Fig. 1. Diagram of the geometric structure of fuel assemblies

Fig. 2. Layout of subchannel numbering and division of 19 pins bundle

Fig. 3. Grid distribution of bundle

Fig. 4. Coolant temperature distributions at measuring location ML1, ML2 and ML3

Fig. 5. Coolant temperature distribution at the outlet of subchannels

Fig. 6. Average outlet temperature of subchannels under different mass flow rates

Fig. 7. Temperature contour plots at the same axial position (z=800 mm) for varying mass flow rates (color online)(a) m=5 kg·s^-1, (b) m=10 kg·s^-1, (c) m=15 kg·s^-1

Fig. 8. Framework of neural network

Fig. 9. Comparison of root mean square error before and after optimization of verification group

Fig. 10. Comparison of subchannel temperature before and after optimization under condition of mass flow at 4.5 kg∙s^-1