In a follow-up to their already successful gold, platinum and titanium cards, Visa metallurgists unveiled the new Polonium Plus credit card Monday. 'With its zero percent introductory interest rate, credit line of up to $500,000 and impressive 962 Celsius boiling point, Polonium Plus is the only choice for the discriminating shopper,' said Visa scientist Dr. Andrew Manzanillo, one of the developers of the radioactive, no-fee card. 'And with more isotopes than any other card, Polonium Plus isn't going to decay any time soon.' Polonium cardholders who maintain a good credit record for three years will automatically be eligible for the inert Xenon Card, which comes in an attractive glass tube. 