Of the remaining five long-lived fission products, selenium-79, tin-126 and palladium-107 are produced only in small quantities (at least in today's thermal neutron, U -burning light water reactors) and the last two should be relatively inert. The other two, zirconium-93 and caesium-135, are produced in larger quantities, but also not highly mobile in the environment. They are also mixed with larger quantities of other isotopes of the same element. Zirconium is used as cladding in fuel rods due to being virtually "transparent" to neutrons, but a small amount of Zr is produced by neutron absorption from the regular zircalloy without much ill effect. Whether Zr could be reused for new cladding material has not been subject of much study thus far.