet high. The mass of the vehicle on takeoff would have been on the order of 10,000 TONS; most of this mass would have gone into orbit.

Project Orion - Life, Death and possible rebirth

(For comparison, the Space Shuttle weighs about 2200 tons on takeoff, and about 26 tons of payload reach orbit.)

Orion will not disappoint. The design achieves an Isp of 12,000 seconds with an Earth-launched payload capacity to LEO of 5500 tons. The fuel would be 800 nuclear bombs.

(for comparison, the best chemical rockets have an Isp of about 450 seconds. Specific impulse (Isp) is defined as pounds of thrust produced per pound of propellant consumed per second.)

This is a late ’50s design that has actually had its economics improve over time. The weight of 800 nuclear bombs has gone way down. For the eight terajoule (two kiloton) explosions required for the 4500-ton version that can carry 20 people to the Moon, Jupiter, and Saturn in the same trip, instead of the 900 tons of nuclear bombs at almost one ton each, there would need to be probably only 50 tons’ worth of bombs since artillery shells were subsequently developed that weighed much, much, less than what could be conceived in the late ’50s. Furthermore, an artillery shell is what is needed because the bomb is going to be shot out the bottom of the Orion ship at intervals as short as one-fourth of a second at first.

From The Space Review

One answer to avoiding fallout for an Orion launch is ludicrously simple. Use a thick metal launchpad. Its as straightforward as that. The Orion team discovered that a thick metal plate can withstand close proximity nuclear blasts very well.

Evidence of this obtained from nuclear tests was the foundation for further research into the feasibility of a pusher plate. It was discovered that ablation (erosion) of the surface of a pusher plate could also be reduced by coating it with graphite. Coating the launch pad in similar fashion would minimise ablation of the surface and therefore create very little fallout indeed. Airbursts are relatively clean from a fallout point of view.

They do generate some fallout from atmospheric dust and water vapor but not as much as legend holds.

Further reduction of fallout can be achieved by using more state of the art pulse units than those proposed back in the 60's. Over 80% of the atmosphere lies within the first 16km from the ground. Its almost completely gone at around 50km. Lessening the concern of bomb shrapnel as a source of residual fallout would make airbursts cleaner still. Casings could be composed of something less amenable to absorbing blast radiation for a start.

Based on more modern research the calculated environmental impact of an Orion launch might cause a single fatality on a global scale. That however is assuming a 10,000 ton Orion designed with 60's fission bombs and launched from Nevada with no effort to minimise groundburst fallout.

From SpaceDaily