RU2219600C2 - Method for reducing nonuniformity of thermal resources distribution in fuel assemblies of nuclear reactor core (alternatives) - Google Patents

Abstract

FIELD: cores of water-cooled thermal reactors. SUBSTANCE: method for reducing nonuniformity of thermal resources distribution in reactor core fuel assemblies incorporating fuel elements of main enrichment with respect to fissionable isotopes includes insertion of thermal resources having lower enrichment in high- nonuniformity fuel elements. Fuel placed in fuel elements in vicinity of higher thermal resources has higher enrichment than in vicinity of lower thermal resources. As one of alternatives of this method, fuel used in lower part of peripheral fuel elements is of higher enrichment than that in their upper part. As another alternative of proposed method, fuel disposed in lower parts of peripheral and pre- peripheral fuel elements is of higher enrichment than that in their upper parts. EFFECT: enhanced thermal reliability of fuel assemblies. 3 cl, 5 dwg

Description

Technical field
The invention relates to nuclear energy, and more specifically to the active zones of water-cooled nuclear reactors with thermal neutrons.

State of the art
A known method of reducing the radial unevenness of energy release by fuel assemblies by means of radial physical profiling, which consists in a complete height reduction of the enrichment of peripheral fuel elements in fuel assemblies [I]. The application of this method provides an approximate equality of energy release for the fuel rods of the peripheral row and the fuel rods inside the fuel assembly without taking into account all kinds of disturbances.

 The disadvantage of this method is that it leaves a rather high degree of unevenness in the distribution of heat reserves in real-life conditions, taking into account the uncertainties and perturbations of the parameters, in particular when the fuel assemblies are bent in the core, which is accompanied by a deviation of the values of the inter-cartridge gaps from the nominal value.

 There is also a method of reducing axial neutron leakage from the core by axial physical profiling, which consists in reducing the enrichment at the ends of the fuel rods in a fuel assembly [2], in particular in the use of thin axial blankets from natural uranium. The application of this method increases the economic efficiency of the use of nuclear fuel.

 The disadvantage of this method is that it is accompanied by an increase in the unevenness of energy release in the core and a decrease in heat reserves.

Disclosure of Invention
The aim of the invention is to increase the safety of a nuclear reactor.

 The objective of the invention is to reduce the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor.

 The technical result of the invention is to increase the heat engineering reliability of fuel assemblies.

 The problem is solved in that the way to reduce the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor containing fuel rods with primary enrichment fuel for fissile isotopes, which consists in the placement of fuel elements of higher enrichment in thermal fuels with higher unevenness of fuel for lower enrichment in fissile isotopes, is different in that in the fuel rods with increased unevenness of heat reserves, in the area of higher heat reserves there is fuel a higher concentration of fissionable isotopes.

 The problem is also solved by the fact that the method of reducing the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor containing fuel elements of the primary enrichment in fissile isotopes, which consists in placing lower enrichment fuel in the peripheral fuel elements in fissile isotopes, differs in The lower part of the peripheral fuel elements contains fuel of higher enrichment in fissile isotopes.

 The problem is also solved by the fact that the method of reducing the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor containing fuel elements of the main enrichment for fissile isotopes, which consists in the placement of lower enrichment fuels for fissile isotopes in the peripheral and pre-peripheral fuel elements, that in the lower part of the peripheral and pre-peripheral fuel rods there is a fuel of higher enrichment for fissile isotopes.

 The problem is also solved by the fact that the method for reducing the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor containing fuel elements of the main enrichment for fissile isotopes, which consists in the placement of fuel elements with increased unevenness of the heat technical reserves of fuel of lower enrichment for fissile isotopes, differs the fact that in the fuel rods with increased unevenness of heat reserves, in the zone of higher heat reserves fuel of higher enrichment in fissile isotopes, and in fuel rods with fuel of the main enrichment, in the zone of lower heat reserves, fuel of lower enrichment in fissile isotopes is placed.

 The proposed method is an effective way to reduce the uneven distribution of heat reserves in the fuel assemblies of the active zone of a nuclear reactor.

Brief Description of the Drawings
The invention is illustrated by drawings, on which:
FIG. 1 is a diagram of the radial placement of profiled fuel rods (with low enrichment fuel) in the peripheral row of the fuel assembly;
FIG. 2 is a diagram of the radial placement of profiled fuel rods (with low enrichment fuel) in two peripheral rows of the fuel assembly;
FIG. 3 is a diagram of an axial distribution of primary and reduced enrichment fuel in profiled fuel rods in the peripheral row of a fuel assembly;
FIG. 4 is a diagram of an axial distribution of primary and reduced enrichment fuel in profiled fuel rods in two peripheral rows of a fuel assembly;
FIG. 5 is a diagram of axial energy release profiles, reflecting the distribution of heat reserves over the height of the active part of non-profiled and profiled fuel rods.

