|CHARACTERISTICS AND COMMON VULNERABILITIES INFRASTRUCTURE CATEGORY: NUCLEAR POWER PLANTS||Law Enforcement Sensitive – Sensitive Homeland Security Information||10 pages||September 22, 2003||Download (PDF)|
|CHARACTERISTICS AND COMMON VULNERABILITIES INFRASTRUCTURE CATEGORY: NUCLEAR POWER PLANTS – COMMERCIALLY OWNED AND OPERATED||Law Enforcement Sensitive – Sensitive Homeland Security Information||11 pages||February 13, 2004||Download (PDF)|
A nuclear power plant is an arrangement of components used to generate electric power. Nuclear power plants used in the United States (U.S.) are either boiling water reactors (BWRs) or pressurized water reactors (PWRs). Boiling water reactors (Figure 1) use a direct cycle in which water boils in the reactor core to produce steam, which drives a steam turbine. This turbine spins a generator to produce electric power. Pressurized water reactors (Figure 2) use an indirect cycle in which water is heated under high pressure in the reactor core and passes through a secondary heat exchanger to convert water in another loop to steam, which in turn drives the turbine. In the PWR design, radioactive water/steam never contacts the turbine. Except for the reactor itself, there is very little difference between a nuclear power plant and a coal- or oil-fired power plant.
Nuclear Power Plant Common Components
The following seven major components are common to all nuclear power plants:
1. Nuclear reactor core, reactor vessel, and containment structure;
2. Heat transfer/working fluid loop;
3. Cooling water system;
4. Plant control room and reactor control system;
5. Spent fuel storage;
6. Generation transformers; and
7. Transmission lines and downstream substations.
Nuclear Reactor Core and Containment
The most commonly used fuel is uranium enriched in the uranium-235 isotope. Typically, uranium is formed into pellets approximately 0.5 inch in diameter and 1.0 inch long. The pellets are stacked into long rods and assembled into fuel bundles; the bundles are then arranged to form the reactor core inside the reactor vessel. Unirradiated (new) fuel is only slightly radioactive, but after fuel is irradiated in the reactor vessel, it becomes highly radioactive. About 99% of the radioactive isotopes at a nuclear power plant are contained in the fuel rods.
The reactor vessel and the containment structure provide substantial barriers and defense-indepth protection against the release of radioactive fission products to the environment. The reactor vessel is located inside the containment structure, which is designed to withstand earthquakes and tornados. The containment structure is also designed to minimize leakage of radioactive gases or fluids. At most PWRs and at some BWRs, the containment structure is a large, reinforced-concrete building with a steel liner (Figure 3). At most BWRs, the containment structure is a steel shell, surrounded by a reinforced-concrete building with a sheet metal top.
Cooling Water System
Excess heat is removed from the heat transfer/working fluid loop to the environment by the cooling water system. Cooling water systems are comprised of three general types: once-through, natural draft cooling towers, and mechanical draft cooling towers. Once-through systems take a large amount of water directly (or via a very large man-made pond) from a natural water body and pump it through an in-plant heat exchanger to remove excess heat from the working fluid loop(s) before the working fluid is returned to the reactor.
In a natural draft cooling tower, the natural buoyancy of the hot air moves the air upward through the tower, drawing in fresh, cool air through the air inlet at ground level where it cools the working fluid. No fan is required. The tower shell is usually constructed in reinforced concrete, can be as high as 650 ft, and is easily recognizable from the air and large distances on the ground.
Mechanical draft cooling towers use a fan to generate the airflow through the tower. Because fans are used, mechanical cooling towers are much smaller than natural draft cooling towers; however, fan diameters of up to 30 ft are commonly used.
Nuclear power plants are likely the most hardened publicly owned infrastructure in the U.S. Like other critical infrastructures and key assets they vary in many characteristics and practices relevant to specifying vulnerabilities. There is no universal list of vulnerabilities that applies to all assets of a particular type within an infrastructure category. Instead, a list of common vulnerabilities has been prepared, based on experience and observation. These vulnerabilities should be interpreted as possible vulnerabilities and not as applying to each and every individual facility or asset.