The swelling of core baffle simulation
The main factor of limitation the operation of NPP's units after designed term is the phenomenon of radiation swelling of pressure vessel internals (PVI) under the influence of the flow of high-energy neutrons. Models of radiation processes of swelling and creep are very sensitive to the temperature distribution in the metal. The value of swelling changes by 2.5 times if the change of temperature equal to 30 degrees.
In IPP-Center Ltd. an analytical model for assessing the necessary boundary conditions was created. It allowed to narrow the boundaries of the calculation model and reduce the requirements for computing resources. The distribution of the coolant flow through the PVI was determined analytically, as well as the upper limit of the values of the coefficient of heat transfer from the inner surface of the core baffle in the assumption of the equivalence to the ring channel and the direction of the heat flow.
The results of calculation for the operation mode at the nominal power level indicate that the most irradiated section situated on 3325 mm from the lower end of the core baffale. The maximum temperature reaches 368.9 ° C.
Separately, the reliability of heat transfer from metal studs has been investigated. Due to the presence of recesses in nuts, which are initially envisaged by the design, the coolant circulates through recesses with the flow rate of 0.1-0.15 m3 / h. The temperature of the coolant around the pins reaches 322 ° C due to the absence of a "stagnant" zone around the recesses. The boiling of the coolant is not achieved.
The obtained temperature fields are less conservative in comparison with the previous results and are used for further calculations of the core baffle swelling. Due to the reduction of conservatism in the calculation of the temperature field, the estimated valur of swelling and strain of creep decreased by 1.5-1.6 times for the two-dimensional model
Temperature of PVI
The obtained input data allowed to carry out a complex of calculations for the static, cyclic and brittle fracture of PVI taking into account the effects of swelling and radiation creep. The strength of the fastening elements was assessed. The possibility of contacts core baffle-core barrel and core baffle-reactor core have been calculated. Additional stresses from the contact have been obtained.
Equivalent von Mises stresses
Equivalent strain of creep