A nuclear reactor incorporates a fully passive decay heat removal system with a modular cold source. The decay heat removal system is configured to remove heat passively, via the outside of a primary vessel included in the nuclear reactor. The cold source has a plurality of modules grouped together in assemblies. Each module is filled with a phase change material. Each module is cooled by a monotube heat exchanger. The decay heat removal system includes a hot collector and a cold collector to ensure the distribution of a heat transfer fluid in the plurality of monotube heat exchangers.