Hundreds of Sellafield staff have this week been told what jobs they could move to when current roles come to an end - and some face losing a quarter of their income.
With reprocessing work set to finish in 2020, Sellafield Ltd is preparing to redeploy staff from the Thorp plant.
Around 470 workers were this week informed of their planned roles, either on the clean up of Thorp or elsewhere on the site.
It is part of a bigger redeployment as around 3,000 existing jobs at Sellafield will come to an end in the next five years. Around 1,500 to 2,000 staff are expected to take on new roles, with others retiring or moving on from the business in the meantime.
The new jobs for Thorp staff will start in April 2019, when reprocessing there ends.
Anyone unhappy about their proposed role can work through options with management before the posts are finalised next April.
But Unite the Union, which has 80 to 90 affected members, says some staff will be moved off shift work and lose unsociable hours payments worth £8,000 to £12,000. The change would mean workers on a £32,000 salary could lose a quarter of their income.
Nick Jeffrey, Unite official at Sellafield, said: "People at the end of the day live hand to mouth. There's not a lot of people can save money. They spend what they earn.
"It's very difficult when you're put in the position of losing that shift money."
Mr Jeffrey said he was disappointed Sellafield Ltd had not followed Unite's request, which would have seen the longest-serving shift workers moved to other shift roles to maintain their income.
Sellafield Ltd said that would have led to age discrimination and instead a fair system was used for all employees.
The company added that anyone who had worked shifts for 20 years or more would have their shift allowance protected after the changes, whether they continued on shifts or not.
It added that anyone else moving from shifts to days would have the allowance protected for one month for every year of service on shifts, meaning someone who had worked shifts for 10 years would receive the shift allowance for another 10 months after April 2019.
Around 60 workers have already moved from their existing roles in Thorp to other jobs in the company.
Once redeployment is complete at Thorp, attention will turn to other areas including the Magnox plant, where reprocessing is set to finish in 2020. A similar redeployment programme is expected there.
Reprocessing of used nuclear fuels from around the world has been carried out at Sellafield on behalf of the Government since the 1950s.
But current contracts are ending and the Government has decided not to take on new deals.
After reprocessing, the site's focus will switch to decommissioning and waste management.
Scott Macaulay, head of Thorp for Sellafield Ltd, said: "We are in the fortunate position of being able to predict accurately how our business is changing.
"This means we can take steps to prepare and use the change to give people new opportunities.
"Our aim is to ensure we offer everyone a rewarding and meaningful future at Sellafield Ltd when reprocessing ends.
"Over the past few months, a ‘best fit’ process has been ongoing in Thorp. This has looked at the skills, experiences and preferences of everyone working in Thorp, to balance these against the business’ need and ultimately get the right people in the right roles."