Method for evaluating a fuel element breakage condition in a pressurized water reactor

Abstract

The application relates to a method for evaluating a fuel element breakage condition in a pressurized water reactor, which comprises the following steps: sampling the coolant of the pressurized water reactor every time interval during operation of the pressurized water reactor, measuring and recording the Xe-133 activity in the coolant sample; generating a curve for indicating the change of the Xe-133 activity with time; and identifying whether the fuel element is broken and the number of the broken fuel elements according to the shape of the curve. The release behavior of the inert gas atoms in the air gap between the fuel element cladding and the fuel element pellets to the coolant is hardly affected by the breakage size and the breakage position of the fuel element. Therefore, the evaluation method in the embodiment of the application takes the activity of Xe-133 as the basis, can effectively evaluate whether the fuel element in the pressurized water reactor is broken and the number of the broken fuel elements during operation of the reactor, and the evaluation result is high in accuracy.

Description

Methods for assessing the damage status of fuel elements in pressurized water reactors Technical Field

[0001] This application relates to the field of nuclear power technology, and in particular to a method for assessing the damage status of fuel elements in a pressurized water reactor. Background Technology

[0002] The integrity of fuel elements (also known as fuel rods) is crucial to the safety, reliability, and economy of a pressurized water reactor (PWR). During normal operation, if a fuel element is damaged, inert gases and other volatile fission products accumulated within the fuel cladding will migrate and be released into the primary coolant, increasing primary coolant radioactivity levels, leading to increased effluent and higher radiation doses for personnel. In cases of severe fuel element damage, the unit must be switched to emergency shutdown mode to prevent the impact on personnel and the environment from exceeding design limits.

[0003] During reactor power operation, the reactor core, which contains fuel elements, is a high-radiation field filled with neutrons and gamma rays, making it inaccessible to personnel. Damaged fuel elements can only be identified through leak detection and ultrasonic testing after the reactor is shut down and the fuel is removed from the core. Therefore, effectively identifying fuel element damage during reactor power operation remains a significant challenge. Summary of the Invention

[0004] Therefore, it is necessary to propose a method for assessing the damage status of fuel elements in a pressurized water reactor, so as to effectively identify the damage status of fuel elements during reactor operation.

[0005] This application provides a method for assessing the damage status of fuel elements in a pressurized water reactor, which includes the following steps:

[0006] Step S10: During the operation of the pressurized water reactor, the coolant of the pressurized water reactor is sampled at regular intervals, and the Xe-133 activity in the coolant sample is measured and recorded.

[0007] Step S20: Generate a curve representing the change of Xe-133 activity over time;

[0008] Step S30: Based on the shape of the curve, identify whether the fuel element is damaged and the number of damaged fuel elements.

[0009] The method for assessing the damage status of fuel elements in a pressurized water reactor according to this application uses the activity of Xe-133 as the basis for evaluating the damage status of fuel elements. Unlike the release of radioactive nuclides from halogens, the release of inert gas atoms from the air gap between the fuel element cladding and the fuel element pellet into the coolant is almost unaffected by the size and location of the puncture in the fuel element. Among these inert gases, Xe-133 has a half-life of 5.2 days, the highest radioactivity in the coolant, and the energy of the gamma rays produced by its decay is within the detection range of conventional nuclear radiation detectors. Therefore, Xe-133 is the most suitable radionuclide among inert gases for assessing the damage status of fuel elements. Therefore, the assessment method in this application, based on the activity of Xe-133, can effectively assess whether fuel elements in a pressurized water reactor have been damaged and the number of damaged fuel elements during reactor operation, and the assessment results are highly accurate.

[0010] In some embodiments of this application, the step of identifying whether a fuel element is damaged and the number of damaged fuel elements based on the shape of the curve includes:

[0011] When the curve exhibits a stepped shape, it confirms that the fuel element has been damaged, and the number of steps represents the number of damaged fuel elements.

[0012] In some embodiments of this application, the evaluation method further includes:

[0013] Record the unit power of the pressurized water reactor at each sampling time;

[0014] If the change in the currently recorded unit power relative to the previously recorded unit power does not exceed the first preset percentage, then the currently recorded Xe-133 activity is valid data.

