CN104575641B - A kind of method and device for improving out-pile Nuclear measurement system Axial power difference estimation precision - Google Patents

Abstract

The invention relates to a method and device for improving the measurement accuracy of the axial power deviation of the nuclear measurement system outside the reactor. Between the outer wall of the pressure vessel and the power range detector, at the middle position of the detector in the longitudinal direction, a moderating absorbing material is arranged to make the reactor The fast neutrons in the upper and lower parts of the core are slowed down to thermal neutrons by the moderator material before reaching the lower detector and the upper detector of the extracore nuclear test respectively, so as to reduce the cross effect of the upper and lower detectors The impact on the calculation accuracy of the axial power deviation of the nuclear measurement system outside the reactor. After adopting the technology of the invention, the cross effect is reduced from the root, so that the variance of the least square coefficient determined under different axial power deviations is significantly reduced, and the measurement accuracy of the axial power deviation of the external core measurement system is improved.

Description

A method and device for improving the accuracy of measuring and calculating the axial power deviation of an external nuclear measurement system

technical field

The invention belongs to the design technology of an out-of-heap nuclear measurement system of a nuclear power plant, and in particular relates to a method and a device for improving the measurement accuracy of axial power deviation of the out-of-core nuclear measurement system.

Background technique

In the reactor protection and control system, for the PWR external power range, the two most important output signals are the total reactor power and the axial power deviation. In the protection system, the power signal is used for shutdown protection of the reactor under different operating conditions, and the power deviation signal is used as the input of the penalty function f(ΔI) in the calculation of overtemperature ΔT (OTΔT) and overpower ΔT (OPΔT). In the control system, the power signal is used for the adjustment of the average temperature and the control of the axial power deviation.

Power range detectors respond to fast neutrons escaping from peripheral fuel assemblies. Neutrons from the central fuel assembly in the core almost never reach the detectors outside the core. For the external power range detector, the upper detector is mainly sensitive to the power in the upper part of the core, and at the same time sensitive to the power in the lower part of the core, and vice versa. This is the cross effect. The principle is shown in Figure 1.

Due to the existence of the cross effect, the power share of the cross effect part of the upper and lower detectors will be different for different axial power deviations. At present, the power plant uses the least square method to determine the different values of the upper and lower detectors under different axial power deviation conditions. Weighting factors so that the result calculated from the power range measurement is as close as possible to the actual axial power deviation. However, this approximate theoretical result corrected for a certain power deviation will inevitably make the calculation results inconsistent with the actual power deviation in the stack during the change of the axial power distribution in the stack, which will lead to deviations in the calculation results of over-temperature and over-power. Only by physically filtering out this cross effect can the uncertainty of axial power deviation measurement be effectively reduced.

Contents of the invention

The purpose of the present invention is to provide a method and device for improving the accuracy of measuring and calculating the axial power deviation of the external nuclear measurement system in view of the deficiencies in the prior art.

The technical solution of the present invention is as follows: a method for improving the measurement accuracy of the axial power deviation of the nuclear measurement system outside the reactor. Between the outer wall of the pressure vessel and the power range detector, the power range detector is located in the middle position in the longitudinal direction, and the slowing absorber is set. materials, so that the fast neutrons in the upper and lower parts of the core are slowed down to thermal neutrons by the moderator material before reaching the lower detector and the upper detector of the power range detector respectively, reducing the detection of the upper detector and the lower detector. Influence of the crossover effect of the reactor on the calculation accuracy of the axial power deviation of the nuclear measurement system outside the reactor.

Furthermore, in the above-mentioned method for improving the measurement accuracy of the axial power deviation of the external core monitoring system, the moderating absorbing material is cross-linked high-density boron-containing polyethylene coated with cadmium on the surface.

