Note that Orion nuclear pulse is perfectly workable and could reach between 1% and 10% of the speed of light.
Sadly... no. Realistic near-term (from the vantage point of 1964) specific impulse for Orion was expected to be around 4,000 seconds. *Really* optimistic thinking for a far-term Orion would be about 12,000 seconds.
Math: assume a delta V of 1% c, or about 2,997,920 m/sec. Assume 12,000 sec Isp. If your ship ends up with a mass of one metric ton, you need to start off with...
m0 = 1000 * Math.exp(2997920 / (9.8 * 12000)) =
117830973530924.19
That's 117,830,973,530.9 metric tons for the initial mass, or 117,830,972,530.9 tons of pulse units strapped to a one-ton spacecraft.
If you want to get to even 1% lightspeed, which would put Alpha Centauri as far away as Ivan the Terrible and Queen Elizabeth I, you need to do *waaaay* better than Orion. Not a measly 12,000 seconds Isp, but something pushing a million. Medusa, with a 400,000 sec Isp, could do it with a mass ratio of 2.147.