This expansion in an engine block is most likely to blow the freeze plugs (small plugs on the engine block designed for this purpose) but could be as bad as cracking an engine block.
And that is Space Monkey's automotive science lesson for the day.
Foundry workers automotive science lesson for the day
There is no such thing as a "freeze " plug in your engine block. those holes are there so that the sand core that the block is poured around, can be shaken out after casting. You are just as likely to crack a block at the back face, if it freezes, as at a core plug, or between a cylinder. There isnt an area of water jacket in your engine thick enough to resist the power of expanding water if it gets cold enough.
Foundry workers automotive science lesson for the day
There is no such thing as a "freeze " plug in your engine block. those holes are there so that the sand core that the block is poured around, can be shaken out after casting. You are just as likely to crack a block at the back face, if it freezes, as at a core plug, or between a cylinder. There isnt an area of water jacket in your engine thick enough to resist the power of expanding water if it gets cold enough.
Down here in FL we have lived with "Water Wetter" for years.
Its perfectly fine to put it in the reservior because the coolant in the radiator gets exchanged with reservoir fluid every time the car heats up and cools down.
When the engine is started the fluid in the radiator heats up and it expands and pushes some of the fluid into the reservoir because even if the entire system is pressurized that is the location where the least amount of pressure is found upon startup. Conversely in the cycle when the car's engine cools down the system seeks to equalize the pressure and fluid from the reservior flows back into the radiator.