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Conclusion

In conclusion the author states that as a result of an extensive literature search combined with very time consuming studies of the acquired literature an understanding of the physical phenomena in the areas of neutronics and thermal-hydraulics which occur in a BWR has been established. With this physical understanding at hand the author created the 1-D steady-state model of the neutronics and thermal-hydraulics of the reactor which was described in the preceding chapters.

The implementation of the three submodels, ie the neutronics model, the hydraulics model and the thermal design model, have been verified with success. A direct verification of the coupled neutronics thermal-hydraulics model has not been possible since it is impossible to obtain results from other sources for comparison. However, since the interface between the three submodels which is very simple can be checked by the results output by the code the correctness of the coupled code is easily established. Furthermore, the results seem reasonable when compared to values given by General Electric (see Table 14.1 for $\mbox{$\dot{m}$}_i$) and obtained by others [54].

The results in the little design study show that there is ample evidence to conclude that the power levels which can be safely attained by a given SBWR setup can be increased appreciable if the steam separator is eliminated from the design.

The tool the author has developed can be used for

• Preliminary design investigations (for orientation purposes) when the code is supplemented by codes for critical heat flux and stability.
• Sensitivity analysis^17.1--which parts of the model have a large impact on the operation state of the BWR.

It is evident that since the time has been rather limited the models presented in this text have been simplified and the author will therefore issue a number of suggestions for further improvement and continued work

• It turns out that it is customary to include a variable contents of burnable Gd poison in the axial direction in order to lower the axial peaking factor (ie to flatten out the power distribution). One has typically three zones with highest Gd contents in the lower part of the core [18]. Therefore, it would be appropriate to let the nuclear cross sections depend also on the number density of Gadolinium.
• Inclusion of the impact of control rods in the nuclear cross sections--the control rods also influence the axial power distribution.
• More accurate model of the steam separator is affordable since the pressure loss of the separator is appreciable^17.2.
• Inclusion of a model of the by-pass flow in the reactor core.
• More comprehensive system model of the core: Different types of fuel channels which have their own axial power distribution.
• Evaluation of the possibilities of eliminating the steam separator.
• Inclusion of a correlation for the boiling transition (critical heat flux). Since the boiling transition phenomenon is closely related to the thermal-hydraulics model in the fuel element it is affordable to include the correlation directly in the code developed by the author instead of performing the boiling transition calculations by another code.
• More elaborate model for the calculation of the gap conductance between the fuel and clad in the fuel rods.
• Implementation of a thermodynamic function for the liquid temperature, $T_\ell = f(h_\ell,p)$.

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