Method and system for monitoring primary circuit water loading in low operating mode

Abstract

The present invention discloses a method and system for monitoring the primary circuit water level in a low-operation mode. The method comprises: determining a nuclear power unit's reference source term and at least one dose monitoring point, using software to fit a reference radiation dose rate-water level relationship formula for the dose monitoring point; installing a dose rate monitoring instrument at the dose monitoring point, and calculating a dose rate correction factor k using the equation k = Dfb / Dfr; using the dose rate monitoring instrument to monitor the actual radiation dose rate at the dose monitoring point; correcting the actual radiation dose rate at the dose monitoring point to a reference radiation dose rate using the dose rate correction factor k, substituting the factor into the reference radiation dose rate-water level relationship formula for the dose monitoring point to calculate the pressure vessel water level; and displaying the pressure vessel water level on a display device. Compared with existing technologies, the present invention enables continuous measurement of the primary circuit water level in low-operation modes such as maintenance cold shutdown or refueling cold shutdown, providing an important basis for monitoring the core cooling status after an accident.

Description

Technical Field

[0001] The present invention belongs to the technical field of monitoring the cooling state of the core after an accident in a nuclear power plant. More specifically, the present invention relates to a method and system for monitoring the primary circuit water loading in a low operating mode of a pressurized water reactor nuclear power plant. Background Art

[0002] The Three Mile Island nuclear accident made the industry realize the necessity of installing primary water level monitoring instruments, especially pressure vessel water level monitoring instruments. The pressure vessel water level provides an important basis for monitoring the core cooling state after the accident, so as to ensure that the core cooling state can be diagnosed in a timely, accurate and convenient manner, and then select the appropriate accident operation strategy. Different schemes have been formed according to the different types of instruments. The most widely used schemes are: (1) Continuous measurement of the pressure vessel water level by measuring the differential pressure between the bottom and top of the pressure vessel, or the differential pressure between the hot pipe section and the bottom of the pressure vessel, such as the differential pressure pressure vessel water level measurement device (CCMS) disclosed in the document "Design of the Core Cooling Monitoring System for the Qinshan Nuclear Power Phase II Expansion Project" (Nuclear Power Engineering, Vol. 29, 2008.2) and the principle and method of calculating the pressure vessel water level by measuring the differential pressure disclosed in the document "Design of the Core Cooling Monitoring System under the SOP Regulations" (Nuclear Power Engineering, Vol. 33, 2012.10); (2) Using thermocouples to intermittently measure the fixed height water level.

[0003] However, whether it is to continuously monitor the water content of the pressure vessel by measuring the differential pressure between the bottom and top of the pressure vessel, or to intermittently measure the water level at a fixed height using thermocouples, neither can be used during the removal of the pressure vessel top cover in low operating mode (maintenance cold shutdown and refueling cold shutdown): (1) For differential pressure pressure vessel water level measurement, its upper pressure lead line is directly derived from the exhaust line on the pressure vessel top cover, and the core outlet coolant temperature measurement thermocouples required for calculating the pressure vessel water level based on the differential pressure are introduced into the core from the instrument tubes at the pressure vessel top cover. During the removal of the pressure vessel top cover, these instrument tubes will be removed, and the differential pressure measurement and the core outlet coolant temperature measurement will be lost; (2) For intermittent thermocouple measurement of fixed height water level, the thermocouples are introduced into the core from the instrument tubes at the pressure vessel top cover and arranged at a fixed height. During the removal of the pressure vessel top cover, these instrument tubes will be removed, and the thermocouple measurements will be lost.

[0004] Therefore, during the period when the pressure vessel top cover is removed in low operating mode, the monitoring of the primary circuit water level can only rely on the loop water level gauge installed in the cold section or hot section. Figure 1As shown, the pressure vessel carrying the core nuclear fuel is located at the bottom of the reactor pool, and the installed reactor pool loop water level gauge has a limited measurement range. The lowest measurable water level is generally at the bottom of the cold and hot pipe sections. When the primary circuit water level continues to deteriorate (for example, the loss of residual heat removal capability causes the coolant in the pressure vessel to continue boiling and evaporating, and the means of recovering the primary circuit water level are unavailable), the pressure vessel water level is lower than the elevation of the bottom of the hot pipe section, and the means of monitoring the primary circuit water level will be completely lost.

