Plans to remove every scrap of radioactive waste from a former nuclear plant are under review, it has emerged.
The former Trawsfynydd site in Gwynedd has been undergoing decommissioning since it ended generation in 1991.
Originally, the power station was due to be left in a state of "care and repair" by 2030 and finally cleared entirely by the 2090s.
But the review could see the remaining structures continue to be removed and low-risk waste left on site.
The details were revealed in presentations to the Snowdonia National Park Authority by the body responsible for cleaning-up the UK's old nuclear plants.
At the moment, the Nuclear Decommissioning Authority (NDA) plans to mothball the Trawsfynydd site by about 2029, leaving any existing radioactive material there to decay naturally over time, before clearing everything.
But the park authority was told that a case is now being developed for continuous decommissioning and for some low-level radioactive waste to be left there permanently.
Officials said the concrete reactor buildings were decaying structurally, and work should get underway to remove them.
But a suggestion that low-level radioactive waste might remain on site has been met with criticism by some in the anti-nuclear lobby.
Robat Idris, from the campaign group People Against Wylfa B, told BBC Radio Cymru: "Once again, we are seeing the nuclear industry changing what they say about this process.
"Originally, the promise was that they would clear the entire site of radioactive material, but now it looks like they are considering keeping some of that material there for a very long time, if indeed they will remove it at all."
The NDA told the park authority that it believed there was little benefit in moving low-level waste off site - and that doing so would pose additional exposure risks to workers if they were given that task.
In a statement, the NDA said it planned to enter a care and maintenance phase at Trwafynydd "where all the mobile hazards on site have been safely dealt with and remaining radiation is left to decay naturally over time, by around 2029 - with final site clearance scheduled for the 2090s."
The statement continued: "To benefit from the latest learning and best practice, and deliver value for the taxpayer, the NDA keeps its strategy under constant review to ensure we have the most appropriate decommissioning plans for all the UK's Magnox nuclear reactor sites. Any change to this strategy would be subject to subject to stakeholder consultation and UK government approval."