Current safety approaches for PWR (generation II reactors)
The fundamental principles of nuclear power plants worldwide are that the operators are responsible for safety. The national regulator f nuclear plants are responsible for ensuring that the nuclear power plants are operated safely by the licensee, they also ensure that the design of the nuclear plant meets the required standards (In Oka, 2014) . The regulators also have a very important mission of protecting the community and the environment around the power plants. The national regulators are also responsible for the design certification of the nuclear reactors. There is an international collaboration among the national regulators however they responsibilities and aims do vary to some degree, but they all have a mutual understanding when it comes to safety and quality.
Since the 1990’s there have been new designs of reactors in an international basis, both industries and the nuclear regulators have come to an understanding that there is a need for greater design standardization and regulation harmony. The groups involved in the standardization of include the World Nuclear Associations CORDEL, the OECD/NEA’s, and MDEP. According to an OECD/NEA report of 2010 claimed that the theoretically calculated frequency for a massive release of radioactivity material from a large power plant accident has reduced by a factor of almost 1600 between the generations of today and the early generations (Bortz, 2013) . It is usually believed that cases of nuclear reactor accidents are near to impossible but if they do there is a high chance the consequences will be devastating. The physics chemistry and engineering put in place have set a good name for nuclear reactors; it is believed that when nuclear reactor accidents do happen, they will not be a server as compared to other industries and energy sources. Facts that prove this point include the Fukushima incident (Bortz, 2013).
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Several industries in the United States like the U.s Navy, the Medical Industry, the aviation Industry and other high-risk organizations have adopted a high human performance training course in order to reduce the chances of human errors in order to reduce chances of industrial accidents. It is approximated that 80 percent of industrial accidents are attributed to human error, while in some industries it is approximated to be 90 percent. With the ingenuity and standards put in place in the creation of these equipment accidents as a result of equipment failure is reduced to 20 percent. While the 80 percent human error is stripped down it is revealed, a majority of the errors is a result organizational weakness. Clearly, if organizations focus on reducing human error the likelihood of industrial errors will reduce drastically. Since the Fukushima accident, a lot of focus has been put on the organizations (Zohuri, 2017) . The United States Navy has an excellent record of safety; this can be attributed to the standardization in all the naval power plants and the quality of the naval training, the Soviet naval also had a good record of safety (Morales, 2017) .
Events that can lead to a small modular reactor meltdown
Small modular reactors are one of the safest reactors in the world today. They are a type of nuclear fission reactors; they are small reactors in comparison to other reactors thus the name small modular reactors. They are usually created in a plant and shipped to an industry to be fully constructed. Modular reactors have less on-site construction, and therefore this increases containment efficiency and high security of the nuclear materials. SMRs are less expensive in comparison to other nuclear reactors, however, there are a lot of critics who do not see the cost-benefit of this reactor over solar energy and wind energy (In Oka, 2014) . Based on the ingenuity and the size of the modular reactor it is hard for them to have a meltdown, however, there is an instance where the reactor can have a meltdown this instances include.
The loss of coolant accident, this may happen when there is an actual loss of coolants; these coolants include liquid sodium, deionized water, or the lack of a sufficient method of administering coolants. Loss of coolants and lack of pressure control are usually common courses of meltdowns among these reactors. In water pressure reactors LOCA can cause steam bubbles to form and hence increase the likely hood of accidents (Bortz, 2013) .
There is the loss of pressure control accidents; this happens when the pressure of the coolant fans fall below the required standards, and there are no means of restoring it. In most cases, this reduces the heat transfer in the reactor and in turn it may form an insulating bubble of steam around the reactors fuel assembly, due to continuous heating of the bubble as a result of the decay heat, the pressure that is required to collapse the bubble may exceed the reactor's capacity (In Oka, 2014) . This type of accident mostly happens in boiling water reactors like those in naval submarines. Gas cooled reactors losses gas pressure in a depressurization fault, this reduces heat transfer efficiency and possess some challenges to the cooling fuel, however on the bright side as long as one gas circulation is available the reactor will be kept cool.
There are also incidents of uncontrolled power excursion accidents. This happens when there is a sudden power spike, and the reactor has not been created to meet such specifications. This occurs when there is a significant change or alteration in the parameters that affect the multiplication of neutrons in a chain reaction (In Oka, 2014) . An example includes the removing of a control rod or increasing the cooling system of the fan, in extreme cases, the reactor can enter a condition known as prompt critical, this is very common among reactors with a positive void coefficient of reactivity. A positive temperature coefficient is usually over moderated, and they can trap surplus quantities of deleterious fission product. The Chernobyl disaster was caused by a similar deficiency however server operator negligence was also blamed for the disaster.
Fires at the core base can put the core at high risks and even cause the fuel assemblies to meltdown. These fires are easily caused by air that enters the graphite-moderated reactor, even in the liquid sodium cooled reactors. Graphite also has some disadvantages since it is easily subjected to the accumulation of winger energy, this form of energy can overheat the graphite (this is the same thing that happened in the wind scale fire.) Light water reactors have an advantage since they are do not have cores or moderators that are flammable therefore they are not subjected to this core fires. Gas cooled reactors like the Magnox, and the AGCR reactors keep their cores covered with a blanket of nonreactive carbon dioxide gases that do not support combustion. Most gas-based reactors use helium, helium is not flammable, and they do have fuels that can withstand high temperatures (Zohuri, 2017) .
Byzantine faults and cascading failures within the control system can cause control systems to have some problems within the workings of the reactor, if the problems are not mitigated at an early stage it may cause severe problems (Zohuri, 2017) . A good example of such a case is the browns ferry fire; it damaged the control cables forcing the plant operator to activate the cooling system manually (Zohuri, 2017) .
References
Bortz, A. B. (2013). Meltdown! The nuclear disaster in Japan and our energy future . Minneapolis, Minn: Twenty-First Century Books.
In Oka, Y. (2014). Nuclear reactor design . Tokyo: Springer
Morales, P. J. (2017). Small modular reactors for electricity generation: An economic and technologically sound alternative . Cham: Springer.
Zohuri, B. (2017). Neutronic analysis for nuclear reactor systems . Cham: Springer International Publishing.
Zohuri, B. (2017). Thermal-Hydraulic Analysis of Nuclear Reactors . Cham: Springer International Publishing.
