A Transport Ministry study group on tunnel safety compiled a final report Monday that puts tunnel inspections into three categories and will serve as a new manual for tunnel maintenance. In line with the report, the ministry the same day issued instructions to the 108 railway operators nationwide to complete thorough inspections based on the manual within one year for tunnels on Shinkansen bullet train lines and within two years for all other tunnels. There are 370 tunnels along Shinkansen routes and 4,840 tunnels on other railway and subway lines, according to the ministry. The new manual says tunnel inspections should be handled differently depending on their category -- initial, regular or special. The manual offers illustrations and photographs that will show maintenance crews how inspections should be carried out and what points to look for when they tap tunnel walls with hammers. The study group said railway operators would be able to locate cracks, borders between older and newer layers of concrete and other conditions of tunnel walls through the complete checkup as stipulated under the manual. Monday's report says railway operators need to conduct thorough inspections on new tunnels soon after they are completed and carry out regular checkups on all tunnels every two years. Special checkups should be carried out on tunnels on Shinkansen lines within a decade of going into use and on all other tunnels within two decades, the report said. The study group also urged railway operators to establish an education system to nurture proper tunnel inspectors and develop technological devises, including lasers and infrared cameras, to speed up the checkups. "Examining major accidents last year (in which huge concrete chunks fell from tunnel walls), we found that the cracks that caused the accidents were formed right after the construction of the tunnels was completed," said Toshihisa Adachi, an engineering professor at Kyoto University who chaired the study group. "It's important to check tunnel walls at the earliest stage and use the records for future inspections." Three major tunnel accidents occurred last year. On June 27, a 200 kg of concrete fell off from the wall of Fukuoka Tunnel and ripped a long hole in the roof of a Hikari super express train. On Oct. 9, a 226 kg block of concrete fell off the wall of the Kitakyushu Tunnel in Fukuoka Prefecture. On Nov. 28, a freight train partly derailed after hitting a 2-ton block of concrete that had fallen off the wall of the Rebunhama Tunnel in Hokkaido.