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NPP, Inspections & Maintenance

Date: 19. 8. 2015

Source: Nuclear Plant Journal

ŠKODA JS is the general supplier of maintenance services for the nuclear islands of both of the Czech Republic's nuclear power plants –Temelín and Dukovany. There are a total of 6 production units at the two plants.

Under the contract with the power plant's operator ČEZ, a.s., ŠKODA JS, using its own employees or qualified staff of technical service providers in the supply chain, ensure the preparation, performance and evaluation of the maintenance of nuclear island equipment, including inspections required by the regulatory authority.

Approximately two-thirds of the activities are performed by the company's own staff; the rest is performed by qualified contractors to the highest standards possible, using the services of manufacturers of a variety of primary circuit equipment.

Maintenance services at power plants include the preparation and performance of planned (preventative and predictive) and corrective maintenance, the majority of which takes place during the outages of production units. These include regular inspection activities, general and regular equipment repairs, the repair or replacement of damaged and dysfunctional components or those that fail to meet inspection requirements, obligatory inspections and tests, as well as activities connected with coordination of the supply chain and expert, consulting and production activities. ŠKODA JS ensures detailed preparation of these activities, whether it be on the technical level (procedures, technical solutions, blueprints, calculations), administrative level (permits, qualification documents), or the inspection level (inspection and test plans, checklists). An integral part of maintenance is also its evaluation, proposals for changes and innovations, the assessment of discovered indications and cooperation in resolving them, as well as close collaboration with the company that operates the power plant in the areas of equipment maintenance planning, technical inspections, investigation of events, scheduling, and job performance quality.

Using its own staff or through contractual partners, ŠKODA JS also conducts a number of innovative activities, such as the replacement of control rod linear stepper drives (the components of the safety and control system of a nuclear reactor), the supply of reactor main flange joint stud tensioners and upper block flange joint stud tensioners, the production and delivery of casks for spent nuclear fuel, special shielded containers, etc.

In-service inspections by ŠKODA JS

The activities of ŠKODA JS include the assembly work itself connected with equipment inspections as well as installations of new technological equipment and the performance of non-destructive tests.

The history of in-service inspections at ŠKODA JS dates back to 1971, when a small group specialized in the in-service inspection of the first Czechoslovak nuclear power plant, A1, was formed.

At present, ŠKODA JS personnel perform in-service inspections of the following primary circuit components:

  • Reactor pressure vessel
  • Reactor pressure vessel closure head
  • Studs and nuts of the main flange connection (M140x6 or M170x6)
  • Reactor internals - core barrel, core shroud and protective tubes unit (PTU)
  • Steam generator heat exchanging tubes
  • Primary piping welds

Three testing methods are used for the automated testing of primary circuit components of Czech nuclear power plants – ultrasonic, eddy current and visual.

Ultrasonic testing

Ultrasonic pulse echo is the principal non-destructive technique used for the automated testing of VVER reactor components. Longitudinal wave and shear wave probes of several different angles are used for the testing. Testing by angle probes is usually done in four directions – two parallel with and two perpendicular to the weld axis, so that the whole tested volume is covered and flaws in all possible orientations can be detected with the highest probability. 

The TOFD diffraction technique is used for the testing of weld roots and for the sizing of volumetric or planar discontinuities which have to be analyzed. Parallel and non-parallel scanning of the tested area is performed if the geometry of the tested part makes it possible. Probe angles and probe separation are chosen so that the tested volume is covered with enough sensitivity.

The phased array technique is used mainly for the testing of austenitic piping welds.

Eddy current testing

The reactor pressure vessel's inner and outer surfaces are tested by surface multi-coil probes, which detect flaws in both orientations, parallel with and perpendicular to the probe movement.

Studs, nuts and threaded holes in the reactor flange are tested using surface probes designed to detect flaws perpendicular to the tested part's long axis.

Bobbin coil probes are used for the testing of steam generator heat exchanging tubes, while the steam generator tube sheet is tested by a cross-probe of the X-point type and rotating probes.

Visual testing

Remote visual inspection is done by color and B/W CCD TV cameras. The cameras can be equipped with different lenses. Remotely-controlled external lights help maintain the best possible conditions for the visual inspection. A camera accessory generating laser dots of a known distance is used to measure the dimensions of displayed objects. The position of the manipulator, the date, time and operator comments can be mixed with the TV picture and displayed on the TV monitor. The entire visual inspection or the inspection of selected parts is recorded on a digital video recorder.

