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Proceedings of the conference

Varovin A.Ya. Minkin A.I. Margolin B.Z.

Experience of measurements and analysis of changes in the geometry of core baffles of russian reactors VVER-1000

Pressure vessel internals (especially core baffle) are the non-replaceable components of WWER-1000 reactor which exposed to the most intense radiation during operation. Dose rate for the core baffle metal is 1.7-2.0 dpa/year and after operation during 30 years damage dose exceeds 50 dpa. The temperature in the inner layers of the core baffle can reach 400?С due to ?-heating. High damage dose and increased temperature lead to radiation swelling and radiation creep of the core baffle metal. Radiation swelling leads to decreasing strength and plasticity of the core baffle metal as well as to sharp decreasing its fracture toughness. In addition radiation swelling and creep lead to changes in the geometry of the core baffle. Such changes leads to a reduction in the gap between the core baffle and fuel assemblies, can complicate a fuel recharging and can lead to jamming and blocking of control rods.

The data on the radiation swelling is a part of the data necessary for the calculation of strength and residual lifetime of PVI. Russian Regulatory Document RD EO 1.1.2.99.0944-2013 prescribes to perform calculations using the temperature-dose dependence of the radiation swelling corresponding to the upper bound for the 95% confidence level. This principle excludes the error to the dangerous side when assessing the strength and safety of the PVI for any NPP Unit but may lead to overly conservative calculations and underestimation of the service life of the Unit.

To decrease conservatism of calculations it is proposed to specify the temperature-dose dependence of the radiation swelling for PVI material of the given NPP Unit. To specify this dependence the comparison should be made for the calculation results of changes in the geometry of core baffle during operation and the results of direct measurements of the core baffle geometry. Procedure for determination of radiation swelling dependence on the basis of direct measurements of the core baffle geometry entered as the mandatory annex in RD EO 1.1.2.99.0944-2013.

For the implementation of this procedure JSC «Diakon» developed a system «STS-C-VKU» for measuring the geometry of the core baffle. The system uses the triangulation method to determine the distance from the TV-probe equipped with a laser and TV camera to the laser impingement point on the inner surface of the core baffle.

The system «STS-C-VKU» has been successfully used for measurements of the core baffle geometry for Balakovo NPP Units №1, №2 and №4. Based on actual experience with this system, its upgrade has been carried out, which increased the service life of system components, measurement accuracy and usability of the equipment. The upgraded system «STS-C-VKU» has undergone procedure of approving measuring means type and it has entered to the list of Federal Information Fund for Support of Unity for the measuring means. The upgraded system «STS-C-VKU» was used for measurements of the core baffle geometry for Kalinin NPP Unit №2.

It is shown that the materials of the core baffles of the above Units have different tendency to the radiation swelling, but the swelling rate for all materials is lower than the swelling rate predicted by equations presented in RD for such materials. Results of the measurements provide information to individualize and decrease the conservatism of calculation of PVI service life of given NPP Units.

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