The policy of each Nuclear Power Plant (NPP) is to ensure safe production of electrical energy as well as security of supply. The operating temperatures in the primary circulation circuit are 270˚C ÷ 320˚C, the pressure reaches 17.5 МРа. The metal of NPP equipment is affected by aging mechanisms formed by the working environment—neutron and thermal brittleness, corrosion, erosion, fatigue and wear [1] . These influencing factors change the mechanical properties of the metal and reduce the bearing capacity of the structures. After several decades of operation of the nuclear unit, it is usually necessary to prepare (or revalidate) strength analyses of the equipment to ensure its safe operation [1] [2] . Corrosion of the metal is caused by the working medium—water with various reagents added (primary circuit fluid). The high values of the reactor temperatures and the operating stresses cause stress corrosion and corrosion fatigue. The radiation environment causes general and intergranular corrosion. Corrosion damage is complemented by the processes of erosive washing away from the flowing fluid. On the other hand, high fluence values average $\left(1,{5.10}^{22}\text{neutrons}/{\text{m}}^2\text{per year}\right)$ from neutrons with an energy greater than 1 MeV can cause changes in the structure of the irradiated material. Figure 1 shows nanostructured images of irradiated materials [3] . Cluster formations of atoms of the chemical elements copper Cu, phosphorus P and silicon Si are observed.

The uneven metal structure is the root cause of the brittleness of RPV Internals metals. (In the state of brittle fracture, metal fracture can occur suddenly and unpredictably, with a small influx of external energy). The neutron flux produced by the chain reaction of the decay of uranium fuel in the core causes the neutron and thermal brittleness of the metal [4] . The essence of the problem is: 1) Swelling is an increase in the external dimension of solid materials after irradiation. 2) Vacations created by the materials that make up the inside of the reactor as a result of irradiation can accumulate in cavities, which can lead to changes in the dimensions of the material. 3) A change in the dimensions of the Internals leads to a decrease in the distance, a violation of the movement of the coolant and the heat balance, and may lead to jamming and blocking of control and protection system units. Swelling is an increase in the external dimension of solid materials after irradiation and is an extremely dangerous condition of the metal structure, because it reduces the bearing capacity of structures. Figure 2 shows diagrams of Reactor Pressure Vessel Internals and Core Barrel.

The Core Baffle is important for safety because it provides a high concentration of the reactor coolant flow in the core region. The Core Baffle is made up of vertical plates called baffles and horizontal support plates called formers. The baffle plates are bolted to the formers by the baffle/former bolts, and the formers are attached to the core barrel by the barrel/former bolts. The Baffles provide a barrier between the core and the former region so that a high concentration of flow in the core region can be maintained [5] . The geometry of the Core barrel is the same as that of the core—it is a shell structure. The change in the size of the Core Baffle would cause a reduction in the distance between the Core barrel and the fuel assemblies, violation of the movement of the coolant and the heat balance; may lead to jamming and blocking of the control roads and protection system. In some operating modes, it is possible in the event of an ECCS incident (Emergency Core Cooling System) to realize fast cooling (Pressurized Thermal Shock). The Core barrel is the most irradiated part of the reactor, it guarantees the safety of the reactor plant. The environmental conditions of the working environment are: 1) Max rate of damaging dose in the metal; 2) The temperature varies 290˚C - 320˚C on the surface and up to 400˚C in the inner layers due to γ irradiation [4] . All these processes of modification of mechanical properties are envisaged in the design of the plant. To ensure the safe operation of a nuclear reactor, different procedures are in force in different countries [6] [7] . However, there is no requirement in the technological regulation to regularly measure the actual geometric dimensions of the internals housing devices. This article discusses the main Core baffle and the Core Barrel as an object of measurement. The metals of the objects were studied for the effect of two mechanisms of degradation of mechanical properties—general corrosion and swelling.

