By Yuka Obayashi and Ritsuko Shimizu TOKYO (Reuters) – Japan’s biggest steelmaker Nippon Steel Corp will be able to secure alternatives for a raw material that it buys from Russia and Ukraine in the event of any supply disruptions amid tensions in Eastern Europe, a company senior executive said. Russia launched an all-out invasion of Ukraine by land, air and sea on Thursday, hours after President Vladimir Putin said he had authorised “a special military operation” to eliminate what he called a serious threat. Nippon Steel, the world’s fifth-biggest steelmaker, buys 14% of its iron ore pellets,…