‘No show-stoppers’ for AP1000 and EPR8 March 2010
Mid-way through the UK’s generic design assessment process, the British regulator has released its reports on the AP1000 and EPR designs. Although it has raised questions about both designs, the NII remains confident that a ‘meaningful’ GDA will be completed by June 2011. By Will Dalrymple
The Nuclear Installations Inspectorate of the UK Health and Safety Executive (the regulator) has released two detailed interim technical reports of its pre-construction ‘generic design assessment’ of the Westinghouse AP1000 and Areva/EDF EPR reactors. It was broadly optimistic that both reactors would finish this first, general phase of the UK reactor licensing process by June 2011, although it would not guarantee there would not be further design changes needed.
In the so-called ‘step 3’ reports, it flagged up several technical issues with each reactor that will require further work. Kevin Allars, the NII’s director of new build, said that Westinghouse has more to do than the Areva/EDF team in the final generic assessment process, when the NII will examine the evidence that back up the vendors’ design claims.
AP1000:?Two key issues
Two key technical issues in the AP1000 reactor revolved around the containment building and a particular type of emergency valve. The AP1000 design calls for a modular containment building made of a steel-concrete-steel sandwich structure. The NII contends that there is a lack of design code for the design, which therefore requires extensive evaluation and testing.
Westinghouse said in a US presentation in November that the initial shield building that was certified by the US regulator, the Nuclear Regulatory Commission, in December 2005, it consisted of a reinforced concrete design. Since then, the NRC challenged vendors to change their designs to meet enhanced aircraft impact standards. As a result, it updated the design, and chose a combination of steel concrete composite and reinforced concrete construction techniques to build it. It said that the steel composite design is ‘common’ in other countries, such as the Japanese nuclear industry. It also said that it is modifying the design to meet criticisms in an NRC assessment and plans to submit a revised report in January.
In a press conference in November, Westinghouse said that a result of the UK feedback, it was considering a ‘number’ of design changes to the containment building, including extra reinforcing across the modular steel sheets to cope with external stresses.
Another issue concerns a new type of fast-acting valve, called a squib, that is actuated by a gunpowder charge to release primary coolant in case of an overpressurisation. The valves are charged so that they will still actuate in the absence of offsite power. Allars said that the valves were quite unique in the UK nuclear industry, and therefore require additional analysis. Westinghouse confirmed that US-based firm SPX Corporation is engineering and manufacturing the valves. SPX said in February that a contract to supply ‘up to’ twelve AP1000 reactors, including for two reactors each at Sanmen and Shandong, would be worth $100 million.
Allars said that a meeting between Westinghouse, the NII and the NRC in the USA in mid-November suggested that a way forward had been found to start to deal with both issues. Westinghouse GDA project director Simon Marshall said that at least initially, both the NII and NRC were satisfied with the plans.
In the GDA report, the NII also requires Westinghouse to supply more information about aspects of reactor design paperwork, including revising its external impacts analysis and the role of human factors in risk. Marshall said that these queries “are really about really understanding the UK’s specific requirements, and translating the US framework to a specific UK framework.”
The NII said that the differences between the US and UK have to do with a difference in regulatory culture: the US has a prescriptive rule-based system, while the UK is less prescriptive and more based on goal-setting. “We are therefore sometimes looking at different aspects in our assessments, but the important thing is that we are both assured that the overall standards of safety are acceptably high. We do this routinely by discussing issues with the USNRC, either bilaterally or through multinational design evaluation programmes,” it said.
In response to the criticisms, Westinghouse has reorganised its balance of power to supply a greater corporate authority to the UK to facilitate the GDA process locally, including at least 14 staff, supported by contractors. That operation is headed up by Mike Tynan.
He admitted that the change, which comes several years into the GDA process, is late, but said that the most critical stage is the last one.
EPR: C&I Solution in sight
Earlier this year, the NII flagged an issue concerning the EPR’s control and instrumentation (C&I) system; it was concerned that the reactor safety system, which shuts the reactor down in emergencies, was interconnected too tightly with the reactor control system that operates the reactor under normal conditions. The amount of interconnection between the two systems risks the possibility of a higher-than-expected failure rate, it says.
