In return for covering about one-third of the costs, the Chinese state-run company China General Nuclear Power Corporation will take about one-third ownership in the project. (A subsidiary of E.D.F. owns the rest.) The British government has also provisionally agreed to let China build a yet-untested Chinese-designed reactor in Bradwell-on-Sea, northeast of London, later.

These deals are a major coup for China, which yearns to become a player on the international nuclear market. But they are a risky move for Britain: They give a Chinese state-owned company access to Britain’s electricity grid, even though China is a longtime strategic competitor.

The British government has also guaranteed that investors in the Hinkley project will get $115 per megawatt-hour over 35 years. This is approximately twice the price of electricity today, and it is more than the prices of most major sources of renewable energy, which the International Renewable Energy Agency predicts will drop over the next few decades.

If the market price of electricity falls below that rate, a government company is contractually bound to cover the difference — with the extra cost passed on to consumers. Price forecasts have dropped since the deal was struck: This summer the government, revising estimates, said differential payments owed under the contract could reach nearly $37 billion.

If the Hinkley plan seems outrageous, that’s because it only makes sense if one considers its connection to Britain’s military projects — especially Trident, a roving fleet of armed nuclear submarines, which is outdated and needs upgrading. Hawks and conservatives, in particular, see the Trident program as vital to preserving Britain’s international clout.

A painstaking study of obscure British military policy documents, released last month by the Science Policy Research Unit at the University of Sussex, demonstrates that the government and some of its partners in the defense industry, like Rolls-Royce and BAE Systems, think a robust civilian nuclear industry is essential to revamping Britain’s nuclear submarine program.

For proponents of Trident, civilian nuclear projects are a way of “masking” the high costs of developing a new fleet of nuclear submarines, according to the report. Merging programs like research and development or skills training across civilian and military sectors helps cut back on military spending. It also helps maintain the talent pool for nuclear specialists. And given the long lead times and life spans of most nuclear projects, connections between civilian and military programs give companies more incentives to make the major investments required.