Embodiments of the present invention provide a heat exchange medium comprising solid particles and a fluid. Embodiments of the present invention also provide a heat exchange system comprising the abovementioned heat exchange medium, a first heat exchanger, a mixing device disposed upstream of the first heat exchanger and configured to mix the solid particles and the fluid of the heat exchange medium and convey the mixed heat exchange medium to the first heat exchanger, a separating device disposed downstream of the first heat exchanger and configured to separate the solid particles from the fluid in the mixed heat exchange medium discharged by the first heat exchanger, a second heat exchanger, and a first conveying device configured to convey the solid particles separated by the separating device to the mixing device after having passed the separated solid particles through the second heat exchanger. In addition, embodiments of the present invention provide a nuclear reactor system comprising the abovementioned heat exchange system. The gas-solid or liquid-solid two-phase flow according to embodiments of the present invention has the following advantages. For example, it has a large thermal capacity, can be used with a low-pressure system, is non-corrosive, and can be processed off-line. The fission reactor according to embodiments of the present invention can be operated safely and reliably at a high power density or at an extremely high power density.