Nuclear Reactor Power Plant
Control Systems

Mechatronics
Professor
13 October 2013
Table of Contents
Heading Page #
Introduction

History

Safety

Control Systems

Coolants

Pressurizer

Moderator

Control Rods

Regulations

Disposal

Conclusion

Works Cited

Introduction
Control systems are an integral part of the nuclear reactor and without the implementation of an effective control system along with constant monitoring and maintenance catastrophic accidents will occur. This report will introduce the important control systems found in many of the most common nuclear reactors along with an explanation on how they work and how they're implemented. First we must go back into the histories to gain a better understanding of why these control systems are so important and consequences that resulted when control systems are not implements or not used properly.

History To know the history of nuclear power plants, one must first understand what a nuclear power plant is. A nuclear power plant is very similar to that of any other steam-electric power plant, in that water is heated and the steam from the water turns turbines, thus creating electricity. The major difference is how the power plant generates heat. The source of the heat from nuclear power plants comes from nuclear fission, rather than from coal, oil or gas. In 1934, a physicist by the name of Enrico Fermi conducted the first experiments that resulted in the splitting of atoms. (“The history of nuclear energy,”pg4) He then proceeded to contact a few other scientist he knew to confirm his results before publishing, among these was Lise Meitner in Copenhagen, who was the one that decided to use Einstein’s theory of relativity to prove the results. Not only did she prove that the fission actually did occur, but was one of the first to prove Einstein’s theory true. (“The history of nuclear energy,”pg5) In 1942, a group of scientists, led by Fermi, gathered at the University of Chicago and began construction on the first nuclear reactor known as Chicago Pile-1 on the floor of the universities squash court. On December 2, 1942, the group, for the first time, created the first self-containing nuclear reaction at exactly 3:25 pm, Chicago time. (“The history of nuclear energy,”pg7) Since this discovery happened during WWII, it was natural that this discovery lead to the creation of The Manhattan Project, but in 1946 Congress created the Atomic Energy Commission (AEC) in the attempt to encourage development of nuclear energy for peacetime use. (“The history of nuclear energy,”pg8) Because of this, the first reactor generated electricity on December 20, 1951.
On March 28, 1979 a nuclear power plant at Three Mile Island, Pennsylvania, experience a cooling malfunction that caused a partial meltdown in the #2 reactor. Apparently, after the #2 reactor was shut down, the reactors fuel core became uncovered, more than a third of the fuel melted. Furthermore, inadequate instrumentation and training, that resulted in the operator's’ inability to respond to the accident in a timely manner. Because of this, a small amount of radiation was released, but it was later confirmed that the amount of radiation released did not pose any serious health or environmental hazards. It appears that the containment building, that reactor #2 was in, had done its job. Although a third of the fuel was melted, the building maintained its integrity and adequately contained the damaged fuel. From 1980 to 1993 extensive studies were done and no adverse side effects were ever found as a result of the accident. (Walker, 2004)
On April 26, 1986, the nuclear power plant in Chernobyl, Ukraine, USSR, experienced a sudden power surge during a reactor test that resulted in the destruction of Unit 4, followed by a fire that ended up releasing massive amounts of radioactive material. The fire was so extensive that emergency crews, by helicopter, dropped huge amount of sand and boron onto the reactors debris. The sand was to stop the fire, while the boron was to prevent any further nuclear reactions. A few weeks later, crews were brought in to completely cover the damaged area with concrete in order to stop any further radioactive contamination. In addition to this, one square mile of forest around the plant was cut down and buried, to further reduce any residual contamination. The Soviets had continued to operate the three remaining reactors for a while, until the last one was finally shut down for good in 1999. After the accident, the Soviet government closed off approximately an 18 square mile area around the plant and initially evacuated 115,000 people, with another 220,000 people in the years that followed. ("Backgrounder on Chernobyl Nuclear Power Plant Accident ")
When the Chernobyl accident ended, over 200,000 workers were exposed to radiation levels between 1 to 100 rem, with the average annual U.S. dose being only .6 rem. Extensive studies of radiation exposure has been conducted since the accident and to date, the only significant effects directly related to the exposure was an increase of thyroid cancer, which has directly been linked to the residence of the surrounding area drinking contaminated milk that contained high doses of radioactive levels of iodine. Fortunately 99% of the children affected were successfully treated. (Fairlie, PHD, 2006)
On March 11, 2011, a magnitude 9.0 earthquake hit northeastern Japan, which then triggered a tsunami with waves of at least 14-meters high, (Tateno) which then started a chain reaction of events that caused a catastrophic meltdown of reactors #1 and #3 at the Fukushima Daiichi Nuclear Power Plant. When the earthquake first hit, all of the emergency procedures operated correctly, that is, the control rods were successfully inserted into the cores, which stopped the nuclear reaction. Even after the plant lost power, the backup generators came online as intended, but, the problem started when the 14 meter tsunami wave struck an hour after the quake. The wave destroyed the outdoor fuel tank that supplied the emergency generator and subsequently flooded the generators located in the basement of the turbine building, which then interrupted power to the plant once again. This loss of power caused the circulation pumps to stop, which in turn resulted in a failure to cool the cores. The temperature of the fuel rods inside of the cores continued to rise, resulting in the melting of the rods themselves. This, in turn, resulted in large amounts of hydrogen being generated, which then began to pool near the ceiling of the reactor building and eventually caused a massive explosion. This explosion is what finally lead to the leaking of both hydrogen and radioactive material into the environment. Approximately 520 tons of “extremely highly radioactive” (Tateno) material has been dumped into the ocean, not to mention the large amounts of iodine 131 and cesium 137 that were released into the atmosphere. It is too soon to tell what the long term effects of this accident will have on the environment, or the residents and it is estimated that it may take up to ten years to fully shut down and clean up the plant. (Tateno)

