Method for judging reliability of pressurized water reactor fuel

Abstract

The invention belongs to the technical field of nuclear power plant nuclear physical fuel and chemical monitoring, and particularly relates to a method for judging the reliability of pressurized water reactor fuel. The method comprises the following steps: 1, regularly monitoring a gamma spectrum of a coolant of a primary loop coolant system of a reactor to obtain the specific activity of I-131 and I-134 nuclides, 2, substituting the iodine specific activity and the purification flow into an index formula to obtain a CF value, 3, comparing the CF value with a constant C, judging whether fuel is damaged, and judging that the fuel is damaged when CF is greater than or equal to the constant C. The method solves the technical problems and consequences that the result of an index formula used in the prior art does not have physical significance actually, and the formula used in the method is not applicable at low power, so that fuel damage identification lags behind sometimes.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear physics fuel and chemical monitoring in nuclear power plants, and particularly relates to a method for judging the reliability of pressurized water reactor fuel. Background Art

[0002] The commonly used method for judging fuel reliability in current nuclear power plants is as follows: First, the nuclear power plant regularly monitors the gamma spectrum of the RCP (reactor primary coolant system, the same below) coolant to obtain the specific activity including nuclides such as I-131 and I-134. Then, the WANO (World Association of Nuclear Operators) fuel reliability index formula (FRI, the same below) is called, and the iodine specific activity, unit power, and purification flow rate (the volume of the main system water is fixed and does not need to be collected each time) are substituted into the formula. Finally, the current integrity of the nuclear fuel is judged according to the obtained FRI value. When the nuclear fuel is intact, this value is close to 0. When the FRI value reaches 19 or more, it can be confirmed that the fuel is damaged.

[0003] The FRI index formula of WANO is described as follows:

[0004] FRI = [AN 131 - 0.0318AN 134 × [(Ln / LHGR) × (100 / Po)] 1.5

[0005] Since the half-life of I-134 is short, it can be assumed that all the measured I-134 radioactivity comes from the fission of the adhering material. Based on this assumption, the I-131 radioactivity from the current fuel defect can be estimated as follows:

[0006] AD = AN 131 - (k) × (AN 134 ) (Formula 1)

[0007] Where:

[0008] AD = I-131 radioactivity from the fuel (normalized to the general purification rate using Equation 2)

[0009] AN 131 = measured I-131 activity (normalized to the general purification rate using Equation 2)

[0010] AN 134 = measured I-134 activity (normalized to the general purification rate using Equation 2)

[0011] k = adhesion correction factor, a constant, depending on the composition of the adhering material

[0012] The discharge or purification rate of the reactor coolant will affect the concentrations of I-131 and I-134 in the steady-state coolant. To make units with different purification rates comparable, the measured radioactivities of I-131 and I-134 are normalized to a common purification rate constant Bn using the following formula:

[0013]

[0014] Where:

[0015] AN = Normalized isotope activity

[0016] AM = Measured isotope activity

[0017] λ = Radionuclide decay constant

[0018] Ba = Actual purification rate constant, defined as follows:

[0019] Ba = Volume flow rate of the discharge flow corrected to the reactor coolant temperature during operation / Coolant volume excluding the pressurizer at the operating temperature

[0020] Bn = 2×10 -5 / s

[0021] Reactor power has a significant impact on the gases released into the reactor coolant. The exponent value corrects the power level (when operating below 100% power) and normalizes it to a common average linear power density (LHGR) to correct for the generation and release of fission products. The following empirical formula developed by a pressurized water reactor fuel supplier is used for reference.

[0022] PN = [(Ln / LHGR)×(100 / Po)] 1.5 (Formula 3)

[0023] Where:

[0024] PN: Power normalization term

[0025] Ln: Common linear power (18.0 kW / m)

[0026] LHGR: Average linear power of the unit at 100% power

[0027] Po: Average reactor power / %

[0028] The parent nuclei of I-131 and I-134 produced after fission in the adhering substances are three nuclides: U-235, Pu-239, and Pu-241. When U-235 and Pu-241 undergo fission, the iodine production fractions are basically the same. When Pu-239 undergoes fission, the iodine production fraction is quite different from the former. The index assumes that among the parent nuclei of the adhering substance fission, U-235 (and Pu-241) accounts for 30%, and Pu-239 fission accounts for 70% of the total fission. It is assumed that based on the fact that the adhering substance generally goes through multiple fuel cycles and has a high burnup. The fission product fractions in Table 1 are cited again to calculate the adhesion correction coefficient (k).

