Nuclear Reactor Core

Introduction

A nuclear reactor core is the part of nuclear reactor that is composed of the nuclear fuel portions where all nuclear reactions are carried out with the heat generating. Commonly, the fuel is low-enriched uranium composed of thousands of fuel pins. The core is also composed of structural components, the parts which moderate the neutrons, thus controlling the reaction, and the parts that transfer the generated heat from the fuel where it should be, beyond the core. There are two types of reactors – water-moderated reactors and graphite-moderated ones. In water-moderated reactors, a pressurized water reactor has nuclear fuel rods which are similar to the diameter of gel type ink-pen, each is nearly 4 m long, which are united in bundles and are called fuel assemblies. Each fuel rod which is considered of uranium oxide are connected end to end. There are also control rods which are located inside the core, filled with hafnium, cadmium, or boron that hold neutrons together. Control rods absorb neutrons when they are lowered into the core, thus preventing the chain reaction. At the same time, if the control rods are increased in tension, more neutrons strike the uranium-235 in fuel rods, which intensify the chain reaction. The core shroud is located within the reactor, which directs the water flowing for cooling down the nuclear reactions within the core. In contrast, the graphite-moderated reactors make use of solid graphite for moderating neutrons and of regular water cooling it down. To enlarge on the issue, carbon dioxide is presented as the coolant that circulates w­­­ithin the core and lowers its temperature. In the advanced gas-cooled reactor, the core is composed of a graphite neutron moderator in which the fuel assemblies are located. These are two major types which are applied for generating energy in various industries and reactors.

There are also specific safety standards for designing the reactor core, which should match the features of the core and compensate the increase in reactivity. The reactor power is monitored by a range of inherent neutron feature of the reactor core including its thermal-hydraulic information as well as the capability of the shutdown and control system to activate all operational conditions and basic accident states. In case rapid acting monitoring and shutdown systems are essential, they capabilities should be justified. According to the standards, “the maximum rate for positive reactivity in operational states and in design basis accidents should be limited in such a way that are the means of reduction of reactor power…are effective in maintaining core coolability, minimizing damage to the core and preventing failure of the pressure boundary for the reactor coolant” (International Atomic Energy Agency, 2005, p. 6). Furthermore, the design for the main fuel elements should be aimed at preventing unfavorable outcomes of reactivity initiated accidents. In this respect, the basis for these initiatives must concern highest standards of fuel control. Further, at least two major and independent shutdown systems must be introduced. The evaluation of the distribution of core power must be carried out in the design for representing operational conditions and providing information to determine operational conditions, limits, and action set point to ensure safety of the protection system. Next, the operating procedures guarantee the adherence to the design limits such as design parameters established in the course of the service period of the reactor core. Finally, reactivity control tools must be used for controlling the reactors in critical conditions with regard to the possible lapses in design and subsequent consequences. Adequate provision must be introduced to the design for maintaining safety for plant states where normal fuel cooling, shutdown, and integrity of the cooling system are out or order, or when the reactor vessel is open for refueling and maintenance.

Main Advantages and Disadvantages

Nuclear energy is a clean, reliable, safe, and competitive source of energy that can serve as a potential substitute for such fossil fuels as gas, coal, and oil, which can contaminate the atmosphere and contribute to the increase of greenhouse effect and reduction of ozone layer. In case people are now more aware of climate change and the reducing amount of oil, the experts should promote a more rational use of energy, especially the renewable one (such as solar and wind energy), when possible as well as adopt a healthy lifestyle. However, such an approach is insufficient to slow down the concentration of atmospheric CO2 and also to meet the needs of the industrial society and goals of developing nations. In response, the nuclear power can replace coal, gas, and oil in the industrial companies, particularly in the developing economies. A refined combination of renewable energy and energy conservation for low-intensity applications as well as for base-load electricity production is among the most sustainable solutions for the future. Nuclear power plants can also deliver power and energy for electric vehicles ensuring cleaner transportation. With the introduction of high temperature, the reactors could be aimed at recovering fresh water from the sea and at supporting hydrogen production. Hence, it should be stressed that the opposition of certain environmental organizations to the civilian applications of nuclear power can be revealed to eliminate previous disadvantages.