[0005] In view of this, it is indeed necessary to provide a method and system for monitoring the water content in a primary circuit under a low operating mode that can solve the above problems. Summary of the Invention

[0006] The purpose of the present invention is to provide a method and system for monitoring the water level in a single circuit under low operating mode, so as to realize continuous measurement of the water level in the pressure vessel during the removal of the pressure vessel top cover under low operating mode, thereby ensuring continuous monitoring of the core cooling state.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides a method for monitoring the water content of a circuit in a low operating mode, comprising the following steps:

[0008] 1) Determine the reference source term of the nuclear power unit and at least one dose monitoring point located above the pressure vessel, and use software to fit the reference radiation dose rate-water level relationship formula of the dose monitoring point;

[0009] 2) installing a dose rate monitoring instrument at the dose monitoring point and calculating the dose rate correction factor k as k = Dfb / Dfr; wherein Dfr and Dfb are the actual radiation dose rate and the reference radiation dose rate at the dose monitoring point when the water level of the pressure vessel is at the same height, respectively;

[0010] 3) Use a dose rate monitoring instrument to monitor the actual radiation dose rate at the dose monitoring point;

[0011] 4) Use the dose rate correction factor k to correct the actual radiation dose rate at the dose monitoring point to the reference radiation dose rate, substitute it into the reference radiation dose rate-water level relationship formula at the dose monitoring point, and calculate the pressure vessel water level;

[0012] 5) The water level of the pressure vessel is displayed on the display device.

[0013] As a preferred embodiment of the method for monitoring the primary water charge in the low operating mode of the present invention, the dose monitoring point is located above the reactor water pool, at a position where a dose rate monitoring instrument can be easily installed to measure the radiation dose rate. Since the dose monitoring point is located outside the pressure vessel top cover, in step 1), for any dose monitoring point, it is necessary to fit a reference radiation dose rate-water level relationship formula according to two situations: with the pressure vessel top cover removed and with the pressure vessel top cover not removed. Correspondingly, in step 4), after the actual radiation dose rate of the dose monitoring point is corrected to the reference radiation dose rate using the dose rate correction factor k, it is necessary to substitute the corrected reference radiation dose rate into the reference radiation dose rate-water level relationship formula under the situation where the pressure vessel top cover is removed or not removed, according to the pressure vessel top cover state corresponding to the actual radiation dose rate, to calculate the pressure vessel water level.

[0014] As a preferred embodiment of the method for monitoring the primary water level in the low operating mode of the present invention, in step 1), the process of using software to fit the reference radiation dose rate-water level relationship formula of the dose monitoring point is:

[0015] The radionuclides stored in the spent fuel assemblies were conservatively considered as naked gamma sources under the reactor pool. Radiation shielding calculation software was used to calculate the baseline radiation dose rates at the dose monitoring points when the pressure vessel was at different water levels, with and without the pressure vessel cover removed. The logarithm of the baseline radiation dose rate was then taken to generate the baseline radiation dose rate-water level relationship equation:

[0016] When the pressure vessel top cover is removed, RPVL1=a1-b1*(log10(Dfb));

[0017] When the pressure vessel cover is not removed, RPVL2 = a2-b2*(log10(Dfb));

[0018] In the above formula, RPVL1 is the water level when the pressure vessel top cover is removed, in m; RPVL2 is the water level when the pressure vessel top cover is not removed, in m; a1, b1, a2, and b2 are the calculation coefficients obtained by fitting; Dfb is the reference radiation dose rate at the dose monitoring point, in mGy / h.

[0019] As a preferred implementation of the method for monitoring the primary water charge in the low operating mode of the present invention, the dose rate correction factor k is calculated as follows: when the water level in the pressure vessel is at a known value, the actual radiation dose rate Dfr at the dose monitoring point is measured, and Dfr and the reference radiation dose rate Dfb when the water level in the pressure vessel is at the same height, calculated based on the reference source term, are substituted into k = Dfb / Dfr to calculate the dose rate correction factor k.

[0020] As a preferred embodiment of the method for monitoring the primary circuit water content in the low operating mode of the present invention, the dose rate correction factor k is calculated as follows: during the refueling overhaul, the primary circuit needs to be lowered to the flange surface water level before the pressure vessel top cover is removed, and the actual radiation dose rate Dfr measured when the water level is lowered to the flange surface is recorded. Dfr and the reference radiation dose rate Dfb at the flange surface water level calculated based on the reference source term are substituted into k = Dfb / Dfr to calculate the dose rate correction factor k.

