Double hole for Doel

A EUR70 million project to replace the two steam generators (SG) at Doel 1 required the creation of openings in the primary and secondary containment for the SG exchange operation. It was just the second time that approach had been used for a SG replacement in a nuclear power plant in Belgium. The project was carried out by two GDF Suez subsidiaries, Electrabel, a leading energy provider and Tractebel Engineering, an established engineering consultancy. GDF Suez group invests an average of EUR100 million each year in its Belgian nuclear power plants.

This article discusses the construction challenges resulting from the atypical SG support configuration at Doel, and explains how the unusual SG replacement projects was carried out. Last year’s project at Doel 1 proved to be slightly tricker than at Doel 2 because it was carried out during the winter. High winds and the less light during the day delayed the lifting of the SG, causing a 5 day delay. The work was also complicated by the presence of asbestos in the SG cubicle, which led to a higher radiation dose during SG replacement (250mSv at Doel 1 compared with 196mSv at Doel 2) . Still, the project team built on their experience at Doel 2 five years earlier for a successful outage. As of mid-December, despite the asbestos, there have been fewer safety incidents at Doel 1; technical aspects of the project have been improved; and there has been better involvement of outage personnel thanks to a the introduction of a two week training programme. At the time of press the outage at Doel 1 was still underway and the was reactor due to come online around 9 January 2010.

Challenges

Each unit of the twin Doel 1 and 2 reactors have a free-standing spherical primary containment vessel that performs a pressure-retaining function. A secondary concrete shielding structure protects the primary containment from impacts and external events.

The Doel 2 steam generators were replaced in 2004. The replacement of those of Doel 1 followed in November-December 2009 and will grant the station a 10% power uprate. The higher tube density of the replacement steam generator (RSG) compared to the old steam generator (OSG) transfers more heat. And because the tubes of the RSG are made of Inconel 690 cupro-nickel, cracks are less likely to occur.

The main particularities of this construction effort are the secondary containment opening and closing using a structural formwork assembly, the use of containment platforms hanging inside the primary containment allowing for parallel primary and secondary containment reconstruction, the de-activation of some of the primary coolant piping and SG restraints following the licensing acceptance of the leak-before-break concept for the primary piping and the removal of asbestos contained in the SG insulation.

For the replacement of the Doel 2 steam generators, Tractebel Engineering performed preliminary studies that mainly focused on the selection of the best containment opening technique for the exchange operation. In these preliminary studies, the benefits of using a through-the-roof replacement option were weighed against the use of the original ground level equipment entry route.

The original entry route was determined to be problematic because of specific features of the containment structure and existing plant, including:

• higher deadweight stresses in the lower portion of the steel primary containment shell and the resulting potential for large deformations during containment opening,

• second-phase concrete interferences with the original ground level entry path,

• limited capacity of the existing polar crane, and complex lifting and handling inside containment for horizontal containment entry.

These considerations pointed to a preferred approach of creating two new openings directly above the steam generators. Lifting and handling of the 20m-long, 4m-diameter steam generators could then be performed using an external crane. Following the successful replacement operation at Doel 2 in 2004, the same technique was been adopted for Doel 1.

The major construction activities that resulted directly from the selected opening option were the cutting and welding of two holes in the 25mm thick plate material forming the primary containment pressure boundary and the concrete removal and restoration for the secondary containment.

Primary Containment

The primary containments in Doel units 1 and 2 are spherical vessels with a diameter of approximately 46m and a wall thickness of 25 mm. The material of construction is fine-grain carbon steel plate with a yield strength of 360MPa. The design pressure is 2.55 barg and actual pressure loading only occurs during pneumatic testing or in case of energy release resulting from failure of a primary or secondary pressure boundary under faulted conditions. During normal plant operation or refuelling the primary containment vessel does not experience any pressure loading of significance. The vessel is subject to the requirements of Section III, Subsection NE, of the ASME Boiler and Pressure Vessel code.

For each SG, a 5.4m hole was required to allow for SG removal and installation. Two aspects of the primary containment opening warranted particular attention: the stability and deformation of the essentially free standing primary containment following introduction of the two openings, and the creation of acceptable working conditions for the construction workers and welders in this area.

To investigate the effects of the modified deadweight distributions after removal of the two cutout pieces, a series of engineering studies were performed by Tractebel Engineering using finite element analysis techniques. The analysis showed that stresses in direct vicinity of the openings are very low as compared to the material yield stress. The general outcome of the engineering studies was that the additional deformation of the containment dome is negligible as compared to the ASME NE-4000 out-of-roundness tolerances of 8% of the diameter of the vessel, which would amount to 3.6m in this case. It was therefore concluded that no reinforcement of the cutting area was required and that the small amount of deformation was acceptable. It is noted that a small out-of-roundness condition would only create secondary stresses if the containment is pressurised. Given the very low number of loading cycles for the assembly, secondary stresses, being only a potential cause of fatigue damage, are of little concern.

