Secretary of Energy Ernest Moniz predicted Monday that major decisions about the nation's nuclear energy sector are going to be coming in the next five years.

Speaking at the Center for Strategic and International Studies, Moniz said there's going to be a great wave of nuclear energy facilities retiring in about 15 years. Decisions about whether to replace them must be made much earlier than that, he said.

"In the utility business to replace that power, especially to replace that low carbon power, calls for capital allocations decisions are certainly on a decadal time scale," he said. "In other words, in the next five years, we're going to start much more facing up to those large capital planning decisions."

Moniz has been a supporter of increased nuclear power production in recent years, seeing it as a potentially lucrative low-carbon energy source that would help the nation meet its climate change goals.

However, public opinion continues to become more and more anti-nuclear power. A Gallup poll conducted in March showed more than half the country opposes nuclear power for the first time since 1994. Support for nuclear power had been growing slowly until 2010, when support quickly eroded away, the pollster found.

That reduced public support leaves a lot of unanswered questions about what to do with the nation's nuclear energy plants, Moniz said.

He said there's a scenario for the country's energy future where nuclear provides much of the nation's power, replacing coal and leading to more carbon cuts in the energy sector. But, many people don't find that appealing, he said.

"There is a certain view that energy efficiency, combined with wind, combined with solar, combined with natural gas to balance those valuable renewables in the near term ... will be the system," he said.

Moniz said the country must develop a "reliable, resilient, decarbonized" electrical system in one form or another. It's still up in the air if nuclear will be a part of that, he said.

There's a chance that the decision might end up being kicked down the road as nuclear power plants age, he added.

"Will most plants be able to go 80 years?" Moniz asked, raising the prospect of most plants retiring after 2050. "If it does, that's an enormous deal to shift that 2030 date to beyond mid-century. That's a very big deal."