Abstract: Nuclear criticality safety is one of the most important security issues in the process of development of nuclear industry, in which evaluation and analysis of critical accidents is the foundation of shielding design and emergency plan after critical accidents. Therefore it has important research significance and engineering value. CAACS is a program developed to analyze criticality accidents of solution system. It can calculate the number of fission, fission power and temperature change with time of the criticality accident and so on. CAACS has been verified by the benchmark experiments. This paper presents the analysis and calculation of two criticality accidents, and compares the calculated results with the accident estimates. The result shows that, the calculation results of CAACS concur with the accident estimates, the study could provide technical means for the design of fuel reprocessing plants, and also lay a foundation for the subsequent research of critical transient.