A molten salt fission reactor. The reactor includes a reactor core, which includes a plurality of fuel tubes. Each fuel tube contains a fuel salt and a gas interface. The fuel salt is a molten salt of one or more fissile isotopes. The gas interface is a surface of the fuel salt in contact with a gas space during operation of the reactor. The reactor also includes a fuel salt cooling system, which is configured to cool the fuel salt. The cooling system includes a heat exchanger and a coolant tank. The coolant tank contains a coolant liquid in which the fuel tubes are at least partially immersed. The heat exchanger is for extracting heat from the coolant liquid. 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


Δ
⁢

H
He




*

ln
⁡
(


P
1


P
2


)


+

1

T
1