<"The article suggests that 7 kilograms of Plutonium can generate 300 Watts of electricity. That means that it would take about 61,833 metric tonnes to generate 2.65 Gigawatts of electricity needed to power electrodynamic stationkeeping engines for a Rotavator that can launch 50 tonnes of payload to orbit every hour."

Probably not that simple, a big factor to take into account here is the ratio between surface area and volume of the radioactive(fissile?) material. The geometery of the setup is an important factor to consider. Plus that much material U in one compact pile and I'm thinking it WOULD go critical, not kaboom but highly radioactive.

Besides, If you're putting up a slug of enriched Plutonium that massive, you may as well use solar arrays or fission (or kerosene ;> )