Complex sentence: Fission produces gases such as iodine-131 or Xenon-133 remain within the fuel rods, but if there were a major accident resulting in fuel failure, small amounts of the gases would be released in to the reactor coolant.
Split into simpler sentences: Fission produces gases such as iodine-131 or Xenon-133. These remain within the fuel rods, but if there were a major accident resulting in fuel failure, small amounts of the gases would be released in to the reactor coolant.