This invention consists of an external radiation-monitoring diagnostic hodoscope system for determining water-coolant levels and reactor-fuel distribution in various regions of a boiling-water-type nuclear power reactor, whether the reactor is shut down or operating. It is specifically intended for the damaged Fukushima Dai-Ichi nuclear-power-station reactors in determining post-accident spatial reconfiguration of both reactor fuel and coolant. Comparative analysis senses changes from the normal condition (core topped with liquid coolant) to advise of changes in the presence and/or density of the coolant at these specific regions. The detectors can also sense the distribution or redistribution of core fuel and some structural constituents. This invention applies to the other boiling-water reactors in Japan, to operational boiling-water reactors around the world, and to commercial pressurized water-cooled reactors.
Patent to Help Decommission Fukushima Nuclear Reactors
Of utmost public concern are technologies to improve or ensure nuclear-reactor safety, including means for providing prompt and appropriate corrective measures, especially technologies employed in the low-probability event of severe reactor malfunction, with such ameliorative and supplementary technologies chosen to minimize reactor damage and maximize public safety.
Related to the invention to be disclosed herein is my expired Patent US 4,092,542 issued May 30, 1978, titled "High-Resolution Radiography by Means of a Hodoscope," which illustrates a nonintrusive radiographic scanning system for use in nuclear research. Also related to the invention to be disclosed herein is my expired Patent US 4,649,015 issued 10 Mar 1987, titled “Monitoring system for a liquid-cooled nuclear fission reactor,” which illustrates a stationary system for monitoring changes of liquid levels in various regions of a pressurized-water-cooled nuclear power reactor while in normal operation.
This invention being disclosed herein relates to improvements in the safety, operation, and decommissioning of nuclear reactors.This application adds inventions that go beyond my two referenced and expired patents. This invention revises, combines, and extends utilization of my two expired patents to a broader class of water-cooled reactors, to a broader range of relevant normal and emergency diagnostic functions for operating and non-operating reactors, and to reactors which are shut down or being decommissioned.
This application applies specifically to the Fukushima Dai-Ichi boiling-water nuclear reactors, but it does not exclude application to other boiling-water reactors around the world, nor does it exclude application to other types of reactors.
The difficulties and malfunctions experienced at Fukushima Dai-Ichi nuclear-reactor station as an aftermath of the 2011 earthquake and tsunami illustrate the need for this radiation-monitoring diagnostic hodoscope invention in order (1) to prevent or minimize further harmful damage to the damaged reactors, (2) to assist the safe and economic decommissioning of the damaged reactors, and (3) to assist in the orderly restart of the undamaged reactors.
If this radiation-monitoring diagnostic hodoscope invention had been installed prior to the reactor accidents, the extent of internal damage might have been greatly reduced. But even now, implementation of certain aspects of this invention at the Fukushima nuclear station will help reduce the hazards, time delay, and cost for decommissioning the damaged reactors and would allow earlier return to operation of the undamaged reactors at the same site.
Also, the invention can be retrofitted to other water-cooled nuclear reactors operating throughout the world.
SUMMARY OF THE INVENTION
In research reactors and in pressurized-water reactors, external instruments have been used for determining certain nuclear-reactor operational characteristics. Previous patented art relevant to this new invention has been directed at obtaining the following types of real-time information: coolant-water levels or complete loss-of-coolant, nuclear-reactor safety or fuel-element failure, nuclear core integrity or loss of integrity, fissile-fuel distribution or redistribution.
This present invention is a unique means of nondestructively determining coolant-water level and core-fuel distribution or redistribution in reactors that have been shut down deliberately or by accident, including specifically the Fukushima Dai-Ichi boiling-water-type reactors. Making use of the art described in the two expired patents referenced above, this invention has novel features that combine and apply directly to the configuration and circumstances of the damaged and undamaged reactors at the Fukushima Dai-Ichi nuclear power station.
This invention fulfills a technical challenge that has yet to be surmounted for the Japanese reactors, and is yet to be recognized or implemented for similar nuclear reactors around the world. Each of two manifestations described in this invention provides its own benefits, context, and limitations — such that each embodiment complements and supplements the other in providing essential data that assists safe and efficient decommissioning of the damaged reactors and in supporting preventative-safety implementation in undamaged reactors of the same type.
My expired 1978 Patent US 4,092,542 "High-Resolution Radiography by Means of a Hodoscope, provides that both neutron and gamma radiographs of high spatial resolution are obtained from the fast-neutron hodoscope by the scanning operation of a collimator, by storing detector data outputs, and by rendering computer reconstruction of the data so obtained. The apparatus is adapted to detect fast neutrons, gamma rays, or both, and to use various combinations of the information obtained from fast neutrons and gamma rays. Considerable field application and publication have occurred for the equipment covered under that Patent.
My expired 1987 Patent US 4,649,015 “Monitoring system for a liquid-cooled nuclear fission reactor” illustrates a system for detecting changes in water-coolant levels at various elevations of a pressurized-water nuclear power reactor operating at full power. A pre-installed vertical array of gamma-radiation detectors was to be mounted at the inside wall of the biological shield, and the detectors were to be collimated so that each received gamma radiation only from predetermined reactor-vessel elevations.