Since then, EDF and Areva have suggested a possible solution. The GDA report summarises it: “They have proposed a way forward which includes provision of a non-computer-based backup system, safety classified displays and manual controls in the control room, reduction of reliability claims for the computer-based systems important to safety (SIS) and other measures such as one-way communication from high to lower classified safety systems. We anticipate that the proposed modifications will be acceptable, but we await the full details.” In a separate interview, Areva UK new-build director Robert Davies said that its solution would guarantee the independence of the two systems.
At the press conference, GDA process deputy head Len Cresswell compared the EPR's C&I system to the interaction between the cruise control and braking systems in a car. In modern cars, both systems are computer-controlled. But once drivers set the cruise control, they can cancel it by pressing the brake pedal. The EPR system needs the same kind of hierarchy, they argued.
According to NII documents, EDF and Areva have reduced the reliability claims for the Teleperm XS system by an order of magnitude to a probability of failure on demand (PFD) of 10-4, and the back-up reactor protection SPPA-2000 system by two orders of magnitude to 10-2 PFD. The non-computerised backup would have a probability of failure on demand of 10-3. Although the enlargement of these figures makes them easier for the regulator to substantiate, it also reflects an increasing likelihood of failure.
In the interim report, the NII also said that more evidence is needed to prove the proposed use of prestressed tendons in the containment. These tendons are tensioned after the concrete has set. Unbonded tendons are then greased, and can be removed for inspection and retension; bonded tendons are grouted for corrosion protection, and cannot be removed, which it is analysing in more detail. It also had queries on construction codes and external and internal hazards.
Areva said that the version of the EPR being used for the GDA process was frozen as of December 2008, despite reports suggesting that the C&I query might force a later version to be used. A spokeswoman said, “Any design changes since then will be incorporated at the end and as part of the GDA.”
“I am confident that we can complete a meaningful GDA by June 2011,” Allars said in the press conference, adding, “I am particularly more confident because we now have the necessary resources to take the project forward.” He said that the next stage of the process, which examines the evidence for safety claims laid out in the reactor applications, would have to gain ground lost earlier in the process from inadequate staffing.
Despite the extra resources being deployed, Tynan at Westinghouse said that the full AP1000 probabilistic safety analysis may not be finished by June 2011. “We have always recognised through the process that certain issues would not be resolved on that timescale. Some may be major, some may not be. At the moment, the probabilistic safety analysis may not be fully complete.”
That does not mean that the GDA is not meaningful, Tynan continued, but would be covered by hold points to be resolved during the site selection process. Although he accepts that the ideal situation is to have a design acceptance situation without exclusions, “the rigour of the challenge placed on requesting parties [vendors] that units be completely safe means that there are likely to be a number of issues transferred into the site-specific licensing process.”
In a response to emailed questions after the NII press conference, the NII admitted that the PSA work is unlikely to finish by 2011, but said that the document would not be needed until operations begin anyway. “The most important thing is that the PSA is giving us confidence that the risks presented by AP1000 are low. The additional work we have identified as necessary is to bring the PSA up to what we regard as modern standards. It is a large programme of work and we are content that not all aspects of it need to be complete for us to be able to come to a conclusion on the safety of the reactors at the end of GDA. However its important to realise that the PSA is a living tool that is used throughout the life of the power station and so we believe it is important that it represents best practice in terms of quality of information, methodologies used, and usability etc. So while we do not need it necessarily to be 100% complete to this standard by the end of GDA, we require it to be before the plant operations commence.”
It also said that some of the EPR C&I work may extend beyond the end of the GDA process.
Davies at Areva argued in favour of finishing everything by the end of GDA. “What we need to happen when we close out the GDA in 2011 is that there are no exclusions, which allows the licensee go forward and apply for a nuclear site licence with a clean GDA. Every exclusion or qualification means potentially extra time to the grantee of the nuclear site licence. That is what we are pressing for.”
Will Dalrymple is editor of Nuclear Engineering InternationalRelated ArticlesAMEC to support EDF with construction in the UK AP1000 containment insufficient for DBA, engineer claims UK regulators query AP1000 ventilation system Horizon signs early work agreements with Areva and Westinghouse EPR and AP1000 pass UK design assessment