Reactor first cycle passive starting method based on MOX fuel assembly

Abstract

The invention belongs to the technical field of nuclear reactors, and particularly relates to a reactor first cycle passive starting method based on MOX fuel assemblies. Comprising the following steps: step 1, after reactor charging is started, firstly, MOX fuel assemblies are arranged at the periphery of a reactor core in a position close to an out-of-reactor high-sensitivity detector according to a certain sequence; 2, sequentially loading the reactor core fuel assemblies according to a diagonal snakelike mode; and 3, for a reactor core scheme that the MOX fuel assemblies are loaded in a reactor core inner area, moving the reactor core positions A08, R08, H01 and H15 of the MOX fuel assemblies loaded near the high-sensitivity detector in the step 1 to the reactor core positions C08, N08, H03 and H13 of the MOX fuel assemblies in the reactor core scheme, and then loading the fuel assemblies determined in the reactor core scheme at the positions near the high-sensitivity detector in the step 1. The method has the beneficial effects that non-blind-area passive starting of the first-cycle reactor can be realized by loading the MOX fuel assembly in the first-cycle reactor core and arranging the out-of-reactor high-sensitivity detector.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear reactors, and in particular relates to a first cycle passive starting method of a reactor based on a MOX fuel assembly. Background Art

[0002] Usually, reactor startup mainly relies on external neutron sources, which are divided into primary neutron sources and secondary neutron sources. Primary neutron sources are mainly used for critical safety supervision of the first cycle. Primary neutron sources can spontaneously produce neutrons. Usually, californium sources are used, and their source neutrons come from the spontaneous fission of Cf-252. From the second cycle, critical safety supervision is carried out by secondary neutron sources. Secondary neutron sources need to be activated by irradiation in the reactor before they can produce neutrons. They are usually placed in the reactor for activation in the first cycle and used for reactor startup from the second cycle. They are usually antimony-beryllium sources (Sb-Be sources), and their source neutrons come from the photon fission of beryllium.

[0003] The use of an external neutron source to start a nuclear reactor has the disadvantages of increasing the cost of handling high-level waste generated by the start-up neutron source, limiting the construction period of the reactor due to the long production and procurement cycle of the primary neutron source and the continuous attenuation of the source strength, and the damage of the secondary neutron source component or the long shutdown time for refueling and overhaul will have a great impact on the loading and startup of the reactor. Therefore, the development of passive start-up technology and its use in existing nuclear reactors provide a way to solve the above problems and have great application prospects.

[0004] Passive start-up technology refers to the technology of using the neutrons spontaneously released by nuclear fuel as the starting neutron source of the nuclear reactor to complete the reactor startup without an external neutron source. When the reactor is passively started, only the neutrons spontaneously released by nuclear fuel are used as the starting neutron source, and the source strength is relatively weak. In particular, the first cycle of the traditional reactor usually only loads new fuel assemblies, and only relies on the neutrons spontaneously released by the new fuel as the starting neutron source, and the source strength is even weaker. During the reactor loading process, the change of the core neutron injection rate during the loading process can be effectively monitored by the temporary detector inside the pile. However, during the reactor startup process, even if two high-sensitivity neutron detectors are added to the detection channel outside the pile, when the critical pre-criticality is deep, there is still a certain range of monitoring "blind spots" in the detector outside the pile. If an unexpected reactive introduction event occurs in the blind spot, it is difficult to ensure the critical safety of the core. Therefore, reducing the blind spot range during passive startup or achieving blind spot-free startup is a difficulty that needs to be solved urgently in the passive startup of the reactor.

[0005] The core of the first cycle of a conventional pressurized water reactor is loaded with new fuel assemblies. MOX fuel is a mixed oxide fuel (Mixed Oxide), which generally refers to PuO obtained by reprocessing spent fuel from a light water reactor. 2 and UO 2The U-Pu mixed oxide fuel is sometimes referred to as Th-U mixed oxide fuel. 2 The neutrons spontaneously released by the new fuel mainly come from 238 Spontaneous fission of U (a small part from 234 U and other nuclides (α, n) reaction). MOX fuel contains more 238 Pu, 239 Pu, 240 Pu, 241 Pu, 242 Pu, 241 The half-life of radioactive isotopes such as Am is 238 U should be short. 238 Pu, 240 Pu, 242 The spontaneous fission of Pu accounts for a high proportion of its decay, and spontaneous fission can release a large number of neutrons. 238 Pu, 239 Pu, 240 Pu, 242 Pu, 241 Am can also release neutrons by releasing α particles through α decay and reacting with O atoms in the fuel ((α,n) reaction). 2 Compared with new fuels, MOX fuel has a stronger neutron source. In the absence of an external neutron source, loading MOX fuel in the first cycle can increase the total neutron source intensity of the reactor and improve the count rate of the off-core detector during the core startup process, thereby reducing the blind area of ​​the first cycle reactor startup process and even achieving passive startup without blind areas.