[0015] If the change in the currently recorded unit power relative to the previously recorded unit power exceeds a first preset percentage, then the currently recorded Xe-133 activity is invalid data.

[0016] In some embodiments of this application, the first preset percentage is 5%.

[0017] In some embodiments of this application, the step of sampling the coolant of the pressurized water reactor at regular intervals includes:

[0018] Coolant samples were taken periodically from the drain line connected to the primary loop of the pressurized water reactor.

[0019] In some embodiments of this application, the evaluation method further includes:

[0020] Record the discharge flow rate at each sampling time;

[0021] If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate does not exceed the second preset percentage, then the Xe-133 activity recorded now is valid data.

[0022] If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate exceeds a second preset percentage, then the current recorded Xe-133 activity is invalid data.

[0023] In some embodiments of this application, the evaluation method further includes:

[0024] Remove the invalid data from the generated curve of Xe-133 activity over time.

[0025] In some embodiments of this application, the step of sampling the coolant of the pressurized water reactor at regular intervals includes:

[0026] The coolant in the pressurized water reactor is sampled every day or every two days.

[0027] In some embodiments of this application, the evaluation method further includes:

[0028] The evaluation method fails when the number of identified damaged fuel elements exceeds a preset number.

[0029] In some embodiments of this application, the step of generating a curve representing the change of Xe-133 activity over time includes:

[0030] The x-axis represents time, and the y-axis represents Xe-133 activity. The x-axis is a linear coordinate, and the y-axis is a logarithmic coordinate, to generate a curve representing the change of Xe-133 activity over time. Attached Figure Description

[0031] Figure 1 is a flowchart illustrating the evaluation method in one embodiment of this application;

[0032] Figure 2 is a graph of Xe-133 activity over time when one fuel element is damaged in an embodiment of this application.

[0033] Figure 3 is a graph of Xe-133 activity over time when two fuel elements are damaged in an embodiment of this application.

[0034] Figure 4 is a graph of Xe-133 activity, unit power, and discharge flow rate over time in one embodiment of this application. Detailed Implementation

[0035] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0036] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0037] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0038] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0039] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0040] It should be noted that when an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0041] The integrity of fuel elements is crucial to the safety, reliability, and economy of a pressurized water reactor (PWR). During normal operation, if a fuel element is damaged, inert gases and other volatile fission products accumulated within the fuel cladding can migrate and be released into the primary coolant, increasing primary coolant radioactivity levels, leading to increased effluent and higher radiation doses for personnel. In cases of severe fuel element damage, the unit must be switched to emergency shutdown mode to prevent the impact on personnel and the environment from exceeding design limits.

[0042] During reactor power operation, the reactor core, which contains fuel elements, is a high-radiation field filled with neutrons and gamma rays, making it inaccessible to personnel. Damaged fuel elements can only be identified through leak detection and ultrasonic testing after the reactor is shut down and the fuel is removed from the core. Therefore, effectively identifying fuel element damage during reactor power operation remains a significant challenge.

[0043] The inventors of this application discovered that when a fuel element breaks, the fission products accumulated in the air gap between the fuel element cladding and the fuel element pellet are released into the coolant of the pressurized water reactor through the break, causing a rapid increase in the activity of the fission products in the coolant. Therefore, by detecting the activity of the fission products in the coolant, it is possible to determine whether the fuel element has broken.

[0044] The inventors further investigated the relationship between the activity of various fission products and the damage state of the fuel element cladding. They discovered that the release behavior of halogens (iodine, bromine, etc.) from the fission products into the coolant is closely related to the size and location of the break in the fuel element. For example, when the break size is relatively large, the release rate of long-half-lived halogens (e.g., I-131, I-133) from the fuel element increases, leading to increased activity of these fission products in the coolant. Therefore, it can be concluded that using the activity of fission products such as I-131 and I-133 as a basis for assessing the damage state of the fuel element may result in significant discrepancies with the actual situation. Thus, it is necessary to find a fission product whose activity is unaffected by the size and location of the break. Using the activity of this fission product as a basis for assessing the damage state of the fuel element would yield much more accurate results.

