Heavy water reactor fuel rod bundle damage positioning and searching method

Abstract

The invention belongs to the technical field of nuclear fuel, and particularly relates to a heavy water reactor fuel rod bundle damage positioning and searching method. Comprising the following steps of: 1, judging reactor entering time according to a change trend of Xe-133 nuclide in a coolant; 2, confirming a reactor core loop where the damaged fuel is located; 3, formulating a damaged fuel positioning system scanning strategy; 4, making and evaluating a damaged fuel reloading scheme; 5, changing and monitoring the damaged fuel; and 6, judging the position of the damaged fuel rod bundle. The method has the beneficial effects that the troubleshooting range of the damaged fuel is further narrowed through coolant nuclide analysis and damaged loop confirmation, delayed neutron scanning is started in combination with a damaged fuel positioning system scanning strategy, and compared with the prior art, the method can shorten the positioning and searching time of the damaged fuel, improve the accuracy of the positioning and searching result of the damaged fuel, and improve the accuracy of the positioning and searching result of the damaged fuel. And the influence of fuel damage is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nuclear fuel, and particularly relates to a heavy water reactor fuel rod bundle breakage positioning and searching method. BACKGROUND

[0002] The conventional heavy water reactor fuel rod bundle breakage positioning method only has a breakage fuel positioning system, and the principle of the breakage fuel positioning system is to measure the delayed neutrons associated with iodine 137 and bromine 87 in the coolant, sample and measure the count of the delayed neutrons of all fuel channels respectively, to determine whether there is breakage in each fuel channel. The breakage fuel positioning system has the following problems:

[0003] 1) There is abnormal data interference: when the breakage fuel positioning system scans, due to the influence of complex factors such as system noise, the count of the delayed neutrons of the normal fuel channel may abnormally rise, and in this case, the normal fuel channel may be misjudged as a breakage fuel channel, which will cause fuel cost loss in its refueling and investigation.

[0004] 2) Difficulty in positioning the breakage of fuel with low nuclide level: the delayed neutrons are easily released into the coolant in the case of large breakage. If the fuel breakage is small, such as fuel air tightness failure, the coolant nuclide level is low, and in this case, it is difficult to locate the breakage fuel channel by scanning the delayed neutrons due to the fact that the delayed neutrons are not released or the amount of the released delayed neutrons is small.

[0005] The above problems will affect the progress of the heavy water reactor breakage fuel positioning and searching, and the breakage fuel cannot be unloaded from the core in time, which increases the influence of the fuel breakage on the environment, personnel and unit. Therefore, a new breakage fuel positioning and searching method needs to be established for the heavy water reactor to help the breakage fuel positioning and searching, shorten the breakage fuel searching time and reduce the influence of the fuel breakage. SUMMARY

[0006] The purpose of the present application is to provide a new breakage fuel positioning and searching method to help the breakage fuel positioning and searching, improve the accuracy of the breakage fuel searching and positioning result, shorten the breakage fuel searching time and reduce the influence of the fuel breakage.

[0007] The technical solution of the present application is as follows: a heavy water reactor fuel rod bundle breakage positioning and searching method, comprising the following steps:

[0008] Step 1: judging the time of entering the reactor according to the change trend of Xe-133 nuclide in the coolant;

[0009] Step 2: confirming the core loop where the breakage fuel is located;

[0010] Step 3: formulating a scanning strategy of the breakage fuel positioning system;

[0011] Step 4: formulating and evaluating a refueling scheme of the breakage fuel;

[0012] Step 5: Broken fuel replacement and its monitoring;

[0013] Step 6: Determine the location of the broken fuel rod bundle.

