Accidents in nuclear reactors

Releases of superheated hquids have to be considered in safety analyses for process plants. Such liquids vaporize on depressurization, which may occur, for example, following leaks or pressure relief of reactors. It is a difficult issue, which is not yet fuUy clarified. Most work on two-phase flow concerns mixtures of water and steam, which play an important role for accidents in nuclear reactors. The more complex task of treating flow processes of mixtures of several components and phases, which are characteristic of process plants, still requires intense research. 

This article does not cover the extensive research carried out to delineate the cause and severity of RPTs possible in nuclear reactor accidents. Although there is not universal agreement, the general consensus is that the superheated liquid concept can explain many of these events and, more importantly, can indicate when RPTs are unlikely to occur in hypothetical reactor accidents. 

Accidents with nuclear reactors or nuclear bombs can expose large numbers of people to several decay products of uranium, and iodine isotopes are among the most abundant compounds released in such reactions. It is therefore logical to use salts of stable isotopes of iodine to prevent the accumulation of radioiodine in a person or population at risk of such exposure. The accidents in Windscale (UK), Three Mile Island (USA), and particularly Chernobyl (Ukraine) drew attention to such problems. The major question is therefore whether the potential adverse effects of stable iodine when given indiscriminately to large... 

Beyond element 92 (U) lie the transuranic elements of the actinide series. These are all artificial but 2< Pu is of interest because it is produced in nuclear reactors from 2 fU and may be released to the environment from accidents or weapons testing. It has a very long half life (2.4 x 104 years) and is a very dangerous alpha emitter, but, like radon, its geochemistry is too specialised to be included in this chapter. Choppin and Stout (1991) have written an overview of the general chemistry of Pu, to mark the 50th anniversary of its original isolation, and Rai et al. (1980) have discussed its soil chemistry. 

It should also be remembered that most evolving technologies, whether boilers during the 19th century, airplanes in this century, or nuclear plants, entail some accidents from which lessons are learned. Both the Three Mile Island accident, from which only limited radioactivity escaped to the environment, and the Chernobyl disaster, have led to the introduction of new safety features in nuclear reactors, in plant operating procedures, and in regulations. 

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... 

Different categories apply to the hydrogen issue in nuclear reactor containments. During normal operation, small quantities of H2 are generated by radiolysis of the coolant, 44 H2 and 22 O2 molecules per 10 eV of neutron radiation energy. Under accident conditions, significant amounts of hydrogen could be produced due to radiolysis and corrosion reactions. 

The present study reports measurement methodology and features of the migration behavior of atmospherically derived radionuclides ( s, Pb, and Be) and includes data from previous studies (Matsunaga et al., 1991, 1995, 1999) of two Japanese watersheds. In addition to the good traceability of Cs, the choice of this radionuclide was rational for us because it is a major radionuclide and important in a nuclear accident because of its abundance in nuclear reactors and its volatility. Though, the present report is for radionuclide elements, the present findings could provide suggestions for the fate of other trace elements. 

In nuclear reactors water used as a coolant or moderator should be as pure as possible to minimize dissociation during the time in the reactor. The formation of an explosive gas mixture of H2 and O2 must be carefully avoided in all reactors in order to prevent accidents. Moreover the deconq)osition products of water can increase the corrosion of fuel elements, structural material, etc. Many reactors use N2 as a protective gas. In this case the radiolysis can lead to the formation of HNO3 unless suppressed by an excess of H2 which preferentially yields NH3. The pH of the water may be regulated by the H2(g) pressure. 

Very large instantaneous doses (> 10 Gy) occur in explosions of nuclear weapons, in accidents involving nuclear reactors, or from carelessness in working with accelerators, X-ray equipment or radioactive installations (e.g. Co sources used for technical and therapeutic purposes), criticality accidents, and in handling unshielded strong radiation sources or unshielded radioactive waste. Such doses are very unlikely to be received in work involving amounts of 1 GBq of radioactivity. 

