Patent Application
20140161218
1. Field of the Invention
Example embodiments relate generally to nuclear Light Water Reactors (LWRs), and more particularly to a system and a method of capturing radioactive material from primary containment during a severe accident. The system may be passive, such that the system may be operated for extended period without the need for monitoring or the application of external electrical power. This system may be considered a filtered venting system or a “filtered vent.”
2. Related Art
During a nuclear power accident, nuclear light water reactors (LWR) predominantly rely on a primary containment structure 3 (see
Example embodiments provide a system and a method for a passive capture of radioactive materials that may be applied to commercial nuclear light water reactors (LWRs). The capture system may be located on-site, and may include a below-grade media that offers a controlled absorption of radioactive materials (gases, liquids and particulates) that may be released from primary containment. The release of radioactive materials from primary containment may be performed to reduce the pressure of the reactor pressure vessel (RPV) and primary containment of the LWR to avoid a more serious accident such as an explosion and/or possible structural damage to primary containment that could cause a release of radioactive materials directly into the atmosphere.
Example embodiments may reduce the liability and size of the exclusion area (per Title 10 of Code of Federal Regulation Article 100) by capturing radio-toxins below-grade to prevent or mitigate a major off-site release. Following stabilization of the LWR, the below-grade capture system may concentrate the radioactive materials through a pump-and-treat process. The mechanics of the example embodiments therefore enable a large-volume, below-grade, passive containment overpressure protection preventing an uncontrolled release during beyond-design-basis events.
The above and other features and advantages of example embodiments will become more apparent by describing in detail, example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.
Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein.
Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.).
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
The capture system 100 may include radioactive capture discharge piping 5 connecting the RPV 2 and portions of primary containment 3 to the media area 4. A discharge valve 6 may be used to allow for the release of radioactive materials to the media area 4. A venting system 7 may be connected to the media area 4 via gas outlet pipe 14, allowing gaseous pressure to be released to the atmosphere to reduce the pressure of the media area 4. The venting system 7 may be a filtered hardened containment vent, or other such well-known system that filters gaseous radioactive materials prior to release into the atmosphere. The venting system may also include a safety relief valve, a rupture disc, or a bursting disc.
A liquid outlet pipe 15 and liquid outlet valve 15a may be included near a bottom portion of the media area 4 to drain liquid that has been filtered via the media 4a. A treatment system 8, such as a temporary pump-and-treat system may also be provided above grade 12. The temporary treatment system 8 may be temporarily installed following stabilization of a severe accident. The temporary treatment system 8 may include a pump 9 that pumps a treatment liquid through treatment inlet pipe 10 and into the media area 4. By pumping a treatment liquid into the media area 4, the media area 4 may be flooded and pressurized, allowing a treatment discharge pipe 11 to discharge the treatment liquid into the treatment system 8 for further processing and neutralization, as described herein in more detail.
In addition to the media 4a adsorbing/absorbing the radioactive material, the media area 4 may also be flooded with water prior to a release of radioactive material into the area 4, in order to serve as a secondary suppression pool. The flooding of the media area 4 would provide additional radioactive material scrubbing as well as reduce the rate of pressurization of the media area 4.
It should be understood that sizing of the media area 4, and a determination of the quantity and surface area of the media 4a within the media area 4, should be determined in order to adequately decrease the radioactivity associated with radioactive material that is associated with a partial or complete meltdown of a nuclear reactor (i.e., worst case nuclear accident). In such an event, the following example fission products may be of most concern to the public.
Maximum radioactivity is determined by the mass of fission products and fuel materials released from containment. Because of the process of radioactive decay, the amount of total radioactivity decreases significantly within the first few days of an accident. Hence the benefit of the media area 4 to hold the release of radioactive materials of on-site until the initial decrease in radioactivity allows easier handling of the release.
Because the unique fission products from a typical LWR number over 700, all with different decay constants and concentrations, the design of the capture system 100 may use an aggregated decay constant, which may be easily calculated using data presented for instance in the Nuclear Regulatory Commission (NRC) Reg. Guide 3.54, ‘Spent Fuel Heat Generation in an Independent Spent Fuel Storage Installation’ (Revision 1). Knowing the decay constant of the radioactive materials, the site meteorological conditions, and the rate of pressure increase in the capture media retaining the radioactive material, the off-site release of radioactive material during the accident may be mitigated or completely avoided. Even in the event that the media 4a becomes pressurized to the design limit of the system, gas may be vented via vent 7 such that an amount of radioactive material released into the atmosphere may be reduced by a factor of between 100 and 10,000,000 (as compared to an expected radioactive release without the use of capture system 100) depending on the design pressure of the capture system 100.
The tank 42 may also be tilted at an angle from horizontal (i.e., sloped relative to a plane that is about perpendicular to the direction of gravity), with a liquid outlet pipe 15 connected to the lowest elevation of the tank 42 to drain captured liquid 21 into a drum 38. This allows liquid to flow counter-current to gas flow and particulate flow within the tank 42.
The vent system 7 may include a pressure gauge 43 providing pressure information to personnel above grade 12. Based on the pressure information, plant personnel may open the vent valve 7a to allow depressurization of the tank 42.
The bottom baffle cell 30 may include a drainage connection 34 that drains liquid 21 into a sloped drain 35 and into drum 38. The bottom surface of the media area 4 may also be slightly sloped (toward drainage connection 34) to further facilitate water drainage. The media 4a may include varying media granule, from a biggest media granule 4a1 to a smallest media granule 4a3, providing increased adsorption/absorption as the radioactive material flows through the media area 4. A gas space area 36 may also be included near the venting system 7 for sampling and inspection.
The media area 4 may include concrete reinforced walls, rubber, steel, or other suitable material (understanding that concrete or steel would be more effective in allowing pressurization of the capture system 100). A sloped (from horizontal) drain 35 and drum 38 may be provided. The longitudinal baffles 84 and the floor of the media area 4 may also be sloped (from horizontal) to facilitate water drainage. A gas space area 36 may be included for sampling and inspection.
Media 4a within the media area 4 may have varying granule sizes, such that the media 4a1 closest to the discharge pipe 5 may have the largest granules, and the media 4an closest to the vent system 7 have the smallest granule sizes. This allows the radioactive material to experience increased levels of adsorption/absorption as the radioactive material travels through the media area 4.
It should also be understood that features of the example embodiments shown in
Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
