**4. Simulation of the CEFR Start-UP Tests with the Serpent Code**

The Neutronics Benchmark of CEFR Start-Up Tests is a CRP proposed by the China Institute of Atomic Energy (CIAE), under the direction and support from IAEA. The CRP was launched in 2018. The main objective of this benchmark is to improve the understanding of the start-up of a SFR and validate the fast reactor analysis computer codes against experimental data obtained at the China Experimental Fast Reactor (CEFR). The CEFR is the first Chinese fast reactor; it is a pool-type sodium cooled reactor, with a nominal thermal power of 65 MWth [14].

NINE, in collaboration with University of Pisa, participated in all the proposed work packages and, in turn, proposed and organized a work package focused on sensitivity and uncertainty analysis of the first criticality test, that, for the time being, has just begun.

The tests included in this benchmark were part of the reactor start-up tests, which included both the fuel loading and the first criticality, the control rod worth measurement, the reactivity coefficients measurement, and the foil activation analysis. All the details of these tests are reported in the Technical specifications [14]. In this chapter, only a sub-set of these tests are analyzed, and their results are compared to experimental measurements.

The first test described is the one referred to the "Fuel loading and criticality" (here and after called work-package 1, WP1). It is focused on the analysis of the first criticality achievement; the tests performed are composed by ten sub-critical steps, with different number of fuel Sub-Assemblies (SAs) loaded, and 3 supercritical steps, which have different RE2 control SA insertion levels. The critical RE2 position was found and reported in the Technical Specification through an extrapolation of the experimental super-critical steps. In this work the results for the super-critical steps, as well as the critical ones, are compared with the experimental measurement; a comparison between two different nuclear libraries is also reported, to exploit the dependence on the nuclear data of the effective multiplication factor.

The second test analyzed is the "Control rod worth measurement", (workpackage 2, WP2). The main goal of this test is to evaluate the control rod worth of each control rod SA and of different group of control rod SAs. Also, in this case a comparison with the experimental measurement is reported in the following sections.

The last test presented is the "Foil activation measurements" (work-package 6, WP6): it concerns the foil activation analysis made through the irradiation of different material samples inside the reactor core in both the axial and radial directions.