Cs-137 is produced by nuclear fission for use in medical devices and gauges. Cs-137 also is one of the byproducts of nuclear fission processes in nuclear reactors and nuclear weapons testing. Small quantities of Cs-137 can be found in the environment from nuclear weapons tests that occurred in the 1950s and 1960s and from nuclear reactor accidents, as in 1986 when wind currents distributed Cs-137 to many countries in Europe after the Chernobyl power plant accident. 

In treating disturbances in process plants problems like the pressure relief of superheated liquids which evaporate on depressurization have to be dealt with. Then two-phase flow results, i.e. the liquid and vapour phase are discharged together. Most work in the area refers to mixtures of water and steam, which are of special importance in nuclear reactor accidents. The much more complex task to treat flow processes of multi-component two phase mixtures, which are a characteristic of process plants, still requires intense research. 

As a result of the analysis of events during the accident at the Three Mile Island nuclear plant (TMI, March 1979), the importance of human error in nuclear plants was better understood. The accident resulted from the confusion of the control room operators with inadequate instrumentation and inaccurate procedures. The most important factor was that they had to act in spite of the weaknesses in the training to respond to unexpected events. Therefore, the United States Nuclear Regulatory Commission (USNRC) called for the improvement of Emergency Operating Procedures (EOPs) and in nuclear reactor operator training. The philosophy of incident response implemented in the improved procedures was to take a symptom-based approach (operators foUow a series of yes - no questions to ensure that the reactor core remains covered and only then determine what was the cause of the problem) (USNRC, 2009). 

Canadian Standards Association (CSA). 1991. Guidelines for Caladating Radiation Doses to the Public from a Release of Airborne Radioactive Material under Hypothetical Accident Conditions in Nuclear Reactors, CAN/CSA-N288.2-M01, Toronto. 

Luxat, J.C. 2007. Thermalhydraulic asjpects of progression to severe accidents in CANDU reactors. In Proceedings of the T2th International Topical Meeting on Nuclear Reactor Thermalhydraulics NURETH-12), September 30-October 4, Sheraton Station Square, Pittsburgh, PA. 

In 1967, E R. Farmer of the United Kingdom proposed that the probabilities as well as consequences of potential accidents need to be estimated to assess the associated risk. Farmer used 1-131 as a surrogate for consequences. By plotting the probability and consequence of each postulated accident, one could distinguish those with high risk from those with low risk. He proposed a boundary line as a criterion for acceptable risk. Farmer s work was a conceptual breakthrough in nuclear reactor safety analysis. Farmer takes full credit as the originator and pioneer of PRA. 

The complexity of the heat transfer processes in nuclear reactor core requires computer codes to handle both local and system-wide behavior under normal, transient, and accident conditions. The code models are assessed with experimental data to ensure that they were working properly. Some of the largest and most widely used codes in United States are as follows ... 

Beahm, E. C., Weber, C. F., Kress, T. S., Shockley, W. E., Daish, S. R. Chemistry and mass transport of iodine in containment. Proc. 2. CSNI Workshop on Iodine Chemistry in Reactor Safety, Toronto, Can., 1988 Report AECL-9923 (1989), p. 251—266 Beard, A. M., Bowsher, B. R., Nichols, A. L. Interaction of molecular iodine vapour with silver—indium—cadmium aerosol. Proc. International Symposium Severe Accidents in Nuclear Power Plants, Sorrento, Italy, 1988 IAEA-SM-298/108, Vol. 2, p. 201—213 Bell, J. T. Chemistry of iodine and cesium, in M. Silberberg (Report Coordinator) Technical Bases for Estimating Fission Product Behavior during LWR Accidents. Report NUREG-0772 (1981), Chapter 5... 

As described earlier in the chapter, the castor plant grows well in soil with small amounts of salt present. It has also been shown to grow in soils contaminated with nickel, chromium, copper, manganese (Gupta and Sinha, 2007), and arsenate (Melo et al., 2009). Because castor has a deep and extensive root Systran, it is possible to use the plant for phytoremediation of soils contaminated with heavy metal ion from mining and manufacturing. Preliminary experiments indicate that castor can also be used for phytoremediation of cesium ion, a process of considerable potential value as the Cs ion is a major contaminant in nuclear reactor accidents. 

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