Unite claimed that two Spanish contractors involved in the building work at Staythorpe, near Newark, would not be employing local labour.
Construction firm Alstom said a small number of its 14 sub-contractors were not from the UK and they employed a small number of UK and non-UK workers.
A spokesman said: "They will actively be looking to employ UK workers and are very open to learning about the capabilities of the local workforce at Staythorpe to deliver the project.
"Their role is to secure the right balance of skills and expertise to deliver the work from both the UK and abroad. Their non-UK workers are sourced from within the EU."
But Unite's joint general secretary Derek Simpson said the situation was a "scandal", adding: "The country is in the grip of a credit crunch and the construction industry is one of the worst-hit sectors.
"We know there are qualified people in the local area who are out of work and they are ready and willing to do the job."
Unite said Alstom, and energy giant RWE, which will run the power station, should put pressure on sub-contractors to give UK workers a fair chance of employment on the project.
Mr Simpson said: "The UK needs to upgrade and build new power stations and there are huge opportunities to create thousands of well paid and highly skilled jobs. It will be a disgrace if UK workers are shut out from building their own power stations."
A spokesman for Alstom said: "Alstom actively encourages the recruitment of local workers where this is appropriate to the skills we need and the project we are tasked to deliver."