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BackgroundMicro-reactors can be used as a lunar surface power or spacecraft power source for space exploration. Before launching the reactor, a safety analysis should be conducted to prevent a launch accident. Currently, the safety analysis of the radioactive isotope power system does not fully include the safety analysis of the reactor. The main critical safety analysis scenario is that the reactor falls and hits the concrete ground from a high altitude. The reactor may return to criticality after high-speed impact.PurposeThis study aims to investigate the nuclear safety characteristics of a space reactor subjected to dynamic shock under high-speed impact conditions.MethodsFirst of all, based on internal and surface unstructured grids, two simplified reactor models corresponding to two high-speed impact scenarios, i.e., pure fuel reactor vertical impact with ground, and cylinder reactor with a reflector layer and shielding layer impact the ground at a 30° angle were established. Then, the ABAQUS finite element method and unstructured mesh Monte Carlo method of particle transport were combined to predict the criticality properties of the pure fuel and cylindrical reactor during high-speed impact. Based on the surface and internal unstructured mesh Monte Carlo transport technology, the criticality safety analysis platform of micro-reactor under high speed impact was established.ResultsThe results show that the keff induced by the deformation may increase with time for the above mentioned two simplified reactors. The maximum increase in the keff of the pure fuel reactor can reach 1 000×10-5, whereas for the cylinder reactor, the keff is improved to a maximum of 200×10-5. Considering the non-uniform density effect, reactivities of -666×10-5 and -132×10-5 are introduced into the two reactors.ConclusionsThe critical safety characteristics of the reactor under different impact conditions should be evaluated to ensure sufficient safety margins under such accident conditions.

Fast-spectrum micro-reactor has many technical advantages, such as high energy density, good mobility, fast deployment speed, long life and so on. Because of the characteristic of compact structure and high role of leakages in the neutron balance in fast-spectrum micro-reactor core, geometric deformation caused by temperature change and mechanical effect under external force is the main source of reactivity feedback. So it is very important to carry out transient analysis based on neutronics-thermo-mechanics coupling to accurately simulate the dynamic characteristics of fast-spectrum micro-reactor core. Based on the open source multi-physics coupling framework MOOSE, a three-dimensional coupled neutronics thermo-mechanics model for the dynamics analysis of fast-spectrum micro-reactors was developed in this paper. The thermo-mechanics coupling was solved by using the JFNK (Jacobian-Free Newton Krylov) method. Then the neutronics model was coupled with thermal-mechanics model by Picard iteration method through the MultiApp and Transfer system based on MOOSE framework. The neutronics solver can directly simulate the neutronics behavior of deformed nuclear reactor configurations including automatic adapting of the cross sections for account for density change. In this paper, this method was applied to the simulation of the supercritical transient of Goidva-Ⅰ with three-dimensional coupled neutronics thermo-mechanics model. Time evolution of fission rate, average temperature rise, surface displacement and surface velocity in the transient with an initial period of 16.2 μs were evaluated. The numerical result of fission rate during the transient was compared with the experimental data, and an overall good agreement was observed. From the numerical results, it can be seen that the fission rate increases, then reaches a maximum value with the positive reactivity introduced. Goidva-Ⅰ is a typical compact, fast-spectrum reactors, reactivity feedback is dominated by core deformation. The quickly accumulated thermal power causes the core temperature increasing, the reactor materials expand, resulting in a larger geometry and reduced material densities. Fission rate then drops due to loss of criticality caused by the thermal expansion. Due to the rapid increase and decrease of fission power, the core is in a state of compression and expansion one after another, and the thermal inertia effect leads to the oscillation of core surface displacement and velocity. At the same time, due to the change of core reactivity caused by deformation, the core power also produces periodic oscillations after 500 μs. The results show that this method can accurately consider the reactivity feedback effect caused by the thermal expansion, which lays a foundation for further multi-physics coupling safety analysis of fastspectrum micro reactors.

Assembly deviation analysis methods, which have been studied and improved over the past 30 years, are necessary and effective for checking functional requirements before manufacturing. Contact deformation was not negligible and significantly affected the final assembly deviation, which prevented the application of the current assembly deviation analysis methods. Considering the contact deformation, we aimed to improve the precision of the 3D deviation analysis methods, which are based on the Jacobian-Torsor matrix and skin model shape (SMS), respectively. In this study, the contact deformation was quantified based on the Hertzian contact theory. The unified Jacobian-Torsor matrix is modified with the location points and contact deformation to increase the precision of the stack-up assembly deviation. In the SMS-based method, the assembly components are represented as non-nominal SMS models. The final posture of each component and stack-up assembly deviation were solved by iteratively mimicking the physical assembly process. The practicality of the improved deviation analysis method was successfully demonstrated through two case studies. The modified Jacobian-Torsor matrix-based method is efficient. However, the improved SMS-based method can provide more precise results by mimicking tight fitting on mating surfaces. [ABSTRACT FROM AUTHOR]