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7. ANALYSIS OF NOZZLES

Within PTS analysis it is necessary to show that the strength criteria are satisfied for the postulateddefects located at the elements of RPV with the worst properties, highest neutron fluence and withmaximum tensile stresses. The coldest water temperatures as well as the maximum heat transfercoefficient under PTS occur at the lower region of the nozzle, leading to maximum tensile stresses

(see the examples presented in Appendix G for WWER-1000 and PWR RPVs). For these reasons PTSanalysis also needs to be performed for this part of the RPV.Irradiation effects are not considered for nozzle area due to negligible fluence values at this location.The integrity assessment results for the RPV nozzles area are the governing ones for unirradiated partsof RPV4. The following sections provide recommendations on nozzle integrity assessment.

7.1. Govern ing transients

Based on experience of PWR and WWER nozzle area calculations the small primary side breakLOCA transient leads to the most severe results due to the high inner pressure during the transient.

Situations with maximum temperature gradients in the RPV nozzle area should also be analysed.Therefore the governing transients to be considered for the nozzle integrity assessment are notnecessary the same as for the beltline region, and specific transient selection should be performed.

7.2. Postulated defect locationThe postulated defect should be located in the region submitted to maximum tensile stresses as well asthe coldest water temperatures during the selected transient. Based on the experience of nozzle

integrity assessment on PWR and WWER (with and without cladding) the defect location at the 6oclock position parallel to the nozzle axis, in the lower nozzle corner (inlet nozzle for PWR, inlet andoutlet nozzle for WWER) leads to the most severe results however the highest stress location in thenozzle can slightly change during the transient. The highest stresses during cooling transient acting onthe outlet nozzle of PWR are located at the lower nozzle area (in the MSL direction and not at the

corner, due to the outlet nozzle geometry of PWR). More details can be found in Appendix G.The size of the postulated defect could be selected with respect to the size of a realistic manufacturing

defect probable to exist in the considered nozzle region or according to standards if available.According to [5] the crack depth can be also connected to the plant specific non-destructive testingqualification criteria, along with specification of safety margins.

7.3. Temperature and stress field

The recommendations in section 4 are also valid for the nozzle area, with some specificconsiderations. Stratification in the nozzle occurs for transients with ECCS water injection. This has to

be taken into account in the thermal load description (T fluid, HTC), in addition to the mechanicalloading due to pressure variation and other loadings due to the attached piping system.The residual stress in the cladding due to the cladding manufacturing should be considered in the

calculation. This stress is usually simulated with the use of the stress-free temperature in the FE code.