A device for improving the calculation accuracy of the axial power deviation of the nuclear measurement system outside the reactor, comprising a bracket arranged in the reactor power range instrument well, and a moderation absorption material is arranged on the bracket, and the moderation absorption material is located on the outer wall of the pressure vessel The vertical middle position of the detector between the detector and the power range detector.

Further, the above-mentioned device for improving the measurement accuracy of the axial power deviation of the excore nuclear measurement system, wherein the moderating absorbing material has a cuboid structure, and the material is cross-linked high-density boron-containing polyethylene with cadmium wrapped on the surface.

Further, the above-mentioned device for improving the measurement accuracy of the axial power deviation of the external nuclear measurement system, wherein the thickness range of the moderating absorbing material is between the position 10-20 mm away from the detector and the outer surface of the pressure vessel distance.

Furthermore, in the above-mentioned device for improving the measurement accuracy of the axial power deviation of the external core measurement system, the unit hydrogen content of the cross-linked high-density boron-containing polyethylene is 1.24 times the hydrogen content of water.

The beneficial effects of the present invention are as follows: the method and device provided by the present invention slow down and absorb the neutrons in the cross-effect part of the power range, eliminate or reduce the influence of the cross-effect, without changing the existing unit algorithm, only adding a physical On the basis of the channel, the purpose of improving the measurement accuracy of the axial power deviation of the external nuclear measurement system is achieved. The present invention slows and absorbs the neutrons of the cross-effect part in a physical way, does not need to add a software processing process, and does not change the existing related software processing process.

Description of drawings

Figure 1 is a schematic diagram of the principle of the cross effect;

Fig. 2 is the schematic diagram of the simplified model of cross effect;

Fig. 3 is the power ratio of the cross effect;

Figure 4-1 to Figure 4-3 are the supporting structure diagrams of the cross-effect moderation module;

In the figure, 1. left support plate; 2. right support plate; 3. left triangular support bracket; 4. right triangular support bracket; 5. triangular support bracket fixing pin; 6. support frame fixing hole; 7. slow chemical absorbent material module.

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a schematic diagram of the principle of cross-effect, which gives the concept of cross-effect. The upper detector is mainly sensitive to the power in the upper part of the core, while the lower detector is mainly sensitive to the power in the lower part of the core. However, due to the existence of cross effects, the upper detector is also sensitive to the power of part of the lower core, and the lower detector is also sensitive to the power of part of the upper detector. As the power decreases, the share of the signal seen by the upper detector from the lower part of the core increases, so that the net signal drop to which the upper detector is sensitive is smaller than the actual drop in the peripheral fuel assemblies. It is conceivable that when the detector is infinitely far away from the core, the upper detector and the lower detector will be sensitive to the power of the entire core, and it will be impossible to calculate the axial power deviation of the core.

Aiming at the impact of the crossover effect of the external nuclear test on the axial power calculation, the present invention proposes to use cross-linked high-density boron-containing polyethylene as the moderating absorbing material to slow down the fast neutrons of the crossover effect, thereby reducing the influence of the crossover effect. After adopting the technology of the invention, the cross effect is reduced from the root, so that the variance of the least square coefficient determined under different axial power deviations is significantly reduced, and the measurement accuracy of the axial power deviation of the external core measurement system is improved.

In the present invention, between the outer wall of the pressure vessel and the power range detector, a cubic installation space is formed in the vertically middle position of the detector; a cross-linked high-density boron-containing polyethylene support structure-bracket is installed in the space; in the installation space Place cross-linked high-density boron-containing polyethylene wrapped with cadmium products on the surface.

The height of the cross-linked high-density boron-containing polyethylene neutron moderation absorption module is about 300mm, the width is the width of the opening of the nuclear testing instrument well outside the reactor, and the thickness range is the distance between the detector working position 10-20mm and the outer surface of the pressure vessel , about 350mm. The hydrogen content per unit of cross-linked high-density boron-containing polyethylene is about 1.24 times that of water, and the high temperature resistance can reach above 350 °C. The use of thermal neutron absorbing material boron is expected to prolong the life of the detector and the calibration cycle, and the content needs further analysis and calculation. The service life should be consistent with the 10-year cycle of the primary circuit pressure test.

