What blew off?
The above BBC footage appears to show a hydrogen explosion of a Mark 1 containment (like the containment at VY). Hydrogen is created when the zircalloy fuel clad gets too hot and chemically reacts by breaking water into hydrogen and oxygen.
Radioactive smoke after the explosion appears to be heading out to sea.
What type of protection do people need? Some photos from Reuters show police and other emergency personnel wearing gas masks. The gas masks will not protect against any radiation. The particles are atomic sized and nothing the emergency personnel are wearing can protect them. The people surrounding the plant and all emergency workers there are also enveloped in a cloud of gamma radiation.
In the US, in about a week, it is possible that a cloud of radiation might reach California, and in 10 days hit VT. Wash your food and take potassium iodide (not now, wait a week until the cloud is near). Potassium iodide fills your thyroid with iodine so that it will not absorb any iodine from the radioactive cloud.
What has happened is that the nuclear fuel cladding is cracking and releasing the radiation held within the nuclear fuel pellets. Fuel pellets are the size of the joint in your pinky and there are tens of thousands of them within every nuclear reactor. The volatile radiation in the form of xenon, krypton, iodine, cesium, and strontium is already being released as a gas through the relief valves in the reactor and out the leaky containment. If you have ever used a pressure cooker, you have seen the relief valve on the top of the pressure cooker, and the feature is similar to the open valve on the top of the reactor, and that valve is fully open right now.