The most likely cause of fuel fleas is when the cladding surrounding the nuclear fuel becomes ruptured or cracked (known as "fuel pin failure"), allowing the fuel particles to escape and allowing the coolant to enter the fuel rod, further accelerating the process. In water-cooled reactors, this can be due to the reaction of the zirconium alloy cladding with the cooling water, which produces hydrogen. The hydrogen can be absorbed into the cladding material, resulting in hydrogen embrittlement. Embrittled cladding is less ductile and more susceptible to cracking. This process is avoided in modern reactors by carefully monitoring the fuel assemblies, limiting operating lifetime of the fuel, and by using alloys developed to resist hydride formation.