[0014] The step 1 comprises:

[0015] Step 11: After discovering the abnormality of the coolant nuclide, the frequency of chemical analysis sampling is increased from regular twice a week to at least three times a week, and the nuclide trend is compared with the online monitoring of the fission gas monitoring system to confirm that the trends are consistent;

[0016] Step 12: The time of the broken fuel into the reactor is preliminarily determined by the rising trend of the Xe-133 nuclide concentration in the coolant, and the rising trend of the nuclide is divided into three cases according to the time of the broken fuel into the reactor:

[0017] Case 1: After 15 minutes of the refueling channel being closed, Xe-133 appears a step rise;

[0018] Search strategy: The remaining fuel rod bundle in the channel is broken, continue to replace the suspected broken fuel, and determine whether the channel is a broken fuel channel by steps 5 and 6;

[0019] Case 2: Under stable state, Xe-133 rises rapidly and reaches equilibrium concentration in a few days, and then maintains stable, indicating that the broken rod bundle has been in the reactor for a long time;

[0020] Search strategy: Slow neutron scanning is performed by the broken fuel positioning system, the results are compared and analyzed to determine the suspected channel, and the suspected channel replacement sequence is determined in combination with the determined broken fuel rod bundle location;

[0021] Case 3: Xe-133 first rises slightly, then continues to rise slowly, and reaches equilibrium concentration in more than 1 month;

[0022] Search strategy: The refueling channel 1-2 weeks before the broken fuel appears is investigated, and steps 5 and 6 are used to determine whether the channel is a broken fuel channel;

[0023] Step 13: If the number of suspected channels is large, auxiliary search is performed by scanning, if the channel in the suspected time range, the slow neutron scanned by the broken fuel positioning system also rises, then the channel is preferentially replaced.

[0024] The step 2 comprises:

[0025] Step 21: Fission gas online monitoring system loop switching

[0026] In the early stage of discovering fuel breakage, the core coolant monitoring loop of the fissile gas online monitoring system is switched between loop 1 and loop 2, each loop measures for more than 0.5 hours, and during the loop switching process, if the concentration of Xe-133, Xe-135, Kr-88 and total gamma of a loop is higher than that of another loop, the breakage fuel is located in the loop of the core;

[0027] Step 22: chemical sampling analysis of each loop

[0028] When the coolant is periodically sampled and analyzed, the coolant of two loops in the core is sampled and analyzed at the same time, and if the concentration of I-131, I-134, Xe-133, Xe-135 and Kr-88 of a loop is higher than that of another loop in continuous multiple sampling and analysis, the breakage fuel is located in the loop of the core;

[0029] Step 23: when the difference between the concentrations of nuclides in the two loops is within 5%, the coolant is isolated and purified or purified for a period of time, and then the chemical sampling of the two loops is compared respectively, and the loop where the breakage fuel is located is determined according to the difference between the concentrations of nuclides in the chemical sampling of the two loops.

[0030] The step 3 comprises:

[0031] Step 31: when the concentration of short half-life nuclides I-134 and Xe-138 in chemical sampling analysis increases by more than 25%, the breakage fuel positioning system full-core delayed neutron scanning should be started;

[0032] Step 32: if there are channels with rising delayed neutrons in the full-core scanning of the breakage fuel positioning system, the channel is judged to exist breakage fuel by combining the step 1 to determine the time of entering the core, the step 2 to determine the loop where the breakage fuel is located, and then the channel is evaluated by step 4, whether it is a breakage fuel channel is confirmed by step 5 and step 6;

[0033] Step 33: if there are no results of step 1 and step 2, but there are channels with rising delayed neutrons in the full-core scanning of the breakage fuel positioning system, then the channels are manually scanned multiple times, if the delayed neutron count of single-channel scanning is still abnormal, the channel is checked for replacement, whether there is suspected breakage fuel, and whether the channel is a breakage fuel channel is confirmed by step 5 and step 6;

[0034] Step 34: for blind channels, whether there is suspected breakage fuel is judged by checking the replacement of blind channels.

