Patent Number: 
Section: description

The methodology of the present invention for determining reduced in-core instrumentation (ICI) patterns is grounded in considerations recommended by the United States Nuclear Regulatory Commission (USNRC) for inclusion in and evaluation of changes to the ICI system. These considerations are made in accordance with federal regulations, specifically, 10 C.F.R. 50.59. The considerations outlined by the USNRC include the following: 1. Detecting inadvertent loading of a fuel assembly into an improper location; 2. Insuring the validity of core power tilt estimates; 3. Maintaining adequate core coverage by instrumentation; 4. Limiting measurement uncertainties to meet plant Technical Specification limits for various measured values, including measured peak linear heat rates, peak pin powers, radial peaking factors, and azimuthal tilts; and 5. Restoring ICI system to full open (or nearly full) service at the beginning of each fuel cycle. Utilizing these considerations, determination of the reduced ICI patterns according to the present inventive methodology includes two main parts. First, candidate ICI patterns having a reduced number of ICIs relative to the existing ICI patterns are selected according to specific selection considerations. Second, the selected candidate ICI patterns are evaluated according to specific criteria. According to the first part of the inventive methodology, candidate ICI patterns having a reduced number of ICIs must be selected. In order to guide selection of the candidate ICI patterns according to the inventive methodology, selection considerations were established to ensure that reduced patterns of ICIs are capable of performing all functions required of a full complement of ICIs. The functions required of the reduced ICI patterns include: i. maintaining the core power tilt measurement capability; ii. providing the capability to perform ex-core detector calibrations; iii. providing full core coverage; iv. detecting fuel misloadings; v. detecting misalignment of the lead control rod bank; and vi. providing sufficient redundancy. First, a reduced ICI pattern maintains core power tilt measurement capability by retaining all ICIs from the existing ICI pattern that belong to tilt groups. In this way, core power tilt measurement capability is not affected by the reduced ICI pattern, because all tilt-related ICIs are retained. Second, the integrity of ex-core detector calibrations is maintained by retaining all ICIs from the existing ICI pattern that provide information about the power distribution in those fuel assemblies that contribute most of the neutron flux impinging on the ex-core detectors. By maintaining measurement capability within those fuel assemblies that contribute most neutron flux to the ex-core detectors, comparisons may be made between the retained ICIs and the ex-core detectors to provide full ex-core detector calibration. Third, full core instrumentation coverage is not impacted through the use of a reduced ICI patterns because those ICIs that provide a uniform distribution of instrumented fuel assemblies from the periphery of the core to the center of the core, in all core quadrants, are retained. In other words, by maintaining a uniform distribution of ICIs, though the number is reduced, full core instrumentation coverage is still provided through the present invention. Fourth, fuel misloadings must be detected by the reduced ICI pattern. According to the inventive method, fuel assembly misleading detection capability of the reduced ICI patterns is verified by simulating fuel misloading during the ICI pattern evaluation process. The evaluation process is discussed in more detail below. Fifth, reduced ICI patterns maintain the capability to detect misalignment of the lead control rod bank. This capability is ensured by retaining those ICIs that are sufficiently close to a control rod location to enable detection of any perturbation in the power distribution associated with movement of the lead control rod bank. By retaining those ICIs that are in a position to detect power distribution changes caused by movement of the lead control rod bank, misalignment of the lead control rod bank is easily detected. Lastly, redundancy of the reduced ICI patterns are proven by evaluating the performance of the reduced ICI patterns in combination with unexpected ICI failures to prove that the reduced ICI patterns still perform their intended functions. The various evaluations are discussed in more detail below. Once a reduced ICI pattern has been selected based upon meeting the various selection considerations set forth above, the reduced ICI pattern configuration is evaluated to ensure that any differences between the predicted core power distributions and those synthesized from the reduced ICI patterns are within licensed limits for the plant. Moreover, the reduced ICI patterns are evaluated to prove that the reduced ICI patterns are able to detect misleading of an improperly located fuel assembly. Further, the reduced ICI pattern is tested with only 75% of the ICIs operable and is forced to meet the full evaluation criteria, even though 25% of the ICIs are inoperable. Thus, under the evaluation portion of the methodology of the present invention, candidate patterns are evaluated according to the following criteria: A. The differences between the predicted core power distributions and those synthesized from the reduced ICI patterns must be in compliance with the limits that have been licensed for the plant; B. The reduced ICI patterns must provide the capability of detecting the misloading of a fuel assembly placed into an improper location; and C. The reduced ICI patterns must be capable of performing all intended functions with only 75% of the ICIs operable, in accordance with the plant Technical Specification. In order to determine that power distribution uncertainties are within licensed limits, comparisons are performed of predicted power distributions obtained for design calculations with measured power distributions obtained from ICI detector signals using accepted licensed methodologies. Two types of uncertainty are evaluated: basic measurement uncertainty and the synthesis uncertainty. Basic measurement uncertainty relates to the local power in instrumented fuel assemblies. Synthesis uncertainty is associated with extrapolating power calculations to non-instrumented fuel assemblies. In order for a reduced ICI