Airborne Safety of Nuclear Reactors in America
- By Duncan Williams -
The events that unfolded on September 11, 2001, caused America to reassess the threat of an airborne attack on our infrastructure, including our nuclear power plants. After September 11, the Nuclear Regulatory Commission (NRC) began a comprehensive security and engineering study as to the risks associated with an airborne attack on nuclear power plants.
This study took into account reactor plants, dry storage containers holding spent nuclear fuel, as well as containment buildings housing spent nuclear fuel in pools of water. The NRC has since concluded that the increased security measures taken by the airlines industry has significantly reduced the threat posed by the impact of a large commercial aircraft with a nuclear power plant.
Nonetheless, the NRC last year adopted regulations requiring all existing and future nuclear power plants to initiate strategies that would mitigate the effects of large fires and explosions from any cause, including aircraft impacts.
Even before the new NRC regulations went into effect, the nuclear industry has been including structures in reactor designs that protect the reactor from airborne collisions. For example, U.S. Patent No. 4,518,561, issued on May 21, 1985, describes a protective structure surrounding the reactor containment building that serves to protect the reactor from an external missile impact as well as provide stability in the event of an earthquake.
As can be seen in the diagram from the patent, the reactor vessel (16) is contained within a concrete or steel vessel well (14), which is itself supported by structures called casemates (18).
All these supporting structures are encased inside a confinement enclosure (4), made from reinforced or pre-stressed concrete. Just inside the confinement enclosure (4) is a steel cylindrical skirt (6) which is sealed by a containment dome (8). The primary function of the confinement enclosure (4) is to prevent the leak of radioactive substances into the environment in the event of an accident.
After constructing the confinement enclosure (4), a ring building (20) made from reinforced concrete is then erected around the confinement enclosure (4). The ring building (20) includes a cylindrical skirt (22) and a roof (24) which is connected to the cylindrical skirt (6) embedded in the confinement enclosure (4). The patent indicates that the ring building (20) can provide protection from incoming projectiles having the mass and speed of a military aircraft.
The structure also distributes the weight over a larger surface area on the ground preventing the development of complex and destructive vibrations that could occur during earthquakes. Internal structures (29) are located inside the ring building (20) and are intentionally disengaged from the cylindrical skirt (22). This ensures that if a projectile strikes the ring building (20), forces will not be transmitted from the cylindrical skirt (22) to the internal structures (29), thus providing protection from secondary impacts.
A more recent patent, U.S. Patent No. 7,578,103, issued on August 25, 2009, describes an interesting energy absorbing structure that is allegedly capable of withstanding high impact loads resulting from missiles, aircraft strikes, as well as other forces.
In this design, the main structure (1) of the reactor containment building is spaced apart from an outer shield having an moveable upper part (3a) by a crushable filling layer (2). The outer shield also includes a lower fixed part (3b) which is connected to the moveable upper part (3a) by way of a plate (4).
The plate (4) has anchors extending horizontally upwards into the moveable upper shield (3a), and anchors extending horizontally downwards into the fixed lower shield (3b). When a projectile collides with the outer upper moveable shield (3a), the upper moveable shield (3a) moves towards the crushable filling layer (2), thereby absorbing most of the kinetic energy of the impact. The anchors serve to maintain the overall structural integrity of the building. The patent explains that the crushable layer (2) can be made from stabilized aluminum foam (SAF), which is highly heat resistant and fire resistant.
SAF is an inexpensive composite that is manufactured by first adding ceramic particles to molten aluminum. Bubbles are then introduced into the mixture and it is allowed to cool, forming a closed cell composite metal. SAF is lightweight and maintains constant physical properties over a wide range of temperatures and pressures.
Aside from buildings that contain the actual nuclear reactor, most reactor designs also include what is known as a missile shield. These thick structures are placed above the entire reactor and function primarily to keep control rods from being ejected from the reactor vessel. Control rods are made from neutron absorbing material and are used to control the power level of the reactor. In a shutdown reactor, the control rods are fully inserted into the reactor vessel so that most of the neutrons that are constantly being emitted from the nuclear fuel are absorbed by the control rods. In order to startup the reactor, the control rods are withdrawn, allowing the neutrons to be absorbed by the nuclear fuel instead of the control rods.
