Eddy Current Inspection of Cladding Material on Vver 440 Reactor Pressure Vessels

Reactor Pressure Vessel (RPV) is classified as a safety related component and continued operation of the NPP for an additional fuel cycle is contingent upon the Nuclear Regulator approving the in-service inspection (ISI) performance. In accordance with applicable Finish regulative the integrity of cladding material on reactor pressure vessel is part of the interest and has to be included in examination scope. Comprehensive deterministic safety analyses have been performed by Fortum Loviisa Power Plant in order to define the critical size of defects which have to be detected, as well as, requirements for their sizing. These analyses included surface breaking and subsurface flaws. On the basis of these specifications INETEC developed eddy current technique which is capable to fulfill the requirements for such examination. The aim was to developed reliable eddy current technique which can be applied in parallel with standard ultrasonic examination. As a common project between Loviisa Power Plant and INETEC, developed technique has been qualified in accordance with ENIQ Methodology. The article will present the requirements on inspection, capabilities of developed eddy current technique and mode of its application. Also, the methodology and results of qualification process will be presented. Introduction: Loviisa NPP has two VVER-440 units of six loop design with reactor pressure vessels (RPV) having a cladding which height is ~9mm. In accordance with applicable Finish regulative the examination of cladding material is of importance from safety point of view and have to be included in the scope of In-service inspection (ISI) of RPV. To insure the integrity of the reactor pressure vessel during operation and during severe accident conditions, it is important that there are no flaws exceeding the plant specific requirements. The flaw type information was based on the manufacturing documents, experiences and engineering judgment on the most potential flaw mechanisms. The flaw types are classified as: a) Specific flaws – flaws specific for inspection areas such as: subsurface volumetric (slag, porosity) and planar welding flaws parallel (also transverse) to welding direction; bonding flaws in cladding/base metal (weld) interface, solidification cracks above wide slag inclusions and slag lines parallel and transverse to welding direction, fatigue cracks (stress cycles in cladding break thin porous cladding ligaments above welding flaws and grow a solidification crack to surface breaking). b) Postulated flaws flaws induced by expected damage mechanism or manufacturing – such as radial fatigue cracks in nozzle inside radius section (pressure and thermal stress cycling increased by nozzle concentration). c) Unspecified flaws these flaw types are neither detected nor postulated for inspection areas of RPV, damage mechanism or process is not identified. Requirements on detection and qualification of flaws are set in accordance with the results of comprehensive deterministic safety analysis for various types of flaws specified above. This included analysis of allowable flaw sizes, taking into account flaw’s position with respect to surface, flaw orientation and morphology. For determination of the different flaw sizes the assessed flaw growth during one inspection interval has been taken into account. Engineering judgment or worst case conditions was used for describing flaw information for qualification and inspection. Requirements on Eddy Current Technique: Due to certain limitations of ultrasonic and visual testing, the eddy current method has been selected as appropriate technique for examination of cladding surface. The general idea for application of eddy current testing was to be used as complementary method to ultrasonic method. Since ultrasonic technique applied for standard RPV examination has a limited capability in characterizing the defects in near surface area, the aim of eddy current was to detect and size the flaws in region 0 – 4 mm from surface. The main goal of eddy current was to define the position of flaw with regard to surface. The flaws are classified as surface in the case that they are breaking surface, or subsurface if flaws do not break the surface. The convention was made that flaws which has ligament ≥ 1mm between the flaw tip and surface will be classified as subsurface flaws. These two types of flaws are presented in Figure 1.