[0006] Currently, there is no passive start-up method for the first cycle of a reactor based on MOX fuel assemblies in the prior art. The physical properties of the core using MOX fuel assemblies are different from those of traditional cores. In order to minimize the impact on the nuclear characteristics of the core, a small number of MOX fuel assemblies with reactivity equivalent to that of traditional assemblies can be arranged in the core near the detectors. By increasing the total source strength of the core, the monitoring blind spots of the external detectors during the startup process can be eliminated. Summary of the invention

[0007] The purpose of the present invention is to provide a method for passive start-up of the first cycle of a reactor based on an MOX fuel assembly, which can eliminate the use of a primary neutron source assembly, eliminate the dependence on the primary source, reduce the output of radioactive waste and its disposal cost, and improve the environmental protection index of the reactor; the method also has the possibility of eliminating the use of a secondary neutron source assembly, which can save the purchase cost of the secondary neutron source, reduce the disposal cost of the secondary source radioactive waste and the tritium emission caused by the secondary source, and is beneficial to environmental protection.

[0008] The technical solution of the present invention is as follows: A method for first cycle passive startup of a reactor based on a MOX fuel assembly comprises the following steps:

[0009] Step 1: After the reactor loading begins, the MOX fuel assemblies are first loaded in a certain order, that is, first in the core near the position of the high-sensitivity detector outside the core, and then loaded into the outer periphery of the core near the position of the high-sensitivity detector outside the core;

[0010] Step 2: The core fuel assemblies are loaded in a diagonal serpentine manner;

[0011] Step 3: For the core scheme in which MOX fuel assemblies are loaded in the inner core area, first complete steps 1 and 2, then move the core positions A08, R08, H01, and H15 of the MOX fuel assemblies loaded near the high-sensitivity detectors in step 1 to the core positions C08, N08, H03, and H13 of the MOX fuel assemblies in the core scheme, and then load the fuel assemblies determined in the core scheme at the positions near the high-sensitivity detectors in step 1.

[0012] In step 1, the MOX fuel assembly is first loaded in the core near the position of the high sensitivity detector outside the core, such as the core scheme A loading sequence or scheme B loading sequence, and loaded into the outer periphery of the core near the position of the high sensitivity detector outside the core.

[0013] The core positions A08, R08, H01, and H15 in the core loading sequence of core scheme A in step 1 are described.

[0014] The core positions of the core loading sequence of core scheme B in step 1 are A08, R08, H01, H15, A07, A09, R07, R09, J01, G01, J15, and G15.

[0015] The step 1 includes the following:

[0016] Step 11: Two high-sensitivity detectors are arranged in the first cycle external pile detection channel to ensure that these two channels obtain effective detector signals during loading;

[0017] Step 12: When loading the above MOX fuel assemblies, assemblies with higher source strength are loaded closer to the high-sensitivity detectors, and new MOX assemblies with an average Pu content of 8.0% are placed at positions A08, R08, H01, and H15 close to the high-sensitivity detectors.

[0018] The core scheme in step 3 is core positions C08, N08, H03, and H13.

[0019] The beneficial effects achieved by the present invention are as follows: by loading MOX fuel assemblies in the first cycle core and arranging high-sensitivity detectors outside the core, the present invention can realize the passive start-up of the first cycle reactor without blind spots, so that the whole process of the neutron injection rate increase during the physical start-up of the reactor is placed under effective supervision, thereby ensuring the safe start-up of the reactor. Compared with the use of new fuel assemblies and primary and secondary source loading in the first cycle of the traditional pressurized water reactor, (1) the use of primary neutron source assemblies can be eliminated, saving the primary source procurement cost, eliminating the dependence on primary source imports, reducing the production of radioactive waste and its disposal costs, which is beneficial to environmental protection; (2) when relying on MOX fuel assemblies to obtain effective counting rate signals during the first cycle core loading process and after the loading is completed, the use of secondary neutron source assemblies can be eliminated. The procurement cost of secondary neutron sources can be saved, while reducing the production of secondary source radioactive waste and its disposal costs, reducing tritium emissions caused by secondary sources, which is beneficial to environmental protection. The present invention can significantly improve the economic efficiency of power plants, help improve the environmental protection indicators of reactors, and avoid dependence on primary neutron source materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of loading 4 MOX fuel assemblies for the first cycle (Scheme A);

