European approval for ABB’s new BWR design30 September 1999
The need for a new generation of nuclear plant acceptable to the general public is becoming ever more pressing as previous generations of power reactors begin to reach the end of their useful lives. A number of possible designs are being developed, with the ABB Atom BWR 90+ in the forefront. The new design addresses the findings of the European Utility Requirements review of ABB's previous model, the BWR 90.
In the early nineties, European nuclear utilities began drafting requirements for the design of future light water reactors. The aim was to establish rules for standardised designs that could be approved as a basis for building new LWRs. The document is known as EUR – “European Utility Requirements”.1 In 1994, a first draft, Rev A, was issued for comments from other utilities and vendors. In 1995, Sweden and Finland became members of the European Union. Shortly thereafter the Swedish utility FKA (Forsmarks Kraftgrupp AB) and the Finnish utility TVO (Teollisuuden Voima OY) joined the effort of developing the EUR document.
The EUR document is split into four volumes. Volume 3 is meant to consist of an assessment of specific designs, ie an analysis of the compliance of these designs with the EUR. Consequently, there is a Volume 3 for each selected design. Currently, the Franco German EPR, the European Passive Plant (EPP), and the ABB Atom BWR 90 designs are the subjects of EUR Volumes 3. The assessments are based on the second draft of EUR, Rev B.
In 1997, the two Nordic utilities mentioned above, joined by KEMA Nederland, volunteered themselves as ‘promoters’ to carry out the assessment of the BWR 90. The plant, used to bid for a fifth nuclear power plant in Finland in 1991, acted as a basis for the assessment, as the design is well described in bid documentation. In 1993 the Finland Parliament decided not to build another nuclear plant for the time being. Nevertheless, ABB Atom and the utilities continue to co-operate in the development of the design under the acronym ‘BWR 90+’.
The structure of the EUR Volume 3 Subset for BWR 90 is as follows:
Plant description (supplied by ABB Atom, the vendor).
Compliance analysis (drafted by the promoters and approved by the EUR steering committee).
Specific EUR requirements on the BWR 90 design (approved by the EUR steering committee).
The assessment of BWR 90 was largely completed in 1998. The EUR group, with Electricité de France (EDF) as a leading partner, is spending a considerable effort in establishing the EUR. So it is likely that this document will have a real impact on future designs of LWRs in Europe.
EUR ASSESSMENT OF BWR 90
In 1997, a secretariat for the project was established at Forsmark, Sweden, and a project manual was drafted. The three promoters together with EDF and ABB Atom formed the EUR/BWR 90 Co-ordination Group.
The project team appointed 'Assessment Performers' to analyse the compliance of the BWR 90 against the various EUR chapters. A pragmatic process evolved in the course of the assessment.
The BWR 90 compliance analysis covers most of the EUR documents:
Vol 1: Main policies and top tier requirements.
Vol 2: Generic nuclear island requirements (all 19 chapters).
Vol 4: Power generation plant requirements (chapter 24).
The inclusion of parts of Volume 4 was important for a BWR because of the tight interconnection between the reactor and the turbine in this type of plant.
The evaluation of BWR 90 to the EUR was relatively straightforward, because the Finish bid design documentation is comprehensive. Modifications of the design in the BWR 90+ project that affected the assessment result, were also taken into account. However, modifications were only considered when they had already been agreed by the organisations participating in the BWR 90+ project.
From autumn 1997 to spring 1998, work on the BWR 90 assessment proceeded intensively. Every “shall” and “should” in the EUR chapters was addressed, and BWR 90 compliance was assessed, corresponding to over four thousand individual requirements. The compliance analysis for each item was documented with information and statements regarding:
•A BWR 90 reference document explaining the design.
•The result of the assessment.
•Assessor's justification for his assessment.
The BWR 90 fulfils the vast majority of the EUR. These include important design items and many aspects of plant construction, quality assurance, and operation.
The EUR reflect the desire to promote standardised designs that, without major changes, can be licensed and built at many European sites. As a result, there are extensive requirements for protection against external hazards such as extreme weather conditions, earthquakes, aeroplane crashes and external explosions.
Clearly, the Finish BWR 90 design will have to be adapted to meet these requirements. The BWR 90 was originally designed for Nordic climatic conditions, with assumptions on low seismicity and a scarce population in neighbouring areas. External hazards have been taken into account, but usually at a lower level than specified in the EUR. However, ABB Atom is sure that it is possible to reach full compliance with the EUR requirements without changes in plant layout. This is true even in the extreme case of a military aircraft crashing. Modifications in structures (eg wall strengths), systems and components would be required, however. As a result, the label "Compliance with objective" (CWO) was attached to the BWR 90 EUR assessment on these items.
