A policeman seals off the Oskarshamn nuclear plant in southeastern Sweden May 21, 2008. Police said on Wednesday they were interrogating the welder who had entered the nuclear plant on Sweden's southeast coast carrying TATP, or triacetone triperoxide, a high explosive which is extremely unstable, especially when subjected to heat, friction and shock. REUTERS/Paul Madej/Scanpix (SWEDEN) NO COMMERCIAL SALES. SWEDEN OUT. NO COMMERCIAL OR EDITORIAL SALES IN SWEDEN. | Foto: Reuters