Federal regulators have asked the officials who operate Oroville Dam — and who are in charge of the $500 million-plus effort to rebuild and reinforce the facility’s compromised spillways — to explain small cracks that have appeared in recently rebuilt sections of the dam’s massive concrete flood-control chute. In a previously undisclosed October letter, the Federal Energy Regulatory Commission told the state Department of Water Resources to document the extent of tiny cracks that have showed up in some of the spillway’s brand-new concrete slabs. FERC also asked DWR what, if any, steps might be required to address the issue.