Correction: Because of a production error, a portion of the third paragraph on page 64 was deleted. The complete paragraph should read:

Vendors like BWXT are unlikely to admit it, but these small reactors may still be too expensive to build and operate in a competitive market. (BWXT declined to comment for this article.) Harnessing nuclear energy remains an enormously complicated engineering feat, and the cost of building mPower’s reactor module alone could run to hundreds of millions of dollars. With natural gas at record-low prices, thanks in part to fracking, and wind and solar energy becoming cheaper every year because of technical and manufacturing innovations, nuclear utilities are finding it harder to sell their electricity. A report last November by Lazard, a financial- advisory firm, compared the prices of nineteen sources of energy. Electricity from a natural-gas combined-cycle plant cost $52 to $78 per megawatt-hour, wind turbines cost $32 to $77, and a utility-scale solar photovoltaic plant cost $43 to $70. Nuclear plants like Vogtle are in a class by themselves, with costs of $97 to $136 per megawatt-hour—and the difference is growing. Lazard estimates that wind and solar power have become 61 percent and 82 percent cheaper since 2009. In the past three years, utilities have shut down five reactors with active operating licenses in the United States, and at least three more will follow suit by 2019.