Advanced Gas-cooled Reactor versus Pressurized Water Reactor debate - Essay Example

? ADVANCED GAS-COOLED REACTOR VERSUS PRESSURIZED WATER REACTOR DEBATE By of the The of the of the school School city and state Date Abstract This research article explores the pros and setbacks of Advanced Gas-Cooled Reactors and Pressurized Water Reactors. The technology, safety, licensing, and environmental considerations in UK are taken into account. The establishment of Advanced Gas-cooled Reactors was probably discontinued due to huge capital investment and high maintenance costs. On the contrary, the Pressurized Water Reactors are cheaper to maintain, environmentally safe and reliable. This prompted the government to commission the Sizewell B project. Future energy generation considerations are likely to favor the use of renewable sources that are cheaper, and environment friendly. Contents 1.0 Introduction From the 1960s, there was a clarion call to develop nuclear energy in UK with Advanced Gas-Cooled Reactors (AGRs) being the preferred choice. The first prototype of the advanced gas-cooled reactor was developed in 1962, but the first commercial AGR became operational in 1976. Complexities in implementation of the prototype delayed completion of the first AGR commercial plants. Other AGR projects were to be commissioned later in the 70s, and the 80s. However, they faced the same challenge (delayed implantation) compounded with a heavy cost implication. This led to an acrimonious debate about the use of AGR technology to generate electricity, when other economically viable means were available. The immense pressure forced the UK government to abandon building AGRs anymore and instead focused on the Pressurized Water Reactors (PWRs). This led to the commissioning of Sizewell B in 1987 to 1995. The PWRs were considered to be cheaper, environmentally safer and more reliable than the AGRs. The need to develop renewable sources of energy, and the carbon emission reduction, is likely to shape future government decisions on the most appropriate means of generating electricity. 2.0 Technology used in advanced gas-cooled reactors and pressurized water coolers Advanced gas-cooled reactor (AGR) refers to a second generation of gas cooled reactors developed in the United Kingdom. The AGR was an improvement of the first generation of gas cooled reactors. The AGR is designed to use carbon dioxide as a coolant and graphite as a moderator. The AGR is a specifically UK breed of reactor developed, from the design for the very first nuclear reactor, to generate electricity for commercial use, a reactor built at Calder Hall in Cumbria, UK (Breeze, 2005, p. 258). Figure 1: Schematic diagram of an AGR Source: World Nuclear Association There are several advanced gas-cooled reactors built in the UK, but they have been found to be costly to operate. Most of the AGR plants cost more at the completion than it was initially anticipated and no further units of the design are planned. Instead, the last nuclear power plant built in the UK employed a United States PWR design (Breeze, 2005, p. 258). Pressurized water reactors (PWRs) are used in light-water reactor power plants. Although, there are different manufacturers in the United States, the fundamental characteristics of the PWRs are the same: the main coolant brings up steam vapor in the heat exchanger, known as the steam generator and this steam drives the turbines. Figure 2: Schematic diagram of a PWR Source: Tennessee Valley authority By the 1970s, United Kingdom was the only major Western Europe country yet to adopt the use of light water reactors (LWR) technology to develop. This was a result of a protracted debate between the protagonist of the AGR and the LWR. It was argued that the LWR was unsafe, thus the promotion of AGR usage. The Magnox stations in UK had for a long time been faced with the problems of low volumetric power density, low operating temperatures and low pressures. The time had come to look for an alternative, and well improved design. This led to the development of the advanced gas-cooled reactors. The technology used in the AGR allows for the use of carbon dioxide as a coolant, with the coolant pressure being around 40 bars and it is possible to attain outlet coolant temperatures of 6500 C (Hewitt & Collier, 2000, p. 40). Therefore, on the basis of attaining high operating temperatures, the AGR has a relative advantage over the PWR. In order to attain the high temperatures and pressure, it was necessary to make some changes in designing the fuel component. In the design of the AGR, it was necessary to provide for generation of steam under conditions that would enable fuel efficiency. This technology enables a steam cycle efficiency of 40 % to be obtained. An outlet temperature of 650 0C allows for this in comparison to the 325 0C in PWRs. This is the best any nuclear reactor could have currently (Hewitt & Collier, 2000, p. 40). On the contrary, the PWR can only attain thermal efficiency of 34 % only. The AGR has a volumetric power density that is up to three times better than that of the best Magnox stations. The average fuel rating is also higher by a factor of 4. Despite these achievements, there are a number of unresolved technological issues in the AGR. For instance, carbon dioxide used as a coolant could react with graphite used, in the moderator, to produce the toxic carbon dioxide. This would also use up the graphite and minimize its strength. CO2 + C 2 CO Scientists came up with a solution of using small concentrations of methane to prevent this reaction. This brought about another challenge; at high concentrations, methane would react with carbon monoxide leading to the formation of carbon. The carbon would interfere with heat transfer in the fuel elements (Hewitt & Collier, 2000, p. 40). On the contrary, the design of the PWR allows for economies of scale hence it is cheaper to develop many reactors on one design. The AGRs come in different designs, which raise production cost. The PWRs also can produce power of up to 1 GW. However, the use of water as a coolant, and as a moderator, cannot allow PWR to operate at high temperatures and pressure which seriously hampers its efficiency. PWR can only attain temperatures of 300 0c and a pressure of 15 MPa. The PWR houses the heat exchanger and steam producing unit in a concrete shield which can allow for control of radioactivity, should a fracture occur. The design of the PWR allows for the use of control rods that can be used to minimize reactivity, and in case reactivity needs to be upped, the rods are withdrawn. 