[0029] Table 1 Decay constants and production fractions of I-131 and I-134

[0030]

[0031] The fission products I-131 and I-134 enter the RCP coolant from the adhering fission substances, and the process is delayed by certain factors. Since the half-life of I-134 is much shorter than that of I-134, the production-release ratios of the two are quite different. In order to correct the delay effect, the following empirical correction model for k is established, which is applicable to the units where the fission products mainly come from the adhering substances.

[0032]

[0033] Where Y = the fraction of the isotope in the fission products

[0034] Substitute the λ and Y values in Table A-1 into the formula to obtain the adhesion correction coefficient k = 0.0318.

[0035] Substitute the specific activities of I-131 and I-134 in the coolant measured in the chemical laboratory into the FRI calculation formula to obtain the FRI value. When the nuclear fuel is not damaged, this value is close to 0. When the FRI value reaches 19 or more, it can be confirmed that the fuel is damaged.

[0036] Technical defect 1: The FRI used in the current technology involves multiple normalizations and is expected to be applicable to reactors with design differences (bleed purification flow rate, operating temperature, fuel linear power density), with empirical corrections superimposed, which results in: ① The FRI formula actually deviates from the nominal meaning of "specific activity of I-131 nuclide from damaged fuel", and the calculation result loses its realistic physical meaning; ② The FRI calculation result is often negative, and it is necessary to artificially stipulate that its value is at least 0.037 (MBq / t);

[0037] Technical defect 2: The FRI formula of this method is artificially stipulated to be only applicable to the condition where the nuclear power is ≥ 85%. The following consequences will occur: ① Forcing the use of the formula at low power results in low accuracy, and the accuracy is almost lost when approaching 0 power; ② Fuel damage mostly occurs under power fluctuation conditions, such as at the initial and final stages of reactor shutdown. It is difficult for this technical formula to identify damage in a timely manner. Summary of the Invention

[0038] The purpose of the present invention is to provide a method for judging the reliability of pressurized water reactor fuel, so as to solve the technical problems and consequences that the result of the index formula used in the current technology actually has no physical meaning, the formula used in the method is not applicable at low power, and sometimes the identification of fuel damage lags behind.

[0039] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0040] A method for judging the reliability of pressurized water reactor fuel. The first step: Regularly monitor the gamma spectrum of the coolant in the primary coolant system of the reactor to obtain the specific activities of I-131 and I-134 nuclides. The second step: Substitute the iodine specific activity and purification flow rate into the index formula to obtain the C F value. The third step: Compare the C F value with the constant C to judge whether the fuel is damaged. When C F ≥ constant C, it is determined that the fuel is damaged. The index formula is as follows:

[0041] C F ×NA = (Am131)(P / λ 131 +V)-(k)×(Am134)(P / λ 134 +V)

[0042] V: Effective water volume of the primary loop; P: Effective letdown purification flow rate; λ: Decay constant of a certain nuclide; Am: Specific activity of measuring a certain nuclide Bq / ton, Am131 is the specific activity of I-131, Am134 is the specific activity of I-134; k: Proportion of I-131 and I-134 nuclides released by adhering fission substances; NA: Avogadro constant; C F : Rate of I-131 nuclide from the fuel entering the primary loop water body to be calculated mol / s.

[0043] k is 0.4811.

[0044] For pressurized water reactors, 1.0E-14 ≤ C ≤ 2.0E-13 mol / s.

[0045] For M310 units, C is taken as 1.0E-13 mol / s.

[0046] The beneficial effects achieved by the present invention are:

[0047] 1. The physical meaning of the indicators used in the method is accurate and intuitive. It is the rate at which the I-131 nuclide from within the fuel cladding enters the primary coolant water during a certain short period before RCP sampling. The result truly presents the rate at which fission products leak out of the cladding at a certain moment after damage. The formula does not require normalization and does not need to be corrected by an empirical model, and the data processing is simple.

[0048] 2. The method has higher determination accuracy and sensitivity. Under the same measurement data, the indicator results of the new method are less likely to be negative. In the existing fuel damage cases in this power plant, the damage response is identified earlier or no later than the existing technology, and there are also no false judgments of damage. The following lists the comparison of the two indicators before and after fuel damage for each of three units once. The bold represents that the current FRI indicator identifies fuel damage, and the italic represents that the method of the present invention identifies fuel damage.

