空泡系数（英语：void coefficient），又称反应性空泡系数（void coefficient of reactivity），是在核工程中一个用于估计核反应堆核连锁反应使用的中子慢化剂和冷却剂对空泡(一般为蒸汽气泡)所产生影响的数字。 在核工程中“反应性”指（非化学反应）反应堆堆芯核连锁反应的变化程度，反应性与反应堆堆芯改变堆芯功率等级的趋势直接相关。 如果反应性为正，堆芯功率倾向于升高；如果为负，堆芯功率倾向于降低；如果为零，堆芯功率趋于稳定。堆芯的反应性可以通过反应堆的控制系统调整以获得所需的功率变化或令反应堆保持在同一功率水平。其可以比作汽车周围环境发生了改变（如风的强度，方向及道路坡度），因此驾驶员做出应对措施以维持车速或执行所期望的反应。

In nuclear engineering, the void coefficient (more properly called "void coefficient of reactivity") is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor moderator or coolant. Reactivity, in the nuclear engineering sense (not to be confused with chemical reactivity), measures the degree of change in neutron multiplication in a reactor core. Reactivity is directly related to the tendency of the reactor core to change power level: if reactivity is positive, the core power tends to increase; if it is negative, the core power tends to decrease; if it is zero, the core power tends to remain stable. The reactivity of the core may be adjusted by the reactor control system in order to obtain a desired power level change (or to keep the same power level). It can be compared to the reaction of an automobile as conditions around it change (for instance, wind intensity and direction or road slope), and therefore the corresponding counter-measure that the driver applies to maintain road speed or execute a desired manoeuvre.