AbstractThis paper examines the potential of the shield and the shield cooling system of a fusion reactor to be an effective heat sink inside the reactor vacuum vessel, thereby improving the performance of the fusion reactor blanket during an undercooling accident such as a loss of cooling accident (LOCA) or loss of flow accident (LOFA). The paper demonstrates that having a shield cooling system capable of natural circulation during such an accident can make the difference between a blanket surviving the accident or not. The paper also demonstrates that it is inexpensive to add natural circulation capability to a shield cooling system.