Taking a new view of internals repair at Qinshan30 September 1999
The lower internals recovery programme for Qinshan 1 required innovative thinking from Westinghouse: the mechanical work was simplified by inverting the internals. Enormous effort went into protecting the internals and the surroundings, and analysing the effect of the process.
Qinshan 1 is a first-of-a-kind Chinese design, consisting of a two-loop PWR rated at 300 MWe. The plant was commissioned in June 1991, and has operated on a 12-month fuel cycle. During the last cycle the Qinshan 1 Nuclear Power Company (QNPC) had difficulty inserting some of the in-core instrumentation thimbles, prompting the company to conduct a remote visual inspection of the lower internals. The inspection was performed during the refuelling outage of July 1998, at which time QNPC discovered damage to some of the components of the lower internals assembly.
In October 1998, QNPC solicited international expert opinion on the best method of performing a repair and recovery programme and returning the plant to power quickly. Subsequently QNPC issued an invitation to bid on the project.
Westinghouse Electric Company was selected to perform the recovery service. Because of the extensive damage to the bottom of the lower internals assembly and QNPC’s desire to return to power quickly, Westinghouse devised an innovative solution. After exploring a number of different methods, it was decided to invert the reactor internals assembly to effect the repair completely and quickly. This first-of-a-kind inversion process meant special equipment had to be designed and qualified for the repair work.
The field implementation team used tools that allowed:
• The lower internals to be placed in an inverted position without damaging or changing critical interface dimensions.
• The refuelling cavity and the reactor vessel to be protected against damage during the upending process.
• The lower internals to be measured or gauged to confirm that critical interface dimensions had not been altered.
• The mechanical repair and hardware replacement to be efficiently and effectively performed.
• Debris generated by the damage and recovery process to be recovered.
The damaged components of the Qinshan 1 plant were the bottom-mounted in-core instrumentation columns and structures. Damage was attributed to flow forces occuring during normal operation. Twenty-three of the 33 guide structures columns and their corresponding fasteners were damaged, and some of them were inoperable.
Other components of the lower internals assembly were also damaged. These were two of the original core barrel bolts, all eight vessel irradiation surveillance capsule holders, and one surveillance capsule holder support bolt. The locking bars on the bolts were missing and the bolts appeared to have rotated, while the surveillance capsule holders showed wear damage at the capsule locating pin. The bottom head of the reactor vessel also showed cladding damage, due to the flow-induced excitation of the loose parts of the lower internals.
The in-core instrumentation structures and damaged areas were at the bottom of the internals. Inverting the lower internals made it easier to reach the work areas, facilitating equipment installation, enhancing repair quality, minimising time on site, and lowering personnel exposure.
SCOPE OF THE RECOVERY
The mechanical repair was completed using conventional tooling, because it was performed with the lower internals inverted. The recovery programme evolved into seven major tasks, necessitating the design and fabrication of specialised equipment. Before the tools were shipped all the new equipment was designed, manufactured, assembled, tested and qualified, and all personnel involved in the on-site implementation were trained.
Westinghouse’s evaluation indicated that the recovery programme schedule would take seven calendar months, and a contractual commitment was made to complete all of the equipment design and site implementation within this time frame. In practice, all site activities were completed on 17 June 1999, eight days ahead of schedule.
The tasks and the newly designed equipment were as follows:
Lower internals repair
To avoid possible hidden deficiencies associated with degraded or damaged equipment on the lower internals, and to help minimise radiation exposure, all of the replacement hardware used during the repair was new. The components replaced were:
• Tie plate and energy absorber with its base plate.
• The 33 in-core instrumentation columns, along with screws, adjustment nuts, adjustment washers, and jam nuts below the lower core support plate.
• Twelve of the 33 columns designed to span the diameter of flow holes in the core support plates and referred to as “cruciform-style”. These 12 columns also included new attachment screws at the core support plate and at the tie plate. A special connection to the lower core plate was added for additional support.
• Two core-barrel former bolts.
• The tops of six worn capsule holder baskets and one capsule holder support screw.
Ultrasonic testing was used to examine the bolts removed from the core barrel and the 60 core barrel to lower core support plate bolts.