[0045] Based on the above research conclusions and concepts, this application proposes a method for assessing the damage status of fuel elements in a pressurized water reactor, as shown in Figure 1. The assessment method includes the following steps:

[0046] Step S10: During the operation of the pressurized water reactor, the coolant of the pressurized water reactor is sampled at regular intervals, and the Xe-133 activity in the coolant sample is measured and recorded.

[0047] Specifically, after sampling the coolant in a pressurized water reactor, the Xe-133 activity in the coolant sample can be measured using a nuclear radiation detector. For example, a high-purity germanium detector can be used. Of course, nuclear radiation detectors can be used not only to measure Xe-133 activity in coolant samples, but also to measure the activity of other radionuclides, such as halogens (e.g., I-131, I-133).

[0048] Step S20: Generate a curve representing the change of Xe-133 activity over time (refer to Figure 2, Figure 3 or Figure 4).

[0049] Each time the Xe-133 activity in the coolant sample is sampled and measured, a Xe-133 activity data point is obtained. This data point can be used as a data point on a graph. When there are many data points, an Xe-133 activity curve can be constructed. Connecting multiple data points will yield the Xe-133 activity curve. The horizontal axis of the graph can specifically represent the operating time of the pressurized water reactor, with the unit being "days".

[0050] Step S30: Based on the shape of the curve, identify whether the fuel element is damaged and the number of damaged fuel elements.

[0051] It is understood that the Xe-133 activity curve in this embodiment can be a curve formed by connecting each Xe-133 activity data point with a line, or it can be a virtual curve used to characterize the trend of Xe-133 activity change, that is, the Xe-133 activity data points are not connected by lines.

[0052] The method for assessing the damage status of fuel elements in a pressurized water reactor according to this application uses the activity of Xe-133 as the basis for evaluating the damage status of fuel elements. Unlike the release of radioactive nuclides from halogens, the release of inert gas atoms from the air gap between the fuel element cladding and the fuel element pellet into the coolant is almost unaffected by the size and location of the puncture in the fuel element. Among these inert gases, Xe-133 has a half-life of 5.2 days, the highest radioactivity in the coolant, and the energy of the gamma rays produced by its decay is within the detection range of conventional nuclear radiation detectors. Therefore, Xe-133 is the most suitable radionuclide among inert gases for assessing the damage status of fuel elements. Therefore, the assessment method in this application, based on the activity of Xe-133, can effectively assess whether fuel elements in a pressurized water reactor have been damaged and the number of damaged fuel elements during reactor operation, and the assessment results are highly accurate.

[0053] In some embodiments of this application, the step of identifying whether a fuel element is damaged and the number of damaged fuel elements based on the shape of the curve includes:

[0054] When the curve shows a stepped shape, it confirms that the fuel element is damaged, and the number of steps represents the number of damaged fuel elements.

[0055] When a fuel element in the core of a pressurized water reactor (PWR) fails (see Figure 2), under stable operating power conditions, the Xe-133 activity in the coolant increases rapidly (typically reaching its maximum within 30 days) and then gradually reaches equilibrium. During subsequent operation, when another fuel element fails (see Figure 3), the Xe-133 activity in the coolant exhibits a rapid jump from the previous quasi-equilibrium state and gradually enters a new quasi-equilibrium state. Therefore, each rapid jump in Xe-133 activity in the coolant can indicate the failure of an additional fuel element. Thus, the failure status of fuel elements can be assessed based on the curve's shape. Specifically, a stepped curve confirms fuel element failure, and the number of steps indicates the number of failed fuel elements (see Figure 4). The assessment method in this embodiment is based on the activity characteristics of Xe-133. Based on multiple verifications and comparisons by the inventors, the assessment results obtained using this method have high accuracy. In addition, this evaluation method is simple to implement and does not require the establishment of a complex evaluation model, thus it has high practical application value.

