The British government has suggested the toxic site, which overlooks the Irish Sea, will not be completely safe and clean until 2120.
As the UK's largest atomic power station, Sellafield has attracted many high-profile Irish opponents.
South Down politician Eddie McGrady MP has insisted: "The nuclear waste is a time bomb. They are not only producing but importing the dirty stuff from the rest of the world, it is incredible."
However, Northern Ireland's Environment Minister Sammy Wilson has suggested increased dependency on nuclear power would reduce the global stretch on natural fossil fuels.
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Reprocessing of spent nuclear fuel is expected to end by 2020 but it will take years for radioactivity levels inside unused reactors to fall to safe limits.
A Sellafield spokesperson said: "Sellafield isn't a place that can just be closed down. It is about the removal of plant and equipment from the building, it is about decontaminating and knocking them down. That takes decades."
"A lot of work has been done but with a site as complex as Sellafield that will take a long time to do carefully and safely, which is the priority and can't be compromised on."
Decommissioning the plant will take a long time, in excess of 100 years, though the quoted cost in some media for the 112-year project of £73bn is inaccurate, as a Sellafield spokesman has pointed out this figure relates to the cost of decommissioning all such nuclear plants across the whole of the UK.
However, a Westminster report has warned these costs could rise, which could result in taxpayers footing the bill.
The British Government is tasked with finding an alternative to the site, amid ever increasing costs of fossil fuels.
At this stage there is no suggestion that a controversial Northern Ireland nuclear site is being considered.
(PR/JM)(BMcC)