The 1987 Patent was based on applicant’s technical analysis that, during normal nuclear-reactor operation to produce steam and thus electrical power, neutrons released in the fission reaction were often captured by the water coolant, or by the steel reactor walls, and/or by nuclear fuel or control structures in the reactor vessel. Nuclear-reactor power production results in many neutrons being captured in the steel structure and containment, thereby often being converted to gamma radiation having an energy level of the energy range of 5-12 MeV (million electron Volts). Meanwhile, neutron capture in water results in a gamma-energy level of approximately 2.2 MeV, while neutron capture in the fission fuel or its cladding mostly provides gamma-rays of 1 MeV or less. Recognition of these intrinsic energy-level differences was essential to implementation of the 1987 Patent described for a reactor operating at rated power level.
This present invention introduces counterintuitive claims that apply specifically to a reactor that has been shut down and is no longer engaged in willful power production. For this invention to be functional, it is necessary to specifically identify and make use of residual effects — known as decay heat and delayed radiation generated as a result of prior high-power operation of the reactor — and further for this invention to be functional, the radiation-monitoring diagnostic hodoscope system must be sensitive to the unique conditions presented by a reactor that has since been deliberately or accidentally shut down, no longer operating at any design power-production level.
It is essential to this new invention to note that the occurrence and rate of neutron capture inside the reactor are altered significantly as the level or density of coolant varies at any water, air, or structural interface, even though the reactor is shut down and no steam is being generated to produce electrical power. Comparative diagnostic analysis of the radiation-monitoring hodoscope gamma-detector data is capable of identifying radical changes from the normal condition, such as liquid coolant present or absent, or to advise of changes in the density of the coolant at these specific regions due to bubbles or steam.
In considering application of this new invention, it is noted that there is substantial lingering uncertainty regarding the damaged Fukushima reactors, as to the height, location, and concentration of water coolant in the reactor pressure vessel and in its external containment vessel. At the same time, there is considerable and crucial safety-related uncertainty as to the distribution or redistribution of fuel and structural components in the damaged Fukushima reactor core and pressure vessels.
Obtaining detailed actionable knowledge of these factors is vital to the safety, cost, procedures, and duration of decommissioning the tsunami-affected reactors. As in the case of the 1987 Patent, this new invention utilizes a multiplicity of exterior gamma-radiation detectors associated with a collimator technique that functions so as to sense separate regions of the reactor vessel, thus providing unique signals for these respective regions, whereby comparative analysis of these signals can be used to advise of the presence and density of cooling water in the vessel.
However, this present invention goes further by also advising of the presence, density, and redistribution of reactor-core materials in the reactor pressure vessel or its immediate surroundings within the freestanding steel dry well of a boiling-water reactor, even if the reactor is no longer deliberately generating steam for electrical power. In some respects, this new invention is similar to the 1987 Patent, which provided “a plurality of gamma radiation detectors arranged vertically along the reactor vessel itself, and collimator means for each detector to limit the detection of gamma radiation from only isolated regions of the vessel, where the detectors cumulatively provide sufficient readout signals from the entire reactor vessel vertically and radially at specific regions in the reactor vessel [such] that comparative analysis of these readouts yields precise indications of the water level and water density therein, all independently of the power level of the reactor."
However, the damaged Fukushima reactors are now inoperative; thus they do not generate radiation source levels that would provide sufficient real-time signal strength as required for the 1987 Patent to be functional. Instead, this new invention is tailored to be functional in a reactor that has been unavoidably shut down. This invention overcomes the sensitivity limitation by detecting residual gamma-radiation emission resulting from delayed and spontaneous fission in fuel that was in the reactor core or has since been redistributed by the accidental uncovering of water coolant in the reactor pressure vessel.
The 1987 Patent provided for detection of “capture by steel of neutrons emanating from the core [which] generates gamma radiation having a high energy level of the order of 5-12 MeV, well above the gamma radiation energy level of fission of the fuel or of the capture of neutrons in the water coolant.” As was the case for the 1987 patent, “The intensity of neutron capture changes significantly at any water interface. Even though the steel walls of the pressure vessel are of considerable thickness (15-20 centimeters), the energetic capture-gamma radiation has a relatively high probability of the order of 1% or higher of penetrating the steel walls and escaping from the reactor vessel."
For this present invention, it is noted that the capture of gamma radiation in water provides an energy level of approximately 2.2 MeV. Moreover, residual radiation associated with the reactor fuel and other core structure contributes to a continuum of gamma rays at lower energies that can be detected.This new invention relies partly on some similar neutronic and radiation phenomena, as well as similar instrument functions as was the case for the 1987 patent, but this invention makes use of new technical insight and analysis to the effect that (1) lower energy gamma rays will also contribute a detectable signal which distinguishes radiation associated with the coolant water as importantly differentiated from radiation associated with nuclear fuel, and (2) modern gamma-ray detector systems and their multichannel data storage and analysis provide more useful, sensitive, and specific diagnostic information than previously available.
Efficient detection and energy sorting of gamma rays can be accomplished in thallium-activated sodium-iodide or bismuth-germanium-oxide scintillation detectors. In water reactors, essentially all excess neutrons produced by the fission reaction that do not leak out of the pressure vessel will be thermalized and captured in the water or the steel within the vessel, thus producing telltale gamma emission. In contrast to the 1987 Patent, this new invention does not directly address or apply to the detection of fission-product gas accumulation in the reactor. However, the new invention remains applicable to “retrofit or new construction of light-water reactors.”
There are two specific embodiments of this radiation-monitoring multichannel diagnostic hodoscope invention, as referred to in this application. The first embodiment is in the form of an array of gamma-ray detectors embedded in the reactor biological shield; this embodiment is called an embedded hodoscope. The second embodiment is in the form of a movable array of gamma-ray detectors that can be moved into and remotely operated within the reactor containment building; this embodiment is called a movable hodoscope.