熔盐堆（英语：molten salt reactor, MSR）是核裂变反应堆的一种，其主冷却剂以至燃料本身都是熔盐混合物，它可以在高温下工作（可获得更高的热效率）时保持低蒸气压，从而降低机械应力，提高安全性，并且比熔融纳冷却剂活性低。

核燃料既可以是固体燃料棒，也可以溶于主冷却剂中，从而无需制造燃料棒，简化反应堆结构，使燃耗均匀化，并允许在线燃料后处理。在许多设计方案中核燃料，如 四氟化铀（UF₄），溶于熔融的氟化物盐。堆芯用石墨做慢化剂，熔盐流体在其中达到临界。许多现代设计方案采用陶瓷燃料在石墨基质中均匀分布，熔盐提供低压高温冷却的形式。熔盐更有效地将热量带出堆芯，减少对泵、管道的需求，并因此而的缩小堆芯的尺寸。

The Molten-Salt Reactor Experiment (MSRE) was an experimental molten salt reactor at the Oak Ridge National Laboratory (ORNL) researching this technology through the 1960s; constructed by 1964, it went critical in 1965 and was operated until 1969.

The MSRE was a 7.4 MW_th test reactor simulating the neutronic "kernel" of a type of inherently safer epithermal thorium breeder reactor called the liquid fluoride thorium reactor. It primarily used two fuels: first uranium-235 and later uranium-233. The latter UF₄ was the result of breeding from thorium in other reactors. Since this was an engineering test, the large, expensive breeding blanket of thorium salt was omitted in favor of neutron measurements.