K. Klučárová, J. Remiš, M. Závodský, V. Petényi V U JE , Inc.

1. 17th Symposium of AER, Sept. 24 – 29, 2006, Yalta, Ukraine K. Klučárová, J. Remiš, M. Závodský, V. PetényiVUJE, Inc. The Influence of Outlet Temperature Profile on Fuel Assembly Power Determination

2. Main results of our previous presented works • The reason of our CFD modelling of coolant flow in FA - discrepancies of temperature measurements at the output of FAs and on the main coolant loops 2 ways of coolant by-pass flow ratio determination: a) Hydraulic by-pass flow ratio - determined from the core pressure gradient of flow area and coefficients of hydraulic resistance b) Thermo-hydraulic by-pass flow ratio - based on enthalpy ratio measured on the core and on the reactor

3. Main results of our previous presented works • Main computational codes used for modelling: - CALOPEA (thermal-hydraulic parameters in fuel bundle of FA) - FLUENT (coolant mixing in the FA head) - BIPR-7 & PERMAK (power distribution in core) • Main factors with influence on quality of the coolant mixing in the FA head: - geometry of FA inner parts (inner size, fuel latttice pitch, fuel diameter, spacer grids, upper mixing grid, central tube) - pin-wise power distribution - FA thermal power

4. CFD Modelling of Coolant Flow in FAs of Real Fuel Loadings FAs selection for CFD modelling of coolant flow: • FA with non-profiled enrichment 3,6 % - geometry 144,2 mm x 2,1 mm, fuel pin outer diameter 9,1 mm, lattice pitch 12,2 mm • FA with profiled enrichment 3,82 % - geometry 145 mm x 1,5 mm, fuel pin outer diameter 9,1 mm, lattice pitch 12,2 mm • FA with burnable absorber and profiled enrichment 4,25 % - geometry 145 mm x 1,5 mm, fuel pin outer diameter 9,07 mm, lattice pitch 12,3 mm 24 variants of FAs operational parameters (FA thermal power, pin-wise power distribution, burn-up) from 4 different fuel loadings

5. Application of CFD Modelling Results The temperature profile in the thermocouple position in relation to average coolant temperature at the fuel bundle outlet can be characterised by parameter: Figure: σ = F(Kk_ave) Kk_ave – average Kk of 36 central fuel pins

6. Application of CFD Modelling Results Application of σ as a corrective factor on real coolant temperatures measured at the FAs outlet: • new values of thermo-hydraulic by-pass flow ratio – better coincidence with hydraulic by-pass flow ratio

7. Application of CFD Modelling Results Application of σ as a corrective factor on real coolant temperatures measured at the FAs outlet: • changes in power distribution in core – corrective factor σ x 100 [%] for individual FAs in different fuel loadings (core sextant) at the beginning of cycle, FA heat-up (by thermocouple readings) correction from –6 % to +5 %

8. Conclusions : • The value of corrective factor on coolant temperature profile in the thermocoupleposition depends mainly on: • ·        geometry of inner parts of FA (FA inner size, fuel lattice pitch, fuel pin diameter, spacer grids and upper mixing grid, central tube), • ·        pin-wise power distribution, • ·        thermal power of FA. • The average coolant temperature at the outlet of fuel bundle is usually overestimatedby thermocouple measurement for fresh fuel assemblies with high thermal power. • For fuel assemblies partly burned-up the measured temperatures at the outlet of FA can be higher or lower than average temperature at the outlet of fuel bundle in dependence on combination of three above mentioned main factors. • The insufficient coolant mixing in the upper part of fuel assembly is possible to model by computational CFD codes with sufficient accuracy for correcting this effect in calculation of real power distribution in core.

9. Acknowledgement This work was supported by Agency for Promotion Research and Development under the contract No. APVV-99-P02405.