[0021] As a preferred embodiment of the method for monitoring the water content in a single circuit under the low operating mode of the present invention, in step 5), the display device provides a numerical display of the water level of the pressure vessel, and also displays the water level information in the form of a bar graph, and identifies the key water level threshold.

[0022] As a preferred embodiment of the method for monitoring the primary loop water loading amount in the low operation mode of the present invention, the measurement range of the pressure vessel water level covers the interval from the lower measurement limit of the loop liquid level gauge to the top of the core spent fuel assembly.

[0023] In order to achieve the above-mentioned object of the invention, the present invention further provides a primary circuit water quantity monitoring system in low operation mode, comprising:

[0024] Dose rate monitoring instrument, used to monitor the actual radiation dose rate at the dose monitoring point;

[0025] A fitting module, used to use software to fit a reference radiation dose rate-water level relationship formula for at least one dose monitoring point;

[0026] a calculation module for calculating a dose rate correction factor k using k = Dfb / Dfr, and for correcting the actual radiation dose rate at the dose monitoring point to a reference radiation dose rate using the dose rate correction factor k, and substituting the reference radiation dose rate-water level relationship formula at the dose monitoring point into the formula to calculate the pressure vessel water level; wherein Dfr and Dfb are the actual radiation dose rate and the reference radiation dose rate at the dose monitoring point, respectively, when the pressure vessel water level is at the same height; and

[0027] Display device, used to display the water level of the pressure vessel.

[0028] As a preferred embodiment of the primary-loop water charge monitoring system in the low operating mode of the present invention, the dose monitoring point is located above the reactor water pool, at a position where a dose rate monitoring instrument can be easily installed to realize radiation dose rate measurement; the fitting module is used to fit a reference radiation dose rate-water level relationship formula for any dose monitoring point according to two situations: with the pressure vessel top cover removed and with it not removed; the calculation module uses the dose rate correction factor k to correct the actual radiation dose rate of the dose monitoring point to the reference radiation dose rate, and then, according to the pressure vessel top cover state corresponding to the actual radiation dose rate, the corrected reference radiation dose rate is substituted into the reference radiation dose rate-water level relationship formula under the situation where the pressure vessel top cover is removed or not removed to calculate the pressure vessel water level.

[0029] As a preferred embodiment of the single-loop water content monitoring system in the low operating mode of the present invention, the display device is used to provide a numerical display of the water level of the pressure vessel, and is also used to display the water level information in the form of a bar graph and identify key water level thresholds.

[0030] Compared with the existing technology, the maintenance path generation method and system of the present invention under the radiation environment of a nuclear power plant can realize the continuous measurement of the primary circuit water filling amount (especially the water level of the pressure vessel) in low operating modes such as maintenance cold shutdown or refueling cold shutdown, providing an important basis for monitoring the core cooling status after the accident, so as to ensure that the core cooling status can be correctly diagnosed in a timely, accurate and convenient manner, and then the appropriate accident operation strategy can be selected to ensure the safety of the core. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The following describes in detail the method and system for monitoring the primary circuit water level in the low operating mode of the present invention in conjunction with the accompanying drawings and specific embodiments.

[0032] Figure 1 This is a schematic diagram of monitoring the water level in a loop through a loop water level gauge.

[0033] Figure 2 This is a flow chart of an example of a method for monitoring the water content in a circuit under low operating mode of the present invention.

[0034] Figure 3 The logarithmic dose rate-water level relationship curve when the pressure vessel top cover is removed is generated based on the data fitting in Table 1.

[0035] Figure 4 The logarithmic dose rate-water level relationship curve is generated based on the data fitting in Table 2 when the pressure vessel top cover is not removed.

[0036] Figure 5 It is a diagram showing the water level of the pressure vessel of the display device.

[0037] Figure 6This is a schematic diagram of an example of a primary circuit water quantity monitoring system in the low operating mode of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and beneficial technical effects of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described in this specification are only for the purpose of explaining the present invention and are not intended to limit the present invention.