[0056] In some embodiments of this application, the method for assessing the damage status of fuel elements in a pressurized water reactor further includes:

[0057] Record the unit power of the pressurized water reactor at each sampling time;

[0058] If the change in the currently recorded unit power relative to the previously recorded unit power does not exceed the first preset percentage, then the currently recorded Xe-133 activity is valid data.

[0059] If the change in the currently recorded unit power relative to the previously recorded unit power exceeds a first preset percentage, then the currently recorded Xe-133 activity is invalid data.

[0060] Under stable operation of a pressurized water reactor (PWR), the Xe-133 activity in the coolant can be used to accurately assess the damage status of fuel elements. However, the Xe-133 activity in the coolant is affected by the PWR's unit power. In other words, if the PWR operates unstablely, the Xe-133 activity will change significantly. In this case, if the assessment of fuel element damage is based solely on the trend of Xe-133 activity changes, the results will deviate significantly from the actual situation.

[0061] To avoid inaccurate evaluation results due to the instability of the pressurized water reactor (PWR), this embodiment records the PWR unit power at the same time as each sampling. The stability of the unit power characterizes the stability of the PWR operation. If the change in the currently recorded unit power relative to the previously recorded unit power does not exceed a first preset percentage, it indicates that the PWR is in a stable operating state at the current sampling time, and the currently recorded Xe-133 activity is valid data that can be used to assess the damage status of fuel elements. Conversely, if the change in the currently recorded unit power relative to the previously recorded unit power exceeds the first preset percentage, it indicates that the PWR is operating unstable at the current sampling time, and the currently recorded Xe-133 activity is invalid data that cannot be used to assess the damage status of fuel elements. For invalid data, the data point on the curve can be considered non-existent.

[0062] In some embodiments of this application, the first preset percentage is 5%.

[0063] In this embodiment, if the change in the currently recorded unit power relative to the previously recorded unit power does not exceed 5%, the pressurized water reactor can be considered to be in a stable operating state; conversely, if the change in the currently recorded unit power relative to the previously recorded unit power exceeds 5%, the pressurized water reactor can be considered to be operating unstable.

[0064] In some embodiments of this application, the step of sampling the coolant of the pressurized water reactor at regular intervals includes:

[0065] Coolant samples were taken periodically from the drain line connected to the primary loop of the pressurized water reactor.

[0066] In related technologies, a drain line is connected to the primary loop of a pressurized water reactor. Therefore, coolant samples can be obtained from this drain line. After obtaining the coolant samples, gamma-ray spectroscopy analysis can be performed to obtain the radioactivity of different radionuclides in the coolant. The Xe-133 activity can be used to assess the damage condition of fuel elements, while the radioactivity of other radionuclides can be used for other analytical purposes.

[0067] In some embodiments of this application, the method for assessing the damage status of fuel elements in a pressurized water reactor further includes:

[0068] Record the discharge flow rate at each sampling time;

[0069] If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate does not exceed the second preset percentage, then the Xe-133 activity recorded now is valid data.

[0070] If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate exceeds a second preset percentage, then the current recorded Xe-133 activity is invalid data.

[0071] The discharge flow rate in the drain line also affects Xe-133 activity. Therefore, if the damage condition of the fuel element is still assessed based on the trend of Xe-133 activity changes when the discharge flow rate changes significantly, the assessment results will deviate from the actual situation. Therefore, in this embodiment, the discharge flow rate at each sampling time is also recorded to determine whether the discharge flow rate at the current sampling time has changed significantly compared to the discharge flow rate at previous sampling times.

[0072] If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate does not exceed a second preset percentage, then the currently recorded Xe-133 activity is valid data and can be used to assess the damage condition of the fuel element. Conversely, if the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate exceeds the second preset percentage, then the currently recorded Xe-133 activity is invalid data and cannot be used to assess the damage condition of the fuel element. For invalid data, the data point on the curve can be considered non-existent.

[0073] In some embodiments of this application, the method for assessing the damage status of fuel elements in a pressurized water reactor further includes:

[0074] Remove invalid data from the generated curve of Xe-133 activity over time.

