Method for detecting degree of cleanliness of reactor core fuel assembly

A technology for a fuel assembly and a detection method, which is applied in the field of cleanliness detection, can solve problems such as the difficulty in determining the surface contamination of the fuel assembly, and achieve the effects of solving the difficulty in determining the surface contamination, being easy to operate, and having a simple method.

Inactive Publication Date: 2015-12-23

CHINA NUCLEAR POWER ENG CO LTD

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[0004] Aiming at the defects existing in the prior art, the present invention provides a method for detecting the cleanliness of the core fuel assembly, which quickly detects the total amount of fuel contamination on the surface of the fuel assembly during the manufacturing process, wit...

Abstract

The invention relates to a method for detecting the degree of cleanliness of a reactor core fuel assembly. The method includes the following steps that first, the total quantity of stains on the surface of the entire fuel assembly and the activity of all nuclides in a cooling agent are obtained through theoretical calculation, and the corresponding relation between the total quantity of the stains and the activity of all the nuclides is found out; then the activity of all the nuclides in the cooling agent is obtained through actual measurement; the activities of all the nuclides, obtained in the above two steps, are compared and analyzed, and accordingly the total quantity of the nuclide stains on the surface of the entire fuel assembly is obtained. By the adoption of the method, the surface contamination condition of the fuel assembly can be determined quickly, no extra measurement or operation to the fuel assembly by a nuclear power plant operating unit is needed, the problem that the surface contamination of a traditional fuel element cannot be determined easily is solved, and a fast and feasible method is provided for detection of the degree of cleanliness of the reactor core fuel assembly. The structure is simple and implementation is convenient.

Application Domain

Nuclear energy generationNuclear monitoring

Technology Topic

Nuclear engineeringNuclear power plant +4

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[0025] The specific embodiments of the present invention will be described in further detail below in conjunction with the accompanying drawings.

[0026] The method for detecting the cleanliness of the core fuel assembly provided by the present invention first uses a method combining theoretical calculation and actual measurement of the fission product source term of the main loop of the power plant, and then the source term of the main loop obtained by the calculation and the measured part The comparative analysis of the source terms of the nuclides can finally quickly determine the total amount of contamination on the surface of the fuel assembly by the new fuel during the manufacturing process.

[0027] Among them, the calculation method of the total surface contamination of the core fuel assembly, the specific theoretical calculation process is as follows:

[0028] First, calculate the contaminated surface area of a single fuel rod by formulas (1) and (2); the total amount of fuel element contamination is calculated by formula (3), and then formula (4) is used to calculate the fission products into the coolant The release rate of, and finally put the result into formula (5), the activity of each nuclide in the coolant can be obtained.

[0029] S 1 =π·d·h(1)

[0030] S 2 = 1 2 · π · d 2 - - - ( 2 )

[0031] N = ( S 1 + S 2 ) · N f · e · A 0 A - - - ( 3 )

[0032]

[0033] dN c i d t = θ - ( λ i + Q W · η i + ( 1 - η i ) · B ′ ( B 0 - B ′ · t ) + L W + σ i · φ · C V ) · N c i + X j = 1 n ( f i j · λ j ) · N c i + X k = 1 n ( σ k · φ · C V R ) · N c k - - - ( 5 )

[0034] among them:

[0035] d = outer diameter of fuel cladding (cm);

[0036] h=length of fuel rod (cm);

[0037] N f = The total number of fuel rods in the core;

[0038] e = enrichment degree of fuel;

[0039] A = atomic weight of nuclide;

[0040] k = assumed fraction of fission products into the coolant;

[0041] N = the total number of atoms contaminated by the fuel element;

[0042] Y = Fission yield (1/s);

[0043] Σ = Macro section (1/cm);

[0044] Φ=neutron flux (n/cm 2 ·S);

[0045] θ = release rate of fission products (1/s);

[0046] N c = The total amount of radionuclides in the reactor coolant;

[0047] ν = the escape rate coefficient of the nuclide (1/s);

[0048] N p = The total amount of radionuclides in the core damaged fuel rod;

[0049] λ = the decay constant of the nuclide (1/s);

[0050] Q=The discharge flow rate of reactor coolant (g/s);

[0051] W = mass of reactor coolant (g);

[0052] η=Removal efficiency of nuclide by chemical and volume control system;

[0053] B'= reduction rate of boron concentration (ppm/s);

[0054] B 0 = Initial boron concentration (ppm);

[0055] t = running time (s);

[0056] L = leakage rate of reactor coolant (g/s);

[0057] σ = microscopic absorption cross section (cm 2 );

[0058] C V = Ratio of core coolant volume to reactor coolant volume;

[0059] f=decay branch ratio (%);

[0060] i = the currently calculated nuclide;

[0061] j = nuclide in the same decay chain as i nuclide;

[0062] k = the nuclide from which neutron capture produces i nuclide;

[0063] For the actual measurement method of the fission product source term in the main circuit of the power plant, there are two cases. For short-lived nuclides, such as 133 The time for measuring samples such as I is 16h after sampling, and the first measurement of the sample; for longer-lived nuclides, the time can be appropriately extended to avoid the influence of short-lived nuclides on the γ spectrum, such as 131 The measurement time of I is 48h after sampling, the first measurement is performed on the sample; after 72h, the second measurement is performed on the two samples respectively, and the measurement results are compared with the measurement results of short-lived and long-lived nuclides. Compare and verify. In order to ensure the accuracy of the measurement result, the present invention uses the following formula to correct the measurement result, and the correction does not consider the decay of other mother nuclei to produce daughter nuclei:

[0064] A 0 =A·e λ·t (6)

[0065] among them:

[0066] A0=The actual activity value of the nuclide in the main coolant (Bq);

[0067] A = the measured activity value of the nuclide in the main coolant (Bq);

[0068] λ = decay constant (1/s);

[0069] t = time interval from sampling to measurement (s).

[0070] Compare the calculated specific activity of the specific nuclide in the main circuit with the actual measured value to get the total amount of contamination on the surface of the fuel assembly, as follows:

[0071] Firstly, the specific activity of the fission product of the main circuit corresponding to the different total amount of contaminated uranium in a specific power plant is calculated and expressed in the form of a curve; then according to the actual measured value, it can be easily obtained by comparing with the existing curve. The total amount of contamination on the fuel assembly surface.

[0072] Such as figure 1 As shown, the present invention is 235 Taking U as an example, the total amount of contamination on the surface of the fuel assembly is approximately 2.4 g. The present invention will take into account the different release mechanisms of fission products and compare Xe-133, I-131 and I-133. On the one hand, it can ensure that the measured activity of the nuclide comes from contamination outside the fuel element, and on the other hand it can avoid only Calculated data, there is no actual measured value. In particular, when there is a big difference between the conclusions of iodine and inert gas, iodine is the dominant one, because the uncertainty of the inert gas in the primary circuit coolant is greater.

[0073] The method for detecting the cleanliness of the core fuel assembly of the present invention is not limited to the above specific embodiments. Those skilled in the art can obtain other embodiments based on the technical solution of the present invention, which also belong to the technical innovation scope of the present invention.

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Method for detecting degree of cleanliness of reactor core fuel assembly

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