[0035] The step 4 comprises:

[0036] Before the suspected breakage channel is checked for replacement of breakage fuel, the following aspects are evaluated:

[0037] Step 41: According to the suspected broken fuel channel and the power and refueling interval distribution of the nearby channel, determine whether to use the standard 8-rod bundle refueling method or the 4-rod bundle refueling method;

[0038] Step 42: Through refueling simulation software calculation, evaluate whether the adopted refueling method meets the channel power and rod bundle power margin requirements after refueling;

[0039] Step 43: According to the liquid level change data of the refueling channel history refueling, evaluate the liquid level change of the liquid region control system during and after the suspected broken fuel channel refueling process, and determine whether it meets the refueling procedure requirements. According to the evaluation and unit operation arrangement, adjust and optimize the refueling sequence of the day;

[0040] Step 44: After completing the above evaluation, prepare and issue the refueling channel selection list, prepare and issue the refueling channel and sequence table and the refueling instruction table of the day;

[0041] Step 45: After the suspected broken channel refueling, if it is necessary to continue to refuel and investigate the remaining fuel rod bundles in the channel, it is necessary to re-execute steps 41 to 45 to evaluate the safety of continued refueling and re-develop a refueling plan.

[0042] The step 5 comprises:

[0043] Step 51: Judge that the broken fuel has been unloaded from the core by comprehensively considering the following steps

[0044] 1) If the broken fuel positioning system single-channel scanning is manually performed on the channel during the refueling process, the delayed neutron count of the channel will be significantly reduced after the broken fuel is unloaded from the core;

[0045] 2) After the broken fuel is unloaded from the core, the downstream refueling machine will cause the gamma dose of the pipeline and valve station in the plant house to be significantly higher than the normal level during the drainage process;

[0046] 3) After the broken fuel is unloaded from the core, there is a tailing phenomenon in the room where the spent fuel unloading pool is located;

[0047] 4) After the broken fuel is unloaded from the core, the Xe-133 isotope in the coolant monitored by the online monitoring system of the fissile gas appears a release transient after the channel is closed;

[0048] 5) After the broken fuel is unloaded from the core, the coolant isotope is reduced to the normal level;

[0049] Step 52: Judge that the broken fuel is still in the remaining fuel rod bundles of the refueling channel by comprehensively considering the following steps, and it is necessary to confirm whether to continue to refuel the channel today through step 4;

[0050] 1) If the damaged fuel positioning system single-channel scanning is manually performed on the channel during the refueling process, and the damaged fuel is still in the refueling channel, the channel buffer neutron count appears the characteristics of peaks and troughs;

[0051] 2) The Xe-133 nuclide and total gamma in the main system appear release transients after 15 minutes of channel closure when the damaged fuel is still in the refueling channel.

[0052] The step 6 comprises the following steps:

[0053] After the damaged fuel is unloaded from the core, the specific position of the damaged fuel rod bundle is confirmed according to the following actions:

[0054] Step 61: When the damaged fuel channel is unloaded, the gamma dose of the spent fuel unloading pool is observed, and if the gamma dose of the unloading pool cannot be reduced to the level before unloading in the first time after any pair of rod bundles is transferred into the pool, and is obviously higher than the level after the previous pair of rod bundles is transferred into the pool, the pair of spent fuel rod bundles is damaged;

[0055] Step 62: On the second day after unloading, the single rod bundle of all suspected damaged spent fuel rod bundles is taken out from the spent fuel unloading pool in turn, so that it is exposed to the water surface, and then is put into the unloading pool after a short time, if the gamma dose of the room where the spent fuel unloading pool is located cannot be reduced to the level before the rod bundle is taken out in the first time after the corresponding rod bundle is put into the unloading pool, the spent fuel rod bundle is a damaged fuel rod bundle;

[0056] Step 63: If the single rod bundle identification cannot determine the specific damaged fuel rod bundle, the underwater inspection of all suspected rod bundles of the damaged channel is performed after two months to find the specific failure point of the damaged rod bundle, so as to determine the specific damaged fuel rod bundle.

[0057] The method has the advantages that the method provided by the application is a new damaged fuel positioning and searching method for a heavy water reactor, the searching range of the damaged fuel is further reduced through coolant nuclide analysis and damaged loop confirmation, the buffer neutron scanning is started in combination with the scanning strategy of the damaged fuel positioning system, the positioning and searching time of the damaged fuel can be shortened compared with the prior art, the accuracy of the positioning and searching result of the damaged fuel is improved, and the influence of the fuel damage is reduced. DETAILED DESCRIPTION

[0058] The application will be further described in detail below with reference to specific embodiments.

