Overheated fuel

The petroleum feedstocks that contain sulfur as an impurity are handled under ambient conditions in carbon-steel tanks and pipelines where the corrosion attack by sulfur is less severe. In the desulfurization step, vaporized feedstock is processed at 400°C in the presence of hydrogen sulfide (H2S) and carbonyl sulfide (COS) - both of which are highly corrosive. Stainless Steels (SS) 304, 316 or 321 (for the fired heater) are used as the material of construction for various pieces of process equipment in this section of the plant. Equipment failures occur because of external corrosion and thinning of fired-heater coils and interior deposition of carbon from coking which leads to overheating. Fuel-gas lines, that contain hydrocarbon vapors and H2S, should be constructed of SS 304 and heat traced to avoid condensation88. 

In addition to these incidents, three relatively minor events involving local channel overheating with graphitemoderated reactors have occurred, at Marcoule, Saint Laurent-des-Eaux and Chapelcross. In each case there was fuel damage and some oxidation of graphite local to the overheated fuel elements. The fuel elements were removed, the affected channels were sealed and the reactors returned to operation. There were no casualties and no release of radiation to the atmosphere. 

Decay heat removal Shutdown cooling system Reactor cavity cooling system Active Passive - If cavity cooling fails, heatup of environment without overheating fuel... 

Similar experimental conditions to those in the Sascha experiments, that is a flowing steam-hydrogen atmosphere at atmospheric pressure, were used in the ORNL high temperature tests, the principal aim of which was to determine the impact of specific accident conditions on the behavior of fuel and fission products. In the early ORNL high-temperature experiments (HT series), fuel rod segments fabricated of power-reactor irradiated fuel and encapsulated in Zircaloy cladding were heated for a short time (a few minutes) to temperatures up to 1900 K in atmospheres of various composition. The results obtained in these tests were the main basis for the assessment of fission product release from overheated fuels made in the NUREG-0772 report (US NRC, 1981). 

The release behavior of the less volatile fission products from overheated fuels was studied in detail in the Sascha experiments (Albrecht, 1987 a). These results demonstrated that the low-volatility elements (e. g. barium, ruthenium, cerium) were essentially released at the highest temperatures realized in the tests, i. e. from the liquid melt. In Fig. 7.11. some of the measured release rates for low-volatility elements are shown and compared with the assumptions made in the NUREG-0772 report. With the exception of barium (and potentially strontium), the release rates of these elements apparently depend only little on the amount of steam supplied. Barium release rates in a pure steam atmosphere have proved to be lower by a factor of nearly 100 than in an Ar + 5% steam atmosphere or in an Ar -H 5% H2 atmosphere. This behavior was assumed to be due to a reduction of the BaO initially present by still non-oxidized Zircaloy, which converted it into the more volatile elemental form. 

The iodine species which are released from the overheated fuel (mainly Csl and iodine atoms, depending on the burnup state of the fuel) are subject in the primary circuit to various chemical reactions, the most important of which are shown sche-... 

A second technical issue related to the ability of the Fukushima BWR s to cope with hydrogen production in the reactor core in fault conditions. If the reactors had been able to disperse the hydrogen produced by the overheating fuel safely, the explosions would have been avoided and the release of fission products would have been much reduced. 

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