[0039] Under normal operating conditions, the nuclear fuel in the reactor core is submerged in water, providing a certain thickness of water layer to achieve biological shielding of radioactive materials, ensuring radiation protection safety for relevant personnel (such as refueling operators during refueling operations). As the water level above the nuclear fuel in the reactor core drops, the biological shielding effect weakens, leading to an increase in monitored radiation levels. In other words, there is a causal relationship between the water level drop and the radiation level increase. Based on this, the inventors of the present invention propose that the radiation dose rate at the dose monitoring point can be measured and the pressure vessel water level can be calculated using a fitting formula for the relationship between the radiation dose rate and the pressure vessel water level, thereby achieving pressure vessel water level monitoring.

[0040] See also Figure 1 The method for monitoring the amount of water in a circuit under low operating mode of the present invention comprises the following steps:

[0041] S1. Determine the reference source term of the nuclear power unit and at least one dose monitoring point located above the pressure vessel, and use software to fit the reference radiation dose rate-water level relationship formula of the dose monitoring point.

[0042] Specifically, the benchmark source term refers to the γ source strength when all the spent fuel in the pressure vessel has not been removed after the unit transitions from normal shutdown to low-power operation mode during refueling overhaul. This γ source strength can be modeled and calculated by mature core design software based on parameters such as core fuel arrangement, shutdown time, coolant boron concentration, control rod position, fuel rod size, fuel enrichment, and fuel burnup. For example, the γ source strength is taken four days after the unit is normally shut down.

[0043] The present invention does not impose strict restrictions on the method for selecting the reference source term. Alternatively, the reference source term can be the design reference source term given in the Final Safety Analysis Report (FSAR). The design reference source term is an envelope source term given during the nuclear power plant design phase and assumes a certain proportion of fuel cladding damage.

[0044] The dose monitoring point is located above the reactor pool, where a dose rate monitoring instrument can be easily installed to measure the radiation dose rate, such as next to the reactor pool or on a refueling crane where a radiation level monitoring instrument is installed.

[0045] The process of using software to fit the baseline radiation dose rate-water level relationship formula of the dose monitoring point is as follows:

[0046] 1) Conservatively treating the radionuclides stored in the spent fuel assembly as a bare gamma source beneath the reactor pool, commercial radiation shielding calculation software (such as Microshield 7.02) was used to calculate the baseline radiation dose rate at the dose monitoring point at different pressure vessel water levels. It is foreseeable that as the thickness of the shielding water layer above the spent fuel assembly gradually decreases, the radiation dose rate near the monitoring channel will increase rapidly.

[0047] Because the dose monitoring point is located outside the pressure vessel top cover, considering the strong shielding effect of the pressure vessel top cover, it is necessary to calculate the corresponding data of the benchmark radiation dose rate and the pressure vessel water level according to the two situations where the pressure vessel top cover is removed and not removed, and then take the logarithm of the benchmark radiation dose rate to fit and generate the logarithmic dose rate-water level relationship curve and the benchmark radiation dose rate-water level relationship formula.

[0048] For example, Tables 1 and 2 respectively give the corresponding data of the benchmark radiation dose rate and the pressure vessel water level of a certain nuclear power unit when the pressure vessel top cover is removed and when it is not removed. Among them, the logarithmic dose rate is the result obtained by taking the logarithm with base 10 of the calculated benchmark radiation dose rate data.

[0049] Table 1. Corresponding data of baseline radiation dose rate and pressure vessel water level when the pressure vessel top cover is removed

[0050]

[0051] Table 2. Corresponding data of baseline radiation dose rate and pressure vessel water level when the pressure vessel top cover is not removed

[0052]

[0053]

[0054] Figure 3 and Figure 4 The logarithmic dose rate-water level relationship curves are generated by fitting the corresponding data of the benchmark radiation dose rate and the pressure vessel water level in Table 1 and Table 2 respectively.

