License Instrument no 547
Magnox Ltd, the site licence company (the Licensee) that operates and maintains Wylfa Power Station, has requested ONR agreement to fitting new GSRV main filter assemblies with integral bursting discs on Reactor 1 and the removal of the auxiliary filters from Reactor 1 in accordance with its arrangements made under Licence Condition 22(1) of nuclear site licence 58A.
Wylfa Power Station has two Magnox reactors, with a pre-stressed concrete pressure vessel for each reactor. The reactor core has been constructed from graphite bricks with vertical channels within them containing magnox-clad fuel. Each reactor has four gas circulators which pump pressurised carbon dioxide gas through the core to provide cooling. The gas leaving the core passes through boilers within the pressure vessel, which consist of a large number of steel tubes containing water. In the boilers, heat is transferred from the carbon dioxide to the water within the tubes, which turns to steam. The steam is used to drive a turbo-alternator, which generates electricity. The cooled carbon dioxide gas returns to the gas circulator inlet and continuously circulates around the reactor.
Boiler tube failure is recognised as a potential fault that needs to be controlled to ensure reactor safety. The water in the boiler tubes is at a higher pressure than the carbon dioxide gas within the reactor and hence if a boiler tube fails, water will enter the reactor. The licensee has produced a consolidated boiler tube safety case which examines the consequences of such an event and identifies improvements to enhance protection against it occurring.
There are a number of possible consequences resulting from boiler tube failure. The most significant of these is that the water or steam mixing with the carbon dioxide gas could increase the gas pressure within the pre-stressed concrete pressure vessel. To protect the pressure vessels integrity, it is fitted with seven gas safety relief valves which open in response to high pressure to allow some of the primary circuit gas to escape to atmosphere and hence control the pressure.
Potentially, this gas could contain particles which would block filters downstream of the valves and prevent the gas pressure being relieved. Magnox Ltd is proposing replacing the current filters with filters with integral bursting discs on Reactor 1. The bursting discs are designed to deploy if the pressure increases above a certain value, in order to allow pressure relief to continue. The licensee is also proposing to remove the auxiliary filters from Reactor 1. These filters are in a line which allows any leakage of carbon dioxide to be vented away from the gas safety relief valve mechanisms to ensure that the valves will always be able to open when required.
Under the licensees arrangements it must seek ONR agreement to the changes to the gas safety relief valve filters.
ONR has assessed the boiler tube failure safety case and specifically the case for the modifications to the gas safety relief valve filters. Assessment has been completed by specialist inspectors in the following technical disciplines:
Since the modification would allow reactor gas to be vented directly to atmosphere in the unlikely event of a significant boiler tube failure and a filter blockage, ONR has consulted the Environment Agency (EA) during the assessment. The Environment Agency is content with the proposals.
There are no significant findings from the work that would prevent agreement to the proposed modifications to the gas safety relief valve filters on Reactor 1.
The ONR programme of assessment gives sufficient confidence to allow agreement to the proposed modifications to the gas safety relief valve filters on Reactor 1.
Agreement to the proposed modifications to the gas safety relief valve filters on Reactor 1 should be given by issuing Licence Instrument Number 547 to the licensee.