I've seen a couple of articles lately referencing the fire in Fukushima's spent fuel pools or the core meltdowns through the containment.
If either had happened, we would be seeing a great many radioisotopes in addition to cesium-137 and iodine-131.
When uranium fissions, its nucleus breaks in half, but not the same way every time. So the fission products are elements around half the atomic mass of uranium-235. The neutrons also add to the nuclei of other fuel components to produce other elements, like to uranium-238 to produce plutonium.
Cesium and iodine are soluble in water. Xenon, another fission product, is a gas. They are the only fission products that have been observed. That is consistent with the release of steam and cooling water from the reactors and the spent fuel pools.
If there had been a fire in the cooling pools or a serious breach of reactor containment, those other fission products, as well as uranium and plutonium, would be released as well. So far, the reports of such elements have been erroneous.
And the data is not just from TEPCO. The cesium and iodine are being detected around the globe. The other elements would be too, if they were there.