Toshiba shares fell 8 percent on Tuesday as the company took a $6.3 billion writedown related to its struggling nuclear power business and delayed the release of its fiscal third-quarter earnings. Shigenori Shiga, Toshiba's chairman and representative executive officer, resigned effective Wednesday, the company said. His resignation in part reflects management taking responsibility for the loss of goodwill and the impairment related to Westinghouse's acquisition of CB&I Stone & Webster from the Chicago Bridge & Iron Co. N.V.