TOKYO (Reuters) - Shares of Toshiba Corp tumbled 10 percent on Wednesday morning after the electronics conglomerate said it would book a $6.3 billion hit to its U.S. nuclear unit, a writedown that wipes out its shareholder equity.<div><a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=tP736f45-sM:bv4PP-Ua8xc:yIl2AUoC8zA"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?d=yIl2AUoC8zA" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=tP736f45-sM:bv4PP-Ua8xc:V_sGLiPBpWU"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=tP736f45-sM:bv4PP-Ua8xc:V_sGLiPBpWU" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=tP736f45-sM:bv4PP-Ua8xc:-BTjWOF_DHI"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=tP736f45-sM:bv4PP-Ua8xc:-BTjWOF_DHI" border="0" /></a></div><img src="http://feeds.feedburner.com/~r/reuters/technologyNews/~4/tP736f45-sM" height="1" width="1" alt="" />