= Solution
For <zirconium alloys>, the practical upper <temperature> is governed by time-dependent <corrosion>, <material creep> and loss of <ductility>, well before bulk melting. In ordinary water-reactor service, cladding surfaces are commonly in the several-hundred-degree range, with normal operation around the low-to-mid $300\,{}^\circ\mathrm{C}$ range in a <pressurized water reactor>. A quoted service limit must specify alloy, <coolant> chemistry, exposure time, <stress> and accumulated hydrogen; there is no universal <temperature> at which every zirconium tube abruptly fails.
Increasing <temperature> accelerates <zirconium-steam oxidation> and creep. Oxide growth increases thermal resistance, and hydrogen uptake can leave a hydride-containing cladding with reduced resistance to cracking. A transient can also cause ballooning or rupture as hot material loses strength under internal <pressure>. Oxygen absorbed beneath the oxide stabilizes a brittle oxygen-rich region; a tube that survives the hot part of a transient can still have poor <ductility> during cooling and quenching.
For scale, the established US emergency-core-cooling criterion discussed by the <United States Nuclear Regulatory Commission fuel-damage criteria> limits peak cladding <temperature> to $2200\,{}^\circ\mathrm{F}$, about $1204\,{}^\circ\mathrm{C}$, together with an oxidation criterion. This is an accident-analysis criterion, rather than a proposed continuous service <temperature>.<United States Nuclear Regulatory Commission fuel-damage criteria>[Issue170, description of the10CFR50.46 peak-temperature and17% oxidation criteria]{ref}
These material constraints limit allowable fuel heat flux and <temperature> excursions and require adequate heat removal and thermal margin. A small increase in <coolant> <temperature> or oxide thermal resistance can raise the underlying metal <temperature> and accelerate degradation. Alloy changes, protective coatings and alternative claddings aim to improve that margin, while still satisfying the neutronic and mechanical requirements.
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