Nikkei set for 7th day of gains on optimism over shareholder returns

* Nikkei's rally longest since last December * SMK, Marubeni up on better shareholder return hopes * Tepco soars on report of contract for reactor in Qatar By Ayai Tomisawa TOKYO, May 25 (Reuters) - Japan's Nikkei share average rose for a seventh day, hitting a fresh 15-year high on Monday morning, helped by ongoing optimism for higher shareholder returns and the recovering economy. The Nikkei 225 gained 0.7 percent to 20,409.05, the highest level since April 2000. The N