French nuclear company Areva announced Wednesday that a joint venture unveiled a day earlier by its former German partner Siemens and Russia's Rosatom breaches a contractual non-competition clause.

The German engineering giant and the Russian state-owned nuclear company said Tuesday that they would form a partnership to "create the world leader" in civilian nuclear technology, a rapidly growing sector led by Areva.

The joint venture would compete directly with Areva.

The value of this expanding market is estimated at 1 trillion dollars. Up to 400 new nuclear power plants are set to be built worldwide between now and 2030.

Siemens and Rosatom plan to develop Russian pressurised-water reactor technology, build new nuclear power plants and modernise existing plants.

Siemens, as Europe’s largest engineering company, brings essential expertise to the table, but the Russian giant will hold the controlling interest in the joint venture – 50% plus one share.

Breaking up is hard to do

Siemens pulled out of a similar joint venture with Areva, in which it held a 34% stake, in January. Siemens and Areva had planned to pool their expertise in the construction of nuclear reactors. The French energy giant is now faced with the prospect of finding 2 billion euros worth of investment to fill the hole left by Siemens when it quit the joint venture.

On Wednesday, Areva said that its former partner would breach a contractual non-competition clause if it went ahead with plans to tie up with Rosatom.

The German-Russian alliance will focus on the development of the VVER reactor, a Russian pressurised-water technology that needs to be further enhanced in order to convincingly compete with Areva.

This is not the first time Siemens and Rosatom have collaborated. The two have already worked together on a nuclear power station in Bulgaria. One  question is whether this alliance will allow them to lead the market as they claim.

Eager to keep the duo at bay will be their major competitors, General Electric (US), Toshiba-Westinghouse (Japan) and, of course, Areva.

A greater question may be how Areva will react. On Wednesday, they already offered a menacing indication.

Areva warns Siemens of 'ensuing consequences'

Areva and Siemens' contract contained a non-competition clause which forbids Siemens from engaging in competitive activity with Areva until 2020, which the French company pointed out in a statement.

"Areva has informed Siemens that by announcing this joint venture it is in breach of contract, with all the ensuing consequences by virtue of the shareholders' agreement," read the statement.

This makes it all the more likely that the two companies will end up airing their differences in the courtroom.