In order to achieve the purpose of moderation, the present invention adopts polyethylene material, which has stronger moderation ability than water, and the number of hydrogen atoms contained is 8.3×10 ²² /cm ³ , while the number of hydrogen atoms contained in water is 6.7×10 ²² /cm ³ .

Since the material is used to be close to the outer wall of the pressure vessel, it is required that the material has high temperature resistance. In order to achieve the purpose of high temperature resistance, the present invention adopts the addition of a crosslinking agent in polyethylene, which will increase the degree of polymerization of the polyethylene material and form high-density polyethylene. The material can withstand high temperature above 350°C.

In order to absorb part of the neutrons of the cross-effect and further reduce the cross-effect, boron 10 material is added to the cross-linked high-density polyethylene material, and its thermal neutron nuclear fission cross-section reaches 3700 targets or more. Thermal neutrons moderated by HDPE.

Therefore, the present invention adopts cross-linked high-density boron-containing polyethylene material for moderating and absorbing neutrons in the cross-effect part, and the materials used are all cheap existing materials, which can be purchased directly on the market.

Since the moderator absorption material used around the insulation layer is about 30-50kg, which is quite different from the glass fiber used in the insulation material, in order not to deform the surrounding insulation material, it is necessary to add a support structure to hold the moderator absorption material. In order not to form pressure on the insulation material.

In order not to affect the installation and disassembly of the insulation material, and in order not to affect the service inspection of the pressure vessel, the bracket structure is designed as a movable structure so that it can be folded and stored after the moderator absorbing material is removed.

Accompanying drawing 2 is the simplified model of cross-effect, in order to estimate the quantitative value of cross-effect, for the neutron that each fuel cell emits, what the detector is sensitive to is the neutron in a fan-shaped cone, because detector and fuel micro-element Due to the limitation of the size of the element, the angle of the fan-shaped cone surface in the radial direction is very small, which can be considered as a fixed value. For simplicity, a sector is used for approximation here. In this way, it is only necessary to know the size of the opening angle of each fuel cell on the upper detector and the lower detector, and the respective power shares of the fuel cell on the upper detector and the lower detector can be determined. The power share of the detector is multiplied by the power of the fuel cell and superimposed to determine the influence of the crossover effect. According to the relevant structural parameters of a certain power station, assuming that the axial power distribution is a cosine distribution, it is calculated and determined that the power share of the lower fuel assembly in the upper detector is about 27.5% of the power of the lower fuel assembly, as shown in the cross-effect power ratio in Figure 3 , where the abscissa is the fuel element shown in Figure 2, and the ordinate is the power ratio. Figure 3 is the ratio of the power share of each fuel element in the lower fuel assembly in the upper detector to the power share in the lower detector under the assumption that the axial power distribution is an ideal condition of cosine distribution. About 27.5% of the power of the lower fuel assembly is sensitive to the upper detector, and 72.5% of the power is sensitive to the lower detector. There is a similar situation for the upper fuel assembly, and it can be seen that the influence of the cross effect is relatively large. It should be noted that the assumption of cosine distribution of axial power is adopted here, but the actual power distribution should be larger in the middle of the axial direction, and considering the length of neutron travel, the actual cross effect should be greater.

