**4.2 Simulation results and preliminary interpretation of the CEFR SMs**

The results presented here are taken from [18]. First, the results of WP1 are analysed. **Table 2** shows the comparison between the calculated and measured values of the effective multiplication factor for the three supercritical steps and the critical position. The relative errors are small and practically constants for all the cases when ENDF/B-VIII.0 nuclear data are used, that suggests a systematic error originating from the nuclear data library can hold. Confirming this, a much better agreement is obtained when use is made of the ENDF/B-VII.1 nuclear data, as the error drops from 0.19% to 0.03% in the case of the RE2 positioned at 190 mm.

Secondly the results of WP2 are presented. **Figure 18** shows the comparison between the calculated and measured integral worth of different control rod SAs, of the two-shutdown system (groups of control rod SAs, considered both fully


#### **Table 2.**

*Recent Advances in Numerical Simulations*

reported in the following section.

surrounding materials.

**Figure 17.**

The Serpent 2.1.31 code [15] was used for all simulations. The ACE format cross-section libraries used have been processed at different temperatures, to match the specification of the various experiment. For the Work Packages (WPs) analysed in this work the data made available by SCK-CEN were used. All the used libraries were based on ENDF/B-VIII.0 [16]; a comparison with the library based on ENDF/B-VII.1 [17] has been performed for one test case of WP1 and has been

*Serpent Geometrical Core Model: (a) Horizontal Section; (b) Vertical Section.*

For each experiment, both whole core geometry and material densities have been adjusted to the experimental temperatures, to consider expansion effects, and, therefore, the leakage variation. For most of the materials involved, the temperature adjusted parameters have been determined making use of the linear thermal expansion coefficients. For the sodium coolant density, the correlation provided in the technical specification has been used, while for the Helium gas, the density has been evaluated dividing the mass of gas at cold condition (at 20°C) by the fuel rods free volume available after the expansion (at hot zero power temperature) of the

For all the experiments, which require a multiplication factor, the Serpent implicit k effective has been considered. In **Table 1** the cycle population, as well as

For each experiment, both whole core geometry and material densities have been adjusted to the experimental temperatures, to consider expansion effects, and, therefore, the leakage variation. For most of the materials involved, the temperature adjusted parameters have been determined making use of the linear thermal expansion coefficients. For the sodium coolant density, the correlation provided in the technical specification has been used, while for the Helium gas, the density has been evaluated dividing the mass of gas at cold condition (at 20 °C) by the fuel

1 All 5.0E+05 500 50 2 SH and SA 5.0E+05 500 50

**cycle**

RE 1.0E+06 500 50

**Active cycle**

2.0E+06 500 50

**Inactive cycle**

the number of active and inactive cycles are reported.

**WP Test cases Particles number per** 

Axial: Au-197 4.0E+06

6 Axial: U-238, Al-27, U-235, Np-237, Ni-58

**258**

**Table 1.**

*Simulations Set-up.*

*WP1, Comparison with Experimental Data.*

**Figure 18.** *WP2, Comparison with Experimental Data, Rod Worth.*

**Figure 19.**

*WP6, Comparison with Experimental Measurements, Axial Reaction Rates.*

operating or with one SA stuck) and of all control rods together. The experimental values are obtained through a rod drop experiment. It appears from the figure that a noticeable good agreement between results and measurements has been achieved.

The last results belong to WP6. In **Figure 19** is reported the comparison between the reaction rates axial distributions evaluated with Serpent and the measured values. The agreement between measurements and simulations is generally quite good. Only the case of 197Au(n,γ) shows a noticeable difference particularly for the positions at the top and bottom of the core. Further investigating and understanding the origin and nature of this discrepancy would contribute to a better understanding of the of the measured activity.
