The worry is that the software is so complex that it will have some undetected design faults and that the secondary safety system will not provide sufficient back-up.
The executive's advisory committee on the safety of nuclear installations raised the issue formally at its meeting last month.
Sizewell B, owned by Nuclear Electric, is the first reactor in the UK to have a computerised primary safety system which shuts the reactor down when faults occur.
There is concern among committee members that the back-up system, based on sensors, does not cover all possible faults.
The Nuclear Installations Inspectorate has also asked Nuclear Electric to consider extending the back-up as a 'prudent' measure. Separately, the advisory committee wants action by the company before it loads fuel into Sizewell B at the end of next year.
A spokesman for Nuclear Electric yesterday rejected allegations that the back-up was inadequate in any way and added: 'We will meet the legal requirement, which is to reduce risk to as low a level as is reasonably practicable.'Reuse content