[0075] After removing invalid data, only valid data is retained on the curve, thus avoiding interference from invalid data points when analyzing the curve.

[0076] In some embodiments of this application, the step of sampling the coolant of the pressurized water reactor at regular intervals includes:

[0077] The coolant in the pressurized water reactor is sampled every day or every two days.

[0078] Understandably, a higher sampling frequency results in denser data points on the curve, leading to higher accuracy and, consequently, a more accurate assessment of fuel element damage within the pressurized water reactor. However, in reality, the probability of fuel element damage occurring daily is not high, and the probability of more than two fuel elements damaging on the same day is even lower. Therefore, there is no need to set the sampling interval to less than one day, as this would waste manpower. Thus, in this embodiment, sampling is performed once a day or every two days, which avoids wasting manpower while ensuring the accuracy of the assessment results.

[0079] In some embodiments of this application, the method for assessing the damage status of fuel elements in a pressurized water reactor further includes:

[0080] The evaluation method fails when the number of identified damaged fuel elements exceeds a preset number.

[0081] When a large number of fuel elements in a pressurized water reactor fail, the activity steps of Xe-133 released into the coolant will overlap, making it impossible to clearly distinguish the number of steps that occur. At this point, the assessment methods can no longer be applied.

[0082] In some embodiments of this application, the step of generating a curve representing the change of Xe-133 activity over time includes:

[0083] The x-axis represents time, and the y-axis represents Xe-133 activity. The x-axis is a linear coordinate, and the y-axis is a logarithmic coordinate, to generate a curve representing the change of Xe-133 activity over time.

[0084] Since the Xe-133 activity value is relatively large and the range of value variation is also relatively large, using a logarithmic coordinate system can better show the trend of Xe-133 activity variation.

[0085] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0086] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for assessing the damage status of fuel elements in a pressurized water reactor, characterized in that, include: During the operation of the pressurized water reactor (PWR), the coolant is sampled periodically to measure and record the Xe-133 activity in the coolant sample. The reactor power at each sampling time is also recorded. If the change in the currently recorded power relative to the previously recorded power does not exceed a first preset percentage, the currently recorded Xe-133 activity is considered valid data; if the change in the currently recorded power relative to the previously recorded power exceeds the first preset percentage, the currently recorded Xe-133 activity is considered invalid data. The discharge flow rate is measured. If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate does not exceed a second preset percentage, then the currently recorded Xe-133 activity is valid data. If the change in the current recorded discharge flow rate relative to the previous recorded discharge flow rate exceeds a second preset percentage, then the currently recorded Xe-133 activity is invalid data. Based on the valid data, a curve representing the change in Xe-133 activity over time is generated. If the curve exhibits a stepped shape, it is confirmed that the fuel element has been damaged, and the number of steps represents the number of damaged fuel elements.

2. The method for assessing the damage status of fuel elements in a pressurized water reactor according to claim 1, characterized in that, The first preset percentage is 5%.

3. The method for assessing the damage status of fuel elements in a pressurized water reactor according to claim 1, characterized in that, The step of sampling the coolant of the pressurized water reactor at regular intervals includes: obtaining coolant samples from the drain pipe connected to the primary loop of the pressurized water reactor at regular intervals.

4. The method for assessing the damage status of fuel elements in a pressurized water reactor according to claim 1, characterized in that, The step of sampling the coolant of the pressurized water reactor at regular intervals includes: sampling the coolant of the pressurized water reactor once every day or every two days.

5. The method for assessing the damage status of fuel elements in a pressurized water reactor according to claim 1, characterized in that, The evaluation method further includes: when the number of identified damaged fuel elements exceeds a preset number, the evaluation method fails.

6. The method for assessing the damage status of fuel elements in a pressurized water reactor according to claim 1, characterized in that, The step of generating a curve representing the change of Xe-133 activity over time includes: using the horizontal axis to represent time and the vertical axis to represent Xe-133 activity, wherein the horizontal axis is a linear coordinate and the vertical axis is a logarithmic coordinate, thereby generating a curve representing the change of Xe-133 activity over time.

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