Meltdown Process

The melting down of the reactor core does not only affect the nuclear fuel, but also the core internals, i. e. the core support structures, control rods, BWR fuel assembly shrouds etc. (see Chapters 1 and 2). At the very high temperatures which are experienced during the meltdown process, radionuclides as well as non-radioac-... 

The anthropogenic radionuclides of most concern are those produced as fission products from nuclear weapons and nuclear reactors. The most devastating release from the latter source to date resulted from the April 26, 1986, explosion, partial meltdown of the reactor core, and breach of confinement structures by a power reactor at Chernobyl in the Ukraine. This disaster released 5 x 107 Ci of radionuclides from the site, which contaminated large areas of Soviet Ukraine and Byelorussia, as well as areas of Scandinavia, Italy, France, Poland, Turkey, and Greece. Radioactive fission products that are the same or similar to elements involved in life processes can be particularly hazardous. One of these is radioactive iodine, which tends to accumulate in the thyroid gland, which may develop cancer or otherwise be damaged as a result. Radioactive cesium exists as the Cs+ ion and is similar to sodium and potassium in its physiological behavior. Radioactive strontium forms the Sr2+ ion and substitutes for Ca2+, especially in bone. 

This concept means that the risk of core meltdown in case the heat from fission processes cannot be led away must be absent. Two examples of proposed inherently safe reactor designs are ... 

The primary source of radionuclides produced in the fission process and found in the environment is atmospheric testing of nuclear weapons. The public has been exposed to these and other radionuclides for five decades, but there has been a substantial decline in atmospheric testing in the past two decades. Therefore the major source of fission product radionuclides in recent years has been from nuclear accidents. A nuclear reactor meltdown could release a spectrum of radionuclides similar to that of a nuclear bomb explosion, but the ratios of nuclides would greatly differ for the two cases. The reason for the differences in ratios of radionuclides is that during the reactor operation the long-lived radionuclides tend to build up progressively, whereas the... 

The risks associated with the operation of nuclear reactors are small but not negligible, as the failnre of the Three Mile Island reactor in the United States in 1979 and the disaster at Chernobyl in the former Soviet Union in 1987 demonstrated. If a reactor has to be shnt down quickly, there is danger of a meltdown, in which the heat from the continning fission processes melts the uranium fuel. Coolant mnst be circulated until heat from the decay of short-lived isotopes has... 

The valuable fertile elements are recovered from the acid solution by extraction with an organic solvent. The acid residue, containing the extremely radioaetive fission products, is processed to convert the waste into a stable solid form. The fission product waste, in a very concentrated form, is stored for ultimate disposal. This waste represents a different problem than the waste from current burner reactors. Because of the chemical concentration step there is less total mass of material. The same concentration process that reduced the mass of the waste concentrates the radiation produced into a smaller more intense package. This waste is so radioactive that it gets hot and must be actively cooled or diluted to prevent meltdown. Safe storage and disposal methods are very difficult to design. 

In contrast to the fission process, nuclear fusion looks like a very promising energy source, at least on paper. Although thermal pollution would be a problem, fusion has the following advantages (1) The fuels are cheap and almost inexhaustible and (2) the process produces little radioactive waste. If a fusion machine were turned off, it would shut down completely and instantly, without any danger of a meltdown. 

The thermal properties of ice cream, such as the heat capacity and the thermal conductivity, are important for several reasons. Parameters of the production process, such as the length of time required to harden the ice cream, depend on the thermal properties, as does the rate at which ice cream warms up and melts. This is important in storage and distribution and also when the ice cream is consumed. It should not melt so rapidly that it falls off the stick before it can be eaten. The thermal properties also affect the sensory properties such as the perception of coldness in the mouth. Furthermore, the freezing point and glass transition lines on the phase diagram, and the ice curve, are obtained from measurements of the thermal properties. The main techniques for measuring the thermal properties are calorimetry, conductivity measurements, thermal mechanical analysis and meltdown. 

The tsunami then overwhelmed the facihty s inadequate 5.5 meter seawall and, most important from a process safety point of view, it knocked out the safety systems designed to keep the reactors cool. Consequently the cores of the reactors overheated leading to partial meltdowns and follow-on problems, such as the generation of hydrogen gas that exploded. A considerable amount of radioactive material leaked to the ground, the sea, and the air— and those leaks appear to be ongoing. 

Thermoplastics are uncrosslinked plastics up to their decomposition temperature. Flow or melting (Fig. 2) occurs above the softening point of the amorphous structure in amorphous thermoplastics and above the melting temperature of semicrystalline thermoplastics. In this thermoplastic state, the viscous liquid can be processed. Form strength is achieved by cooUng. Meltdown, solidification, and crystallization can be repeated any number of times. 

Material in drying hopper caking or meltdown occurring Process temperature set too high Check resin data sheet for meltdown temperature Make sure operators know correct process temperature set point... 

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