A molten salt fission reactor comprising: a reactor core comprising a plurality of fuel tubes (101, 205, 301), each fuel tube containing: a fuel salt (102, 302, 206), the fuel salt being a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, which is a surface of the fuel salt in contact with a gas space (106, 207, 306) during operation of the reactor; a fuel salt cooling system configured to cool the fuel salt, the fuel salt cooling system comprising a coolant tank containing a coolant liquid (103, 203, 303) in which the fuel tubes are at least partially immersed, and a heat exchanger (104, 208, 304) for extracting heat from the coolant liquid; characterised in that: the fuel salt cooling system is configured such that during operation of the reactor, for all points within the fuel salt within each fuel tube except at the respective gas interface: T 2 > 1 − R He ΔH He ∗ ln P 1 P 2 + 1 T 1 where: T 1 is the temperature of the fuel salt at the gas interface; T 2 is the temperature of the fuel salt at the measured point; P 1 is the absolute pressure at the gas interface; P 2 is the absolute pressure at the measured point; R He is the gas constant of Helium; ΔH He is the enthalpy of solution of Helium in the fuel salt a reactor core comprising a plurality of fuel tubes (101, 205, 301), each fuel tube containing: a fuel salt (102, 302, 206), the fuel salt being a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, which is a surface of the fuel salt in contact with a gas space (106, 207, 306) during operation of the reactor; a fuel salt (102, 302, 206), the fuel salt being a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, which is a surface of the fuel salt in contact with a gas space (106, 207, 306) during operation of the reactor; a fuel salt cooling system configured to cool the fuel salt, the fuel salt cooling system comprising a coolant tank containing a coolant liquid (103, 203, 303) in which the fuel tubes are at least partially immersed, and a heat exchanger (104, 208, 304) for extracting heat from the coolant liquid; a coolant tank containing a coolant liquid (103, 203, 303) in which the fuel tubes are at least partially immersed, and a heat exchanger (104, 208, 304) for extracting heat from the coolant liquid; characterised in that: the fuel salt cooling system is configured such that during operation of the reactor, for all points within the fuel salt within each fuel tube except at the respective gas interface: T 2 > 1 − R He ΔH He ∗ ln P 1 P 2 + 1 T 1 where: T 1 is the temperature of the fuel salt at the gas interface; T 2 is the temperature of the fuel salt at the measured point; P 1 is the absolute pressure at the gas interface; P 2 is the absolute pressure at the measured point; R He is the gas constant of Helium; ΔH He is the enthalpy of solution of Helium in the fuel salt T 1 is the temperature of the fuel salt at the gas interface; T 2 is the temperature of the fuel salt at the measured point; P 1 is the absolute pressure at the gas interface; P 2 is the absolute pressure at the measured point; R He is the gas constant of Helium; ΔH He is the enthalpy of solution of Helium in the fuel salt