[0049]

[0050] 3. The applicable conditions are loose. This method has no specific requirements for the reactor power and does not need to deliberately obtain power parameters. There are cases where fuel damage is successfully identified when the power is close to 0. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of a method for judging the reliability of PWR fuel. DETAILED DESCRIPTION OF THE INVENTION

[0052] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0053] The new technology can solve the problem, and the fundamental lies in the new indicator. The indicator calculates "the rate at which the I-131 nuclide from the fuel enters the primary coolant water". Whether there is damage to the fuel is judged according to the calculation result. The first step of the present invention is the same as the original technical method: regularly monitor the gamma spectrum of the RCP coolant to obtain the specific activities of the I-131 and I-134 nuclides. In the second step, the iodine specific activity and the purification flow rate are brought into the new indicator formula to obtain the C F value. In the third step, the obtained C F value is compared with the constant C to judge whether the fuel is damaged. The technical indicators of the present invention are more suitable for identifying fuel damage in PWR nuclear power plants. The indicators include the following formula.

[0054] C F ×NA = (Am131) (P / λ 131 +V)-(k)×(Am134) (P / λ 134 +V) (Formula 4)

[0055] Where

[0056] V: Effective water volume of the primary loop

[0057] P: Effective discharge purification flow rate

[0058] λ: Decay constant of a certain nuclide

[0059] Am: Specific activity of a certain nuclide measured (Bq / ton), Am131 is the specific activity of I-131, and Am134 is the specific activity of I-134

[0060] k: Proportion of I-131 and I-134 nuclides produced (released) by adhering fission substances, generally taken as 0.4811

[0061] NA: Avogadro constant

[0062] C F : Rate (mol / s) of I-131 nuclide from fuel entering the primary coolant water to be calculated

[0063] When C F ≥ constant C, it is determined that the fuel is damaged. Generally, for a pressurized water reactor, 1.0E-14 ≤ C ≤ 2.0E-13. According to the experience of the M310 unit, it is recommended that C be taken as 1.0E-13 (mol / s).

[0064] The key points and points to be protected by the present invention are: the index used in the method determines the fuel integrity based on the rate of I-131 nuclide from fuel entering the primary coolant water. The index formula 4 used in the method and its equivalent transformation. The range and value of the determination constant C used in the method.

[0065] The new technology does not involve changes in external systems compared with the current technology, only changes the fuel damage determination index formula and the final determination method. The implementation of the new technology is as follows:

[0066] 1. Obtain the specific activities of I-131 and I-134 at equilibrium in the primary coolant through sampling analysis (the same as the current technology); 2. Obtain the system operation parameters, including: effective water volume V of the primary coolant, effective discharge purification flow rate P (the current technology also additionally requires the unit nuclear power Po)

[0067] 3. Substitute the above analysis data and operation parameters into index formula 4 to calculate C F (Current technology, substitute parameters into index formula FRI)

[0068] 4. When C F ≥ C (the recommended value of C for the M310 unit is 1.0E-13), the fuel is damaged (in the current technology, when FRI > 19, the fuel is damaged).

Claims

1. A method for judging the reliability of a pressurized water reactor fuel, characterized in that: Step 1: Regularly monitor the gamma spectrum of the coolant in the primary coolant system of the reactor to obtain the specific activities of I-131 and I-134 nuclides. In step 2, substitute the iodine specific activity and purification flow rate into the index formula to obtain the C F value. In step 3, compare the C F value with the constant C to determine whether the fuel is damaged. When C F ≥ constant C, it is determined that the fuel is damaged. The index formula is as follows: C F ×NA = (Am131)(P / λ 131 + V)-(k)×(Am134)(P / λ 134 + V) V: The effective water volume of the primary loop; P: The effective letdown purification flow rate; λ: decay constant of a certain nuclide; Am: specific activity of a certain nuclide measured in Bq / ton, Am131 is the specific activity of I-131, and Am134 is the specific activity of I-134; k: proportion of I-131 and I-134 nuclides released by adhering fission materials; NA: Avogadro constant; C F : rate of I-131 nuclide from fuel entering the primary coolant water to be calculated in mol / second.

2. The method for judging the reliability of a pressurized water reactor fuel according to claim 1, wherein: k is 0.4811.

3. The method for judging the reliability of a pressurized water reactor fuel according to claim 1, characterized in that: For a pressurized water reactor, 1.0E-14 ≤ C ≤ 2.0E-13 mol / s.

4. The method for judging the reliability of a pressurized water reactor fuel according to claim 3, characterized in that: For the M310 unit, C is taken as 1.0E-13 mol / s.

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