Thousands of staff are expected back at work at the Sellafield nuclear plant on Tuesday after staying at home for four days following the London bombings.
Many workers are still at home including clerical staff
The Cumbrian site employs 10,000 workers, but on Friday it was manned by a skeleton staff of just 1,600.
This was because access to the site had to be restricted and all cars searched.
However, on Tuesday all personnel are expected to clock back in, although managers are hoping they will share cars to aid security checks.
The visitors' centre at the plant will also be closed until Friday to avoid traffic problems.
Park and ride
The decision to increase the level of alert to the second highest, Amber, at all nuclear sites across the UK was made by the government's Office of Civil Nuclear Security.
But a spokeswoman for Sellafield stressed the heightened security was a precautionary measure and no specific threat had been made to the plant.
She said: "On Monday about 4,000 workers returned to work including technical support, catering staff and the secretaries to senior managers.
"Cleaning staff, clerical staff and finance workers remained at home but obviously we cannot do without these workers for ever.
"Everyone is expected to return on Tuesday, but we will operate a park and ride scheme for drivers as all vehicles will have to be searched.
"We also want to encourage staff to give each other lifts to restrict the number of vehicles coming to the site."