[0059] The method for positioning and searching the damaged fuel rod bundle of the heavy water reactor comprises the following steps:

[0060] The fissile gas monitoring system applied in the present application is used for monitoring the concentration of Kr-88, I-131, Xe-133 and Xe-135 in the coolant and the total gamma activity on line and continuously, so as to monitor the total radioactivity level in the coolant, and can be used for judging whether there is fuel breakage in the core, which belongs to the prior art.

[0061] The broken fuel positioning system adopts the principle of measuring the delayed neutrons associated with I-137 and Br-87, measures the count of delayed neutrons in each fuel channel by sampling 380 fuel channels respectively, so as to judge whether there is broken fuel in each fuel channel, which belongs to the prior art.

[0062] Step 1: judging the time of entering the reactor according to the change trend of Xe-133 nuclide in the coolant, including the following:

[0063] Step 11: after finding that the coolant nuclide is abnormal (the monitoring data continuously has burr and is higher than 0.4 MBq / kg, or the monitoring data continuously rises and the rising slope is greater than 0), the sampling frequency of chemical analysis is increased from regular twice a week to at least three times a week, the change trend of the coolant nuclide is strengthened, and the change trend is compared with the online monitoring nuclide change trend of the fissile gas monitoring system to confirm that the change trends are consistent.

[0064] Step 12: judging the time of entering the reactor of the broken fuel according to the rising trend of the Xe-133 nuclide concentration of the coolant, and the rising trend of the nuclide is divided into three cases according to the different time of entering the reactor of the broken fuel:

[0065] Case 1: after the refueling channel is closed for 15 minutes, the Xe-133 appears stepwise rising (the nuclide monitoring result rises vertically, and the rising range is usually more than one time).

[0066] Finding strategy: there is broken fuel in the remaining fuel rod bundle in the channel, suspected broken refueling is continued, and whether the channel is a broken fuel channel is confirmed according to steps 5 and 6;

[0067] Type 2: under the stable state without refueling disturbance, the Xe-133 appears rising, and reaches the equilibrium concentration within one week, and then maintains stable, which indicates that the time of entering the reactor of the broken rod bundle is long (usually more than 2 months);

[0068] Finding strategy: since the number of channels with the time of entering the reactor more than 2 months is too large, the delayed neutron scanning finding is needed through the broken fuel positioning system, the suspected channels are determined according to the scanning results, the possibility of fuel breakage of each suspected channel is determined according to the loop where the broken fuel rod bundle is located and the rising range of the delayed neutron count rate, the sequence of suspected channel refueling is determined. If the broken fuel positioning system is not suitable for scanning finding, suspected broken channel refueling is tried combined with the refueling time, the loop and the fuel manufacturing information.

[0069] Type 3: The online monitoring of Xe-133 shows spikes, followed by a slow increase (usually a gentle sloping line with a slope less than 1 and greater than 0), and it takes more than one month for Xe-133 to reach an equilibrium concentration.

[0070] Search strategy: Since the time after the damaged rod bundle is put into the pile or after the refueling movement is short, the Xe-133 in the fuel has not reached a balanced state. The refueling channel 1-2 weeks before the damage occurred can be investigated, and the channel can be confirmed as the damaged fuel channel according to steps 5 and 6.

[0071] Step 13: Based on historical experience, the longer the irradiation time of the damaged rod bundle, the higher the power of the damaged rod bundle, and the greater the slight increase in Xe133 in the early stage of damage. If the number of suspicious channels exceeds 10, scanning can be used for auxiliary location. If the slow-emission neutrons scanned by the damaged fuel location system also increase in channels that entered the reactor within the suspicious time range, then the refueling of that channel should be prioritized.

[0072] Step 2: Identify the core loop containing the damaged fuel.