Control Systems
Nuclear reactor power plants have the potential to be very dangerous and the effects of what could go wrong will have a major impact on the surrounding region and even globally. It is important for every system within the reactor to be closely monitored and controlled at all times to not only optimize the efficiency of the reactor but to ensure the safety of the equipment, workers and the people. This section will discuss the purpose, design and use of important control systems within the nuclear reactor.
Coolants - Nuclear reactors use a variety of coolant types to which all serve the same purpose of transferring the heat created by the reactor core to generators for conversion into electrical energy. The amount of cooling required by electrical generation plants, both nuclear and fossil fuel, can be determined by their thermal efficiency. As stated by the World Nuclear Association (2013) Nuclear power plants often have lower efficiency ratings than their counterparts but have greater flexibility in deciding their site locations which can be determined by their cooling requirements. Various coolants include light and heavy water, molten metal, molten salt, gas and hydrocarbons.
Light Water Coolant: Light water simply put is ordinary water, but is considered light water when compared to heavy water which is known as deuterium oxide and from this point on we will refer to light water as just water. Water is used as a coolant in Light Water Reactors (LWRs) to not only control the heat created by the reactor core, but it also serves as the primary moderator for the nuclear reaction. LWRs can be broken down into two main categories: Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). PWRs have water in their primary cooling/heat transfer circuit that generates steam in a secondary circuit. (World Nuclear Association, 2013). In other words, PWRs use the water in a closed loop system to transfer the heat from the core to a secondary closed loop system also made up of water. The water in the secondary loop is permitted to boil and the steam created is used to rotate turbines for generating electricity. The water in the primary closed loop is maintained at a steady 600 degrees Fahrenheit and is kept from boiling by a water pressurization system. BWR’s on the other hand, allow the water to boil when it comes in contact with the reactor and just as the steam in the secondary loop of the PWR, it rotates turbines for generating electricity.
Heavy Water Coolant: Heavy water is used in Pressurized Heavy-Water Reactors (PHWRs) and works in the same way that water is used in PWRs except that instead of ordinary water, Deuterium Oxide is used which is a much heavier element due to the addition of a neutron in the hydrogen atom. Heavy water is also used as the moderator for the reactor core just as water is used in the PWR.
Pressurizer - Another important control system within the PWR and PHWR is the pressurizer which is used to maintain a steady pressure of 155 bar (AREVA, 2004) to prevent the primary coolant (water) from boiling to maintain a liquid state. This allows the water to be pumped through the reactor core and through piping for the transfer of it heat energy to the secondary coolant so that it may boil. The pressurizer also has a safety feature in its overpressure release valve that is intended to relieve pressure if it gets too high.
Moderator - The moderator in any thermal nuclear reactor serves the purpose of controlling the reactivity of the core by controlling the chain reaction caused by the fast fission neutrons. It does this by slowing down the neutrons without absorbing them. (Muller, 2001) The moderator comes in many forms, some of which have been mentioned earlier such as light and heavy water. Another common moderator is graphite and is sometimes built onto the tips of control rods. No matter the form a moderator comes in, it is used for the same purpose of controlling the chain reaction of neutrons moving throughout the core of the reactor. In the case of water, the moderation is controlled by the rate the water is flowing through the system in a PWR or by how much the water is allowed to boil in the BWR. For graphite moderation, the carbon used in the reactor reflects the neutrons back to the core, preventing them from escaping or introduced to the core to increase the reactivity.
Control Rods - Control rods are vital to nuclear reactors due to their ability to control the reactivity of the core as well as an important safety feature if the core should overheat. Control rods are usually constructed of silver, indium or cadmium and are used to control the fission within the nuclear reactor. With the absence of the control rods, fission is optimal and neutrons are able to freely move within the core increasing the chain reaction and further increasing the reactivity. With the introduction of the control rods, the neutrons are absorbed and the chain reaction slows. This will decrease the reactivity of the core and reduce the power output of the reactor. In emergency situations the control rods will fully inserted into the core, virtually halting the fission process and shutting down the core.