The Core Baffle materials are austenitic steels and ferrite-pearlite steels with austenitic surfacing. Austenitic steels are corrosion-resistant, have appropriate technological properties, work up to temperatures of 700˚C; steels of type 08X18H10T are radiation-resistant (for WWER reactors 440). Alloyed Pearlite Chromium-Molybdenum-Vanadium Steel 15Х2НМФА (15H2NMFA) has two layers of austenitic overlay (for VVER 1000 reactors). Steels 15Х2MФA (15H2MFA), 15Х2НМФА (15H2NMFA), А542, А543, А508 have resistance to radiation brittleness, high strength and good plasticity (R_e = 500 ÷ 900 MРa), are not corrosion-resistant. The mechanisms of degradation of mechanical properties are determined, Table 1 . The assessment is carried out on the basis of the technical condition of the facilities after more than 30 years of operation, and information from other NPPs is also taken into account.

This study investigates the effects of aging induced by the mechanisms of corrosion and swelling. Corrosion is examined by a visual method of inspection by remote means [8] . The specific objects of study by visual method are from the equipment of three NPPs, type WWER, Table 2 . The study period covers 35 years.

The equipment and the visual inspection scheme are presented at Figure 5 .

The measurement of geometric dimensions of the Core Barrel is carried out on the basis of the triangulation method, Figure 6 [9] .

The known dependencies of such triangles are used. In this method, a television probe (camera) is used, which has a fixed part and a rotating part. A light source (laser) is attached to the rotating part, Figure 7 . The laser beam scans the wall of the Core barrel step by step. Through the camera, the image of the metal is monitored and recorded. The distance from the probe to the Core Barrel wall (AB) is measured. Any deviation from the initially set value of this distance (reference value) is registered.

This method of geometrical dimension research is applied to two WWER 1000 units. The period of operation of one unit is 32 years, and the other unit is 30 years.

The results of a visual examination of the Core Barrel metal of three WWER blocks show that no corrosion spots are observed on the surface of the objects. Corrosion ulcers are noticeable only on the inner surface of the reactor vessel in the places of upper attachment of the elements of the Core baffle, Figure 8 .

Individual surface inconsistencies (defects) in the shell surfacing are observed and their parameters are determined. An analysis is being carried out whether the defects are permissible according to the regulatory requirements.

A measurement of the geometric dimensions of the Core barrel was carried out during operation. The specific indicators of the measurement carried out are:

1) Coverage of the measured distance, 1450 ÷ 1650 mm; Discrete distance value, 0.1 mm; absolute distance measurement error, ±0.5 mm;

2) Rotation Angle Measurement Range, 0 ÷ 360˚; Discrete Angle Value, 0.0001˚; Absolute Rotation Angle Measurement Error, ± 0.5˚.

The measurement results obtained are given in Figure 9 .

The presence of the zones with maximum displacement is shown in Figure 10 .

Measured results by height of the Core barrel are presented in Figure 11 .

The assessment of resistance to corrosion and erosion wear (general corrosion) is made by the aging index—the type, size and location of surface defects of the metal from the inner surface of the reactor vessel. The defects found are admissible according to the regulatory documents, therefore, the requirement for resistance of the metal to corrosion and erosion wear is met.

The assessment of swelling resistance is carried out by the indicator change in dimensions. The results obtained show that there is no significant change in the geometric distances of the Core Barrel, compared to the passport data. Тhe displacement dimensions are less than 1 mm. The metal is resistant to swelling. It is assumed that until the next such measurement (after 4 years) there will be no (significant) change in the geometric distances.

The metal of the internal casing devices of the nuclear units, type WWER, is resistant to corrosion and swelling. This conclusion can be made under the very important condition that nuclear units are operated (must be operated!) in accordance with the requirements of the technological regulation. The presented method for measuring the swelling of the Core Barrel metal caused by neutron and thermal effects is an easy and affordable way to regularly measure the actual geometry of the Baffle walls. It is easy to make comparisons with previous data and accordingly track the trend of geometry change. At present, there are no standards (procedures) for measuring the swelling of the Core barrel metal caused by neutron and thermal effects. The presented method would contribute to the development of similar standards (procedures).