I think your jaw would have to be made of tungsten or something for that to happen, but I'm not a physist; I've just played one in a video game. 🤷‍♂️

It would not make a chain reaction because the nuclei of typical atoms are unlikely to split when irradiated (that is to say, hit by the pieces of the original atom that split) and will more likely ionize (have an electron knocked off) or transmute (turn into a different element by changing the number of protons).

In nuclear weapons or reactors the uranium must be "enriched" because regular uranium atoms are too stable to sustain a fission reaction. We must have a high amount of rare uranium-235 (the easily splittable isotope) compared to U-238 (the more common stabler isotope) which is why uranium needs to be enriched. Most of the atoms in your salad are stable isotopes, so there will likely not be any cascading fission reactions even if your teeth were somehow able to trigger one (or even 1000) every single time you took a bite.