VC Summers nuclear project could see an increase of $421 million owed after a state audit found the utilities behind it owe sales tax on the materials they bought for the unfinished plant.

The utilities weren’t initially required to pay sales tax on the reactor parts because South Carolina exempts major manufacturers from paying tax on the materials they use to build plants. But after the power companies called off construction, the S.C. Department of Revenue questioned whether they owed sales tax.

The assessment covers every item that South Carolina Electric & Gas and Santee Cooper bought for the massive construction project.

Both SCE&G and Santee Cooper have said they will challenge the findings of the audit but both will likely take different approaches.  Santee Cooper spokeswoman Mollie Gore said the state-owned power provider plans to contest the audit separately. Unlike SCE&G, Santee Cooper hasn’t formally abandoned the V.C. Summer project, potentially creating an opening for it to hold off the tax bill.