New potential for potentiometry10 May 2018
Russia has developed a new method of nondestructive testing, known as scanning contact potentiometry. Vitaly Surin reviews the features and prospects of the technology.
The need for a new method of nondestructive testing was identified after studies into the behaviour of carbonitride nuclear fuel (carbonitride steel is surface-hardened with nitrogen) under irradiation carried out in the late 1980s. After publication of results in The Journal of Nuclear Materials, a lot of effort has been put into developing electro-physical methods of nondestructive testing. The method will work reliably in high radiation fields and high temperatures.
In 2003 the laboratory of functional electro-physical diagnostics and non-destructive testing (ElphysLAB) was established at the National Research Nuclear University in Moscow, Russia. The technologies developed for in-reactor nuclear fuel research were subsequently transferred to laboratory conditions for studying the strength and plastic properties of materials. During the process, a new approach was applied to deal with the problem of electrical contact materials with metallic conductivity. In addition, more effective information-measuring systems, software and analytical systems were applied as IT technologies were developed.
Description of the process
The method of scanning contact potentiometry (SCP) is based on the following basic provisions and models:
- The processes of formation of dynamic waviness and roughness occur on the surface of a solid under load.
- The electrical contact of the sample with a sensor represented by a quantum object. Here, under load, electron energy levels degenerate, and there is dynamic change of surface dipole barrier at the interface and the work function of the electrons.
- There is a causal relationship between the change in the mechanical and electrical properties of materials.
To develop the measuring technology, we used pure aluminium, copper, molybnedum and lead as objects of investigation; we also used alloys ??20 and ??10, and low and medium carbon steel, steel grades.
For tensile tests, we used wire (diameter 0.1–0.5mm; length 100-450mm); round and rectangular rods, and strip and sheet samples (using copper and steel). Cylindrical aluminum samples with a diameter of 5-7mm and a height of 8-12mm were tested for compression. For fatigue testing for a three-point bend, steel samples were used in the form of beams of rectangular cross section (4.5×4.5×36mm). Over the period of the studies, more than 200 different samples were tested.
The method has a high local resolution. Different types of defects radiate waves (infra-red and ultrasonic) in different frequency ranges and with different amplitudes. The intensity of this radiation will be higher whenever the transducer is located close to the source. If the elastic deformations or stresses are present in the volume of the object being diagnosed, then we also obtain the characteristics of the inhomogeneity.
Nuclear power plant equipment undergoes diagnostic testing to determine the real condition of the equipment and ensure proper operation, maintenance and repair if necessary, and also to identify possible malfunctions at an early stage of development.
The electro-physical flaw detector (defectoscope) EDSS-1RD is designed for electro-physical diagnostics of welded joints of large and medium-diameter pipes. It was demonstrated at the Kalinin nuclear power plant in early 2016. NDT specialists appreciated its functionality and saw that it could be used in operating conditions.
An ‘approval of method’ was carried out late last year in the National Research Nuclear University MEPhI on the site of the JSC AEM-Technologies or Atommash in Volgodonsk city. The object of nondestructive testing was the welded joints of the primary loop to the steam generator PGV1000. The SCP method performed point-by-point measurements using a portable electro-physical sensor, and then carried out the surface scan using the manual physical flaw detector EDSS-1RD.
Subsurface disturbances detected in the strip SSL “6” (SSL – the structural level of the signal), are identified as transverse or longitudinally-transverse small-size discontinuities. In the welded joint of the collector sample, in the regime of direct current transmission, a singularity defect was detected, the presence of which was confirmed by specialists who had performed ultrasonic inspection at the factory, as well as by the results of frequency-time analysis of the signals. V.G. Beketov, (coordinator of the Resource Center of National Research Nuclear University MEPhI) and A. E. Dembitski (head of laboratory in Research institute – Nuclear energy engineering) took an active part in organising the work with the support of the management of Atommash.
The SCP method has already prompted interest from several enterprises of the nuclear industry. The novelty of the method is confirmed by an application an international patent.
This SCP electro-physical method of non-destructive testing provides effective diagnostics for reactor equipment, providing reliable measurement results even in conditions close to an operating reactor. This includes the reactor’s active zone, where other methods of diagnostics and control do not give the real picture because the accuracy of the measurements depend directly on external factors, for example, temperature, dose rate, influence of an aggressive medium, etc.
The electro-physical sensors do not use high-current circuits and heated areas, therefore measuring systems are characterised as having high fire safety. It should also be noted that the electrical power level of the useful signal is very low so there is no sparking from mechanical contacts.
The SCP method can permit multipoint and distributed measurements, including using multiplexing of sensing elements located at different parts of the monitoring object. To process the flow of incoming information, software codes of frequency-time and spectral analysis are applied.
Several tasks have already been completed during the development and verification of the SCP method:
- Determining the changing shape of articles during mechanical tests for tension, compression and bending, including in-reactor conditions.
- Evaluating the technical status of metal plates in fatigue tests.
- Evaluating the technical condition of the tooth surface in a gear mechanism.
- Gauging the thickness of oxide films on the surface of metal plates.
Electro-physical non-destructive testing of materials and products was carried out and the following results were obtained:
- The value of the non-uniform deformation distributed along the length of a wire during tension, and search and localisation of the place of wire breakage.
- The magnitude and distribution of thermo-elastic stresses in a steel pipe while passing through the coolant.
- Values of local narrowing (neck) in fractographic studies of metals and alloys.
- Values of porosity of sintered nuclear fuel pellets.
- Electro-physical properties of Cu-Nb nano-materials.
One of the main advantages of the SCP method is that the measuring system transducers have small linear dimensions (diameter 1-2mm), provide high reliability and reproducibility of measurement results, have a high noise immunity and low level of intrinsic noise, and can be located, practically, in any inaccessible place.
The SCP method will allow us to improve the safety of nuclear power plants in operation.
The latest achievement is the verification of the SCP method at the pulsed fast reactor IBR-2 in the FLNP JINR (Dubna, Russia). A unique method of neutron correlation diffractometry was used. The experiment was successful. Scientists also believe that the proposed method is useful and will find wide application in high-quality diagnostics of reactor equipment.
The author expresses his gratitude to the Government of the Russian Federation, the leadership of State Atomic Energy Corporation Rosatom and the National Research Nuclear University MEPhI for the support extended over a long period of development and verification of the SCP method.
Vitaly Surin is Scientific Supervisor of ElphysLAB at the National Research Nuclear University, Moscow, Russia.