The following sections summarise some important points of non-compliance (NOC) of BWR 90 with the EUR. Comprehensive lists are found in the synthesis reports mentioned above.
The 12 hour rule
Perhaps the most conspicuous new set of requirements in the EUR is in the area of reactor safety. The European utilities have introduced a '12-hour rule'.
The rule implies that, in the event of a severe accident, it is assumed no operator action is necessary during the first 12 hours. Furthermore, no containment venting is allowed within the first 12 hours. For the BWR 90, early containment venting might lead to dose rates from released noble gases in the near vicinity of the plant in excess of EUR limits.
For BWRs, filtered venting systems were introduced in European countries in the 1980s2. They are believed to greatly promote safety, since containment pressure relief will prevent failure. Such systems are installed in all Swedish nuclear power plants and the two BWRs at Olkiluoto in Finland. Filtering aerosols and iodine before venting greatly reduces any release of radioactive matter; only the noble gases are vented. Above all, land contamination is avoided. This was a priority in the policy leading to the Nordic filtered containment venting concept. The concept also allows the reactor and primary containment to reach a safe and stable final state, as the containment soon reaches ambient pressure and fills with cooling water.
Nevertheless, the EUR does not allow the dose rates that may arise from noble gases (particularly Xe 133) following early containment venting. The underlying EUR principle is that no emergency measure should be needed in the vicinity of the plant (800 m from reactor) following a severe accident. The EUR group identified this as the only specific requirement for modification in the BWR 90 design.
With the BWR 90+, the primary containment volume has been increased, in order to ensure that early venting can be avoided, and it fulfils the EUR requirements in this respect.
THE HARMONISATION PROCESS
The BWR 90 Co-ordination Group identified a number of other important BWR 90 design items that do not comply with the EUR. They are summarised below together with ABB Atom's views regarding these deviations. Final determination of the issues is still under discussion. For this reason, additional points of deviation may be added, and some current points may be deleted.
The EUR requires a plant to cope with quite severe disturbances on the external electric grid without loss of its own generating capability. The BWR 90 was designed to specific Nordic requirements (NORDEL) which are less stringent with respect to the plant's capability to sustain grid voltage variations. However, the EUR grid requirements are about to be adjusted towards those of NORDEL.
ABB welcomes a revision of EUR grid requirements that acknowledges NORDEL requirements in order to reduce costs. Grid operators, rather than plant operators, should carry the responsibility for coping with severe grid disturbances.
Instrumentation and control
Software qualification and diversity between the process automation system and the protection system in the BWR 90 do not fully comply with the EUR.
The Advant and Master product families used for the BWR 90 have not been qualified to specific nuclear standards (IEC 880) as required by the EUR, and the process automation system (PAS) and the protection system are not fully diversified. ABB Master and ABB Advant equipment with the same hardware and basic software is used for both functions. The execution of the codes in the two systems is different, however, and this implies that some functional diversity exists.In the BWR 90+ design I&C functions will be adapted to the EUR.
The BWR is associated with radiation conditions not recognised or accepted by the EUR. 'Skyshine' from the turbine in a BWR leads to higher dose rates at the plant perimeter than those accepted by EUR. The requirement should be modified according to the International Commission of Radiological Protection limits in order to take into account BWR technology. The same is true with respect to normal operation radiation rates in the turbine compound which are higher in a BWR than EUR limits. In the view of ABB Atom, lower dose rate limits than those established world-wide by the ICRP are not cost-effective and no valid reason for them can be found. The limits requested by the EUR are lower than variations in dose rate caused by natural background sources.
Sub-criticality during handling
According to the EUR the reactor core and associated components and systems should be designed such that keff is lower than 0.95 during fuel handling, inside and outside the reactor, including planned intermediate fuel configurations, for any single failure or single procedural error.
In the BWR 90, keff <0.95 applies to the dry fuel storage and the fuel pools. For the core, keff <0.95 applies with all control rods inserted in the core. No single failure or single procedural error can lead to keff >0.99. In ABB Atom's view, the EUR document should be adapted to BWR technology on this point.
Spent fuel storage capacity
The EUR requires space in the plant for 10 years' storage of spent fuel (15 years for MOX fuel). The spent fuel pool in the BWR 90 design is sufficient for 5 years of operation plus one full core. Storage for 10 years has not been considered cost-effective, partly because of interim storage facilities used in the Nordic countries. For the same reason, the design does not accommodate the storage of MOX fuel for 15 years. Obviously, it is possible to accommodate the requirement by providing space in a separate on-site compartment or building. In the BWR 90+ design, the layout provides the possibility of increasing the storage capacity.