3.0 Safety considerations AGRs come with design incorporations that allow for the first and second level of defense. The first level entails prevention while the second level entails detection and control of failures. The first level of defense is in-depth and takes into account the need to eliminate a series of accidents through the use of the intrinsic capability of the systems in a more effective way, reducing the power density, increasing the design margins increasing the time constraints for the overall reactor system in order to slow down the transients response of the system (Nag, 2008, p. 48). This helps to prevent the occurrence of level two of defense-in-depth and allows for more time for automatic control and operator actions and avoiding these failures to develop into accidents. They also simplify the design of control system and the actions required from the operators, and they decrease the number and severity of challenges to structures and safety systems. Limited size of some reactors allows achieving the decay heat removal function with simple and reliable systems. PWR reactors are safer compared to the AGRs. They have a higher safety margin through the use of passive safety systems, relying more on reactor designs and less on operator action to prevent accidents. The pressurized water reactor has safety relief valves that limit excessive pressure. There are also control rods in the pressurizer; they range from 50 to 100 depending on the reactor, which control reactivity using neutron absorbers such as Boron. In order to control reactivity, control rods are released into the core by a scram to absorb neutrons and stop the chain reaction (Nag, 2008, p. 50). The PWR can also allow for emergency cooling. In case of a pressure drop, the High Pressure Safety Injection pumps cooling water. The use of water as a cooler and as a moderator acts a safety measure. However, the steam generator tubes of the PWR are vulnerable to accelerating deterioration of steam generator tubes. This is because the metal in the component is quite vulnerable. The pressure vessel is another area of concern in the PWR because if it fails there is nothing to back up the system and cool the reactor. This may lead to unwarranted nuclear meltdown. 4.0 Licensing considerations AGRs have to be inspected after a given period, usually after two years. The main features that have to be monitored include the pressure and reactor sections. UK safety guidelines require total frequency that could lead to unchecked release of radioactivity to be more than 10-6 per reactor in a year. The guidelines also set a limit of 10-7 per year for a single accident. Design of the AGR, therefore, takes into consideration these guidelines thus protective features are put, in place, to ensure that the likelihood of an uncontrolled release of radioactivity is limited. The design of the AGR, with a good design, should be able to protect the occurrence of faults. The AGRs come in different designs and different degrees of innovation. This makes it difficult to develop valid safety and licensing considerations without being too general. This is in contrary to the PWRs whereby specific licensing considerations can be made. The considerations include: whether the AGRs has the capacity to control radiological consequences in case of an accident. The licensing regulations also take into account the ability to control the occurrence of severe accidents, right from the early phases of the design. It is expected that the possibility of core damage taking place is low. This has to be demonstrated through testing of the integral reactor. The design of the AGR should have realistic assumptions on measures to take in case of severe accident due to late containment. The structure of the graphite core together with its components determines the ability of the AGR to shut down safely and cool the fuel. It is not certain how long the graphite core can last given that the behavior of graphite is uncertain. For instance, the graphite component is vulnerable to cracking raising safety issues about the AGR. In the future, it would be imperative to demonstrate that adequate in-depth defense could be attained through the use of cheap and straightforward technology. In setting up a PWR there has to be evidence that the radioactive materials released would be less than the amounts specified in the regulation. This prevents failures instead of mitigating consequences. 5.0 Reliability and independence of energy supply The AGR achieves reliability by using the suitable design, as well as redundancy. Redundancy has the capacity to protect occurrence of frequent faults. Consistent use of safety guidelines protects against failures and ensures reliability. In the PWR, reliability is maintained by having two to four generators and coolant pumps. PWR is considered the best in terms of reliability especially in large scale use. It is essential to protect the integrity of pressure systems, which are crucial to maintaining the safety of atomic reactors. Attention is, therefore, paid to the failure of the pressure vessel. The AGR does not require a phase change of the coolant, unlike in the PWRs. Moreover, the fuel can perform well at all pressures, both in the air and the carbon dioxide. This gives the AGR an element of reliability. It is essential to note that the fuel rating is relatively low, and the graphite mass usually acts as a sink. This makes fuel element temperatures to rise slowly. This is both an element of safety and reliability. 