An evaluation method for monitoring damage of fuel assemblies in nuclear power plants

Abstract

The present invention provides a method for evaluating damage monitoring of nuclear power plant fuel assemblies. The method comprises: determining whether a fuel assembly has been damaged based on the measured Xe-133 activity in the reactor coolant under stable pressurized water reactor (PWR) operation, correcting for the contribution of core sparse matter and power, and normalizing the Xe-133 activity value to a 100% linear power coefficient. The method is widely applicable to PWR fuel assembly damage assessment, and is particularly suitable for damage cases characterized by xenon growth alone without iodine growth.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear power technology and relates to an evaluation method, in particular to an evaluation method for monitoring damage of fuel assemblies in nuclear power plants. Background Art

[0002] Fuel assemblies serve as the first line of defense for nuclear power plants, and their integrity is directly linked to their safety, reliability, and economic viability. In recent years, advancements in fuel assembly manufacturing have significantly improved fuel reliability. However, with the continued expansion of my country's nuclear power fleet, fuel assembly damage is a growing concern during actual reactor operation.

[0003] Currently, during normal operation, nuclear power plants regularly conduct radionuclide chemical analysis of the coolant in the primary circuit of the unit. By analyzing the radioactivity levels of relevant nuclides such as I, Xe, Kr, and Ce in the coolant, trend analysis and tracking of these nuclides are established to enable timely assessment of the integrity of the fuel assembly cladding. Existing cladding integrity assessment technology primarily relies on qualitative analysis and tracking of changes in radionuclide values ​​to achieve a qualitative assessment of cladding integrity.

[0004] Researchers analyzing recent fuel assembly failures at various nuclear power plants have found that, once a fuel assembly failure occurs, the size and morphology of the failure are closely correlated with changes in the number of radionuclides in the primary circuit. If the cladding failure is extensive, growth in both Xe-related and I-related nuclides can be detected in the primary circuit. However, if the cladding failure is minor, such as a "tight" rupture defect, growth in nuclides such as Xe and Kr may be detected, but not necessarily in I-related nuclides.

[0005] Therefore, the present invention considers developing a calculation method that processes data on relevant nuclides in the primary circuit, normalizes the relevant parameters based on the characteristics of different units, and uses the calculated results to assess whether a unit has experienced fuel assembly damage and the extent of the damage. Given that both small and large fuel assembly breaches can result in an increase in xenon levels in the primary circuit, the present invention considers using Xe values ​​for this calculation and analysis. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide an evaluation method for monitoring the damage of nuclear power plant fuel assemblies. The evaluation method can be widely applied to the damage evaluation of pressurized water reactor fuel assemblies, and is particularly suitable for damage types where only xenon growth occurs but no iodine growth occurs.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] The present invention provides an evaluation method for monitoring damage of nuclear power plant fuel assemblies. The evaluation method comprises the following steps: when a pressurized water reactor is in stable operation, the Xe-133 activity in the reactor coolant is measured, corrected for the contribution of core haze and power, and normalized to a 100% linear power coefficient to obtain a Xe-133 activity value, and whether the fuel assembly is damaged is determined based on the Xe-133 activity value.

[0009] The present invention quantitatively calculates and evaluates fuel assembly damage based on the radioactive activity of Xe-133 and Xe-138 nuclides measured during chemical sampling and analysis. This evaluation method is widely applicable to damage assessment of pressurized water reactor fuel assemblies, particularly for damage involving xenon growth alone without iodine growth. Nuclear power plants can use this method to assess the condition of their fuel assemblies. Furthermore, because the evaluation method incorporates normalization, the damage levels of fuel assemblies at different nuclear power plants can be compared horizontally based on the calculated results.

[0010] Preferably, the stable operating condition is specifically: the power plant operates stably for at least 3 consecutive days at a power step with a power variation not exceeding ±5%.

[0011] In the present invention, the power change of the power plant under stable operating conditions can be ±1% power level, ±2% power level, ±3% power level, ±4% power level or ±5% power level, and the number of days of continuous stable operation can be 3 days, 4 days, 5 days, 6 days, 7 days, 8 days, 9 days or 10 days, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0012] Preferably, the calculation formula for the Xe-133 activity value is:

[0013] FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5

[0014] Among them, FRI Xe is the calculated value of Xe-133 activity after normalization;

[0015] c 133 is the percentage of Xe-133 in the gas phase of the downflow system, which is taken as 1.3;

[0016] A 133 is the measured value of Xe-133 concentration in the next circuit at steady state, in Bg / g;

[0017] k138 is the Xe-133 / Xe-138 recoil ratio when the fission source is 30% U-235 and 70% Pu-239, which is 1.36;

[0018] A 138 is the measured value of Xe-138 concentration in the next circuit at steady state, in Bg / g;

[0019] L N is the baseline power density, and the uniform value for pressurized water reactors is 18KW / m;

[0020] LHGR is the actual average linear power density of the unit at full power, which is determined by each nuclear power plant based on the actual unit design parameters and is expressed in KW / m2.

