The effect of feedback reactivity on the safety of a fast reactor has been studied based on the asymptotic approximation for unprotected transients. The asymptotic approximation adopts the steady state heat transfer equation for calculating temperatures of coolant and fuel with the use of the relevant core power and mass flow. The feedback reactivity can be approximately estimated from the temperatures. This approximation is applied to the UTOP (unprotected transient overpower) and ULOF (unprotected loss of flow) accidents of a sodium-cooled fast reactor with a sodium plenum and GEM (gas expansion module). The reactivity feedback has been estimated, and the constraints of the feedback reactivity for maintaining the safety of the core have been predicted. For the UTOP accident the constraint for the control rod reactivity and the Doppler (+fuel expansion) reactivity coefficient is presented to prevent the fuel melting. For the ULOF accident the constraint of the parameter R is introduced which is the normalized ratio of the power to mass flow rate. The parameter does not exceed the critical point corresponding to the onset of the coolant boiling, and the coolant boiling does not occur because of the large negative reactivity insertion by the GEM. Thus the core safety can be approximately estimated by the present method without detailed calculations, though the accurate analyses are required by the use of transient analyses.

已经基于无保护瞬变的渐近近似研究了反馈反应性对快堆安全性的影响。渐近近似采用稳态传热方程，利用相关的堆芯功率和质量流量计算冷却剂和燃料的温度。反馈反应性可以从温度近似估计。该近似值适用于具有钠气室和 GEM（气体膨胀模块）的钠冷快堆的 UTOP（无保护瞬态超功率）和 ULOF（无保护流量损失）事故。已经估计了反应性反馈，并预测了维持堆芯安全的反馈反应性约束。对于 UTOP 事故，提出了控制棒反应性和多普勒（+燃料膨胀）反应性系数的约束，以防止燃料熔化。对于 ULOF 事故，引入了参数 R 的约束，它是功率与质量流量的归一化比率。该参数不超过对应于冷却剂沸腾开始的临界点，并且由于 GEM 的大量负反应性插入而不会发生冷却剂沸腾。因此，尽管需要使用瞬态分析进行准确的分析，但通过本方法可以近似估计堆芯安全，而无需进行详细计算。对于 ULOF 事故，引入了参数 R 的约束，它是功率与质量流量的归一化比率。该参数不超过对应于冷却剂沸腾开始的临界点，并且由于 GEM 的大量负反应性插入而不会发生冷却剂沸腾。因此，尽管需要使用瞬态分析进行准确的分析，但通过本方法可以近似估计堆芯安全，而无需进行详细计算。对于 ULOF 事故，引入了参数 R 的约束，它是功率与质量流量的归一化比率。该参数不超过对应于冷却剂沸腾开始的临界点，并且由于 GEM 的大量负反应性插入而不会发生冷却剂沸腾。因此，尽管需要使用瞬态分析进行准确的分析，但通过本方法可以近似估计堆芯安全，而无需进行详细计算。