Safety concerns have been raised about Oldbury Nuclear Power station in South Gloucestershire.
Oldbury Power Station is set to close by 2008
Documents obtained through the Freedom of Information Act show that inspectors were worried about the strength of some parts of the reactor's graphite core.
In letters obtained by the BBC, the Nuclear Installations Inspectorate asked if Oldbury should remain open until its 2008 closing date.
The operators BNFL say regular tests ensure safety is not being compromised.
The letters by the Health and Safety Executive's Nuclear Installations Inspectorate stated that the blocks of granite at the core of the reactor were becoming weaker after constant exposure to heat and radiation, and weight reduced by a third.
The inspectorate has said that it has been given assurances about the safety and expertise of the station since the letters were written in 2004 and 2005.
In 2001, a leaked report also raised questions about the structural safety of the power station core.
It is scheduled to be closed in 2008, as part of a national decommissioning programme.