Reactor vessel cladding examination
The vessel bottom head was examined in several places for signs of wear. Cladding wear was examined and measured. Damaged regions of the reactor vessel bottom head were examined for cracking. Finally an engineering evaluation of the reactor vessel was carried out to qualify it for continued operation with damaged cladding.
Internals inversion and protection
To prevent damage to the lower internals during the inversion process, a handling assembly was designed and fabricated, consisting of:
• Upender assembly with integral shield plates.
• Lower internals spider brace assembly.
• Baffle-former shim.
• Lower internals top structure .
Refuelling cavity protection
Protection equipment was designed and fabricated to prevent damage to the refuelling cavity during the upending process. This included:
• Reactor vessel cover assembly.
• Reactor cavity floor support.
• Lower internals support adapter.
The inversion and protection became the largest part of the repair activities, and they are detailed below.
Lower internals integrity
To ensure that the lower internals would not be affected by the inversion, a three-dimensional finite element analysis was performed, focusing on the handling of the reactor internals in the reactor cavity and their interaction with the handling equipment (see below). This analysis indicated that the internals structure remained within elastic stress limits, well below the material yield strength.
Foreign object retrieval
To help remove any existing loose parts produced when the internals were damaged, an additional foreign object search was conducted in the reactor vessel, primary piping (from reactor pressure vessel to steam generator, and reactor coolant pump to reactor pressure vessel) and upper internals.
The design of the replacement lower structures for the in-core instrumentation system at Qinshan 1 focused on those areas that were considered to have failed or were possibly involved in the failure of other components produced by the flow forces at normal operation. The replacement design objectives were to increase the strength of the in-core instrumentation column structures and their corresponding bolted connections, and simultaneously to increase the response resonant frequencies of the overall assembly, without significantly altering the flow and pressure drop through the core of the reactor coolant system.
The design selected by Westinghouse addressed these goals by using new columns of greater diameter, and increasing the size of the bolted connections for the columns from those bolts in the original. The upper end of the new cruciform-style columns that protruded through the lower core plate was held in place against lateral motion by means of a special hollow bolt, through which the in-core thimble could pass towards the core. The jam nuts that were on the original design on all other columns were replaced with screws.
The size and thickness of the energy absorber columns and the energy absorber base plate were also modified. The energy absorber assembly is designed to control the impact loads transferred into the reactor vessel during the design-basis postulated-event of a core drop accident. The energy absorber assembly components were modified to lower the corresponding weight of the base plate, thus increasing the frequency and reducing the flow-induced displacement. The tie plate design, which provides an intermediate support for the replacement columns, was essentially unchanged except for increasing its thickness and adding new larger attachment screws at all of the new columns. This accommodated the moments and forces induced from the calculated flow-induced vibration.
Stainless steel was used for the replacement plates and columns and strain-hardened stainless steel was used for the fasteners. A mechanical crimp-style locking device was used to lock all of the replacement screws. No remote underwater welding was required. New core-barrel bolts were used that are the same size as the originals, but a separate mechanical crimp-style locking device was used in lieu of the original locking bar.
Supports for the vessel material irradiation surveillance capsule holders were provided with new bolts of a larger screw diameter, made of strain-hardened stainless steel. As above, the locking device that was used was also a mechanical crimp-style device that did not require welding.
The other components at Qinshan 1 that had to be refurbished were the irradiation surveillance capsule holder tubes. The upper end of these tubes had been worn by irradiation capsule vibration. Westinghouse removed the worn part of the capsule holder tube and replaced it with a sleeve that was clamped around the lower part of the existing holder tube.
Field implementation was performed in three phases, two with the lower internals in the normal position, and one with the lower internals in the inverted position.
With the internals in the normal position, the core barrel bolts, surveillance capsule holder sleeves, and surveillance capsule holder support bolt were all machined and replaced using divers. The lower internals were measured remotely for the actual critical dimensions, debris on the lower internals was removed and protective devices for the internals and the vessel and the refuelling cavity were installed.
With the lower internals moved to an inverted position, mechanical repair of the lower internals instrument columns was completed and the debris removed.
