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Nuclear Power Plant Design
Through the industry's deliberate, conservative, advance planning for safety, the potential hazards of nuclear reactors have been reduced to an extremely low level of actual risk.1
Nuclear power plants in the U.S. and the West are built with multiple barrier systems and redundancies, a concept called "defense-in-depth," to protect against equipment failure, human error, and severe natural events. In no other commercial venture has safety been considered with such deliberate attention, painstaking detail and broad conservatism.2
A nuclear reactor can be designed with engineering safeguards and materials that virtually eliminate the possibility of any accidental release of radioactive elements.3
Nuclear power plants are designed to withstand damage from jet airliner crashes, tornados and earthquakes. In fact, earthquake standards are several times more stringent for nuclear power plants than for hospitals, apartment buildings, fossil fuel plants and other structures.4
The accident at Three Mile Island Unit 2 in 1979 proved just how well the defense-in-depth system works; although there was extensive damage to the core of the reactor, there were no deaths or injuries to the public or to plant workers. Despite mechanical failures and human errors, the damage at TMI was contained.5
The margins of safety in plant design and the defense-in-depth approach have prevented design or manufacturing deficiencies from creating and undue risk to public health and safety.6
Nuclear power programs are subject to a high standard of quality assurance at every step.7
The safe design of plants is ensured by the licensing process.8
There is nothing in the design of nuclear power plants which involves significant technological unknowns and no technical problems incapable of being solved.9
Unless further steps are taken to reduce substantially the likelihood of a core meltdown accident, we can expect to see such an accident at a U.S. plant within the next 20 years. There are accident sequences for U.S. plants that can lead to rupture or bypassing of the containment which would result in the off-site release of fission products comparable or worse than the releases estimated by the NRC to have taken place during the Chernobyl accident.10
"Defense-in-depth" is a fiction. The depth does not exist because the different layers of protection involved all have known weaknesses, and the backups are highly unreliable.11
Because we are in a relatively early stage of commercial experience with nuclear power, statistics used to support the claim that reactors are safe are of little significance.12
Meteorological and geological events can make all reactor safeguards useless. Natural disasters such as tornados and floods can be fatal to nuclear power plants.13
Nuclear reactors are allowed to operate despite the fact that scores of "generic" safety problems have not been resolved. "Generic" is the term used by the NRC to refer to unresolved problems that affect either all nuclear reactors or large groups of reactors.14
Operating experience has revealed flaws in plant design that were previously unknown, rendering plants unsafe.15
Safe reactors require that each level of industrial workmanship, engineering, inspection and quality control be raised well above conventional standards. However, routing manufacturing and engineering practices prevail while considerations of quality, service, and safety succumb to the industry's priorities of meeting delivery dates and profit margins.16
The industry, with the concurrence of the NRC, has overemphasized a theoretical approach to reactor design analysis, without paying sufficient attention to prototypes, lab or field test verification.17
Safety problems related to reactor design have translated into massive cost escalations for the construction and operation of nuclear reactors.18
Nuclear plants in the United States have become much more complex in the past decade. Efforts to enhance reactor safety account for much of the complexity. New regulations implemented since the early 1970's have affected virtually all aspects of plant design and construction. Systems and components have multiplied; design and construction reviews, along with regulatory documentations, have grown more elaborate. The NRC has applied many new requirements retroactively, making it necessary to redo much completed work.19
All of the conventional nuclear power plants in operation today use steam turbines and generators to produce power. They differ from fossil-fueled plants in using a nuclear reactor to produce the heat to make steam, rather than boil-fired by coal or some other fuel.20 They also differ from fossil-fueled plants by using and generating large quantities of radioactive material.21
There are many varieties of reactor designs (Click here to see more about reactor designs), but most share some common features. All reactors operate on the principle of nuclear fission or atom-splitting. When a neutron encounters the nucleus of uranium-235 or certain other fuels, the fuel atom is split (fissioned) into different elements (fission products) plus neutrons and heat. The heat must be transferred out of the reactor to keep it from overheating, and to produce steam to power the turbine. The newly released neutrons keep a chain reaction going when they encounter other fissionable atoms.22
Control rods contain a material that absorbs neutrons and prevents them from hitting fissionable atoms, enabling the rods to speed up or slow down the chain reaction. Operation of the reactor is controlled by varying the number of rods withdrawn and the amount of their withdrawal.23
The cooling fluid serves a dual purpose. It removes the intense heat generated by the nuclear chain reaction, and it delivers the heat, either directly or indirectly, to the electrical generating part of the plant.24
