A new Didcot Science Bridge and road will connect the A4130 to the Collett Roundabout through the former Didcot A Power Station site.
A new Didcot to Culham road bridge will also be built between the A4130 and A415.
The UK Atomic Energy Agency (UKAEA), which is based at Culham Campus, said last year that the successful delivery of HIF1 was “pivotal” to its work.
The leaders of South Oxfordshire and Vale of White Horse District Councils, David Rouane and Bethia Thomas, were both in favour of the project.
But county council committee members said in summer 2023 that they worried about potential traffic impacts on Abingdon and Didcot and about the Science Bridge’s design, among other reasons.
Last year, Chris Church, from Oxford Friends of the Earth, said the road would “generate 510,000 tonnes of carbon dioxide (CO2) in its construction”.
“This is 40 times what the council produces in all its operations every year”, Mr Church said.
“It’s a complete tearing up of its climate commitments.”
Some Green Belt land will be used for some of the project but planning inspector Lesley Coffey said the benefits of using it will outweigh the harms.
Mr Pennycook, acting on behalf of deputy prime minister Angela Rayner, agreed.