This causes the nuclear fuel to undergo a fission reaction which releases even more neutrons, eventually resulting in a sustained chain reaction. In order to shutdown the reactor, the control rods are again inserted into the reactor vessel, causing the neutrons to be absorbed by the control rods instead of the nuclear fuel, halting the fission chain reaction.
Control rods are controlled by an electronic system located above the reactor vessel called the control rod drive mechanisms (CRDMs). Placing the CRDMs above the vessel ensures that in the event of a loss of electricity gravity will cause the control rods to fall downward, immediately shutting the reactor down. In the event of a reactor accident, the temperature and pressure in the reactor vessel could increase to a level that causes a control rod to be ejected from the reactor at a high velocity. To prevent this, missile shields are located above the CRDMs in order to prevent a breach in the containment building of the reactor.
Although the missile shield was designed to keep projectiles from being ejected from the reactor, it can also serve to prevent external projectiles from penetrating the reactor vessel. U.S. Patent No. 6,061,415, issued on May 9, 2000, describes a missile shield design owned by Westinghouse Electric. As can be seen in the diagram taken from the patent, the main component of the missile shield is a polyhedron-shaped plate (164) made from at least 2 inches of steel.
The steel plate (164) is supported by an inner beam support (168) as well as an outer beam support (170) attached to one another by interconnected beam supports (172). Additionally, chord supports (180), in conjunction with cross supports (182), provide further stability for the steel plate (164). The steel plate (164) itself includes ventilation holes (162) in order to allow for proper cooling of the CRDMs located beneath it.
As seen in the diagram showing the top of the reactor vessel, the missile shield (26) is located above the CRDMs (22) which are attached to the control rods (not shown in the diagram).
Since the CRDMs contain numerous electrical components, it is essential that enough cooling air is provided to them for proper operation. Fans (30) located at the top of the structure causes air to flow downwards in the direction of the arrows, and through the holes (60) in the missile shield (26). After cooling the CRDMs (22), air exits the structure via air ports (96).
Another missile shield design is described in U.S. Patent No. 6,546,066, issued on April 8, 2003, and currently assigned to Advent Engineering Services, Inc. Similar to the missile shield described in the Westinghouse patents, the missile shield design owned by Advent Engineering also sits above the CRDMs.
This missile shield (400) has extending tab portions (430) for attachment to the top of the reactor vessel. The shield (400) also includes left and right portions (420) that are shaped to allow for cooling air ventilation ducts (not shown in the diagram).
Designers have also incorporated protective shielding into storage casks that contain spent nuclear fuel which has been removed from a reactor. U.S. Patent Publication No. 20090198092, published on August 6, 2009, and currently assigned to Holtec International, Inc., describes a cask for storing spent nuclear fuel that incorporates a missile shield.
A diagram from the patent application shows a sleeve-like missile shield (200) that fits around a storage cask (100) containing spent nuclear fuel. The shield is made of at least 5 inches of steel or lead, and also prevents gamma rays from radiating out of the cask (100). A ring-like top plate (210), having a central opening, sits on top of the shield (200). This design also allows for a space between the shield (200) and the cask (100) so that air can circulate over the inner cask (100) in order to remove the decay heat from the spent fuel.
Conclusion
Despite the safeguards already incorporated into nuclear plants, at least one company has been told by the NRC that it must resubmit design documents in order to be in compliance with the new regulations issued last year. The NRC has informed Westinghouse that it must demonstrate that the containment building housing its AP1000 reactor design complies with the new regulations. However, documents submitted to and issued from the NRC regarding this issue suggest that the NRC is concerned about a passive cooling water tank stored near the top of the containment building and the tank’s susceptibility to earthquakes (not airborne projectiles). It is very likely that this hurdle will be overcome, as the passive cooling tank was intended to be used in emergencies and not during normal operations. Representatives from Westinghouse have confirmed that this issue will not delay the design certification process with the NRC. However, the NRC’s willingness to raise this issue publicly with Westinghouse should serve as a warning to other companies seeking design certification that the NRC is taking these new rules seriously.
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About Duncan WilliamsDuncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics. Upon graduation, he was commissioned in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program. He then served onboard an 
aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer. Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems. This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers. He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.
Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets. While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School. After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP. While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies. He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights. Duncan is currently an intellectual property associate at Blank Rome LLP.
If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> duncan@nuclearstreet.com