I own a Kenmore microwave, not sure if there is a radiation leak.

No! this could not be further from the truth, you can buy a microwave test card and check for leakage around the door seals, or take it to a service professional to do a quick test.

Silicone will not block microwave radiation. However, it isn't only cosmetic. The silicone keeps liquid that spills inside your microwave (or even steam) from getting into the guts of your microwave and rusting and causing shorts.

why is it rusting?

First of all, is the hole a factory one that you never noticed before? If so, leave it alone. I know that microwaves generate a lot interior heat and it needs to escape someway. Secondly, if it is not a factory made vent hole and you can't be sure if it caused from a lot of use, and if the oven still works fine, I would not worry about it if it were me.

THe microwave has to be removed from the wall and then the outer cabinet has to be removed to get to the bulb. Do not do this with the unit plugged in and do not do this if u don't know how to work on a microwave as there is a shock hazard.

the only thing you can do is replace your microwave

Obviously, heat escaping from anything heated in the oven will indirectly heat the walls and the magnetron is not 100% efficient so it radiates a good deal of heat but that is normally carried off by the cooling fan and exhausted into the room air.

A microwave oven does not generate ionizing radiation, it's just radio waves. All it can do is burn you.

But the spot should be repaired so that food particles and grease do not accumulate there and cause arcing or rust.

It will have to be sanded smooth and clean and dried thoroughly.

Then you can apply some high-temperature appliance paint, available from places like Wal-Mart, Lowe's or Builder's Square.

We're happy to help and we appreciate your thoughtful rating of our answer.

There is a good possibility that microwave energy is leaking near the magnetron tube, or some other high voltage electrical problem is occurring. I would recommend having it tested by qualified microwave repair technician. A few things you can do....
-Make sure the door seal is clean and no corrosion.
-Check the interior for rust spots or holes and make sure it's clean inside.

You can find parts lists and diagrams on this Sears Web Page. There is a good possibility that your magnetron tube is getting weak. Make sure the interior is clean and doesn't have any rust or holes in it. Also check the seal around the door for corrosion. If you're not familiar with electrical and electronic repair work, you may need to take it to an experienced microwave service technician to get an accurate diagnosis of all the needed parts.