[0021] 18000, new assembly with 1.8% enrichment; 24004, new assembly with 2.4% enrichment and 4 gadolinium rods; 24008, new assembly with 2.4% enrichment and 8 gadolinium rods; 31000, new assembly with 3.1% enrichment; 31008, new assembly with 3.1% enrichment and 8 gadolinium rods; 31012, new assembly with 3.1% enrichment and 12 gadolinium rods; 31016, new assembly with 3.1% enrichment and 16 gadolinium rods; 39000, new assembly with 3.9% enrichment; 640 m , a new MOX assembly with an average Pu content of 6.4% (containing 264 MOX fuel rods);

[0022] Figure 2 Schematic diagram of the core loading sequence for loading 4 MOX fuel assemblies in the first cycle (Scheme A);

[0023] The numbers in the figure indicate the loading steps, and the capital letter (M) indicates the MOX fuel assembly.

[0024] Figure 3 Schematic diagram of loading 12 MOX fuel assemblies for the first cycle (Scheme B);

[0025] 18000, new assembly with 1.8% enrichment; 24004, new assembly with 2.4% enrichment and 4 gadolinium rods; 24008, new assembly with 2.4% enrichment and 8 gadolinium rods; 31000, new assembly with 3.1% enrichment; 31008, new assembly with 3.1% enrichment and 8 gadolinium rods; 31012, new assembly with 3.1% enrichment and 12 gadolinium rods; 31016, new assembly with 3.1% enrichment and 16 gadolinium rods; 39000, new assembly with 3.9% enrichment; 640 m , new MOX assembly with an average Pu content of 6.4% (containing 264 MOX fuel rods); 800 m , a new MOX assembly with an average Pu content of 8.0% (containing 264 MOX fuel rods);

[0026] Figure 4 Schematic diagram of the core loading sequence for loading 12 MOX fuel assemblies in the first cycle (Scheme B);

[0027] The numbers in the figure indicate the loading steps, and the capital letter (M) indicates the MOX fuel assembly;

[0028] Figure 5 This is a schematic diagram of the layout of the off-pile detectors used in the passive start-up of the first cycle. DETAILED DESCRIPTION

[0029] The specific implementation of the present invention is given as follows with reference to the accompanying drawings, but is not limited to the implementation, and the scope of rights of the present invention cannot be limited by this implementation. Therefore, equivalent changes made according to the scope of the patent application of the present invention still fall within the scope covered by the present invention.

[0030] The present invention provides a method for passive start-up of a reactor in the first cycle based on MOX fuel assemblies, which aims to improve the total neutron source strength in the core, eliminate the blind area of ​​critical monitoring during the critical start-up of the first cycle reactor, and realize the passive start-up of the first cycle reactor without blind areas. Specifically, it includes the following steps:

[0031] Step 1: After the reactor loading begins, firstly, according to the first cycle core loading plan for loading MOX fuel assemblies, that is, the plan for loading MOX fuel assemblies in the first cycle core, such as Figure 1 Plan A Figure 2 Plan B), first load the MOX fuel assemblies in a certain order, that is, first load the MOX fuel assemblies in the core near the high-sensitivity detector outside the core, as shown in the loading sequence of core plan A Figure 3 Or the charging sequence of Scheme B Figure 4 , loaded into the core periphery near the high sensitivity detector outside the core, as shown in the loading sequence diagram of core scheme A Figure 3 Core positions A08, R08, H01, H15 or loading sequence diagram for Scheme B Figure 4 The core positions are A08, R08, H01, H15, A07, A09, R07, R09, J01, G01, J15, and G15;

[0032] Further, step 1 includes the following:

[0033] Step 11: Arrange two high-sensitivity detectors (such as He-3 detectors) and two source range detectors in the first cycle external detection channel, and ensure the effectiveness of these four detectors before loading;

[0034] Step 12: When loading the above MOX fuel assemblies, assemblies with higher source strength are loaded at positions closer to the high-sensitivity detectors. For example, the neutron source strength of the new MOX assembly with an average Pu content of 8.0% in Scheme B is greater than that of the new MOX assembly with an average Pu content of 6.4%. In order to maximize the neutron flux at the high-sensitivity detector outside the pile, the new MOX assembly with an average Pu content of 8.0% is arranged at positions A08 and R08 closer to the high-sensitivity detectors, or the new MOX assembly with an average Pu content of 6.4% is arranged at positions A08 and R08 closer to the high-sensitivity detectors as in Scheme A.

