Optimization of the heterogeneous GFR 2400 control rod design

The biggest challenge of implementing GEN IV technologies in the nuclear fuel cycle is their safety. The challenges are very broad, but one of the most significant ones is the effectiveness of the reactivity control system. The Gas-cooled Fast reactor requires a reactivity control system which provides sufficient reactivity worth to compensate the local neutron zones in the core. Due to the early development stage of the pin type GFR 2400 core, only the homogenous material composition of the system of control rods was published so far. In this paper the performance of the reactivity control system of the GEN IV Gas-cooled Fast Reactor (GFR), was investigated. The presented analyses were performed for the pin type GFR 2400 core by the MCNP5 and KENO6 stochastic calculation codes. To analyse the effects of geometry self-shielding, a simple heterogeneous control rod design was created and evaluated through a special procedure. Since the worth of the created control rod design is lower compared to the homogeneous configuration, moderator materials were accommodated in the control rod structure. The main goal of this study was to create a design which is, in terms of reactivity, comparable with the reference homogenous design. The presented paper deals with 6 proposed geometry configurations in combination with 8 moderator materials. The best properties were found for the hydride moderators, ZrH2.0, ZrH1.5 and LiH, unfortunately these materials have undesirable properties at high temperatures. The BeO and Li2O moderators showed worse properties but their impact on the control rod could be enhanced by increasing their mass proportion in the assembly.