BRUSSELS, Sept 15 - Sweden's resounding rejection in a referendum to join the euro is not a catastrophe as the European single currency is already powerful, the European Commission said on Monday. "We would naturally want that everybody joins it, in time, when proper circumstances are met. But if somebody decides not to join, it's not a catastrophe for a currency that is already powerful," chief Commission spokesman Reijo Kemppinen told a daily briefing.//