[0035] Step 2: Follow the diagonal serpentine loading method, as shown in the loading sequence of core scheme A Figure 3 The charging sequence of scheme B is shown in the figure Figure 4 The core fuel assemblies are loaded in a diagonal serpentine manner. Through this optimized fuel assembly loading order, the movement of the core detectors is reduced, the efficiency and safety of core loading are improved, and the loading of the remaining fuel assemblies in the first cycle core is completed.

[0036] Step 3: For core plan B in which MOX fuel assemblies are loaded in the inner core area (e.g., core positions C08, N08, H03, and H13), first complete steps 1 and 2, then move the MOX fuel assemblies loaded near the high-sensitivity detectors in step 1 to positions C08, N08, H03, and H13 of MOX fuel assemblies in core plan B, and then load the fuel assemblies determined in the core plan at positions near the high-sensitivity detectors in step 1. At this point, the loading of all fuel assemblies in the entire core is completed.

[0037] The final position of the MOX fuel assembly in the loading diagram must meet the requirements of the core loading plan on the design criteria and limits of the first cycle length, power peak factor, moderator temperature coefficient, etc. The final position of the MOX fuel assembly (and secondary source) in the loading diagram must also enable the two high-sensitivity detectors to obtain effective count rate signals after loading is completed.

[0038] Example:

[0039] Figure 1 This is a schematic diagram of loading MOX fuel assemblies in the first cycle of a pressurized water reactor. The core consists of 177 boxes of fuel assemblies. In the first cycle, 4 boxes of MOX fuel assemblies are loaded on the periphery of the core according to the 1 / 4 rotational symmetry principle, and the remaining fuel assemblies are new fuel assemblies with different enrichments. Figure 1 When the loading diagram is shown, the MOX fuel assembly meets the design criteria and limit requirements of the loading scheme regarding cycle length, power peak factor, moderator temperature coefficient, etc. at the loading position.

[0040] According to the method for monitoring the criticality of loading MOX fuel assemblies in a pressurized water reactor in the first cycle provided by the present invention, the loading steps are as follows: Figure 2 Solution A enables the high-sensitivity detector outside the pile to obtain a count rate signal of about 2cps, and Solution B enables the source range detector to obtain a count rate signal of about 0.5cps.

[0041] The specific steps of scheme A (high sensitivity detector) are as follows:

[0042] Step 1: According to Figure 1 The first cycle core loading scheme for loading MOX fuel assemblies (new MOX assemblies with an average Pu content of 6.4% are loaded at core positions A08, R08, H01, and H15) is shown in FIG. 4. Four boxes of MOX fuel assemblies are selected and loaded into the outer periphery of the core near the position of the high-sensitivity detector outside the core in a certain order. Specifically, Figure 2 The middle loading steps 1 to 4 (loading into the core positions A08, R08, H01, and H15 in sequence) are shown;

[0043] Step 3: Complete the loading of the remaining fuel assemblies in the first cycle core in a diagonal serpentine loading manner, as follows: Figure 2 The loading steps are shown in 5-77, 79-98, 100-119, and 121-174.

[0044] At this point, the loading of all fuel assemblies in the entire core has been completed. Calculation and analysis show that at this time, relying on the spontaneous release of neutrons from the entire fuel assembly, the two high-sensitivity detectors outside the core can obtain a count rate signal of about 2cps, meeting the requirements of the "Nuclear Reactor Instrumentation Guidelines Part I: General Principles" (GB12789.1-1991) (the neutron count rate of the high-sensitivity channel outside the core after loading is not less than 2cps).

[0045] The specific steps of Solution B (source range detector) are as follows:

[0046] Step 1: According to Figure 3The first cycle core loading scheme for loading MOX fuel assemblies (MOX new assemblies with an average Pu content of 8.0% are loaded at core positions A08, R08, H01, and H15, and MOX new assemblies with an average Pu content of 6.4% are loaded at core positions A07, A09, R07, R09, J01, G01, J15, and G15) is shown. 12 boxes of MOX fuel assemblies are selected and loaded into the outer periphery of the core near the position of the external source range detector in a certain order, as shown in FIG. Figure 4 The middle loading sequence is shown in steps 1 to 12 (loading into the core positions A08, R08, H01, H15, A07, A09, R07, R09, J01, G01, J15, G15 in sequence);

[0047] Step 2: During the actual fuel loading process, the average count rate of the two symmetrically located source range detectors is measured as the reference count rate, and the alarm setting value of the source range detector is set on this basis for the reactor criticality safety monitoring in the subsequent fuel loading process;

[0048] Step 3: Complete the loading of the remaining fuel assemblies in the first cycle core in a diagonal serpentine loading manner, as follows: Figure 4 The loading steps are shown as 13-24, 26-77, 79-119, 121-162, and 164-175.

