Xcel Energy won state approval on Thursday to store more nuclear waste at its Monticello Nuclear Generating Plant, a requisite step to extend the plant's life by at least 10 years.

The Minnesota Public Utilities Commission unanimously allowed Xcel to expand its storage space at Monticello to house an additional 36 canisters of high-grade nuclear waste.

Xcel says Monticello currently houses 30 steel casks of waste in concrete storage structures.

The plant was built in 1970 with a 40-year federal operating license, and it was relicensed in 2010. Xcel has asked the U.S. Nuclear Regulatory Commission (NRC) to re-up Monticello for another 20 years after its current license expires in 2030.

The federal licensing review is extensive, and Xcel doesn't expect a decision from the NRC until late next year.

Minneapolis-based Xcel says it plans to add 14 waste casks at Monticello between 2030 and 2040 if its federal license is renewed. But the company wants space for more if the plant continues running beyond 2040, hence its request for 36 more casks.

Relicensing by the NRC comes in 20-year increments, so Xcel would have to go back the PUC for any further approvals to extend Monticello's life beyond 2040.

Xcel, the state's largest electricity provider, is also expected to seek relicensing for its two nuclear generators at Prairie Island; their current licenses expire in 2033 and 2034. Xcel says its three nuclear generators provide more than 30 % of its power in the Upper Midwest.

The company sees nuclear energy as critical to meeting its own — and the state's — goals of producing 100 % carbon-free energy. A law passed earlier this year in Minnesota calls for state electricity producers to meet that goal by 2040.

"Nuclear power is crucial to achieving those goals because of its unique combination of reliability, affordability and zero emissions," Christopher Clark, Xcel's president for Minnesota and the Dakotas, said in a statement.

Still, the industry has an intractable problem of permanently storing toxic waste. Plans for a national nuclear waste depository stalled long ago, so casks of spent fuel keep growing at nuclear plants across the country.

PUC Commissioner John Tuma, while he voted for the Monticello measure Thursday, acknowledged misgivings about the waste legacy, apologizing "to our great-great-grandchildren down the road."