pattern to be acceptable, both calculated basic measurement uncertainty and synthesis uncertainty must be within the limits licensed for the plant. In the second part of the evaluation, candidate ICI patterns that satisfy the uncertainty analysis are tested to ensure the ability to detect fuel misloadings. Testing of the fuel misloading detection ability with a reduced ICI pattern is verified by simulating a spectrum of fuel misloadings and examining the differences in the power distributions between a properly loaded core and a simulated misloaded core, as measured by the reduced ICI pattern. Such simulations may be performed using conventionally known simulation techniques. In order to be acceptable, the differences in power distributions obtained from ICI detector signals for the properly loaded core and the simulated misloaded core must be discernable. If the differences are discernable, it is assumed that the reduced ICI pattern is able to detect fuel misloadings. Finally, each reduced ICI pattern is evaluated for its capability to perform its intended functions under a 75% operability Technical Specification requirement. That is, each reduced ICI pattern is further reduced to 75% of the reduced pattern number to demonstrate that the reduced ICI pattern can perform its intended functions with only 75% of the ICIs operable. The ICI operability limit is derived from the plant Technical Specification and the plant license. Existing plant Technical Specifications set forth a 75% ICI operability limit. If the ICI operability limit is ever changed, e.g. increased to 90% or decreased to 70%, then each reduced ICI pattern would be evaluated for its capability to perform its intended functions under that particular Technical Specification ICI operability limit. Applying the methodology described above, several reduced ICI patterns have been determined to be acceptable for various core configurations. Several of these acceptable reduced ICI patterns are set forth below as examples and are shown in FIGS. 1-8. It should be understood that the following Examples are exemplary only, and do not limit the scope of the invention in any way. In FIGS. 1-8, boxes representing fuel assemblies that contain an ICI in existing cores of this configuration are shown with both light and dark cross-hatching, and include a corresponding numeric designation in parentheses. Those boxes including dark cross-hatching correspond to those assemblies from which an ICI has been removed using the methodology of the present invention. Thus, those boxes including light cross-hatching correspond to those assemblies wherein ICIs have not been removed. FIG. 1 shows a plan diagram of a 241 fuel assembly PWR core. Under existing plant designs, a 241 fuel assembly core would ordinarily have an existing ICI pattern of 61 ICIs distributed as shown in FIG. 1. Using the methodology described herein, 12 ICIs (numbers 13, 22, 23, 25, 27, 30, 34, 35, 37, 43, 44 and 47) are removed, leading to a reduced ICI pattern of 49 ICIs. The net ICI reduction in FIG. 1 by the claimed methodology is 20%. A 217 fuel assembly core is shown having an original ICI pattern of 56 ICIs. Using the methodology described herein, 12 ICIs (numbers 2, 4, 9, 15, 18, 20, 37, 39, 42, 48, 53 and 55) are removed from the core. The reduced ICI pattern thereby includes only 44 ICIs, for a net reduction of 21% FIG. 3 shows an alternative 217 fuel assembly core configuration having an original ICI pattern of 45 ICIs. The original ICI pattern in FIG. 3 is different from that shown in FIG. 2. Using the methodology described herein, 10 ICIs (numbers 9, 12, 18, 19, 20, 27, 28, 33, 35 and 37) are removed from the core, yielding a reduced ICI pattern of 35 ICIs, for a net reduction of 22%. In FIG. 4, another configuration is shown for a 217 fuel assembly core with an original ICI pattern of 45 ICIs. Again, the existing ICI pattern shown in FIG. 4 is different from the patterns of either FIGS. 2 or 3. Using the inventive methodology, 10 ICIs (8, 12, 18, 19, 20, 27, 31, 33, 35 and 35) are removed, yielding a reduced ICI pattern of 35 ICIs is shown, yielding a reduction of 10 ICIs, or 22%. In FIG. 5, another 217 fuel assembly core pattern is shown having a 45 ICI original pattern that is different from that shown in either FIGS. 2, 3 or 4. Using the inventive methodology, 10 ICIs (9, 12, 18, 19, 20, 27, 28, 33, 35 and 37) are removed, yielding a reduced ICI pattern of 35 ICIs, yielding a net reduction of 22%. In FIG. 6, a 177 fuel assembly core is shown that includes an original ICI pattern of 45 ICIs. Using the inventive methodology, 6 ICIs (16, 17, 20, 26, 32 and 33) are removed, reducing the number of ICIs to 39, yielding a net reduction of 13%. In FIG. 7, another 177 fuel assembly core configuration is shown having an original ICI pattern of 44 ICIs. Using the inventive methodology, the number of ICIs is reduced by 8 (Numbers 3, 5, 14, 20, 25, 31, 40 and 42) to 36 ICIs, yielding a net reduction of 18%. FIG. 8 shows a 133 fuel assembly core having an original ICI pattern of 28 ICIs. Using the inventive methodology, the number of ICIs is reduced by 3 (Numbers 10, 18 and 20), yielding a net reduction of 11%. In each of examples 1-8, the reduced ICI patterns shown satisfy all requirements of the ICI system while providing substantial reductions in the number of ICIs. Reducing the number of ICIs reduces both plant operating and maintenance costs. If the present methodology is implemented during new plant construction, the amount of additional equipment necessary to support the ICI system would be reduced because fewer ICIs would have to be supported. As a result, a significant reduction in overall capital cost due to construction as well as due to operating and maintenance is realized through implementation of the present invention. Moreover, associated costs from reactor down time during refueling outages may be reduced because a shorter time needed to replace a reduced complement of ICIs. Moreover, because the time along the critical path is correspondingly reduced, potential radiation exposure to plant personnel is also reduced. Various embodiments of this invention have been disclosed. But it should be realized that the various changes and modifications that are possible will be self-evident to those of skill in the art in which the present invention pertains, and may be made without departing from the scope of the invention, which is limited only by the appended claims.