Costs have also climbed from $19 billion in 2016 to around $23 billion early last year at 2015 prices. Under its contract with the U.K. government, the project developer absorbs construction cost hikes in return for a guaranteed electricity price for the first 35 years of operation.

"This is the first time such a financing approach has been used for nuclear power anywhere in the world," noted the National Audit Office. For subsequent nuclear plants, the UK government will use a financing system employed in water and other utilities.

Under the proposed regulated asset base system, an operator would finance work through regulated charges to electricity consumers from the start. According to the energy department, the system reduces costs by passing some construction and operating risk to consumers.

In its home country, EdF has again delayed fuel loading at its 1,650-MW Flamanville 3 project from the end of this year to mid 2023. It has also announced another cost hike to $13.4 billion, about four times more than first forecast when the developer started the project's 4.5 years of planned construction in 2007.

Using the same basic EPR technology, the Olkiluoto III plant in Finland is running even later. In April, utility Teollisuuden Voima Oy said electricity production would start this September. Construction began in 2005 with a 2009 completion deadline.