[0055] according to Figure 3 and Figure 4 From the logarithmic dose rate-water level relationship curve, it is easy to find that the logarithmic dose rate and water level change approximately linearly. Based on this, the relationship between the logarithmic dose rate and water level when the pressure vessel top cover is removed and not removed can be approximately fitted into a linear formula:

[0056] When the pressure vessel top cover is removed, RPVL1=a1-b1*(log10(Dfb))---Formula (1);

[0057] When the pressure vessel top cover is not removed, RPVL2=a2-b2*(log10(Dfb))---Formula (2);

[0058] In the above-mentioned reference radiation dose rate-water level relationship formulas (1) and (2), RPVL1 is the water level (m) when the pressure vessel top cover is removed, and RPVL2 is the water level (m) when the pressure vessel top cover is not removed; a1, b1, a2, and b2 are the calculation coefficients obtained by fitting, and the calculation coefficients a1, b1, a2, and b2 obtained by fitting will be different for different nuclear power units, different reference source terms, different dose monitoring points, etc.; Dfb is the reference radiation dose rate at the dose monitoring point, in mGy / h.

[0059] For example, according to Figure 3 and Figure 4 The curve (or the data in Table 1 and Table 2) can be fitted as follows:

[0060] When the pressure vessel top cover is removed: RPVL1 = 12-5.0 / 9.68*(log10(Dfb)+5.07), that is, a1 is 12-5.07*5.0 / 9.68, b1 is 5.0 / 9.68;

[0061] When the top cover of the pressure vessel is not removed: RPVL2 = 12-5.0 / 8.99*(log10(Dfb)+6.76), that is, a2 is 12-6.76*5.0 / 8.99, and b2 is 5.0 / 8.99.

[0062] It can be seen that the water level of the pressure vessel can be calculated based on the baseline radiation dose rate at the dose monitoring point and formulas (1) and (2).

[0063] The present invention does not limit the specific measurement range of the pressure vessel water level. Preferably, the measurement range of the pressure vessel water level should cover the interval from the lower measurement limit of the loop liquid level gauge to the top of the core spent fuel assembly (such as the pressure vessel water level interval from the bottom of the hot pipe section to the top of the core), so as to achieve continuous monitoring of the pressure vessel water level during the period when the water filling amount in the first loop continues to deteriorate until the core begins to be exposed in the low operating mode.

[0064] S2. Install a dose rate monitoring instrument at the dose monitoring point and calculate the dose rate correction factor k as k = Dfb / Dfr; wherein Dfr and Dfb are the actual radiation dose rate and the reference radiation dose rate at the dose monitoring point when the water level of the pressure vessel is at the same height, respectively.

[0065] The present invention does not specify the monitoring method or the placement of dose monitoring points for radiation level monitoring instruments. Existing radiation level monitoring instruments can be used to monitor radiation dose rates, typically gamma dose rate monitoring instruments installed near the reactor pool or on refueling cranes. While the present invention uses the gamma dose rate as an example, in practice, the reactor vessel water level can be calculated by monitoring the gamma dose rate, or by monitoring the alpha and beta dose rates, depending on the specific situation.

[0066] The reason for introducing the dose rate correction factor k is that Dfb in formulas (1) and (2) is the reference radiation dose rate calculated based on the reference source term, while in practice, the dose rate monitoring instrument measures the actual radiation dose rate Dfr. Due to factors such as core fuel burnup and fuel cladding status during each refueling overhaul of the unit, there is a certain deviation between the actual source term and the reference source term. There is also a deviation between the actual radiation dose rate Dfr measured at the dose monitoring point and the theoretically calculated reference radiation dose rate Dfb. In order to ensure the accuracy of the pressure vessel water level calculation, the actual radiation dose rate measured by the radiation level monitoring instrument cannot be directly substituted into formulas (1) and (2) as the reference radiation dose rate to calculate the pressure vessel water level. Instead, it is necessary to set the dose rate correction factor k and correct the measured actual radiation dose rate to the reference radiation dose rate before it can be substituted into the reference radiation dose rate-water level relationship formulas (1) and (2) to calculate the pressure vessel water level.

[0067] It should be noted that the dose rate correction factor k in formulas (1) and (2) is the same because the deviation between the actual radiation dose rate and the reference radiation dose rate is caused by the deviation between the actual source term and the reference source term, and has nothing to do with whether the pressure vessel top cover is removed.

[0068] The dose rate correction factor k is calculated as follows: when the water level in the pressure vessel is at a known value, measure the actual radiation dose rate Dfr at the dose monitoring point, and substitute Dfr and the reference radiation dose rate Dfb when the water level in the pressure vessel is at the same water level calculated based on the reference source term into k = Dfb / Dfr to calculate the dose rate correction factor k.