[0021] Po is the average reactor power when the Xe-133 is measured.

[0022] Preferably, the value of Po is ≥85% of the power level to obtain a more reliable index value, for example, it can be 85% of the power level, 86% of the power level, 87% of the power level, 88% of the power level, 89% of the power level, 90% of the power level, 91% of the power level, 92% of the power level, 93% of the power level, 94% of the power level or 95% of the power level, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0023] Preferably, the specific evaluation criteria for determining whether the fuel assembly is damaged based on the Xe-133 activity value are:

[0024] (1) When FRI Xe When ≤3.7Bg / g, the fuel assembly is judged to be undamaged;

[0025] (2) When FRI Xe When the concentration is ≥3700Bg / g, the fuel assembly is judged to be damaged;

[0026] (3) When 3.7Bg / g<FRI Xe When the concentration is less than 3700 Bg / g, it is impossible to determine whether the fuel assembly is damaged.

[0027] By analyzing the radiochemical data of fuel assembly damage in nuclear power plants at home and abroad in recent years, it is found that the growth of Xe-133 caused by fuel assembly damage generally shows FRI. Xe The calculated value is greater than 3700Bg / g; if the fuel assembly is not damaged, FRI Xe It is at a very low level, generally less than 3.7Bg / g, or even negative.

[0028] As a preferred technical solution of the present invention, the evaluation method is: when the pressurized water reactor is in stable operating conditions, the Xe-133 activity in the reactor coolant is measured, corrected for the contribution of core mist matter and power size, and normalized to a 100% linear power coefficient to obtain a Xe-133 activity value, and whether the fuel assembly is damaged is determined based on the Xe-133 activity value.

[0029] The stable operating condition is specifically: the power plant operates stably for at least 3 consecutive days at a power step with a power variation of no more than ±5%.

[0030] The calculation formula of the Xe-133 activity value is:

[0031] FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5

[0032] Among them, FRI Xe is the calculated value of Xe-133 activity after normalization;

[0033] c 133 is the percentage of Xe-133 in the gas phase of the downflow system, which is taken as 1.3;

[0034] A 133 is the measured value of Xe-133 concentration in the next circuit at steady state, in Bg / g;

[0035] k 138 is the Xe-133 / Xe-138 recoil ratio when the fission source is 30% U-235 and 70% Pu-239, which is 1.36;

[0036] A 138 is the measured value of Xe-138 concentration in the next circuit at steady state, in Bg / g;

[0037] L N is the baseline power density, and the uniform value for pressurized water reactors is 18KW / m;

[0038] LHGR is the actual average linear power density of the unit at full power, which is determined by each nuclear power plant based on the actual unit design parameters and is expressed in KW / m2.

[0039] Po is the average reactor power when measuring Xe-133, and is taken as ≥85% power level.

[0040] The specific evaluation criteria for determining whether a fuel assembly is damaged based on the Xe-133 activity value are as follows:

[0041] (1) When FRI Xe When ≤3.7Bg / g, the fuel assembly is judged to be undamaged;

[0042] (2) When FRI Xe When the concentration is ≥3700Bg / g, the fuel assembly is judged to be damaged;

[0043] (3) When 3.7Bg / g<FRI Xe When the concentration is less than 3700 Bg / g, it is impossible to determine whether the fuel assembly is damaged.

[0044] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The present invention quantitatively calculates and evaluates fuel assembly damage based on the radioactive activity of Xe-133 and Xe-138 nuclides measured during chemical sampling and analysis. This evaluation method is widely applicable to damage assessment of pressurized water reactor fuel assemblies, particularly for damage involving xenon growth alone without iodine growth. Nuclear power plants can use this method to assess the condition of their fuel assemblies. Furthermore, because the evaluation method incorporates normalization, the damage levels of fuel assemblies at different nuclear power plants can be compared horizontally based on the calculated results. DETAILED DESCRIPTION

[0047] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0048] Example 1

[0049] This embodiment provides an evaluation method for monitoring damage to nuclear power plant fuel assemblies. The evaluation method comprises the following steps: when a pressurized water reactor is in stable operating conditions, the Xe-133 activity in the reactor coolant is measured, corrected for the contribution of core haze and power, and normalized to a 100% linear power coefficient to obtain a Xe-133 activity value. Whether the fuel assembly is damaged is determined based on the Xe-133 activity value.