6.0 Fuel production and fuel cycles Fuel cycle refers to a series of steps through which nuclear fuel passes through. Fuel burn-up, cooling time before reprocessing, and reprocessing losses are some of the most the most influential parameters when considering the efficiency of fuel production and a fuel cycle. The AGR can attain a high burn-up of 18,000 MW for one tonne of fuel. Thus, they should berefueled less frequently. The AGR plants have the capacity to produce between 550MWe to 670MWe. The AGRs use uranium dioxide as the fuel. The uranium dioxide pellets are usually enriched to 2-3.5%. Enrichment subsequently rises up the cost of generating power using AGR. Despite the high thermal energy, AGRs needs to be larger than a PWR in order to produce the same level of output. Also, when the fuel is discharged, the burn-up is lower which means that efficiency of fuel use is low. This counteracts the gains made in terms of high thermal efficiency (Breeze, 2005, p. 77). In order to fuel an AGR, it does not have to be shut down. This contradicts the use of PWRs that have to be shut-down before refueling. Reprocessing fuel from the AGR is difficult as it is filled with graphite. The graphite should be removed from the fuel before reprocessing takes place. Layers of silicon carbide and pyrolytic carbon must be broken up. Unless these steps are taken, fuel cannot be dissolved and reprocessed chemically. The fact that AGRs could be refueled with the reactor on load gives them a relative advantage over the PWRs. PWRs utilize uranium that is enriched to around 4%. The Sizewell B PWR plant operates an 18-month cycle before shutting down to be maintained and refueled. The commercial life of Sizewell B was initially estimated to be 40 years (up to the year 2035). Environmental considerations There are distinct offices mandated to monitor discharge of radioactive effluent and gasses from the AGR stations. Matters to do with the environment are covered in the Radioactive Substances Act of 1960. The office in charge has to monitor and approve the methods used to discharge effluent. Radioactive gases come from the reaction between impurities and additives in the reactor coolant carbon dioxide. The main ones include nitrogen, hydrogen and argon. Sulpur is also released. The AGR have an issue to do with the graphite bricks in the core of their reactors. Pressurized water reactors are environmentally safe. The fact that the primary coolant is separated from the secondary coolant in the PWR eliminates the chances of mixing the two liquids, a process which could be extremely radioactive. The design enables heat transfer to be achieved without mixing of the primary and the secondary coolant. This is in contrast to the boiling water reactors, just lie the as the ones currently in Japan that are prone to water boiling in the reactor due to low pressure (Cacuci, 2010, p. 67). The likelihood of ‘degraded core accidents’ occurring at Sizewell B was termed as improbable after an analysis. This proves that PWRS are environmentally safe. Therefore, the chances of release of nucleotide into the environment thereby causing harmful effects are close to zero. The analysis proved that the occurrence of such an accident was 10-6 in a year. The building containing the reactors would remain intact in the event of a ‘degraded core accident’ thereby no radioactive material would be emitted to the environment. Decommissioning has to be approved by the regulatory body, the Nuclear Decommissioning Authority. An extension maybe granted or rejected depending on the state of the plant (Breeze, 2005, p. 55). 7.0 Conclusion The AGR project proved too complex to establish on site. This was complicated by poor labour relations. The first station, the Dungeness B scheduled to begin to be completed in 1970 was delayed and began producing electricity in 1983. The other stations also experienced similar delays. The money expended in building the stations became too much hence watered down the claims that it would be an economically viable project. Out of the seven nuclear stations, two of them are AGRs. After the AGRs were adopted for commercial purposes, companies that came up with the prototype erroneously underestimated the cost. In the end, the projects did cost more than projected. For instance, it was an estimate that the Hunterston ‘B’ would cost 97 million pounds, but in the end the project 143 million pounds. This was attributed to delays in the project, problems in the implementation of the prototype and unforeseen inflation. Recently, there increased cases of Leukemia around nuclear facilities. This is especially for children under the age of five living within 5 kilometers of the sites. Although the link is bound to be disputed, it is bound to raise questions on safety. If UK is going to pursue production of nuclear energy, then this is bound to be a challenge. Costs consideration and provisions to go green and produce renewable forms of energy seems to be at the forefront of the drive to decommission most of the AGR pants by 2037. Bibliography Breeze, P. (2005). Power Generation Technologies. London : Newnes. Cacuci, D. G. (2010). Handbook of Nuclear Engineering. Verlag: Springer. Hewitt, G. F., & Collier, J. G. (2000). Introduction to Nuclear Power. New York : Taylor & Francis. Nag, P. (2008). Power Plant Engineering 3e. London: Tata McGraw-Hill Education. Read More