Nuclear Fusion / Nuclear Fission

Size: px

Start display at page:

Download "Nuclear Fusion / Nuclear Fission"

Natalie Stanley
6 years ago
Views:

1 Nuclear Fusion / Nuclear Fission Fission and Fusion are two of methods where atomic scale energy can be acquired in the vast sums needed for commercial power generation. It is primarily based on the Binding Energy seen below, both fission and fusion systems can be seen below. Nuclear Fusion When light nuclei undergo fusion, great amounts of energy are released in the sun. Fusion is just that in that light elements are combined to form heavier elements. Due to the nature of the binding energy of the nucleons, combining two light atoms such as deuterium and tritium result in the release of a large amount of energy (about 14 MeV). This energy is typically in the form of kinetic energy in the new nucleus and a large number of high-energy neutrons.

2 The proverbial phrase associated with this limitless power source is that it is twenty years from being a commercially viable power generating resource. In sharp contrast with fission products, products from fusion are typically not highly radioactive. In the perfect world we could harness this endless supply of energy and be free of our bonds to fossil fuels. But the world is not perfect, and even when fusion power is commercially viable, there will be highly radioactive materials produced from the neutrons associated with this process. The First Wall of the system will be exposed to high fluences of energetic neutrons, that will damage the materials at the atomic level. The damage to the materials will lead to the degradation in the strength and capability of the materials to confine the plasma (the temperature in a fusion plasma is on the order of our sun). The materials used in the construction of the first wall will have similar characteristics to that of the highly radioactive spent nuclear fuel from fission reactors. So once again we learn that there is no free lunch. Just as we cannot

3 create or destroy matter, there is not easy way to make energy without producing any sort of pollution. Nuclear Fission Nuclear fission power is a here and now technology. The first fission reactor was the Fermi-Pile at the university of Chicago. Enrico Fermi was the architect of this reactor on December 2, 1942, less than one year after Pearl Harbor was attacked. This is where the Acronym Safety Control Rod Axe Man or SCRAM was first used. This is the person who was to shut the reactor down if anything went wrong with the experiment. The man was standing on top of the pile with an axe to cut a rope that would allow a neutron absorbing material to drop into the pile and absorb neutrons and shut down the pile. The reaction that was unleashed in the pile is depicted below, a neutron hits a target nucleus of U-235 and produces neutrons and fission products. Fission products are typically highly radioactive and remain so for hundreds of years after the fuel has been removed from the reactor. This is where questions arise regarding the production of a material that will remain highly-radioactive for thousands of years. Risk Benefit can be applied in this case as one can use items such as the risk of global climate change or the political instabilities of the

4 Middle East. The benefit being not impacting our environment or not having to get involved in the messy politics of the region. This said, from this simple pile of uranium, we have a wide range of technologies that are used to produce electric power from a fuel that is domestically abundant, and does not pollute the air we breath or water we drink. The primary classes of reactors are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Just as indicated in the names, the different types of reactors operate on different schemes. The PWR s use the heat from fission to raise the enthalpy of the water that is then used to heat up a secondary cooling loop. Part of this loop is called a steam generator, because the high temperature primary reactor flow is used to produce steam for a turbine. The steam is allowed to expand inside the turbine and is later condensed using the massive cooling towers typically associated with nuclear power plants. The signature cooling towers of a nuclear are merely completing the thermodynamic cycle necessary to efficiently produce power in this case. If cooling water is not a problem, as in the case of Seabrook Station, ocean cooling water is used to cool the steam and allow it to continue its cycle through the secondary system. In the case, the reactor is always running without any boiling of water within the reactor core. The Boiling Water Reactor or BWR has some significant differences to the scheme of the PWR, in that water is boiled directly in the core

5 of the reactor. The product of the boiling, steam is then removed from the core and sent to a turbine where power can be generated. This class of system utilizes a similar mechanism to generate power and to cool the steam back into water at the completion of the thermodynamic cycle. During operation, and even after shutdown, a large, 1,000-megawatt (MW) power reactor contains billions of curies of radioactivity. Radiation emitted from the reactor during operation and from the fission products after shutdown is absorbed in thick concrete shields around the reactor and primary coolant system. Other safety features include emergency core cooling systems to prevent core overheating in the event of malfunction of the main coolant systems and, in most countries, a large steel and concrete containment building to retain any radioactive elements that might escape in the event of a leak. Although more than 100 nuclear power plants were operating or being built in the United States at the beginning of the 1980s, in the aftermath of the Three Mile Island accident in Pennsylvania in 1979

6 safety concerns and economic factors combined to block any additional growth in nuclear power. No orders for nuclear plants have been placed in the United States since 1978, and some plants that have been completed have not been allowed to operate. In 1996 about 22 percent of the electric power generated in the United States came from nuclear power plants. In contrast, in France almost threequarters of the electricity generated was from nuclear power plants. In the initial period of nuclear power development in the early 1950s, enriched uranium was available only in the United States and the Union of Soviet Socialist Republics (USSR). The nuclear power programs in Canada, France, and the United Kingdom therefore centered about natural uranium reactors, in which ordinary water cannot be used as the moderator because it absorbs too many neutrons. This limitation led Canadian engineers to develop a reactor cooled and moderated by deuterium oxide (D 2 O), or heavy water. The Canadian deuterium-uranium reactor known as CANDU has operated satisfactorily in Canada, and similar plants have been built in India, Argentina, and elsewhere. In the United Kingdom and France the first full-scale power reactors were fueled with natural uranium metal, were graphite-moderated, and were cooled with carbon dioxide gas under pressure. These initial designs have been superseded in the United Kingdom by a system that uses enriched uranium fuel. In France the initial reactor type chosen was dropped in favor of the PWR of U.S. design when enriched uranium became available from French isotope-enrichment plants. Russia and the other successor states of the USSR had a large nuclear power program, using both graphite-moderated and PWR systems. The number of radioisotopes released to the environment from fossilfuel power plants is greater than that released from nuclear power plants. (Chemistry in the Community 4th Ed., American Chemical Society, p. 428) What does number mean? The average annual dose to an individual living within 50 miles of a nuclear power plant is.009 mrem/yr from routine releases.