Embodiments of the invention
The local heat supply is approximately estimated by the margin to the maximum energy release - the ratio of the maximum energy release for a given coordinate along the fuel rod height to the actual energy release at this point. In many cases, there is a significant decrease in the local heat supply in the upper part of fuel rods 1 and 5, which determines the high unevenness of the heat supply over the height of these fuel rods. And in some cases, the need to provide a local heat supply of more than 1.0 forces to reduce the energy release in the fuel rods 1, 2 and 5 by reducing the integral power of the reactor. To avoid this, the existing method [1] provides a decrease in linear energy release in the most energy-intense fuel rods 1 by placing in them the entire height of the fuel low enrichment for fissile isotopes. This reduces the energy production in the fuel assemblies containing thus profiled fuel rods 1.

 The proposed method to reduce the uneven distribution of heat reserves in the fuel assemblies of a nuclear reactor involves the addition of the existing radial profiling scheme of fuel assemblies, according to which among the fuel rods (2) with the main enrichment for fissile isotopes in the areas of increased energy release there are fuel rods (1) with low enrichment fuel for fissile isotopes [I], axial profiling of peripheral fuel elements (1). Such a total use of profiling schemes leads to the fact that in the most energy-intensive fuel rods (1) in the lower half fuel of the main enrichment (3) is located in fissile isotopes, and in the upper half - fuel of low enrichment (4) in fissile isotopes. As a result of this, the energy release in these fuel rods is redistributed in such a way that it increases in the lower half and decreases in the upper half. This, in turn, leads to a corresponding redistribution of local heat engineering reserves along the height of the fuel rods. In the lower part (3) of the fuel rods under consideration (1), the previously existing large thermal reserves are slightly reduced, and in the upper part (4), small reserves increase. Such a redistribution of heat reserves allows us to maintain the required power of energy release both in the fuel rods themselves (1) and in the fuel assembly as a whole. And it follows that the total energy production in the fuel rods and fuel assemblies with the proposed profiling is higher than in the fuel rods and fuel assemblies profiled by the method [I].

 The increase in local heat reserves in the upper part as a result of the application of the proposed profiling of the most energy-intensive fuel elements 1 also allows you to compensate for the adverse effect of bursts of energy in the zone of increased cassette clearances on the reliability of cooling of the fuel elements. The maximum value of the cassette gap in the core and its axial shape are usually estimated by complex thermomechanical calculations, the error of which is quite large (up to 40%); So for the predicted value of the maximum inter-cartridge gap of 13 mm, the range of actual inter-cartridge gaps, taking into account the error, can be from 9 to 18 mm. The location of the maximum gap between the cassettes in the core is also uncertain. In addition, as a rule, one thing is predicted - the average value of the inter-cartridge gap along each face of the fuel assembly in the plan, which in reality is unstable due to "rotational" deformations, as a result of which the local inter-cartridge gap can be twice as large as the average along the face. This implies the requirement of optimality of profiling for a wide range of gaps in the entire core. This cannot be ensured by the known method of profiling [I], increasing or decreasing the enrichment of peripheral fuel rods 1 for fissile isotopes along the entire height, since when there is an actual deviation of the inter-cassette clearances from the predicted maximum value, there is a risk of exceeding the maximum permissible power release line within the calculated error 8, and when they are deviated to the smaller side, a tendency arises of an undesirable underload of the peripheral row of fuel rods 1 in the fuel failure in comparison with pre-peripheral row 5 and internal fuel rods 2. In addition, according to calculations [3-5], even with more accurate prediction of the maximum inter-cartridge gap in the core with fuel assembly profiling according to the scheme [1], there will be many inter-cartridge gaps with reduced down to zero value, which also gives an underload of the array of peripheral fuel elements 1.

 It is proposed to use combined - radial-axial local profiling, consisting in reducing fuel enrichment only in those small areas of the active zone (areas of "hot spots") where minimal heat reserves are realized taking into account perturbations of inter-cartridge gaps in a wide range, and also taking into account technological uncertainties and design errors, reducing the degree of unevenness in the distribution of heat reserves and thereby increasing these reserves. The proposed method is an effective way to compensate for the adverse effect of increased cassette clearances when the fuel assemblies are bent on heat reserves in conditions of incomplete knowledge of the magnitude and location of the maximum cassette clearances in the core.

 To identify areas of "hot spots" in the core, which can only be correctly done taking into account perturbations of inter-cassette gaps, an analysis is carried out according to a specially developed technique (see [3-5], with the further development of this approach, which allows one to directly evaluate the quantitative indicators of distributions energy release in the core with perturbed inter-cassette clearances for various fuel elements groups, for various fuel assemblies and burnup times. The technique provides an unambiguous detailed and formations, with an assessment of the degree of reliability, in conditions of incomplete certainty of knowledge of the magnitude and location of the maximum inter-cassette gap in the active zone.The technique allows with sufficient sensitivity to analyze the effectiveness of various radial and axial profiling profiles of fuel assemblies and generate initial data for a separate thermal-hydraulic safety analysis, in which quantitatively determined increase in heat reserves.

 In FIG. Figure 5 shows a typical example of extremely realizing axial energy distribution distributions for the traditionally used profiling scheme (curve 9) and the local combined profiling method proposed here (curve 10). Quantitatively, safety reserves are estimated in terms of creating and increasing the stock before the boiling crisis (DNBR) from 1.28 and less than 1.0 - the “crisis” (curve 9) to 1.38 and 1.05 (curve 10), respectively, for normal operating conditions and for modes with violations (complete blackout).