[0069] Preferably, because the primary circuit must be lowered to the flange level before the pressure vessel top cover is removed during each refueling overhaul, and the elevation of 11 meters is a known value, the actual radiation dose rate Dfr measured when the water level is lowered to the flange level can be recorded and substituted together with the reference radiation dose rate Dfb at the flange level calculated based on the reference source term into k = Dfb / Dfr to calculate the dose rate correction factor k. For example, for the nuclear power unit related to Table 2, the actual radiation dose rate recorded when the water level was lowered to the flange level was 8.42E-06 mGy / h. From Table 2, it is found that the reference radiation dose rate calculated based on the reference source term at a water level elevation of 11 meters is 9.26E-06 mGy / h. Therefore, the dose rate correction factor for this nuclear power unit is k = Dfb / Dfr = 9.26E-06 / 8.42E-06 = 1.1.

[0070] S3, using a dose rate monitoring instrument to monitor the actual radiation dose rate at the dose monitoring point.

[0071] S4. Use the dose rate correction factor k to correct the actual radiation dose rate at the dose monitoring point to the reference radiation dose rate, substitute it into the reference radiation dose rate-water level relationship formula at the dose monitoring point, and calculate the water level of the pressure vessel.

[0072] It is easy to understand that after using the dose rate correction factor k to correct the actual radiation dose rate at the dose monitoring point to the reference radiation dose rate, it is necessary to substitute the corrected reference radiation dose rate into the reference radiation dose rate-water level relationship formula when the pressure vessel top cover is removed or not, based on the pressure vessel top cover status corresponding to the actual radiation dose rate, to calculate the pressure vessel water level.

[0073] In practice, to facilitate calculation, the dose rate correction factor k can be first substituted into the reference radiation dose rate-water level relationship formula to obtain the actual radiation dose rate-water level relationship formula at the dose monitoring point, and then the actual radiation dose rate at the dose monitoring point can be substituted into the actual radiation dose rate-water level relationship formula to calculate the water level of the pressure vessel.

[0074] Specifically, according to the calculation formula of the dose rate correction factor k, Dfb = k*Dfr --- formula (3);

[0075] Substituting formula (3) into formulas (1) and (2), we can obtain the actual radiation dose rate-water level relationship formula at the dose monitoring point:

[0076] When the pressure vessel top cover is removed, RPVL1=a1-b1*(log10(k*Dfr))---Formula (4);

[0077] When the pressure vessel top cover is not removed, RPVL2=a2-b2*(log10(k*Dfr))---Formula (5);

[0078] Wherein, RPVL1 is the water level when the pressure vessel top cover is removed, in m; RPVL2 is the water level when the pressure vessel top cover is not removed, in m; a1, b1, a2, and b2 are the calculation coefficients obtained by fitting, respectively; k is the dose rate correction factor, which has no unit; Dfr is the actual radiation dose rate at the dose monitoring point, that is, the monitoring value of the dose rate monitoring instrument, in mGy / h.

[0079] S5, displaying the water level value of the pressure vessel on the display device.

[0080] like Figure 5 As shown, in addition to providing a numerical display of the water level in the pressure vessel, the display device can also display the water level information in the form of a bar graph in order to intuitively reflect the water content in the pressure vessel and identify key water level thresholds for the operator's use.

[0081] In addition, when the dose rate monitoring instrument fails (such as over-range), the display device can give an indication of measurement failure, and the water level value is displayed as the "effective value at the last moment".

[0082] As can be seen from the above description, the primary water level monitoring method under the operating mode of the present invention is based on the actual radiation dose rate at the monitoring point measured by the dose rate monitoring instrument, and the actual radiation dose rate is corrected to obtain a reference radiation dose rate. The pressure vessel water level is obtained according to the fitted actual radiation dose rate-water level relationship formula. Finally, the obtained pressure vessel water level is reasonably displayed. The present invention measures the pressure vessel water level by monitoring the radiation level above the pressure vessel. This can achieve continuous measurement of the pressure vessel water level during low operating modes such as maintenance cold shutdown or refueling cold shutdown, when the conventional differential pressure thermal vessel water level measurement or thermocouple pressure vessel water level measurement is unavailable due to the removal of the pressure vessel top cover. This provides an important basis for monitoring the core cooling status after an accident, ensuring that the core cooling status can be correctly diagnosed in a timely, accurate, and convenient manner, and then selecting an appropriate accident operation strategy to ensure core safety. The present invention can serve as a supplement to the existing primary water level monitoring instrument system.