Safety From the beginning, there has been a strong awareness of the potential hazards of running a nuclear power plant. Since the potential of radioactive leakage has always been a major concern, the safety of these power plants had always begun with their designs. As three major nuclear reactor accidents have plagued the civil nuclear power industry; Three Mile Island, Chernobyl, and Fukushima over the years, it has been proven that even with the best precautions built into the design of these buildings, it’s still very difficult to ensure the safety of all. Although, over the course of 7 decades and over 30 countries, there still have been only three major nuclear reactor accidents that can be spoken of, so perhaps the designs are pretty sound after all. When it comes to safety, there are some key aspects that are addressed in achieving optimum nuclear safety, these are:

· The design and construction of the facility.
· Equipment that is designed to prevent human failures.
· Comprehensive monitoring and testing to detect possible failures.
· Redundant systems to control damage to the fuel and prevent radioactive release.
· Comprehensive plans to confine the effects of any possible fuel damage or release.

“These can be summed up as Prevention, Monitoring, and Action.” (World Nuclear Association, 2013)

The prevention portion of this starts with the physical barriers that are designed to accommodate human error. First, the fuel is put into the form of ceramic pellets, where the radioactive fission occurs, and then these pellets are packed inside sealed zirconium alloy tubes that form the fuel rods. These rods are then “confined inside large steel pressure vessels with walls up to 30 cm thick.” (World Nuclear Association, 2013) This, of course, is then wrapped inside a “reinforced concrete structure with walls that are at least one meter thick.” (World Nuclear Association, 2013) These barriers are all monitored continuously for any differences in the amount of radioactivity in the cooling water, any leaks in the cooling system, as well as any leakage of air into the atmosphere. The three basic functions used to ensure safety within a nuclear power plant is the ability to control reactivity, to be able to cool the fuel, and the ability to contain any radioactive substances. So, in summary, the safety of the power plant is achieved by using the control rods, while inserted, to absorb neutrons and regulate the fission while utilizing a back-up emergency core system to remove excess heat, and then finally the containment of any radioactive materials. The safety features within a nuclear power plant are so well designed and redundant that during the Cold War, neither the U.S. nor Russia had targeted any of each other's nuclear power plants because the damage caused by them would be minimal. (World Nuclear Association, 2013)

Disposal
Spent nuclear fuel is a by-product of nuclear power plants and requires a control process to safely handle, transport and store the highly volatile and radioactive material. Failure to control this process can result in a release of deadly radioactivity to the atmosphere harming the environment and the surrounding community. Regulations have been developed by the Nuclear Regulatory Commission (NRC) and the International Atomic Energy Agency (IAEA) to ensure that the safety of the public is not compromised and the environment is protected from contamination. These regulations help to make certain the continued availability of clean energy to the public is without risk or as close to it as possible.
Although the discussion here focuses on spent fuel from Nuclear reactors, when conversing on the subject all types must at least be mentioned. Spent nuclear fuel is often referred to as waste and can be categorized in two basic categories; High Level Waste (HLW) and Low Level Waste (LLW). Waste, which fall into these groups, are produced from both medicine and industry. Bodansky (2004) tells us that HLW is the most highly radioactive and plentiful as it is produced by nuclear power plant reactors and reactors that produce plutonium for nuclear weapons (P. 231). The radioactivity of the waste produced through electric generation plants is much greater than that of military applications however; the volume is less due to the waste remaining in solid form (Bodansky, 2004, P. 232).
Approximately every 12 to 18 months the spent rods of a nuclear power plant are removed from the reactor core where they are stored in spent fuel pools. According to Storage (2013) The NRC regulates spent fuel and the storage pools through a combination of regulatory requirements, licensing and safety oversight, including inspection. The rods are moved into the pool from the reactor along the bottom of water canals and are kept under at least 20 feet of water to provide adequate shielding from radiation for any workers. To increase storage capacity the NRC allows the spent rods to be re-racked after a period of time using neutron absorbing panels between the rods (Storage, 2013).
The storage pools are constructed of reinforced concrete with steel liners and are usually about 40 feet deep. The water in the pool serves to both cool the rods and to shield workers from radiation exposure. Several control processes are required by the NRC to mitigate damage to the spent fuel pools from fire, explosion or other accidents such as natural disasters and possible terrorist activity. These mitigating measures include: 1. Controlling the configuration of fuel assemblies in the pool to enhance the ability to keep the fuel cool and recover from damage to the pool. 2. Establishing emergency spent fuel cooling capability. 3. Staging emergency response equipment nearby so it can be deployed quickly.