Testing Systems

ŠKODA JS uses several manipulators for the testing, some provided by the NPPs, some designed and produced or adapted by the company.

Reactor pressure vessels have so far been tested from the inner surface by a system consisting of a SKIN manipulator, the 48-channel UT instrument Microplus 2, the eddy current instrument MS5800 and a color CCD TV camera, TV CC 510TL. The camera has a lens with a horizontal viewing angle of 5° - 49° and can operate in a light intensity of only 1.5 lux.

The testing from the outer surface in Dukovany is done by a system consisting of an upgraded manipulator USK-213, the 16-channel UT instrument Microplus 2, the ET instrument MS-5800 and a color CCD TV camera, the SONY XC-999. The camera has a lens with horizontal view angle 59° and can operate in a light intensity of only 4.5 lux.

The system for the testing from the outer surface in Temelín is composed of an upgraded manipulator SK-187, the 16-channel UT instrument Microplus 2, the ET instrument MS-5800 and a SONY XC-999 TV camera.

Reactor internals at Temelín are inspected by the TEKOR manipulator equipped with a radiation tolerant TV camera with remote control and lighting head with remotely controllable halogen lights. The camera lens is interchangeable; usually a lens with a viewing angle of 20° is used. This manipulator was designed for the inspection from the outer surface; after a modification performed in 2010 it is also used for the inspection of reactor internals' inner surfaces.

New MKS manipulator

The new MKS manipulator for the testing of reactor pressure vessels from the inner surface has been designed and produced at ŠKODA JS as a substitute for SKIN. The manipulator has a main support with three legs, a telescopic mast and transverse track with two trolleys. Two testing arrangements can be installed on each trolley, so that at any one time this manipulator is prepared to perform four different testing tasks.

The testing system consists of the MKS manipulator, the 48-channel UT instrument Microplus db-POD, the ET instrument MS 8500 and HDTV cameras and monitors. This manipulator can be used on several types of pressurized water reactors – VVER 440, VVER 1000, VVER 1200, AP 1000 and others.

This manipulator shall have the following advantages in comparison with SKIN

  • smaller workplace in the reactor hall
  • shorter assembly and disassembly time
  • shorter inspection time
  • lower demand for cranes

Production of the manipulator was finished in 2014 and the testing including a practical qualification trial on a qualification sample was finished in February 2015. The first in-service inspection of this manipulator shall be performed on Temelin NPP Unit 2 in May 2015.

Increasing equipment lifetime

ŠKODA JS further deals with activities in the area of the evaluation of and changes in parameters of strength pressure testing and optimization of the scope of in-service inspections conducted on the primary circuit of VVER-type reactors. The parameters of a strength pressure test are to be understood as decreasing the pressure and extending the period compared to the design values, which means a significant improvement in the wear lifetime of the main components. Closely connected with this is optimization of the performance of in-service inspections, mainly in terms of determining equipment integrity, i.e., that quality non-destructive methods can to a certain extent substitute strength pressure tests thanks to the quality evaluation of indications of faults in the integrity of the component's material.

In terms of increasing the lifetime of structures, systems and components, ŠKODA JS a.s. focuses mainly on the components included in its production portfolio (e.g., connecting rods, control rod drive mechanisms for VVER 440 reactors, compact spent fuel storage racks, etc.).

Design, production and operation of special manipulators

ŠKODA JS also develops and produces devices whose utilization allows customers to not only reduce the length of outages, but also achieve higher quality. One such example is the upper block flange joint stud tensioner for VVER 440 and VVER 1000 reactors. For example, use of the EZ250 flange joint stud tensioner for the VVER 1000 results in a reduction in the time necessary for loosening and tightening the flange joints of 35 hours per individual reactor inspection.

Special repairs

ŠKODA JS in the past carried out and is currently working on a number of unique repairs of equipment, such as:

  • Removal of a jammed PTU from the RPW in VVER 1000
  • Refurbishment of rotors of MCP motors in VVER 1000
  • Repair of the SG heterogeneous welding joint in VVER 400
  • Developing technology for repairs of welding joints No. 111 (SG) in VVER 1000
  • Replacement of the upper part of the SG collector in VVER 440

You can see the original of this article here:

NPJ article