[0083] See also Figure 6 The first-loop water quantity monitoring system in the low-operation mode of the present invention includes:

[0084] a dose rate monitoring instrument 20, for monitoring the actual radiation dose rate at the dose monitoring point;

[0085] A fitting module 22 is used to fit a reference radiation dose rate-water level relationship formula of at least one dose monitoring point using software;

[0086] a calculation module 24 for calculating a dose rate correction factor k as k=Dfb / Dfr, and for correcting the actual radiation dose rate at the dose monitoring point to a reference radiation dose rate using the dose rate correction factor k, and substituting the reference radiation dose rate at the dose monitoring point into a water level relationship formula to calculate the water level of the pressure vessel; and

[0087] The display device 26 is used to display the water level of the pressure vessel.

[0088] Specifically, the dose monitoring point is located above the reactor water pool, at a position where a dose rate monitoring instrument can be easily installed to measure the radiation dose rate; the fitting module 22 is used to fit a reference radiation dose rate-water level relationship formula for each dose monitoring point according to two situations: with the pressure vessel top cover removed and with the pressure vessel top cover not removed; the calculation module 24 uses the dose rate correction factor k to correct the actual radiation dose rate of the dose monitoring point to the reference radiation dose rate, and then, based on the pressure vessel top cover state corresponding to the actual radiation dose rate, substitute the corrected reference radiation dose rate into the reference radiation dose rate-water level relationship formula under the situation where the pressure vessel top cover is removed or not removed to calculate the pressure vessel water level.

[0089] Specifically, the display device 26 is used to provide a numerical display of the water level of the pressure vessel, and is also used to display the water level information in the form of a bar graph and identify key water level thresholds.

[0090] In combination with the above detailed description of the present invention, it can be seen that, compared with the prior art, the present invention can realize the continuous measurement of the primary circuit water charge (especially the pressure vessel water level) in low operating modes such as maintenance cold shutdown or refueling cold shutdown, providing an important basis for monitoring the core cooling state after the accident, so as to ensure that the core cooling state can be correctly diagnosed in a timely, accurate and convenient manner, and then the appropriate accident operation strategy can be selected to ensure the safety of the core.

[0091] Based on the disclosure and teachings of the above description, those skilled in the art may also make appropriate changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and any modifications and variations of the present invention should also fall within the scope of protection of the claims of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for monitoring the amount of water in a circuit under low operating mode, characterized in that: The following steps are involved: 1) Determine the reference source term of the nuclear power unit and at least one dose monitoring point located above the pressure vessel, and use software to fit the reference radiation dose rate-water level relationship formula of the dose monitoring point; 2) installing a dose rate monitoring instrument at the dose monitoring point and calculating the dose rate correction factor k as k = Dfb / Dfr; wherein Dfr and Dfb are the actual radiation dose rate and the reference radiation dose rate at the dose monitoring point when the water level of the pressure vessel is at the same height, respectively; 3) Use a dose rate monitoring instrument to monitor the actual radiation dose rate at the dose monitoring point; 4) Use the dose rate correction factor k to correct the actual radiation dose rate at the dose monitoring point to the reference radiation dose rate, substitute it into the reference radiation dose rate-water level relationship formula at the dose monitoring point, and calculate the pressure vessel water level; 5) Displaying the water level of the pressure vessel on the display device; In which, the dose monitoring point is located above the reactor water pool and is at a position where a dose rate monitoring instrument can be easily installed to measure the radiation dose rate. Since the dose monitoring point is located outside the pressure vessel top cover, step 1) requires fitting a reference radiation dose rate-water level relationship formula for each dose monitoring point according to two situations: with the pressure vessel top cover removed and with the pressure vessel top cover not removed. Correspondingly, step 4) requires using the dose rate correction factor k to correct the actual radiation dose rate of the dose monitoring point to the reference radiation dose rate. Then, based on the pressure vessel top cover state corresponding to the actual radiation dose rate, the corrected reference radiation dose rate needs to be substituted into the reference radiation dose rate-water level relationship formula for the situation where the pressure vessel top cover is removed or not removed to calculate the pressure vessel water level.