[0049] At this point, the loading of all fuel assemblies in the entire core has been completed. Calculation and analysis show that at this time, relying on the neutrons spontaneously released by the entire fuel assembly, the two external source range detectors can obtain a count rate signal of about 0.5cps, which meets the requirements of the "First Fuel Loading Test of Pressurized Water Reactor Nuclear Power Plant Reactor" (EJ / T 1114-2000). If the source range detectors are to obtain a count rate signal of ≥2cps in order to meet the requirements of the "Nuclear Reactor Instrumentation Code Part I: General Principles" (GB12789.1-1991), more MOX fuel assemblies or secondary neutron source assemblies must be loaded.

[0050] The present invention discloses a method for passive starting of the first cycle of a reactor based on MOX fuel assemblies, comprising the following steps: selecting a number of boxes of MOX fuel assemblies according to a core loading plan and loading them into a position near a high-sensitivity detector or a source range detector on the periphery of the core; and completing the loading of the remaining fuel assemblies in a diagonal serpentine manner. The present invention can eliminate the use of a primary neutron source, and has the possibility of eliminating the use of a secondary neutron source according to the specific conditions of the unit, thereby saving procurement costs, reducing dependence on imported materials, and reducing the output of radioactive waste. While retaining the secondary neutron source, the decay time allowed for the secondary neutron source before entering the stack can be extended, thereby increasing the flexibility of the nuclear power plant construction period and reducing transportation and storage costs.

Claims

1. A method for first cycle passive startup of a reactor based on MOX fuel assembly, characterized in that: The following steps are involved: Step 1: After the reactor loading begins, the MOX fuel assemblies are first loaded in a certain order, that is, first in the core near the position of the high-sensitivity detector outside the core, and then loaded into the outer periphery of the core near the position of the high-sensitivity detector outside the core; Step 2: The core fuel assemblies are loaded in a diagonal serpentine manner; Step 3: For the core scheme in which MOX fuel assemblies are loaded in the inner area of ​​the core, first complete steps 1 and 2, then move the MOX fuel assemblies loaded near the high-sensitivity detectors in step 1 to positions C08, N08, H03, and H13 of the MOX fuel assemblies in the core scheme, and then load the fuel assemblies determined in the core scheme at positions near the high-sensitivity detectors in step 1.

2. A method for first cycle passive startup of a reactor based on MOX fuel assembly according to claim 1, characterized in that: In step 1, the MOX fuel assembly is first loaded in the core near the position of the high sensitivity detector outside the core, such as the core scheme A loading sequence or scheme B loading sequence, and loaded into the outer periphery of the core near the position of the high sensitivity detector outside the core.

3. A method for first cycle passive startup of a reactor based on MOX fuel assembly as claimed in claim 2, characterized in that: The core positions A08, R08, H01, and H15 in the core loading sequence of core scheme A in step 1 are described.

4. A method for first cycle passive startup of a reactor based on MOX fuel assembly as claimed in claim 2, characterized in that: The core positions of the loading sequence of Scheme B in Step 1 are A08, R08, H01, H15, A07, A09, R07, R09, J01, G01, J15, and G15.

5. A method for first cycle passive startup of a reactor based on MOX fuel assembly according to any one of claims 1 to 4, characterized in that: The step 1 includes the following: Step 11: Two high-sensitivity detectors are arranged in the first cycle external pile detection channel to ensure that these two channels obtain effective detector signals during loading; Step 12: When loading the above MOX fuel assemblies, assemblies with higher source strength are loaded closer to the high-sensitivity detectors, and new MOX assemblies with an average Pu content of 8.0% are placed at positions A08 and R08 away from the high-sensitivity detectors.

6. A method for first cycle passive startup of a reactor based on MOX fuel assembly as claimed in claim 5, characterized in that: The high-sensitivity detector is a He-3 detector.

7. A method for first cycle passive startup of a reactor based on MOX fuel assembly as claimed in claim 1, characterized in that: The core scheme in step 3 is core positions C08, N08, H03, and H13.

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