[0073] Step 2 includes:

[0074] Step 21: Loop switching of the online fission gas monitoring system

[0075] In the early stages of fuel damage detection, the core coolant monitoring loop (LOOP) of the fission gas online monitoring system is switched back and forth between loop 1 (LOOP1) and loop 2 (LOOP2), with each loop measuring for more than 0.5 hours. During the loop switching process, if the concentrations of Xe-133, Xe-135, Kr-88, and total gamma in one loop are higher than in the other loop, then the damaged fuel is located in that loop of the core.

[0076] Step 22: Chemical loop sampling and analysis

[0077] When the coolant is chemically sampled and analyzed periodically, the coolant in both loops of the reactor core is sampled and analyzed simultaneously. If the concentration of nuclides such as I-131, I-134, Xe-133, Xe-135, and Kr-88 in one loop is higher than that in the other loop after multiple consecutive sampling and analysis, then the damaged fuel is located in that loop of the reactor core.

[0078] Step 23: For minor damage with very small increases in nuclide concentration, where the difference in nuclide concentration between the two loops is not significant (within 5%) and the phenomena observed by the above two methods are not obvious, it is necessary to isolate the coolant for purification or to bypass the purification process for a period of time (usually 12-24 hours) to reduce the mixing effect of the coolant in the two loops. Then, chemical samples are taken from each of the two loops for comparison. The loop in which the damaged fuel is located is determined based on the difference in nuclide concentration in the chemical samples of the two loops (the coolant nuclide sampling results and increases are higher in the loop in which the damaged fuel is located).

[0079] Step 3: Scanning Strategy for Damaged Fuel Locator

[0080] Step 3 includes:

[0081] Step 31: When the concentration of short half-life nuclides I-134 and Xe-138 in chemical sampling analysis increases by more than 25%, the whole-core slow neutron scan of the damaged fuel location system should be initiated.

[0082] Step 32: If the damaged fuel location system scans the entire core and finds a channel with slow-emission neutron rise, and combines the reactor entry time determined in Step 1 and the loop where the damaged fuel is located determined in Step 2, if it is determined that there is damaged fuel in the channel, then after evaluation in Step 4, it will be replaced and investigated, and then confirmed whether the channel is a damaged fuel channel according to Steps 5 and 6.

[0083] Step 33: If there are no judgment results in Step 1 and Step 2, but the whole core scan of the damaged fuel positioning system shows a channel with a slow-emission neutron increase, then manually perform multiple single-channel scans on these channels. If the slow-emission neutron count of the single-channel scan is still higher than the background count rate, then replace the fuel in the channel to check for suspected damaged fuel, and confirm whether the channel is a damaged fuel channel according to Step 5 and Step 6.

[0084] Step 34: For channels that are not sensitive to delayed neutron scanning counts (blind channels), when the degree of fuel damage is small and the level of delayed neutrons in the damaged channel is low, it is necessary to check whether there is suspected damaged fuel by replacing the fuel in the blind channel.

[0085] Step 4: Development and Evaluation of Damaged Fuel Replacement Plan:

[0086] Step 4 includes:

[0087] Before replacing the damaged fuel in a suspected damaged channel, the following assessments should be completed:

[0088] Step 41: Based on the power and refueling interval distribution of the suspected damaged fuel channel and nearby channels, and combined with the results of the physical program simulation, determine whether to adopt the standard 8-bar bundle refueling method or the 4-bar bundle refueling method.

[0089] Step 42: Calculate and evaluate whether the material replacement method adopted meets the channel power and rod bundle power margin requirements after material replacement using material replacement simulation software.

[0090] Step 43: Based on the historical data of liquid level changes during refueling in the refueling channel, assess the liquid level changes in the liquid area control system during and after refueling in the suspected damaged fuel channel, determine whether the refueling procedure requirements are met, and adjust and optimize the refueling sequence for the day based on the assessment and the unit's work schedule.

[0091] Step 44: After completing the above assessment, compile and publish the material change channel selection list, the material change channel and sequence table, and the material change instruction table for the day.

