**3.4 Neutron spectra in three assemblies**

The neutron spectra in PE, DU, and ThO2 assemblies are simulated by using the MCNP4B code [21] with ENDF/B-VII.0 [22], in which the S (α, β) thermal scattering model in PE is considered. The angular dependences of the source neutron

**Figure 4.** *ThO2 powder cylindrical assembly.*

**Figure 5.** *Neutron spectra at foils in three assemblies.*

energy and intensity are calculated by "DROSG-2000" code [23]. The neutron spectra at foils with different distances *d* to the neutron source in three assemblies are relatively compared, as shown in **Figure 5**. The ordinate is a normalized neutron fraction, that is, the proportion of the neutron number in each energy segment to the one in the whole energy range [11, 13]. The results show that the differences of the fractions are very obvious, especially in the low-energy region.
