Before revision: Principle Using uranium-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the former uranium-235 nucleus.
After revision: Principle Using uranium-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed uranium-236 nucleus.