Additionally, the scientists and experts in the field assume that the industrial civilization consumes over 85 % of energy delivered by oil, gas, and coal. The latter began to be utilized extensively in Britain as soon as the forests failed to provide energy requirements. It should also introduce subsequent steps in managing environment in the sites where the foci of industrial activities are located. Coal is easily mined, and the earth has sufficient amounts of buried fuel for several centuries. Further, oil has replaced whale oil at the end of the nineteenth century, and its use has become popular since then. The discovery of new supplies and deposits do not meet the growing consumptions needs. Currently, the consumption rate is much higher as compared to recent decades. Over half reserves of oil are situated in environmentally fragile and unstable areas, and the initiation of the mining activities there could be detrimental for the regions in environmental terms. Finally, gas was first introduced as the side product of oil extraction, and it has later been transformed into the most popular energy source. However, the sources are limited, and it has been already estimated that they last out for few decades only. The environmental consequences of utilizing all the resources presented above are detrimental as well because they have a negative influence on climate change that leads to the aggravation of green house effect, reduction of ozone layer, and overall heating of the atmosphere, thus resulting in the general temperature rise. According to Comby (n. d.), “in burning fossil fuels, we inject 23 billion tons of carbon dioxide every year into the atmosphere – 730 tons per second. Half of it is absorbed in the seas and vegetation, but half remains in the atmosphere” (p. 2). As a result, the composition of the atmosphere has altered significantly, affecting the overall climatic conditions on the planet.

With regard to the above-presented characteristics, nuclear power could be considered as safe, reliable, clean, and almost inexhaustible. Currently, over 400 nuclear reactors create base-load electric power in 30 countries. The nuclear power has been used for over half a century, and it is comparatively mature technology with the evident improvements for the next generation with specifically promoted shipboard power. Although there are many cases of disasters and accidents, the strict and accurate safety measures can make this source of energy the most effective and clean. Nuclear energy produces no carbon dioxide, nitrogen oxide, and sulfur dioxide. This type of energy is produced in large amounts when fossil fuels are introduced. The nuclear waste is minimized. As an example, one gram of uranium produces the energy which could be produced by a tone of oil or coal. Nuclear waste is approximately about million times smaller in comparison with fossil fuel waste, and it is absolutely confined. It should be stressed that in Sweden and the United States, spent fuel is stored away. In other countries, spent fuel is recycled to split out the 3 % of radioactive fission outputs and heavy components to be vitrified either for permanent storage or for sale. The remained 97 % of the uranium and plutonium is recycled and recovered into the new elements which also produce energy (Cohen, n. d). The volume of nuclear waste is relatively small. Hence, it could be be geological storage locations that do not contact with the biosphere. Its influence on the ecological environment is insignificant. Additionally, nuclear waste is dangerous for over a certain period of time whereas stable chemical waste decay could last much longer. In contrast, most fossil waste is presented in the form of gas, which turns into smog. It is impossible to see it, but it has a tangible effect on acid rain happening, global warming, and other types of atmospheric pollution.

Apart from evident advantages, the use of nuclear reactor core as the underpinning for generating energy could still be dangerous. The first and the most threatening issue about nuclear power are the radiation and its influence on health conditions. The radiation is composed of subatomic particles which are transferring almost at the speed of light. As a result, they can penetrate into the human body, damage biological cells, and cause cancer. They can also damage sex cells leading to genetic modifications and disorders in progeny. Radiation could also become a natural outcome of environmental processes; a typical person is also affected by 15,000 particles of radiation each second coming from nature. It has also been defined that the average medial x-ray can involve 100 billion of particles for one session, which could be dangerous; however, the probability of receiving cancer from this influence is almost minimal (Cohen, n. d). Nevertheless, the nuclear power technology manufacture materials which are more active in emitting radiation, and these emissions are called radioactive. These materials can interact with human body through small incidents during daily plant operations, disasters at nuclear plants, or accidents in delivering radioactive products. Since natural radiation causes only 1 % of cancer cases, the nuclear technology radiation can increase its risk by 0.002 % white reducing life expectancy. Although the risk of cancer development among people whose body was damaged by nuclear technology radiation is not significant, the case of Hiroshima and Nagasaki has proved that there were 400 extra cancer deaths among people who suffered from the disaster. Since there are not possible methods for the cells to distinguish between types of radiation, the nuclear industry does not cause the spread and emergence of genetic modifications threatening the human race. Other consequences of genetic disease involve delayed parenthood, spontaneous mutation, and other genetically predetermined disorders. The genetic risks of nuclear reactor core influence are equal to 2,5 parenthood delay.

Apart from severe health outcomes, the radioactive waste should not be underestimated either. Specifically, the radioactive waste products from the nuclear plant operations should be isolated from human contact and activities for a long period. The point is that radioactivity consists in the spent fuel, which is relatively small in amount and, as a result, easily controlled with great care. The high amount of waste could be transformed into a rock-like type and embedded deep inside into the natural habitat or rocks. The average lifetime of the rock in the environment equals to billion years. In case the waste is presented in the form of rock, it can be easily demonstrated that the waste produced by one industrial plant will definitely cause negative health outcomes for the humans (Cohen, n. d.). In comparison, the emissions from coal plants can lead to thousands of deaths while generating equal amount of electricity. Larger volumes of less radioactive emissions from nuclear plants are buried at shallow depths in soil. Assuming that the material can be dispersed through the soil in water, it can lead to the significant damages to the nature and to the human organism as a result.

