Two utilities companies announced Monday that they will abandon construction of a pair of nuclear reactors in South Carolina, Brad Plumer of The New York Times reports. The project has been beset by construction difficulties, regulative hindrances, and financial struggles resulting from decreasing demand for electricity amidst improvements in energy efficiency.

The reactors are 40% complete and have already cost a combined $9 billion.

The V.C. Summer project, as the effort to build the reactors is known, was proposed in 2007 with a view toward reviving the United States’ nuclear power program. At that time, the country had not built a new nuclear reactor since the 1970s.

Originally, the project was scheduled to be complete by 2018, but earlier this year, officials said they did not expect the reactors to be functional until 2021. Moreover, project administrators revised their cost estimates to $25 billion; the initial figure was $11.5 billion.

“This was a first-of-a-kind project, so it was always going to be hard,” said Rich Powell, executive director of the ClearPath Foundation, a clean-energy group in Washington, per The New York Times. “But you can also see this as a symptom of a broader problem. We’ve let our nuclear industry atrophy for 30 years, and we’ve lost the robust supply chains and expertise needed” to build reactors.

Customers of the two utilities companies spearheading the project, SC Electric & Gas and Santee Cooper, were funding it. Before the abandonment plans, the project accounted for 18% of the bills of the former company’s customers. The latter company, which is state-owned, had raised rates five times to cover the ballooning costs.

The reactors were AP1000 models designed by Westinghouse Electric Company, a subsidiary of Toshiba. The Westinghouse website touts the AP1000 as “the safest and most economical nuclear power plant available in the worldwide commercial marketplace.”

But, officials broke ground on the VC Summit project before the AP1000 model was finalized, and as a result, costly safety modifications had to be made during the construction process.

In 2015, Westinghouse bought out its partners and became the leading contractor on the South Carolina project, but according to Plumer, “analysts say the company did not have sufficient expertise in handling large construction projects.”

In March, buried under the costs of its nuclear development programs and reeling in a market in which demand for electricity was stifled by a glut of natural gas, Westinghouse filed for bankruptcy. Parent company Toshiba paid $2.2 billion to be “released from the project” (Plumer’s words), but SCEG and Santee cited “the [insufficient] amount of anticipated guaranty settlement payments from Toshiba” as a principal factor in their decision to abort the project.

Westinghouse was also spearheading construction of the two other nuclear reactors being built on US soil, a pair of AP1000s at the Vogtle nuclear power plant in Georgia. Southern Company has agreed to take the lead on those projects in the wake of the Westinghouse bankruptcy, but the nuclear efforts in Georgia face their own obstacles.

According to Plumer, “the [Vogtle] reactors will have to come online before 2021 to qualify for federal tax credits, although Congress is working on a bill to extend that deadline.”

President Obama’s Clean Power Plan would have provided incentives to finish the nuclear projects, Plumer says, but the Trump administration is “dismantling” the CPP initiative.

Nuclear power is a leading contender as an environmentally-friendly fossil-fuel alternative, but the potential hazards, the inconvenient size, and the high costs of today’s nuclear power plants make wind energy and solar energy more attractive to utility companies. Nuclear power has been further pushed to the back burner by the affordability of natural gas.

However, companies like Oregon-based NusScale Power are working to make reactors smaller and safer. NuScale submitted plans for the first ever small modular reactor to the US Nuclear Registry Commission (NRC).

Still, the abandonment of VC Summit, which comes as over a dozen nuclear plants across the USA are being retired, is widely regarded as a major setback to the nation’s nuclear power efforts.