High energy lines
The comments column of the EUR states that "No high energy lines (with high pressure and temperature) with reactor coolant should penetrate the Primary Containment". In a BWR, steam lines and residual heat removal lines (RHR), including a connection to the reactor coolant clean-up system, penetrate primary containment. Modern BWRs have RHR systems outside the containment, and this technology should be recognised by the EUR.
The one point of non-compliance the EUR group considers essential concerns dose rates caused by noble gases close to the plant in the event of early containment venting following a core melt accident. This deviation as well as the other items discussed above can be resolved through the following measures: the adaptation of the EUR to BWR technology; revision of the EUR according to alternative standards; and modifications of the BWR 90 in the BWR 90+ design.
THE BWR 90+ PROJECT
Since 1994, ABB Atom has worked with the Finnish utility TVO in further developing the design offered to Finland in 1991. Recently, the Swedish power companies FKA and OKG, representing the two nuclear utility groups in Sweden, Vattenfall and Sydkraft, joined in this effort. For TVO, the main interest is to maintain the ABB Atom design as a viable option for a new plant in Finland, and for the Swedish companies to develop prototype designs for future modernisation of operating BWR plants.
The design is based on operating ABB Atom plants in Sweden and Finland. Key features include internal recirculation pumps, fine motion control rods, the SVEA Optima fuel bundle, and a pre-stressed concrete primary containment, with a steel liner to ensure tightness. The following table shows some key parameters and design goals for the BWR 90+, and corresponding EUR targets.
The number one goal of the BWR 90+ development is to maintain the BWR as a competitive option for a revived market in Europe. From an economic perspective, the above parameters are obviously important. Experience shows the goals in the table are realistic. For example, operating ABB Atom internal pump plants have exhibited an energy availability above 90% during the past ten years and the operating BWRs in Finland have regularly had annual refuelling outages below 20 days, although with a smaller core size. Consequently, electricity generating costs for these plants have been low. This is demonstrated by published production costs for the Forsmark 1, 2 and 3 plants3.
Aside from economic considerations, two sets of requirements are most important in the BWR 90+ development; those given by the Finnish regulator, STUK, and those given by the EUR.
In both cases, requirements related to severe accidents have the greatest impact. The primary containment volume is increased considerably in size, in comparison with the BWR 90, so that it can accommodate the large amounts of hydrogen and other gases, which are postulated to be generated during core melt sequences, without the need for pressure relief through filtered venting. The size of the pressure suppression pool is also increased to allow residual heat removal for 12 hours without reliance on active cooling. These modifications meet the EUR requirements discussed in Section 3.
The containment is illustrated in Figure 4 and in reference 4. The core catcher arrangement differs significantly from the BWR 90, in which core debris penetrating the reactor pressure vessel would fall into the suppression pool, causing potential steam explosion problems. In the BWR 90+, this phenomenon is avoided, since core melt products will fall onto a dry core catcher. Also, one can see that no core debris will affect the structure of the containment, ie impacting parts that guarantee containment integrity and leak tightness.
Today there is a keen interest in passive safety systems, shared by the EUR. In the development of the BWR 90+ a balance between redundancy and separation, diversity, and passivity is sought, and probabilistic safety assessment (PSA) is used as a tool to find the best solutions. The main passive system introduced in the BWR 90+ is an isolation condenser for residual heat removal. Incidentally, such a device is being used in Oskarshamn 1, the first ABB Atom BWR.
In summary, the strategy for the BWR 90+ design is to comply with all relevant EUR.
The EUR assessment of the BWR 90 represents 10 man-years of engineering effort among the EUR utilities and ABB Atom. It involved a thorough review of the design, including all essential characteristics of the plant, by several independent assessors. While the utilities issued no unconditional approval, their message is a clear 'go ahead' to the BWR 90+ design.
The assessment is a first step in a commercial process aimed at quotations for new LWRs in Europe. In the near-term, it will be useful for the efforts of the Nordic utilities to focus on finding suitable prototypes to upgrade and modernise operating plants in Sweden and Finland. The analysis results are also used by ABB Atom as input to the further development of the BWR 90+. Finally, as a correlate to the analysis, several items were identified where the EUR document itself needs amendment to become better adapted to BWR technology, or it may need revision for other reasons.