There are also many other potential problems related to high levels of radioactivity from nuclear plants. The potential problems from disasters and accidents in transporting the radioactive materials are neutralized by the packaging. A great amount of transport could take place for over 50 years, and there have been a great amount of accident which proves that the risk of disaster and its consequences are much more serious than those caused by other types of energy generating industries. Mining uranium for nuclear plants leaves mill tailings, the remnants from chemical processing of the gold that leads to radon exposures.

Limitations

The nuclear power industries, which are in service nowadays, were designed in the 1960s. At the same time, it has been predetermined to reach the maximum effectiveness and the minimum cost to complete successful substitute for coal and oil industries. In order to increase the effectiveness of nuclear plants including pressures, power, and temperatures, it was essential to reach the highest practical limits. Safety measures were sufficient for that period, and the current state of industrial development also approves the previously established safety standards. However, they are not still adjusted to the current demands of high safety of nuclear industry. As the community has become more concerned with the safety issues, the Nuclear Regulatory Commission has been organized to develop new safety measures and equipment and provide the amount of materials for these applications, which were conceived previously (Cohen, n. d). The complexity of these aspects shows that plants could become difficult and expensive for construction, maintenance, and operation. Furthermore, the level of safety was still restricted by the genuine conceptual design.

Current State of Technological Development

Currently, nuclear power reactors have turned with much more sophisticated mechanisms with the significant growth in supporting technology. University programs provide separate sources which cover basic topics such as reactor physics, materials, and thermal hydraulics. The overall availability of powerful computers has turned design procedures into sophisticated methods. Although the accepted principles of managing the power reactor coincide with those of PWRS, the diversity in core design has a tangible impact on the certain approaches applied. As such, an overview of the core composition that forms the fuel assembly control perspective is beneficial. An important trait involves the placement of a group of four major assemblies with the control elements. In power reactors, the fuel rods in the assembly possess similar enrichment, but this is not the case for other boiled water reactors, where the assemblies are of smaller size. For compensating the water gap influence in case of withdrawal of control blades, the fuel rods in each assembly should have a lower enrichment. In general, there are four stages of enrichment in each assembly. The level of enrichment should be from 1,7 % to 21 % in the first layer and 2.5 to 3 % in the subsequent layers (Glasstone & Sesonske, 2012). Boiling-water reactors are monitored so that the fuel economy is enriched by changing the neutron spectrum for controlling the reactivity. During the first half of the cycle, the cooling flow percentage is decreased as the core is cooling down for the relatively high percentage of influence. The core is under-moderated. The result of the resonance region is the increased conversion. When reactor operation carries on, the consumption of fuel takes place and product waste is accumulated. The cooling flow is enhanced reducing the core coolant void fraction.

Many researchers and scientists in the sphere are currently working on advancing the quality and effectiveness of the nuclear reactor core. As an example, Norouzi, Aghaie, Fard, Zolfaghari, and Minuchehr (2013) have explored the fuel and operation management of nuclear reactors, which importance has been emphasized. Specifically, core performance evaluation focuses on an important image of core fuel management optimization. In this respect, the new algorithm of core optimization to safety standards of nuclear reactors has been presented. The authors have paid attention to the Parallel Integer Coded Genetic Algorithm (PICGA) that has been employed for obtaining the most effective configuration related to the maximum multiplication factors. The PICGA could be utilized for over two search algorithms along with sharing their information in a synchronous way. The algorithm is associated with WIMSD4 and PARCS codes that are used for counting the fuel depletion, group constant, and power peaking factors as well as for corresponding multiplication factors. The application of the algorithm is essential for understanding how different codes could be improved.

Future Challenges in Developing the Nuclear Reactor Core

Currently, nuclear power delivers nearly 13 % of general electric power, and it has been presented as a valid and reliable base-load source of energy and electricity. A range of material problems could be resolved for nuclear energy generation to focus on further improvements in terms of economics, reliability, and safety. The operating setting for the materials should be used for developing major operating components. In this respect, Zinkle and Was (2013) have emphasized that the material challenges related to power extensions and uprates of the operating lifetime of the nuclear reactors must be taken into consideration. There are three major material obstacles for the next generation of the nuclear; water-cooled reactors are presented with regard to two structural material aging and degradating aspects along with the improved fuel system of accident tolerance. Additionally, the scholars have assumed that the materials for degradation issues of Zralloy-clad UO2 system currently applied in the majority of nuclear power industries and are presented for both normal and abnormal conditions. In the future, there are five major radiation degradation effects, such as low temperature embrittlement and hardening, radiation induced segregation, irradiation creep, and high temperature embrittlement. There are also stress and multitude of corrosion which can have a specific effect on the performance of the materials.

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