Design of GAMA-SHP microreactor for outer space application: Neutronic study

Outer space researchs and explorations require reliable electrical supplies. For outer space explorations far from the sun, nuclear reactors become reliable for electricity generation. The space reactor must be simple, safe, and long live. GAMA Space Heat Pipe (GAMA-SHP) is a nuclear reactor designed to generate electricity for space applications with 1.2 MWe output power, fueled with ThF₄ and UF₄ mixture. The reactor is cylindrical shape with 100 cm diameter, 100 cm height and has 20 cm radial and axial graphite reflectors. Calculations using Open MC software have been performed to calculate reactor criticality, temperature reactivity coefficient, void reactivity coefficient, optimation of control rod position, control rod worth, power distribution and burn up. The GAMA-SHP cannot achieve critical condition without reflector. With reflector, GAMA-SHP can achieve critical condition at more than 90 % mole fraction of UF₄. The reactivity difference with and without the reflector is 6648 pcm. The effective fuel temperature reactivity coefficient is $-4.790$ pcm/K. The fuel void reactivity coefficient is $-297.52$ pcm/%. With negative fuel temperature and fuel void reactivity coefficients, GAMA-SHP is inherently safe. The control rod optimal configuration is one central rod at the axial axis of the reactor (Ring 0) and six peripheral rods at Ring 6. At the beginning of the lifespan of the reactor, the criticality value when all rods are fully inserted at the optimal configuration is 0.937150 (reactor reactivity of $-6707$ pcm). With a thermal power of 5 MWth, the GAMA-SHP can maintain the critical condition for 10 years.