 The desired effect is obtained in conditions of real possibilities and without significant complications of the production process. So in FIG. 5 shows an example (curve 10) in which the enrichment of peripheral fuel elements 1 is reduced only in the upper part 4 by a length of 40%. The lower 60% of the length of these fuel rods 3 is composed of primary enrichment tablets. Thus, for curve 10, as well as for curve 9, in fuel assemblies there are two types of fuel pellets for enrichment with fissile isotopes.

The main embodiments of the invention:
- a decrease in enrichment in the upper part 4 (along the length H1) of the peripheral row of fuel elements 1, while in the lower part 3 of these fuel elements the enrichment is reduced less or remains equal to the main enrichment for fissile isotopes (see Figs. 1, 3);
- a decrease in enrichment in the upper part 4 (along the length H1) of the peripheral row of fuel rods and in the upper part 6 (along the length H2) of the pre-peripheral row of fuel rods 5, while in the lower part of these fuel rods the enrichment is reduced less or remains equal to the main enrichment for fissile isotopes (see Fig. 2, 4).

 In this case, the following should be ensured: the axial profile of the worst distribution of energy release is shifted to the lower part of the core and better corresponds to the maximum permissible power release line of energy 8.

 This ensures the creation or increase of the reserve to the maximum energy release in the upper part of the core, while in the lower part the large reserve to the maximum energy release can be reduced. This also shifts the total axial offset of energy release in the core zone averaged over the campaign of the reactor downward, which increases the safety of fuel operation.

 A reserve should be provided for peripheral 1 (see Fig. 1, 3) or two rows (see Fig. 2, 4) of fuel elements 1 and 5 in fuel assemblies to compensate for a possible increase in cassette clearances.

 Axial profiling of one or two peripheral rows of fuel rods in fuel assemblies, used as a control action, is carried out according to a specially developed technique. With the right choice of control action, its goals are achieved - compensation of perturbations of inter-cartridge gaps and increasing the safety of fuel operation.

Thus, a method is obtained for:
a) compensation for the effect of disturbances of inter-cartridge gaps;
b) reduce the uneven distribution of heat reserves;
c) reducing the neutron fluence to the lower sections of the absorbing rods of the control and protection system of a nuclear reactor;
in the active zone of a nuclear reactor, which consists in placing low-enrichment fuel in the fuel rods of one or two peripheral rows of fuel assemblies, characterized in that fuel of lower enrichment is placed in the upper part of these fuel rods than in their lower part.

Industrial applicability
It is most expedient to use the proposed solutions for operation in nuclear power thermal-neutron water-cooled water reactors.

Sources of information
[1] F.Ya. Ovchinnikov, V.A. Voznesensky, V.V. Semenov et al. "Operational modes of nuclear power plants with VVER-1000". Moscow., Energoatomizdat, 1992.

 [2] Nuclear technology abroad, 1987, 9, l. 17-19.

 [3] Article by G.L. Ponomarenko "Taking into account the influence of the warping of VVER-1000 fuel assemblies on the power of fuel elements." In the journal Atomic Energy, September 1999, vol. 87, no. 3, p. 210-213.

 [4] Report of G.L. Ponomarenko, "Profiling of cartridge enrichment to compensate for perturbations of inter-cassette clearances in the VVER-1000 core," MEPhI-2001 Scientific session. Collection of works. Volume 8. Nuclear power. S. 142-143.

 [5] Article G.L. Ponomarenko, A.V. Voronkova, A.K. Gorokhova: "A probabilistic method for assessing the influence of inter-cartridge gaps on energy release in the VVER-1000 core". Sent to the journal "Atomic Energy", ref. OKB Gidropress 12-10 / 20 from 13.02.01.

Claims (3)

1. A method of reducing the uneven distribution of heat reserves in fuel assemblies of the active zone of a nuclear reactor containing fuel rods (2) with primary enrichment fuel for fissile isotopes, which consists in placing fuel rods (1) with increased non-uniformity of heat reserves of lower enrichment fuel, characterized in that that in fuel rods (1) with increased unevenness of heat reserves, in the zone of higher heat reserves, fuel of higher enrichment is placed (3) than in the zone no Warm heat engineering stocks. 2. A way to reduce the uneven distribution of heat reserves in the fuel assemblies of the nuclear reactor core containing fuel rods (2) with primary enrichment fuel for fissile isotopes, which consists in placing lower enrichment fuel (4) in peripheral fuel rods (1), characterized in that fuel of higher enrichment (3) is placed in the lower part of the peripheral fuel elements (1) than in their upper part. 3. A way to reduce the uneven distribution of heat reserves in the fuel assemblies of the nuclear reactor core containing fuel rods (2) with primary enrichment fuel for fissile isotopes, which consists in placing lower enrichment fuel in peripheral and pre-peripheral fuel rods (1) and (5) (4) ) and (6), characterized in that in the lower parts of the peripheral and pre-peripheral fuel elements (1) and (5) fuel of higher enrichments (3) and (7) is placed than in their upper parts.

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