If hydrogen accumulates in sufficient quantities - concentrations of 4% or more in the air - then it can explode, as has apparently occurred at Fukushima Daiichi reactors No.
Thus, the reactivity of the nuclear reactor can be easily adjusted by changing the boron concentration in the coolant.
That is, when the boron concentration is increased (boration) by dissolving more boric acid into the coolant, the reactivity of the reactor is decreased.
Conversely, when the boron concentration is decreased (dilution) by adding more water, the reactivity of the reactor is increased.
Additionally, this effect must be taken into account for longer cycles of nuclear reactor operation and thus requires higher initial concentration of boron in the coolant.
Molten salts share with metals the advantage of low vapor pressure even at high temperatures, and are less chemically reactive than sodium.
Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity, Organically moderated and cooled reactors were an early concept studied, using hydrocarbons as coolant.