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.

“These cracks are not abnormal nor do they cause a concern,” Erin Mellon, spokeswoman for the California Department of Water Resources, said in an email.

The dam has been under heavy scrutiny since the main spillway partially collapsed in February, forcing 180,000 people in downstream communities to evacuate due to fears that water could spill uncontrollably from the reservoir.

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The Department of Water Resources recently wrapped up the first phase of its two-year plan to rebuild the main spillway as well as a back-up emergency chute that was also damaged amid heavy rains last winter. Officials say both spillways are ready to handle whatever weather comes this winter.

The cracks in the partially reconstructed main spillway were revealed in a series of letters between state officials and federal regulators, which were first reported by KQED.

On Oct. 2, the Federal Energy Regulatory Commission asked the state to investigate the apparent cracking of the new spillway and report back. On Nov. 7, the state responded that the “hairline cracks are a result of some of the design elements included to restrain the slabs and produce a robust and durable structure.”

State officials elaborated Tuesday that the fissures developed when the newly laid concrete settled, and that the problem commonly occurs with large pours, especially when steel girders are placed within the concrete to strengthen it.

The state declined to release a full report on the cracks, citing security concerns.

Federal regulators, in a letter sent last week, said they agreed with the Department of Water Resources that the cracking presented no immediate problem, but expected the state to monitor the spillway surface once it resumes water releases from the reservoir.

Mellon said the state will work with its contractor, Kiewit Corp., to revise the concrete mixture to reduce cracking in future pours at the dam. However, she said that any changes come with the risk of weakening the concrete.

“Considering these hairline cracks do not cause a concern, the mixture may remain the same going forward,” she said.

Lake Oroville, on the Feather River about 75 miles north of Sacramento, is California’s second largest reservoir. In addition to providing water for cities and farms across the state, the facility offers flood control for Oroville and many communities to the south.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander