Departure from nucleate boiling (DNB) may occur in the process of reactor accident. The departure from nucleate boiling (DNB) design basis is one of the most important bases for reactor core thermo-hydraulic design. In order to evaluate whether the DNB design basis meets the demand of thermal hydraulic design, the departure from nucleate boiling ratio (DNBR) should be calculated. The traditional DNBR calculation method is the direct substitution method (DSM). However, the research has pointed out that the DSM was inaccurate and non-conservative to represent the real safety margin. There is also a modified DNBR method by combining with the heat balance equation, which was called heat balance condition method (HBM). The essence of both DNBR methods is to evaluate the reactor safety margin by the difference between the heat fluxes of the actual condition and the critical condition. So the critical heat flux (CHF) value needs to be measured by CHF tests in actual condition parameters, which results in high heating power. To avoid this deficiency of DNBR methods, a new safety margin evaluation method has been developed in the research, which was called quality margin method. In this method, the safety margin of the reactor was evaluated by the difference between the critical quality and the actual quality. The method required lower test power, so greatly decreased the difficulty and cost of CHF experiments.

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