Several small cracks have been discovered on the Oroville Dam’s newly rebuilt concrete spillway, prompting federal regulators to express concern about the $500 million construction project under way at the troubled facility.  But state water officials said Tuesday that the series of millimeter-wide cracks on the surface of the main spillway pose no structural problems for the nation’s tallest dam. Such tiny fissures, officials said, commonly develop after massive amounts of concrete are poured.