The 5.4m diameter cuts were made using a oxygen-acetylene cutting process. The re-welding of the primary containment was performed manually using shielded electrodes (shielded metal arc welding), as the unique installation work and low radiation levels at this location of the containment do not warrant development and qualification of a machine or automated weld system.

Access to the containment cut-out areas for the manual weld process can be created through the erection of scaffolding on the polar crane or the use of aerial work platforms from the refuelling floor. However, considering the intensive crane usage during the construction effort, neither of these methods was retained for the project. Therefore, to allow for access of the welders to the inside diameter of the containment vessel, specially-designed containment platforms were installed directly underneath the primary containment openings.

The two platforms, which make up an area of about 30 sq m, hang from the primary containment. They are hinged at the far edges; their near corners are attached to manual chain hoists that raise and lower the platform like a set of trap doors. The platforms are designed for deadweight and live loads, but also for seismic and LOCA loads. The containment platforms can be opened and closed during the respective SG movements. In this manner, access to the primary containment weld joint is restored almost immediately following lowering of the RSG on its support columns, which obviously benefits overall project schedule. Given the manual weld process, stable and comfortable access conditions are essential for weld quality. For the Doel 2 replacement, a minimal number of weld defects were noted over the roughly 32 meters of weld joint. The optimised weld joint access created by the platform contributed to overall weld quality.

A third opening was created at the base level of the primary containment that, in combination with a temporary dress-out facility, provided extra personnel entry and exit. This is critical at times when the man loading of the project peaks and the normal plant facilities are insufficient to accommodate the 700 workers on site. Workers pass first through a temporary air lock, and afterwards through the additional opening in the containment to enter the reactor building. The third opening is equipped with a special door in order to make an isolation of the containment possible if required. This door allowed to open the containment at the very beginning of the stop.

During the period with the two main containment openings, the reactor vessel was empty and the reactor building was kept at 25Pa less than atmospheric pressure, through existing reactor ventilation, to reduce the risk of a radiation leak. Tarps covered the steam generator containment holes to help stabilise internal pressure and reduce the chance of external objects falling into the containment.

Secondary containment

The secondary (concrete) containment opening and restoration work used several innovative construction methods. The containment opening was performed using diamond wire cutting methods. Each opening was divided in four cut-out pieces that were supported during cutting using a temporary steel frame installed on the roof of the containment. In this manner the crane usage period was limited to the time required to actually move the pieces down to the laydown area rather than blocking the crane during the whole duration of the concrete cutting work. The steel frame was also used to provide deadweight support to the formwork during concrete cure.

Following SG exchange, concrete restoration required re-assembling the upper and lower re-bar beds. To avoid extensive overhead manual labour and scaffolding work in the annulus between the primary and secondary containment, the lower reinforcement was re-assembled using a structural and permanent formwork. The formwork was designed to generate equivalent tensile capacity as the original lower reinforcement and anchored to the secondary containment using through-wall studs on the side of the cutout area.

SG restraints

Most of the existing PWRs were designed with due consideration to the USNRC General Design Criterion (GDC) No. 4. Per this design criterion, the SG and reactor coolant system (RCS) piping restraints shall be able to resist the effects of pipe breaks of, for example, the primary coolant piping or the main steam line. The restraints designed to minimise pipe whipping or SG motion almost always pose a major construction challenge in SGR projects. Heavy steel structures have to be demounted and re-assembled to allow for exchange of the SGs, as the structures are in close vicinity to the coolant piping or SG vessel to minimise equipment and pipe motion in case of a postulated break.

For the Belgian SGR project, a detailed leak-before-break analysis is always performed prior to SGR implementation based on which most of the large coolant loop piping breaks can be eliminated from the structural design basis of the plant (see Ref. 2). This has the tremendous advantage of simplifying some of the demolition work and eliminating the need for re- installation of the SG and RCS piping restraints. It also simplifies the adjustment of some of these restraints (the so-called gap shimming) and reduces the risk of thermal interferences during thermal expansion of the primary coolant loop. The restraint assembly transfers loads, for example those resulting from earthquake or pipe break, from the SG shell to the civil structures surrounding the SG cubicles. The tie rods are 170mm in diameter and their removal and re-installation are work–intensive. In this particular case, the tie rods parallel to the hot leg piping are obsolete following application of the leak-before-break concept and need not be re-installed after the installation of the new steam generator. This simplifies both the disassembly and the re-assembly of the lower lateral SG restraints. Other RCS piping restraints such as the SG inlet elbow restraint and the crossover leg horizontal run restraint are also de-activated by removal of shims or re-adjustment. De-activation is not required for the SG exchange operation, but performed anyway to minimise the risks for thermal inference or binding between the RCS circuit piping and the restraints during normal plant operation.