[0050] In this embodiment, the stable operating condition specifically refers to: the power plant continuously and stably operates for more than 3 days at a power step with a power variation of no more than ±5%.

[0051] The specific parameters selected in this embodiment during normal operation of a pressurized water reactor nuclear power plant are:

[0052] LHGR: 18.76KW / m; the power Po at the sampling time is 99.5.

[0053] The actual activity data of Xe-133 and Xe-138 obtained from the measurements are:

[0054] A 133 =36.8Bg / g; A 138 =66.3Bg / g.

[0055] Substituting into the formula, we can get:

[0056] FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5

[0057] =(1.3×36.8-1.36×66.3)[(18 / 18.76)(100 / 99.5)] 1.5

[0058] =-40.08Bg / g

[0059] Since FRI Xe =-40.08Bg / g≤3.7Bg / g, indicating that the fuel assembly is not damaged, which is consistent with the actual situation.

[0060] Example 2

[0061] This embodiment provides an evaluation method for monitoring damage to nuclear power plant fuel assemblies. The evaluation method comprises the following steps: when a pressurized water reactor is in stable operating conditions, the Xe-133 activity in the reactor coolant is measured, corrected for the contribution of core haze and power, and normalized to a 100% linear power coefficient to obtain a Xe-133 activity value. Whether the fuel assembly is damaged is determined based on the Xe-133 activity value.

[0062] In this embodiment, the stable operating condition specifically refers to: the power plant continuously and stably operates for more than 5 days at a power step with a power variation of no more than ±5%.

[0063] The specific parameters selected in this embodiment during normal operation of a pressurized water reactor nuclear power plant are:

[0064] LHGR: 18.76KW / m; the power Po at the sampling time is 99.

[0065] The actual activity data of Xe-133 and Xe-138 obtained from the measurements are:

[0066] A133 =4802.9Bg / g; A 138 =69.5Bg / g.

[0067] Substituting into the formula, we can get:

[0068] FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5

[0069] =(1.3×4802.9-1.36×69.5)[(18 / 18.76)(100 / 99)] 1.5

[0070] =5866.99Bg / g

[0071] Since FRI Xe =5866.99Bg / g≥3700Bg / g, it is determined that the fuel assembly has been damaged, which is consistent with the actual situation.

[0072] Example 3

[0073] This embodiment provides an evaluation method for monitoring damage to nuclear power plant fuel assemblies. The evaluation method comprises the following steps: when a pressurized water reactor is in stable operating conditions, the Xe-133 activity in the reactor coolant is measured, corrected for the contribution of core haze and power, and normalized to a 100% linear power coefficient to obtain a Xe-133 activity value. Whether the fuel assembly is damaged is determined based on the Xe-133 activity value.

[0074] In this embodiment, the stable operating condition specifically refers to: the power plant continuously and stably operates for more than 7 days at a power step with a power variation of no more than ±5%.

[0075] The specific parameters selected in this embodiment during normal operation of a pressurized water reactor nuclear power plant are:

[0076] LHGR: 18.76KW / m; the power Po at the sampling time is 99.

[0077] The actual activity data of Xe-133 and Xe-138 obtained from the measurements are:

[0078] A 133 =450.6Bg / g; A 138 =78.8Bg / g.

[0079] Substituting into the formula, we can get:

[0080] FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5

[0081] =(1.3×450.6-1.36×78.8)[(18 / 18.76)(100 / 99)] 1.5

[0082] =456.6Bg / g

[0083] Since FRI Xe =456.6Bg / g, which is between 3.7Bg / g and 3700Bg / g. It is impossible to determine whether the fuel assembly is damaged.

[0084] Comparative Example 1

[0085] This comparative example provides an evaluation method for monitoring damage to nuclear power plant fuel assemblies. The evaluation method comprises the following steps: regularly sampling and analyzing the fission products I-131, I-132, I-133, I-134, Xe-133, Xe-135, Xe-138, Kr-85, Kr-87, Cs-134, and Cs-137 of the primary coolant in a pressurized water reactor nuclear power plant, and performing trend analysis. If, during a sudden power surge (power change greater than 20%) or a shutdown and depressurization of the unit, one or more of the above nuclides exhibits a peak value, i.e., an increase of more than five times compared to the previous steady state, it can be determined that the fuel assembly is damaged.

[0086] However, during normal operation, the growth of these nuclides is not entirely due to fuel damage. Adjustments to the unit's purge and downflow capabilities also affect radiochemical data. Therefore, relying solely on radiochemical data trends to determine fuel assembly damage is unreliable. Furthermore, in the case of a damaged "compact" fuel assembly, it's highly likely that only xenon will be observed, with no or minimal growth in other nuclides.

