Khmelnitsky NPP-retrofitting reactor

In the framework of the Tacis-96 program for Khmelnitsky NPP (KhNPP) the Consortium DTN, EDF and GKN (with DTN as consortium leader) provided On-Site Assistance (OSA) to the Ukrainian NPP (On site utility). The Terms of Reference (TOR) for this service project U1.04/96 “Retrofitting of Reactor Vessel SK-187 surveillance system” was elaborated by the OSA Utility team. A restricted tender was initiated in October 1999 and the Contract with the Consortium led by Siemens AG was signed in March 2000.

The increasing demand of a higher level of safety in the operation of the nuclear power plants, requires the utilization of more demanding procedures to perform In Service Inspection (ISI) of the reactor pressure vessel. The objective of the project was to provide KhNPP technical staff with the required skills and background knowledge to be able to manage the modern ISI technology with the inspection equipment available at KhNPP.

The objective was further to develop procedures, studies and training programs related to the modern ISI technology focused on KhNPP. The activities for the project were based on the already existing, generic experience of ultrasonic inspection of reactor pressure vessel welds and the other related studies. Also the studies performed in the framework of the Tacis project U1.02/92A for South Ukraine NPP were taken into account. Finally one objective was also to provide proper calibration blocks for selected RPV parts and geometries.

A. Task 1 Safety assessment studies and proposals for NDT

RPV stress analyses

Preservation of Reactor Pressure Vessel Integrity is an essential factor to assure Plant's safety and long time reliability. High stressed areas of the RPV are sensitive areas for integrity and they have therefore to be subjected to In-Service Inspection (ISI) in order to detect and monitor any development of defects.

The first part of TASK 1 compared western requirements to those which were the Russian design base for VVER-1000 Reactor type. It was intended to do this study in a detailed way, but as there was no deeper information for more detailed analysis all activities to fulfil Task 1 were done on the base of more general information about the work of VVER-1000 reactor, which was received from KhNPP and from other projects. Comparison of Ukrainian codes with codes with western codes showed, that operating rules in Western Europe could be used as a base for safety analysis of VVER-1000 reactors. Therefore the proposed procedure for stress analysis was based on ASME code and KTA Safety Standards.

Indication assessment manual

The manual for assessment of indications found in ISI was based on the Russian and German (KTA 3201.2, Edition 6/96) and Russian (PNAE G-7-002-86) standards as well as on IAEA recommendations for VVERs (IAEA-EBP-WWER-08, Vienna, April 1997). The manual was divided into two parts; Analyses of thermal stresses and fracture mechanics analyses. First a description of the ISI methodology and flaw configuration was given. Procedure for defining relevant load cases as well as postulated flaws in critical zones were developed.

The following list of initiating events recommended to be considered for VVER-1000/320 was elaborated:

• Spectrum of postulated piping break within the reactor coolant pressure boundary;
• Rupture of the line connecting the pressurizer and a pressurizer safety valve;
• Inadvertent opening of one pressurizer safety valve;
• Leaks from the primary to the secondary side of the steam generator (SG):

1. SG tube rupture;
2. Primary collector leaks up to – collector sealing component lift-up.

• Inadvertent opening of one check or isolation valve separating reactor coolant boundary and low pressure part of the system;
• Inadvertent actuation of emergency core cooling system during power operation;
• Chemical and volume control system malfunction that increases reactor coolant inventory;
• Inadvertent opening of one steam generator safety or relief valve or urbine bypass valve;
• Spectrum of steam system piping break inside and outside of containment;
• Break of feed water piping.

Finally recommendations regarding temperature and stress field calculations were elaborated. This part was mainly based on the above mentioned IAEA report.

B. Task 2 Inspection Procedures

There were two separate inspection procedures foreseen:

• one manual with instructions related to the mechanical aspects of the RPV inspection;
• one manual with procedures related to ultrasonic testing techniques for the RPV.

The ultrasonic testing equipment SK-187 used at KhNPP was described. The system consists of 3 separate parts. The first part is a system for testing of the RPV bottom and cylindrical weld below the supporting ring of the RPV. This system is based on a platform which is located below the RPV in a special storage room. For the inspection procedure the testing platform will be transferred from the a storage room to a service room just below the RPV. There is one weld seam in the RPV bottom and it is welded by electro-slag method. This weld is inspected with a special bottom testing mechanism carriage and manipulator. On the testing platform there is also a telescopic bar with another mechanism carriage which can be pushed up along and rotated around the RPV cylindrical part for inspecting the cylindrical welds from the outside. The carriage mechanism includes fasteners for ultrasonic probes, grinding device, TV camera sensors for localization etc.

The manuals also describe a subsystem for testing of the cylindrical welds in the nozzle area of the RPV. Adjacent to the welds in the nozzle zone there are permanent rails along which the inspection carriage, “tractor”, can travel around the vessel cylinder. Similar inspection systems are used for inspection of the 8 primary piping and 4 ECCS piping nozzle welds.

The manuals also describe the search heads and the transducers used for the ultrasonic inspection of the above mentioned vessel and nozzle welds by using the SK-187 inspection equipment. It is shown how the areas to be inspected have to be scanned with overlapping, which transducers are used and other details related to the implementation of ultrasonic inspection.

C. Task 3 Training program

An important part of the project was to perform transfer of know-how from the EC Consultant to the concerned organisations in Ukraine in order to prepare them to perform similar in their NPP in the future and integrating western experience. One objective of the project was to provide KHNPP staff with required skill and background knowledge to be able to manage modern ISI technology with the equipment available at site. The training of the inspectors was implemented in 2 separate parts. The first training took part in Rungis in France and the second part at site in KhNPP. The training focused on all inspection aspects related to preparation of inspection, performance and evaluation of results obtained during the ISI of a RPV. The technical content of the training program was based mainly on the results of Task 2 above, including the inspection procedures and the ultrasonic technique to be used. the training was divided into two topics: one for shift leaders, field managers and analysts and the other for data acquisition system operators. One part of the training was devoted to phased array application even if the existing equipment at KhNPP did not support the phased array technique as yet. The phased array training was based on the SIROCO manipulator control unit, SAPHIR UT-equipment with phased array technique and SAPHIR data acquisition and analyses system (IntelligeNDT equipment. ex Siemens NDT).

D. Task 4 Supply of a set of calibration and reference blocks

All work under task 4 was planned for modifying existing calibration and reference blocks according to requirements of the ISI could be fulfilled according to proposal of Task 1. Originally it was planned to utilise the existing and available set of reference and calibration blocks in KhNPP as in other equivalent TACIS projects, which had been performed in South Ukraine NPP, in order to avoid to have a double set of test blocks.

KhNPP informed NIKIMT and the consortium that the concerned set of calibration and reference blocks was contaminated and could not be cleaned well enough.

Therefore NIKIMT looked for appropriate material on the Russian and Ukrainian market in order to manufacture new reference blocks which can be handled outside the radiation controlled area. After the material had been found the manufacturing of the blocks started. New reference blocks were manufactured and delivered and they were qualified and finally accepted by all stakeholders. The new delivered reference and calibration blocks covered the MCL and ECCS nozzles, the RPV bottom weld as well as the RPV cylindrical shell welds.

Comments

The project showed that the phased array UT inspection technique would provide a large increase of volume coverage for the RPV welds. Therefore there was a strong recommendation to improve the inspection technique at KhNPP by using the phased array technique.