Abstract: The k_eff of the US Kilopower space reactor during launch failure accident cannot meet the criticality safety requirement of our country space reactor. During the launch failure accident, the reactor may run into a disastrous situation of prompt super criticality if the B₄C outside the radial reflector is missing. Three different schemes were proposed in order to solve the problem in this paper. All of these schemes can meet the criticality safety requirement. Besides, the comprehensive comparison of these schemes related to the volume, mass, and the physical and thermal-hydraulics performance was carried out, and a best scheme was chosen and proposed.