[0087] Thus, the present invention quantitatively calculates and evaluates fuel assembly damage based on the radioactive activity values ​​of Xe-133 and Xe-138 nuclides measured during chemical sampling and analysis. This evaluation method is widely applicable to damage assessment of pressurized water reactor fuel assemblies, particularly for damage characterized by xenon growth alone without iodine growth. Nuclear power plants can use this method to assess the condition of their fuel assemblies. Furthermore, because the evaluation method incorporates normalization, the damage levels of fuel assemblies at different nuclear power plants can be compared horizontally based on the calculated results.

[0088] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for evaluating damage monitoring of fuel assemblies in nuclear power plants, characterized in that: The evaluation method comprises: when the pressurized water reactor is in stable operation, the Xe-133 activity value in the reactor coolant is obtained by correcting the Xe-133 activity by the contribution of the core mist material and the power, and normalizing the Xe-133 activity value to 100% linear power coefficient, and determining whether the fuel assembly is damaged based on the Xe-133 activity value; The calculation formula of the Xe-133 activity value is: FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5 Among them, FRI Xe is the calculated value of Xe-133 activity after normalization; c 133 is the percentage of Xe-133 in the gas phase of the downflow system, which is taken as 1.3; A 133 is the measured value of Xe-133 concentration in the next circuit at steady state, in Bg / g; k 138 is the Xe-133 / Xe-138 recoil ratio when the fission source is 30% U-235 and 70% Pu-239, which is 1.36; A 138 is the measured value of Xe-138 concentration in the next circuit at steady state, in Bg / g; L N is the baseline power density, and the uniform value for pressurized water reactors is 18KW / m; LHGR is the actual average linear power density of the unit at full power, which is determined by each nuclear power plant based on the actual unit design parameters and is expressed in KW / m2. Po is the average reactor power when the Xe-133 is measured.

2. The evaluation method according to claim 1, wherein The stable operating condition is specifically: the power plant operates stably for at least 3 consecutive days at a power step with a power variation of no more than ±5%.

3. The evaluation method according to claim 1, wherein: The value of Po is ≥85% of the power level.

4. The evaluation method according to claim 1, wherein: The specific evaluation criteria for determining whether a fuel assembly is damaged based on the Xe-133 activity value are as follows: (1) When FRI Xe When ≤3.7Bg / g, the fuel assembly is judged to be undamaged; (2) When FRI Xe When the concentration is ≥3700Bg / g, the fuel assembly is judged to be damaged; (3) When 3.7Bg / g<FRI Xe When the concentration is less than 3700 Bg / g, it is impossible to determine whether the fuel assembly is damaged.

5. The evaluation method according to any one of claims 1 to 4, characterized in that: The evaluation method comprises: when the pressurized water reactor is in stable operation, the Xe-133 activity value in the reactor coolant is obtained by correcting the Xe-133 activity by the contribution of the core mist material and the power, and normalizing the Xe-133 activity value to 100% linear power coefficient, and determining whether the fuel assembly is damaged based on the Xe-133 activity value; The stable operating condition is specifically: the power plant operates stably for at least 3 consecutive days at a power step with a power variation of no more than ±5%; The calculation formula of the Xe-133 activity value is: FRI Xe =(c 133 A 133 -k 138 A 138 )[(L N / LHGR)(100 / Po)] 1.5 Among them, FRI Xe is the calculated value of Xe-133 activity after normalization; c 133 is the percentage of Xe-133 in the gas phase of the downflow system, which is taken as 1.3; A 133 is the measured value of Xe-133 concentration in the next circuit at steady state, in Bg / g; k 138 is the Xe-133 / Xe-138 recoil ratio when the fission source is 30% U-235 and 70% Pu-239, which is 1.36; A 138 is the measured value of Xe-138 concentration in the next circuit at steady state, in Bg / g; L N is the baseline power density, and the uniform value for pressurized water reactors is 18KW / m; LHGR is the actual average linear power density of the unit at full power, which is determined by each nuclear power plant based on the actual unit design parameters and is expressed in KW / m2. Po is the average reactor power when measuring Xe-133, which is taken as ≥85% power level; The specific evaluation criteria for determining whether a fuel assembly is damaged based on the Xe-133 activity value are as follows: (1) When FRI Xe When ≤3.7Bg / g, the fuel assembly is judged to be undamaged; (2) When FRI Xe When the concentration is ≥3700Bg / g, the fuel assembly is judged to be damaged; (3) When 3.7Bg / g<FRI Xe When the concentration is less than 3700 Bg / g, it is impossible to determine whether the fuel assembly is damaged.