2. The method for monitoring the primary circuit water level in low operating mode according to claim 1, characterized in that: In step 1), the process of using software to fit the reference radiation dose rate-water level relationship formula of the dose monitoring point is: The radionuclides stored in the spent fuel assemblies were conservatively considered as naked gamma sources under the reactor pool. Radiation shielding calculation software was used to calculate the baseline radiation dose rates at the dose monitoring points when the pressure vessel was at different water levels, with and without the pressure vessel cover removed. The logarithm of the baseline radiation dose rate was then taken to generate the baseline radiation dose rate-water level relationship equation: When the pressure vessel top cover is removed, RPVL1=a1-b1*(log10(Dfb)); When the pressure vessel cover is not removed, RPVL2 = a2-b2*(log10(Dfb)); In the above formula, RPVL1 is the water level when the pressure vessel top cover is removed, in meters; RPVL2 is the water level when the pressure vessel top cover is not removed, in meters; a1, b1, a2, and b2 are the calculation coefficients obtained by fitting; Dfb is the reference radiation dose rate at the dose monitoring point, in mGy / h.

3. The method for monitoring the primary circuit water level in low operating mode according to claim 2, characterized in that: The dose rate correction factor k is calculated as follows: when the water level of the pressure vessel is at a known value, the actual radiation dose rate Dfr at the dose monitoring point is measured, and Dfr and the reference radiation dose rate Dfb when the water level of the pressure vessel is at the same height calculated based on the reference source term are substituted into k=Dfb / Dfr to calculate the dose rate correction factor k.

4. The method for monitoring the primary circuit water level in low operating mode according to claim 3, characterized in that: The dose rate correction factor k is calculated as follows: during the refueling overhaul, the primary circuit must be lowered to the flange surface water level before the pressure vessel top cover is removed, and the actual radiation dose rate Dfr measured when the water level is lowered to the flange surface is recorded. Dfr and the reference radiation dose rate Dfb at the flange surface water level calculated based on the reference source term are substituted into k = Dfb / Dfr to calculate the dose rate correction factor k.

5. The method for monitoring the primary circuit water level in low operating mode according to claim 1, characterized in that: In step 5), the display device provides a numerical display of the water level of the pressure vessel, and also displays the water level information in the form of a bar graph, and identifies the key water level threshold.

6. The method for monitoring the primary circuit water level in low operating mode according to claim 1, characterized in that: The measurement range of the water level in the pressure vessel covers the interval from the lower measurement limit of the loop liquid level gauge to the top of the spent fuel assembly in the core.

7. A system for monitoring the amount of water in a circuit under low operating mode, characterized in that: The primary circuit water quantity monitoring system in the low operating mode includes: Dose rate monitoring instrument, used to monitor the actual radiation dose rate at the dose monitoring point; A fitting module, used to use software to fit a reference radiation dose rate-water level relationship formula for at least one dose monitoring point; a calculation module for calculating a dose rate correction factor k using k = Dfb / Dfr, and for correcting the actual radiation dose rate at the dose monitoring point to a reference radiation dose rate using the dose rate correction factor k, and substituting the reference radiation dose rate-water level relationship formula at the dose monitoring point into the formula to calculate the pressure vessel water level; wherein Dfr and Dfb are the actual radiation dose rate and the reference radiation dose rate at the dose monitoring point, respectively, when the pressure vessel water level is at the same height; and A display device for displaying the water level of the pressure vessel; The dose monitoring point is located above the reactor water pool, at a position where a dose rate monitoring instrument can be easily installed to measure the radiation dose rate. The fitting module is used to fit a reference radiation dose rate-water level relationship formula for each dose monitoring point according to two situations: with the pressure vessel top cover removed and with the pressure vessel top cover not removed. The calculation module uses a dose rate correction factor k to correct the actual radiation dose rate of the dose monitoring point to a reference radiation dose rate. Then, based on the pressure vessel top cover state corresponding to the actual radiation dose rate, the corrected reference radiation dose rate is substituted into the reference radiation dose rate-water level relationship formula under the situation where the pressure vessel top cover is removed or not removed to calculate the pressure vessel water level.

8. The primary circuit water quantity monitoring system in low operation mode according to claim 7 is characterized in that: The display device is used to provide a numerical display of the water level of the pressure vessel, and is also used to display the water level information in the form of a bar graph and identify key water level thresholds.

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