The rods are typically cooled in the pools for at least five years where out of necessity they can be moved to dry storage in containment packages. The NRC has approved the transfer of spent rods as soon as three years but the industry standard is ten years (Storage, 2013).
Transportation of new and spent nuclear fuel begins with containment packages that maintain a high level of long term effectiveness under extreme mechanical forces and thermo mechanical conditions. Packages for spent nuclear fuel transportation are required to meet three specifications (Committee, 2006, P. 56).

1. Prevent an unsafe configuration of the spent nuclear fuel that could lead to establishing a nuclear reaction.
2. Prevent the release of radioactive material.
3. Limit radioactive dose rates on the packages external surfaces to an acceptable level.

Although there are several types of transportation containment packages, depending on the type of transportation used e.g. truck or rail, the packages use a series of fail-safe’s to negotiate hazardous conditions and maintain their structural integrity. Containment packages are leak tested, filled with inert gasses, use impact limiters, contain borated metals as neutron absorbers and are monitored for changes in internal pressure (Committee, 2006, P. 58).
Once the spent nuclear fuel has been transported, the storage or possible disposal process begins. Bodansky (2004) states that “spent fuel is commonly referred to as waste, although some of the radionuclides have potentially useful applications” (p. 232). Reprocessing of spent nuclear fuel is possible and can recover fissionable plutonium but is not cost effective due to the availability of inexpensive uranium. Storage at facilities such as the Yucca mountain nuclear waste repository are the desired method of dealing with spent nuclear fuel in the United States but In reality the waste remains at the reactor facilities Bodansky (2004). It can take from hundreds of years for low level waste to thousands of years for high level waste to become inert.

Conclusion
Control systems within the reactor serve important roles in the safety and efficiency of the power plant. Pressurization systems maintain a constant pressure which may be used to prevent the water from boiling when it isn’t supposed to. Control rods work hand in hand with the coolant system to act as moderators as well as control the reactivity of the core while the coolant system plays an additional role of preventing the core from overheating and transferring the heat for use. All in all, Control systems are important to any electronic or mechanical system and as made prevalent in this report, are especially important when it comes to the nuclear reactor. Control systems are found throughout the entire plant and it is critical that these systems are implemented properly, used correctly and not ignored when they raise an alert.

Works Cited

AREVA. (2004). Pressurized Water Reactor. [Video file]. Retrieved from http://www.youtube.com/watch?v=MSFgmLW1Crw.
Bodansky, D. (2004). Nuclear Energy : Principles, Practices, and Prospects. New York: Springer.
Committee on Transportation of Radioactive Waste National Research Council. (2006) Going the Distance? : The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Retrieved from http://site.ebrary.com/lib/devry/docDetail.action?docID=10132069. Fairlie, PHD, I. (2006). The Other Report On Chernobyl (TORCH). The Greens/EFA in the European Parliament, Retrieved from http://www.chernobylreport.org/U.S, Department of Energy, Office of Nuclear Energy, Science and Technology. (n.d.). The history of nuclear energy (DOE/NE-0088). Retrieved from U.S, Department of Energy website: http://energy.gov/sites/prod/files/The History of Nuclear Energy_0.pdf.
Muller, Richard A. (2001). Nuclear Reactors, the China Syndrome, and Waste Storage. Retreived from http://muller.lbl.gov/teaching/physics10/old%20physics%2010/chapters%20(old)/8-ChinaSyndrome.html.
Storage of Spent Nuclear Fuel. (2013) Retrieved from http://www.nrc.gov/waste/spent-fuel-storage.html.
Tateno, Jun. "What Happened at the Fukushima Daiichi Nuclear Power Plant?" Yomiuri.co. The Japan News, 23 May 2011. Web. 5 Oct 2013. <http://www.yomiuri.co.jp/adv/chuo/dy/opinion/20110523.htm>.
Walker, J. S. (2004). Three mile island : A Nuclear Crisis in Historical Perspective. Los Angeles: The University of California press.
World Nuclear Association. (2013, August). Safety of Nuclear Power Reactors. Retrieved from http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Safety-of-Nuclear-Power-Reactors/.