[0092] Step 45: After replacing the fuel in the suspected damaged channel, if it is necessary to continue to check and replace the remaining fuel rod bundles in the channel, steps 41 to 45 need to be repeated to assess the safety of continuing the fuel replacement and to formulate a new fuel replacement plan.

[0093] Step 5: Replacement and monitoring of damaged fuel

[0094] Step 5 includes:

[0095] Step 51: Based on the following steps, determine that the damaged fuel has been removed from the reactor core.

[0096] 1) If the damaged fuel positioning system is manually scanned in a single channel during the refueling process, the delayed neutron count in the channel will show a significant decrease after the damaged fuel is unloaded from the core, and the continuous decrease can be monitored until it stabilizes.

[0097] 2) After the damaged fuel is discharged from the reactor core, during the drainage process of the downstream refueling machine, the gamma dose of the pipelines and valve stations that the drainage passes through will be significantly higher (more than 10 times) than the level during normal refueling (normally about 0.01 to 0.02 mGy / h).

[0098] 3) After the damaged fuel is discharged from the reactor core, the gamma dose in the chamber where the spent fuel discharge pool is located cannot drop to the level before discharge (normally about 0.05 to 0.07 mGy / h) immediately (usually within 30 seconds). There is a step that is significantly higher than the normal level and it takes more than 3 minutes to recover to the normal gamma dose level (tailing phenomenon). The larger the rupture, the longer it takes to recover to the normal gamma dose level.

[0099] 4) After the damaged fuel is discharged from the reactor core, about one hour after the passage is closed, the Xe-133 nuclide in the coolant monitored by the online fission gas monitoring system will show a release transient.

[0100] 5) After the damaged fuel is removed from the reactor core, the coolant nuclides gradually decrease to normal levels.

[0101] Step 52: Based on the following steps, if it is determined that the damaged fuel is still in the remaining fuel rod bundle in the refueling channel, it is necessary to confirm in Step 4 whether to continue refueling in this channel that day.

[0102] 1) If the damaged fuel positioning system is manually scanned in a single channel during the refueling process, and the damaged fuel is still in the refueling channel, the slow-emission neutron count in the channel will show peak and trough characteristics.

[0103] 2) The damaged fuel is still in the refueling channel, and the Xe-133 nuclide and total gamma in the main system are released transiently 15 minutes after the channel is closed.

[0104] Step 6: Determine the location of the damaged fuel rod bundle

[0105] Step 6 includes:

[0106] After the damaged fuel is removed from the reactor core, the following actions are taken to confirm the exact location of the damaged fuel rod bundle:

[0107] Step 61: When unloading fuel through the damaged fuel channel, observe the gamma dose in the spent fuel unloading pool. If, after any pair of fuel rod bundles is transferred to the water pool, the gamma dose in the unloading pool cannot drop to the level before unloading (normally about 0.05 to 0.07 mGy / h) immediately (usually within 30 seconds), and is significantly higher than the level after the previous pair entered the water (normally about 0.05 to 0.07 mGy / h), then that pair of spent fuel rod bundles is damaged.

[0108] Step 62: The day after unloading, remove all suspected damaged spent fuel rod bundles one by one from the spent fuel unloading pool, exposing them to the water surface. After a short time, put them back into the unloading pool. If the gamma dose in the room where the spent fuel unloading pool is located cannot drop to the level before removal immediately (usually within 30 seconds) after the corresponding rod bundle enters the unloading pool, then the spent fuel rod bundle is a damaged fuel rod bundle.

[0109] Step 63: If the single-bar bundle identification cannot determine the specific damaged fuel bar bundle, it is necessary to wait 2 months and then conduct underwater inspections on all suspected bar bundles in the damaged channel to find the specific failure point of the damaged bar bundle in order to identify the specific damaged fuel bar bundle.

Claims

1. A method for locating and finding damaged fuel rod bundles in heavy water reactors, characterized in that, Includes the following steps: Step 1: Determine the reactor insertion time based on the changing trend of Xe-133 nuclide in the coolant; Step 2: Identify the core loop containing the damaged fuel; Step 3: Develop a scanning strategy for the damaged fuel location system; Step 4: Develop and evaluate a damaged fuel replacement plan; Step 5: Replacement and monitoring of damaged fuel; Step 6: Determine the location of the damaged fuel rod bundle.

2. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 1 includes: Step 11: After the coolant nuclide anomaly was detected, the sampling frequency for chemical analysis was increased from twice a week to at least three times a week, and the trend of nuclide change was compared with that of the fission gas monitoring system to confirm that the two trends were consistent. Step 12: By observing the rising trend of Xe-133 nuclide concentration in the coolant, the initial reactor insertion time of the damaged fuel is determined. The nuclide rising trend is divided into three categories based on the different reactor insertion times of the damaged fuel: Scenario 1: Xe-133 showed a step increase 15 minutes after the material change channel was closed; Search strategy: The remaining fuel rod bundle in this channel is damaged. Continue to replace the suspected damaged fuel, and proceed to steps 5 and 6 to confirm whether this channel is a damaged fuel channel. Scenario 2: Under stable conditions, Xe-133 rises rapidly, reaches equilibrium concentration after a few days, and then remains stable, indicating that the broken rod bundle has been in the pile for a long time. Search strategy: Use the damaged fuel location system to perform delayed neutron scanning to search for the damaged fuel. Based on the scanning results, conduct comparative analysis to identify suspicious channels. Combine this with the identified loop where the damaged fuel rod bundle is located to formulate a refueling and investigation sequence for the suspicious channels. Scenario 3: Xe-133 initially shows a slight increase, then continues to rise slowly, taking more than a month to reach equilibrium concentration; Search strategy: Investigate the material change channel 1-2 weeks before the damage occurred, and confirm whether the channel is the damaged fuel channel by proceeding to steps 5 and 6. Step 13: If there are many suspicious channels, use scanning to assist in the search. If the slow-emission neutrons detected by the damaged fuel positioning system also increase in channels that entered the reactor within the suspicious time range, then prioritize replacing the fuel in that channel.

3. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 2 includes: Step 21: Loop switching of the online fission gas monitoring system In the early stages of fuel damage detection, the core coolant monitoring loop of the fission gas online monitoring system is switched, switching back and forth between loop 1 and loop 2. The measurement time for each loop is greater than 0.5 hours. During the loop switching process, if the concentration of Xe-133, Xe-135, Kr-88, and total gamma in one loop is higher than that in the other loop, then the damaged fuel is located in that loop of the core. Step 22: Chemical loop sampling and analysis When performing regular chemical sampling and analysis of the coolant, the coolant in both loops of the reactor core is sampled and analyzed simultaneously. If the concentrations of I-131, I-134, Xe-133, Xe-135, and Kr-88 in one loop are higher than those in the other loop after multiple consecutive sampling and analysis, then the damaged fuel is located in that loop of the reactor core. Step 23: If the difference in nuclide concentration between the two loops is within 5%, after the isolation coolant is purified or the purification is bypassed for a period of time, chemical samples are taken from the two loops for comparison. The loop where the damaged fuel is located is determined based on the difference in nuclide concentration in the chemical samples of the two loops.

4. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 3 includes: Step 31: When the concentration of short half-life nuclides I-134 and Xe-138 in chemical sampling analysis increases by more than 25%, the whole core slowed neutron scan of the damaged fuel location system should be initiated. Step 32: If the damaged fuel location system scans the entire core and finds a channel with slow-emission neutron rise, and combines the reactor entry time determined in Step 1 and the loop where the damaged fuel is located determined in Step 2, if it is determined that there is damaged fuel in the channel, then after evaluation in Step 4, it will be replaced and investigated, and Steps 5 and 6 will be performed to confirm whether the channel is a damaged fuel channel. Step 33: If there are no judgment results in Step 1 and Step 2, but the whole core scan of the damaged fuel positioning system shows a channel with a slow-emission neutron increase, then manually perform multiple single-channel scans on these channels. If the slow-emission neutron count in the single-channel scan is still abnormal, then replace the fuel in the channel to check if there is any suspected damaged fuel, and proceed to Step 5 and Step 6 to confirm whether the channel is a damaged fuel channel. Step 34: For blind channels, check the material replacement process in blind channels to determine if there is any suspected damaged fuel.

5. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 4 includes: Before replacing the damaged fuel in a suspected damaged channel, the following assessments should be completed: Step 41: Based on the power and refueling interval distribution of the suspected damaged fuel channel and nearby channels, determine whether to use the standard 8-bar bundle refueling method or the 4-bar bundle refueling method; Step 42: Calculate and evaluate whether the adopted material replacement method meets the channel power and rod bundle power margin requirements after material replacement using material replacement simulation software; Step 43: Based on the historical data of liquid level changes during refueling in the refueling channel, assess the liquid level changes in the liquid area control system during and after refueling in the suspected damaged fuel channel, determine whether the refueling procedure requirements are met, and adjust and optimize the refueling sequence for the day based on the assessment and the unit's work schedule. Step 44: After completing the above assessment, compile and publish the material change channel selection list, compile and publish the material change channel and sequence table, as well as the material change instruction table for the day; Step 45: After replacing the fuel in the suspected damaged channel, if it is necessary to continue to check and replace the remaining fuel rod bundles in the channel, steps 41 to 45 need to be repeated to assess the safety of continuing the fuel replacement and to formulate a new fuel replacement plan.

6. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 5 includes: Step 51: Based on the following steps, determine that the damaged fuel has been removed from the reactor core. 1) If the damaged fuel positioning system is manually scanned in a single channel during the refueling process, the delayed neutron count in the channel will be significantly reduced after the damaged fuel is unloaded from the core. 2) After the damaged fuel is discharged from the core, the downstream refueling machine will cause the gamma dose in the pipelines and valve station buildings through which the drained fuel passes to be significantly higher than the level during normal refueling. 3) After the damaged fuel is discharged from the reactor core, there is a trailing phenomenon in the room where the spent fuel unloading pool is located; 4) After the damaged fuel was unloaded from the reactor core and the passage was closed, the Xe-133 nuclide in the coolant monitored by the online fission gas monitoring system showed a release transient. 5) After the damaged fuel is removed from the reactor core, the coolant nuclides are reduced to normal levels; Step 52: Based on the following steps, if it is determined that the damaged fuel is still in the remaining fuel rod bundle in the refueling channel, it is necessary to confirm in Step 4 whether to continue refueling the channel that day; 1) If the damaged fuel positioning system is manually scanned in a single channel during the refueling process, and the damaged fuel is still in the refueling channel, the slow-emission neutron count in the channel will show peaks and troughs. 2) The damaged fuel is still in the refueling channel, and the Xe-133 nuclide and total gamma in the main system are released transiently 15 minutes after the channel is closed.

7. The method for locating and finding damaged fuel rod bundles in heavy water reactors as described in claim 1, characterized in that, Step 6 includes: After the damaged fuel is removed from the reactor core, the following actions are taken to confirm the exact location of the damaged fuel rod bundle: Step 61: When unloading fuel through the damaged fuel channel, observe the gamma dose in the spent fuel unloading pool. If the gamma dose in the unloading pool cannot be reduced to the level before unloading immediately after any pair of fuel rods are transferred to the water pool, and is significantly higher than the level after the previous pair entered the water, then the pair of spent fuel rods is damaged. Step 62: The day after unloading, remove all suspected damaged spent fuel rod bundles one by one from the spent fuel unloading pool, so that they are exposed above the water surface. After a short time, put them back into the unloading pool. If the gamma dose in the room where the spent fuel unloading pool is located cannot be reduced to the level before removal immediately after the corresponding rod bundle enters the unloading pool, then the spent fuel rod bundle is a damaged fuel rod bundle. Step 63: If the single-bar bundle identification cannot determine the specific damaged fuel bar bundle, wait 2 months and then conduct underwater inspections on all suspected bar bundles in the damaged channel to find the specific failure point of the damaged bar bundle in order to identify the specific damaged fuel bar bundle.