Reactor safety features are either "intrinsic" or "engineered." Intrinsic safety features are inherent in the physical nature of the reactor design. For example, if an abnormal rise in the chain reaction rate overheats the coolant fluid, the resulting reduction in the coolant's density should cause the chain reaction to stop. Engineered safety features are those added to the basic reactor concept. The emergency shutdown-control-rod system is regarded as an engineered safety feature.25
For a reactor to operate safely, the coolant's capacity to remove the heat must match the heat produced by the fuel. With the fundamental design goal to prevent any damage to the fuel elements, the melting point of the fuel rod or "cladding" is critical. The two keys to the prevention of cladding damage are the ability to shut down the chain reaction rapidly and dependably when required, and cooling systems with enough redundancy to carry away the heat generated in the core.25
To achieve reactor safety, the nuclear industry has adopted a strategy called "defense-in-depth." With this approach, a series of independent barriers are established between the radioactive material in reactors and the environment. Most of these barriers consist of passive, or physical design elements built to prevent the release of radioactivity.26
Other components of the defense-in-depth strategy include:
The overall effect of these measures is a system so complex that it is difficult for the designer to calculate safety factors in a quantitative way. Instead, the designer replaces an overall margin of safety with a "conservative" design that includes a number of independent safety barriers that must be breached or bypassed before a serious accident can occur.28
Although the defense-in-depth strategy has succeeded in preventing a catastrophic nuclear accident in the U.S., it has fundamental weaknesses. The sum of conservative design decisions is not necessarily conservative. Design problems stemming from this approach, such as excess weight or difficult access, can impair overall safety.29
The NRC currently uses an analytical technique called Probabilistic Risk Assessment (PRA) to evaluate plants and plant systems. Despite significant uncertainties in the risk estimate derived from PRA, when PRA is properly applied it can assist in interpreting operating experience, in analyzing data on the reliability of components and plant systems, and in identifying potential contributors to severe accidents. Even so, further research is necessary to improve PRA methodology, especially in the areas of analyzing human factors and external accident initiators, among others.30
Nuclear plants in the United States average many more reactor shutdowns per year than their Japanese and French counterparts. The failure of plant components and systems are significant contributing factors in a number of these shutdowns. In many cases the components and systems that fail (valves, valve operators, pumps, small turbines, control equipment) are not designed specifically for the nuclear industry. They are conventional equipment designed for many different industrial applications.
Component failures can degrade plant protection systems and challenge the capabilities of the operating staff. The relatively large number of significant failures of components and systems illustrates the need for increased component and system reliability. Development work is also needed to determine whether simplifications or other changes in the design of plant systems can be made to increase plant reliability, safety, and economy.31.
1 Atomic Industrial Forum, Inc., pamphlet.
3 Richard Curtis and Elizabeth Hogan with Shel Horowitz, Nuclear Lessons, (Wellingborough: Turnstone Press Ltd., 1980), p. 14.
4 Atomic Industrial Forum, Inc., "Safety and the U.S. Nuclear Industry," Fact Sheet, December 1986.
6 Investigation of Charges Relating to Nuclear Reactor Safety, Testimony before the Joint Committee on Atomic Energy, February 19, 1976, p. 722.
7 Curtis and Hogan, op. cit., pp. 95-96.
8 Technological Concerns in Nuclear Reactor Safety, Part 1, Hearings on Proposition 15: The Nuclear Initiative, Assembly Committee on Resources, Land Use and Energy, Sacramento, CA, October 21, 1975, p. 42.
9 Investigation of Charges Relating to Nuclear Reactor Safety, op. cit., p. 86.
10 Commissioner James K. Asselstine, U.S. Nuclear Regulatory Commission, statement before Subcommittee on Energy Conservation and Power, Committee on Energy and Commerce, May 22, 1986, pp. 2-3.
11 NRC Action to Shut Down Nuclear Power Plants for Safety Inspection, Hearings before the Joint Committee on Atomic Energy and the Committee on Government Operations of the U.S. Senate, 94th Congress, 1st session, February 5, 1975, pp. 80-83.
12 Ibid., p. 74.
13 Curtis and Hogan, op. cit., pp. 75-77.
14 Union of Concerned Scientists, Safety Second: A Critical Evaluation of the NRC's First Decade, (Washington, D.C., February 1985), p. 15.
15 Investigation of Charges Relating to Nuclear Reactor Safety, op. cit., p. 722.
16 Curtis and Hogan, op. cit., pp. 95-96.
17 Investigation of Charges Relating to Nuclear Reactor Safety, op. cit., p. 501.
18 Technological Concerns in Nuclear Reactor Safety,op. cit., p. 501.
19 Richard K. Lester, "Rethinking Nuclear Power," Scientific American, Vol. 254, No. 3, March 1986, p. 33.
20 Pierce, op. cit., p. 212.
21 Atomic Energy Commission, The Safety of Nuclear Power Reactors (LWRs), July 1973, p. 3.
22 Ibid., and Atomic Industrial Forum, Inc., "How Nuclear Plants Work," information pamphlet.
23 Atomic Industrial Forum, Inc., "How Nuclear Plants Work," op. cit.
24 Anthony V. Nero, Jr., A Guidebook to Nuclear Reactors, (Berkeley: University of California Press, 1979), pp. 3-4.
25 Ibid., p. 13.
26 Ibid., p. 52.
27 R.R. Ferber, ed., Nuclear Plant Safety, (IEEE Power Engineering Society, 1971), pp. 61-62.
29 National Academy of Sciences, Committee on Nuclear Safety Research, Revitalizing Nuclear Safety Research, National Research Council, (Washington, D.C.: National Academy Press, December 1986), p. 36.
30 Ibid., pp. 35-36.