Under the first phase, the companies will launch preparatory work for generic design assessment of a VVER-type power plant and site licensing, which form part of the wider approvals process required for new nuclear reactors in the UK.
The companies have also signed commercial contracts under the agreement.
Fortum executive vice president Matti Ruotsala said the company will support the project by bringing on board its nuclear competence in various areas such as safety and waste management on commercial terms.
"We see the nuclear programme in the UK as a really interesting opportunity to utilise our wide expertise," Ruotsala added.
Fortum has not made any investment decisions related to new nuclear projects in the UK.
Britain is currently preparing mechanisms that facilitate development of new nuclear plants as the existing fleet is nearing the end of its service life.
The collaboration agreement represents a continuation to the strategic cooperation between Rosatom and Fortum, based on the memorandum of understanding signed in 2010.
