In general, fuel melting must be excluded for condition III and condition IV accidents as well. This provides enough margin to fuel melting and to loss of fuel integrity. Expansion of the fuel pellet upon centerline melting may cause the pellet to stress the cladding to the point of failure.Īlthough the melting point of UO2 is over 2,800☌,fuel is usually operated at a much lower peak centerline temperatures (less than 1,400☌). Overheating of the fuel is prevented by maintaining the steady state peak linear heat rate (LHR) or the Heat Flux Hot Channel Factor – F Q(z) below the level at which fuel centerline melting occurs. This low thermal conductivity can result in localised overheating in the fuel centerline and therefore this overheating must be avoided. The thermal conductivity is one of parameters, which determine the fuel centerline temperature. The thermal conductivity of uranium dioxide is very low when compared with metal uranium, uranium nitride, uranium carbide and zirconium cladding material. Special reference: Nuclear Power Reactor Core Melt Accidents ISBN: 978-2-7598-1835-8, IRSN 2015. Depending on the local temperature levels, degradation may result in more or less severe hydrogen production, fission product (FP) release, and molten corium formation and propagation towards the lower head. The mechanisms of this degradation are both chemical and mechanical.
If the reactor core remains dry for a considerable length of time, the temperature of the fuel rods rises and may locally reach levels that cause significant and irreversible core degradation. The heat causing the melting of a reactor may originate from the nuclear chain reaction, but more commonly decay heat of the fission products contained in the fuel rods is the primary heat source. It occurs when the heat generated by a nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. The core melt accident is a severe nuclear reactor accident that results in core damage from overheating. This type of accident is known under term a nuclear meltdown (core meltdown), but this is not officially defined by the International Atomic Energy Agency or by the Nuclear Regulatory Commission. Especially, common (usually 3×100%) failure of the Emergency Core Cooling System (ECCS) must occur after severe loss of coolant accident. There are many and many barriers that have to be breached. Although this event is very unlikely, it cannot be ruled out. Reactor core melt accident is an event or sequence of events that result in the melting of part of the fuel in the reactor core.
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