After adopting the method and device provided by the present invention, the fast neutrons in the upper part of the core are moderated into thermal neutrons by cross-linked high-density boron-containing polyethylene before reaching the detector in the lower part of the nuclear test outside the reactor, and the energy is reduced, and the absorption is in progress. The fast neutrons in the lower part of the core are slowed down to thermal neutrons by cross-linked high-density boron-containing polyethylene before reaching the detector in the upper part of the nuclear test. The energy is reduced and the neutrons are absorbed. The thermal neutrons moderated by the cross-linked high-density boron-containing polyethylene in the upper part of the core are further moderated by the polyethylene inside the detector, the energy is further reduced, the nuclear absorption cross section is reduced, and the influence of cross effects is reduced; the lower part of the core is passed through The thermal neutrons moderated by the cross-linked high-density boron-containing polyethylene are further moderated by the polyethylene inside the detector, the energy is further reduced, the nuclear absorption cross section is reduced, and the influence of the cross effect is reduced. After the detector signal is processed by the coefficient algorithm after verification, the total power of the reactor and the axial power deviation signal are obtained, and then sent as a control signal to the protection and rod control system and used as a display signal for alarm and operation control.

The device is to install a moderator absorbing material bracket in the power range instrument well, and a cuboid solid cross-linked high-density boron-containing polyethylene wrapped with cadmium products is installed on the bracket. In a specific embodiment, it is necessary to replace a piece of insulation material in the middle of each power range channel, and replace it with a cross-linked high-density boron-containing polyethylene moderation absorption material. As shown in Figure 4, in this embodiment, the bracket structure is installed on the walls on both sides of the power range instrument well through the left support plate 1 and the right support plate 2, and the left support plate 1 and the right support plate 2 are fixed by the support frame The hole 6 is fixed, and the bracket is two separate movable tripod structures, including the left triangular support bracket 3 and the right triangular support bracket 4, which are respectively fixed on the left support plate 1 and the right support plate 2 by the triangular support bracket fixing pin 5 When in use, the bracket structure is placed in the working position to support the moderating absorption material module 7, and when not in use, it is placed in a fixed position against the wall.

It should be noted that the above description is only exemplary. Under the above teaching of the present invention, those skilled in the art can make various improvements and deformations on the basis of the above examples. Measures to reduce cross-effects all fall within the protection scope of the present invention. Those skilled in the art should understand that the above specific description is only for explaining the method of the present invention, and is not intended to limit the present invention. The protection scope of the present invention is defined by the claims and their equivalents.

Claims (6)

1. A method for improving the calculation accuracy of the axial power deviation of the nuclear measurement system outside the reactor, characterized in that: between the outer wall of the pressure vessel and the power range detector, in the middle position of the power range detector in the longitudinal direction, a moderating absorbing material is set , so that the fast neutrons in the upper and lower parts of the core are moderated into thermal neutrons by the moderator material before reaching the lower detector and the upper detector of the power range detector respectively, reducing the number of the upper detector and the lower detector The influence of cross effect on the calculation accuracy of the axial power deviation of the nuclear measurement system outside the reactor. 2. The method for improving the accuracy of measuring and calculating the axial power deviation of the external core monitoring system according to claim 1, characterized in that: the moderating absorbing material is cross-linked high-density boron-containing polyethylene coated with cadmium on the surface. 3. A device for improving the calculation accuracy of the axial power deviation of the external nuclear measurement system, characterized in that: it includes a bracket arranged in the reactor power range instrument well, and a moderator absorbing material is set on the bracket, and the moderator The absorbing material is located in the longitudinal middle position section of the detector between the outer wall of the pressure vessel and the power range detector. 4. The device for improving the calculation accuracy of the axial power deviation of the external nuclear measurement system according to claim 3, characterized in that: the moderating absorbing material is a cuboid structure, and the material is a cross-linked high-density boron-containing material wrapped with cadmium on the surface polyethylene. 5. The device for improving the calculation accuracy of the axial power deviation of the external nuclear measurement system according to claim 3 or 4, characterized in that: the thickness range of the moderating absorbing material is between 10 and 20 mm from the detector distance from the outer surface of the pressure vessel. 6. The device for improving the measurement accuracy of the axial power deviation of the external nuclear measurement system according to claim 4, characterized in that: the unit hydrogen content of the cross-linked high-density boron-containing polyethylene is 1.24 of the hydrogen content of water times.