Reactor coolant piping

The primary side inlet and outlet nozzles of the steam generators are welded to the PWR circuit piping, cast austenitic stainless steel elbows, with an outside diameter of roughly 37 inches (940mm) and a wall thickness of 3 inches (76mm). The progress in RCS cutting and re-welding techniques has lead to significant reductions in primary performance windows, i.e. the time period between the start of RCS cutting and completion of primary side welding inclusive of non-destructive examination and restoration of primary system cleanliness. In this case a 15-day primary window was foreseen and this important milestone was met for during SG replacements at Doel 2 and, more recently Doel 1.

As the reactor coolant pipes are relatively short and of large diameter, cold springing of the coolant pipe to achieve fit-up with the new SG would require use of large hydraulic actuators that need to be installed in a congested, radioactive environment. Similar restraints have been proposed in the past to address concerns over ‘pipe springing’ when the RCS piping is breached during RCS cutting. Based on observations on past SGR projects and the alignment criteria of Section III of the ASME B&PV Code, installation of temporary supports for the Doel 1 and 2 projects, as well as for the other recent SGRs has been reduced to a minimum, thereby minimising radiation exposure for the workers, engineering effort and hardware cost.

Based on a statistical evaluation of a series of hot and crossover leg pipe-end movements on past projects, and the precision of OSG and RSG laser metrology and machining, the overall risk of unacceptable alignment was found very small. Based on this low risk, the number of temporary RCS pipe supports during RCS cutting is limited to a single vertically-oriented support installed underneath the horizontal run of the cross under piping. One important caveat is that the reactor coolant pump should be supported in a stable manner (that is, not spring-hung as is the case for some of the very early PWRs). Contingency actuators are brought to the construction site, so that they can be used in case of unexpected, large pipe end movements.

Subarticle NB-4200 of Section III of the ASME Boiler and Pressure Vessel Code provides rules for forming, cutting and aligning of components during assembly. Per NB-4232, and for double sided welds, an offset of 1/8T is allowed for primary side inlet and outlet nozzles of the steam generators welded on both sides. As long as these alignment conditions are met, the NB-3680 stress indices may be applied for the stress analysis of the piping joint which greatly reduces the analysis effort. In practice, the actual offsets achieved in the as-fitted condition of the RSG to the existing loop piping are typically less that 5% of the wall thickness, as the engineer leading the laser metrology has several means to optimize the fit-up. Options for optimization of RSG to the reactor coolant pipe end fit-up are use of a slight SG rotation around its vertical axis or dedication of the RSG to a reactor coolant loop that best suits the as-built condition of the reactor coolant loop piping and the OSG. The amount of extra welding to achieve an acceptable finish for the future in-service inspection of the joint is generally not a concern and has little impact on the RCS window and the radiation exposure of the workers. Adequate weld root conditions are achieved through the use of so-called ‘floating’ weld bevel lands. This means that, even though the SG nozzle and RCS piping centrelines have a limited amount of offset, the weld bevel lands on both sides of the joint are perfectly concentric resulting in an optimal weld root configuration.

Asbestos

A complicating factor was that a survey had shown that insulation material on the steam generator contained asbestos. As a result, the project team had to create a tent to take out the insulation without releasing any asbestos fibres. Inside this sealed work environment was perhaps one of the first shower facilities ever installed in a nuclear reactor containment structure. Sampling measurements taken outside the tent found no traces of asbestos.

Project structure

Tractebel Engineering’s team for the Doel 1 SGR project consisted of about 40 staff. It managed the 20 different contractors involved on the project, and liaised with the station operator Electrabel, which is carrying out its own outage activities unrelated to the upgrade. A core team of four project managers handled the overall project, aided by teams for each specific work discipline: mechanical, civil, electrical/instrumentation & control, radiation monitoring, asbestos, quality control and security.

Westinghouse was responsible for replacement activities on the RC in the SG cubicle. This work included reconditioning of supports and restraints of the RCP and SG, optical survey and templating, cutting and machining of the RCP, SG nozzles and SG column supports, decontamination of the RC pipe ends, fitting up the SG to the RCP, welding of the RCP, inspection and cleaning of the SG channel head and the RCP. Its other work included transport and rigging of the SGs, and opening and closing of the primary containment. Belgian contractor Fabricom was responsible for cutting, fitting and welding secondary lines, the main steam lines, the main feedwater lines, all small pipes and level monitoring. Kaefer carried out insulation work. Interboring handled concrete cutting.

Author Info:

This article is based on ‘Construction Aspects Of The Doel 1 And 2 SGR Projects’ presented at the Icone 17 international conference on nuclear engineering, July 2009, Brussels (ICONE17-75671), by Bert Kroes of Westinghouse Electric Belgium, Rue de l’Industrie 43, BE-1400 Nivelles, Belgium, and Edmond Gobert, Xavier Delhaye, Peter Devolder and Michel Sonville, all of Tractebel Engineering GDF SUEZ GROUP, Avenue Ariane 7, BE-1200 Brussels, Belgium. Also: Danny Thierens of Tractebel Engineering and Jan Defloor and Louis Maesen of Electrabel GDF SUEZ GROUP. Additional reporting by Will Dalrymple.

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