HOUSTON, April 28 (Reuters) - Lower natural gas prices and stagnant growth in electric demand will lead to the loss of 10,800 megawatts of U.S. nuclear generation, or around 10 percent of total capacity, by the end of the decade, the U.S. Energy Information Administration said in a report issued on Monday.

About 6,000 MW of nuclear capacity will shut by 2020 in addition to six reactors totaling 4,800 MW that have already shut or plan to shut in that time period, the EIA said in its 2014 annual electric output study.

“Retirements often are the result of unique circumstances, but some owners of nuclear power plants have voiced concerns about the profitability of their units,” the EIA said in its report.

Lower natural gas prices that have pared wholesale power prices will hurt profitability for nuclear units and some high-cost reactors will shut as economic challenges mount, the EIA said.

“When faced with declining profitability, plant owners may choose to retire their units rather than make additional investments to keep them operating,” the report said.

“Those projected retirements are represented by derating of existing capacity for plants in vulnerable regions, not by retiring specific plants,” EIA added.

Rising natural gas prices after 2020 may support continued operation of U.S. nuclear plants for several years, but many reactors will reach the end of their 60-year operating license beginning in 2029 and shut permanently.

The EIA outlook changed after four reactors shut in 2013 - Edison International’s San Onofre 2 and 3, Dominion Resources’ Kewaunee and Duke Energy’s Crystal River. A fifth reactor - Entergy Corp’s Vermont Yankee - is also set to retire by the end of 2014.

A sixth reactor, Exelon’s Oyster Creek, is scheduled to shut in 2019.

In its 2013 report, the EIA projected only 7,700 MW, or about 7 percent, of nuclear capacity would retire by 2040. The report did not mention the number of units that are likely shut after operating for 60 years after 2029.