During the final phase, with the lower internals instrumentation structures in their normal position, the special bolts on the cruciform-style columns were installed, and the lower internals were finally gauged and cleaned.
PROTECTION AND UPENDING
The upending process evolved into the major task of the entire recovery programme. Devices had to be designed and supplied that would allow upending to take place, but would also provide protection for the lower reactor internals assembly, as it was moved from its normal upright position to an inverted position, and then back to its normal upright position.
Protective devices for the refuelling cavity floor and associated equipment, reactor vessel, and lower internals storage stand also needed to be designed, fabricated, and supplied to meet the overall project schedule.
The protective devices that were designed and supplied for the lower reactor internals were composed of two major assemblies: the top structure, spider, and baffle support; and the upender assembly.
The top structure, spider, and baffle support were used to ensure that the inside of the internals was stabilised and supported. The internals spider assembly was composed of a series of large horizontal plates that held eight radial jacks, each applying a small nominal radial force on the baffle assembly that was always perpendicular to the longitudinal axis of the internals. Supports were also applied axially at the bottom and top baffle plates.
The top structure assembly was composed of a large fabricated ring and two cross beams. Also included in this structure were axially oriented columns intended to provide a restraining axial load on the baffle plates. The top structure carried the weight of the baffle plates when they were inverted and provided protection for the internals head and vessel alignment pins. The second major assembly – the upender assembly with integral shields – was designed so that the internals were protected.
The upender assembly – basically a space frame truss – was designed to provide a support to the exterior of the lower internals and to provide shielding against personnel exposure during the inversion process. The upender structure was designed to lift the entire weight of the lower internals with the top structure and internals spider bracing (a total of some 140 000 lb). The upender was designed to be lifted with and without the reactor internals.
The reactor cavity floor was protected by floor reinforcement that essentially covered the cavity floor and equipment around the reactor vessel. The purpose of this reinforcement was to prevent damage to the cavity liner and concrete abutments at the bottom of the cavity floor.
The reactor vessel was also covered to prevent debris from entering the vessel during the recovery process.
A special adapter for the lower internals storage stand provided an augmented structure to hold the combined upender assembly and lower internals assembly weight, as well as provide a guidance feature for locating and supporting the lower internals in the inverted position.
Several extensive analyses were performed for this operation.
The lower internals replacement components were analysed to establish the operational flow velocities and forces during normal operation and transient conditions. They were shown to be acceptable with respect to ASME code limits.
As indicated previously, the reactor internals were also analysed using a 3-D finite-element structural model that incorporated a model of the lower internals and its components, and included the upender assembly and its components, along with the internals spider braces and top structure.
This analysis showed that the stress level on the lower internals and components would remain within elastic range during the upending process and consequently not be affected by the inversions.
The integrity of the lower internals, the maintenance of the alignment of critical interface and guidance devices on the lower internals, and the prevention of damage, were of prime concern during the recovery process. To this end, a programme of plant equipment measurements was undertaken. This programme required that unique measurement tools be designed, fabricated, and delivered to the site before beginning any work on the reactor internals.
Gauging tools were supplied to:
• Measure the “as-built” location of all of the vessel penetrations at the bottom of the reactor vessel that guide the in-core instruments during plant operation .
• Measure the location of the fuel core baffle plates relative to the lower core plate.
• Tie the plate and energy absorber base plate relative to the lower core plate of the lower internals.
• Measure the flatness of important features that were machined into the core support plate.
These measurement tools were designed to operate remotely underwater, providing dimensional measurements to an accuracy of ±0.010 in. A total of seven such tools was supplied.
The success of this programme can be measured by its low personnel exposure, short on-site schedule, and low cost. Advanced planning, such as doing the repair with the internals inverted, using all new internals components, and special rigging for heavy lifts contributed immensely to this success. “First of a kind” tools such as the upender had to be designed and fabricated to accomplish new tasks such as inverting the internals and protection the refuelling cavity.
Finally, close co-operation between Westinghouse and Qinshan 1 Nuclear Power Company during the on- site work, for example in daily planning sessions, and the commitment of Westinghouse management and resources to make this a high priority project were essential to the programme’s success.