7 The average annual dose to an individual living within 50 miles of a coal-fired plant is.03 mrem/yr from routine releases. (Chemistry in the Community 4th Ed., American Chemical Society, p. 430) So if given the choice where would you rather live? Three Mile Island 2 Nuclear Power Plant Summary of The accident began about 4:00 a.m. on March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, caused by either a mechanical or electrical failure, which prevented the steam generators from removing heat. First the turbine, then the reactor automatically shut down. Immediately, the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve (a valve located at the top of the pressurizer) opened.

The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open.

8 The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open. As a result, cooling water poured out of the stuck-open valve and caused the core of the reactor to overheat. Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes which hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt. It was later found that about one-half of the core melted during the early stages of the accident. Although the TMI-2 plant suffered a severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the worst-case consequences that reactor experts had long feared. In a

9 worst-case accident, the melting of nuclear fuel would lead to a breach of the walls of the containment building and release massive quantities of radiation to the environment. But this did not occur as a result of the Three Mile Island accident. The accident caught federal and state authorities off-guard. They were concerned about the small releases of radioactive gases that were measured off-site by the late morning of March 28 and even more concerned about the potential threat that the reactor posed to the surrounding population. They did not know that the core had melted, but they immediately took steps to try to gain control of the reactor and ensure adequate cooling to the core. The NRC_s regional office in King of Prussia, Pennsylvania, was notified at 7:45 a.m. on March 28. By 8:00, NRC Headquarters in Washington, D.C. was alerted and the NRC Operations Center in Bethesda, Maryland, was activated. The regional office promptly dispatched the first team of inspectors to the site and other agencies, such as the Department of Energy and the Environmental Protection Agency, also mobilized their response teams. Helicopters hired by TMI's owner, General Public Utilities Nuclear, and the Department of Energy were sampling radioactivity in the atmosphere above the plant by midday. (1 of 6) By the evening of March 28, the core appeared to be adequately cooled and the reactor appeared to be stable. But new concerns arose by the morning of Friday, March 30. A significant release of radiation from the plants auxiliary building, performed to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core, caused a great deal of confusion and consternation. In an atmosphere of growing uncertainty about the condition of the plant, the governor of Pennsylvania, Richard L. Thornburgh, consulted with the NRC about evacuating the population near the plant. Eventually, he and NRC Chairman Joseph Hendrie agreed that it would be prudent for those members of society most vulnerable to radiation to evacuate the area. Thornburgh announced that he was advising pregnant women and pre-school-age children within a 5-mile radius of the plant to leave the area.

10 Within a short time, the presence of a large hydrogen bubble in the dome of the pressure vessel, the container that holds the reactor core, stirred new worries. The concern was that the hydrogen bubble might burn or even explode and rupture the pressure vessel. In that event, the core would fall into the containment building and perhaps cause a breach of containment. The hydrogen bubble was a source of intense scrutiny and great anxiety, both among government authorities and the population, throughout the day on Saturday, March 31. The crisis ended when experts determined on Sunday, April 1, that the bubble could not burn or explode because of the absence of oxygen in the pressure vessel. Further, by that time, the utility had succeeded in greatly reducing the size of the bubble. Health Effects Detailed studies of the radiological consequences of the accident have been conducted by the NRC, the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the State of Pennsylvania. Several independent studies have also been conducted. Estimates are that the average dose to about 2 million people in the area was only about 1 millirem. To put this into context, exposure from a full set of chest x-rays is about 6 millirem. Compared to the natural radioactive background dose of about millirem per year for the area, the collective dose to the community from the accident was very small. The maximum dose to a person at the site boundary would have been less than 100 millirem. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various groups monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. However, comprehensive investigations and assessments by several well-respected organizations have concluded that in spite of serious damage to the reactor, most of the radiation was contained and that the actual release had negligible effects on the physical health of individuals or the environment.

11 Statistically speaking, it is likely that one person will dies as a result of the accident at TMI. Summary The bottom line on TMI is that the proven safety systems were capable of securing and containing the vast majority of the radiation from the accident. The containment systems that are required by law in the U.S. served their intended function. These structures can withstand a direct hit from a fully laded aircraft, in fact, Seabrook Station in Seabrook NH had a second 18 inch thick containment added to its structure to ensure that a plane from Pease Air Force base did not cause any damage to the facility. This accident actually demonstrated the effectiveness of our engineering. In later lecture notes we will contrast the systems used and the impact that they had under accident conditions.

BACKGROUNDER Office of Public Affairs Phone:

BACKGROUNDER Office of Public Affairs Phone: 301-415-8200 Email: opa.resource@nrc.gov Three Mile Island Accident The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near Middletown,