1998 Hyundai Accent COOLENT SPITTING

Hi

I have a hyundai accent, s reg 1998, but recently had to get a cylinder head replaced after a blown gasket, is few months on from then and car has been ok until recently I had been hearing hissing from the engine, so decided to open the hood and this small plastic container to the left of the vehicle, where I frequently top up coolent/antifreeze mixed with some cold water as been told down the years, only to notice the hissing noise was this, it was literally spitting piping hot, the water/coolent wasnt to cool lol and im scared if this will again blow my gaskets again, wot can I do and is there a quick fix to this, someone said it could be that the radiator is not flitering, maybe leaking or something, because when I turn on air condition all is smelling of antifreeze. Please could you help,

thanks david