CONTROL ROD IN A WATER-COOLED NUCLEAR REACTOR

Abstract: The invention relates to the nuclear technology, especially, to facilities in the control and protective system of water-cooled vessel reactors, and can be used among the control mechanisms realized in the form of rods, either single or integrated to an assembly. The control rod includes an cladding having a neutron absorber stack disposed within the cladding and comprised of two parts, of which one is made of a material having (n,α) reaction with neutrons, for example, of boron carbide. The other part, inserted first into the core, is of a length determined by a mathematical expression including a length of the neutron absorber stack and efficiency values of the control rods with various parameters. A length of the part made of a dysprosium-based material, determined by the above-mentioned mathematical expression, permits to manufacture the rod with a maximum weight, taking into account the change in its absorbing properties resulting from an increase in the specific share of the dysprosium-based material, the density of which is clearly greater than that of boron carbide.

Description: Field of the Invention The present invention relates to the nuclear technology, especially to facilities in the control and protective system (CPS) of water-cooled nuclear reactors, and can be used in control mechanisms realized in the form of single rods having various cross-section or integrated to assemblies including a set of control rods or a set of fuel and control rods, designed to compensate for excess reactivity, to control reactivity in the process of power operation, especially as safety rods, as well as when used in control systems with combined functions. Background of the Invention To provide proper and secure operation of a nuclear reactor, a required reactivity level should be maintained during reactor start-up, arrest, transitional processes, with reactivity being sharply reduced during reactor shutdown. For this purpose, the reactor is equipped with control rods of various design, which are connected to a driving mechanism for displacing the rods along the height of the core to vary reactivity within a required range. A specific feature of water-cooled reactors is that, due to a sufficiently long operation of nuclear fuel without recharging and to high fuel rating, they have a comparatively high excess reactivity accounted for fuel burnup. Further, such reactors exhibit considerable temperature and power effects. Taken together, these factors ultimately result in that the core of a water-cooled vessel reactor, at the start of campaign, may contain up to several tens of critical masses, for compensation of which a considerable combined efficiency of all CPS mechanisms is required, thereby posing a problem of accommodation of the mechanical control system. In modern water-cooled reactors, in particular, VVER-1000, reactivity margin for fuel burnup is compensated for by boric acid dissolved in a primary system coolant, the boric acid being gradually withdrawn with burnup of fuel charges. As this takes place, the operating group of control mechanisms (approximately 10% of the total number) is in a semi-inserted state. The remaining control mechanisms (about 90% of their total number) are taken out from the core and stay in the emergency protection mode (EPM). In this mode, a lower part of absorbing elements is actually in the top reflector zone and vigorously burns up. This feature of employment of absorbing elements in water-cooled reactors causes a substantial irregularity of absorber burnup in control mechanisms along their length, depending on the position relative to the core. For this reason, in designing and improving the absorbing elements for this type of reactor, great emphasis is placed on selection of neutron-absorbing materials and compositions. Especially as in reactor operation, a part of control mechanisms is with time elevated from the core, the other part of the control mechanisms changes its function, and the control mechanisms suspended above the core may be introduced therein by various reasons. Therefore, in designing control rods, account must be taken of their characteristics under different conditions of reactor operation, specifically, to also provide a required absorption capacity profile when the rod operates in the emergency protection mode. A required absorption capacity profile along the rod length can be provided by various designs. For instance, the use may be made of pellets made of a material with a lower neutron-absorbing capacity, such as zirconium dioxide, placed in the rod lower part between boron carbide pellets. (See US Patent No.4624827, Int.Cl. G21 C 7/10, 1986). The required reduction in absorption capacity in the lower part of the rod can be provided by reduction in the pellet diameter from the top down with simultaneous decrease in concentration of absorbing material, boron carbide. (See FR Application No.2570214, Int.Cl. G21C 7/10, 1986). Described in US Patent No.4062725, Int.Cl. G21C 7/10, 1977) is a rod including three sections, wherein the upper section has the greatest absorption cross-section and is made of boron carbide, the lower section accommodates an absorber material with a lower absorption cross-section than that in the upper section, a middle section disposed between them being of a material which does not essentially absorb neutrons. This design also realizes a predetermined absorption capacity along the rod. All of the designs mentioned above assume the use, in the upper rod part, of boron carbide or a material including boron-10, which are excellent neutron absorbers, as this is important when the rod operates in the emergency protection mode. However, when absorbing neutrons in (n,α) reaction, boron carbide swells and intensively gasses, thereby reducing life of control mechanisms and requiring design and process improvements in the rods. Location of silver alloy (Ag-In-Cd) having (n,γ) reaction with neutrons in the rod lower part, inserted first into the core, ensures essential reduction in swelling and gassing of boron carbide owing to a shielding effect upon boron carbide by withdrawal of the same from areas with high neutron fluxes when the rod is disposed, in operation, in the core upper part or above the same (see US Patent No.4699756, G21C 7/10, 1985). However, in the radiation process during the reactor campaign, the efficiency of neutron absorption by the silver alloy changes, leading to variation in its shielding effect upon boron carbide and, as a consequence, to alteration in characteristics of the rod during its operation, in particular, to variation in the combined physical weight of the rod. Further, the efficiency of neutron absorption by Ag-In-Cd alloy varies by a nonlinear law as a function of flux, practically eliminating the possibility of exact calculation of efficiency of the rod as a whole by the time of residence in different areas of the core and above the same. This hampers forecasting the rod life and designing the rod displacing systems (monitoring devices, driving mechanisms, etc.) since two parts of the rod irregularly burn up in unpredictable manner. Most closely approaching the present invention is a control rod of a water-cooled vessel reactor, comprising a cladding, in which a stack of neutron absorber material is located, said stack being comprised of two parts along length L, one of the parts including a material having (n,α) reaction with neutrons, while the other part, inserted first into the core, includes a dysprosium-based material (see V.M.Chernyshov, V.I.Ryakhovskikh et al., Improved Absorption Rods. Reactor VVER-1000. Report at VVER Fuel Reliability and Flexibility, Czechia, Rzhezh, June 17-22, 1996). The selection of dysprosium compounds (Dy 2 O 3 ·TiO 2 , Dy 2 O 3 ·TiO 7 , Dy 2 O 3 ·HfO 2 ), as a material having (n,γ) reaction with neutrons, provides stability of rod parameters since the aforementioned dysprosium compounds, first, cause only a slight change in the neutron absorption efficiency in the radiation process, second, the law of changing neutron absorption efficiency is of a pronounced linear nature. A reasonably reliable shielding of boron carbide against swelling is provided when the part occupied by the dysprosium-based material has a length no less than 2% of the entire length of the neutron absorber stack. Further, availability of a dysprosium-based material in the part of the stack substantially increases the combined rod weight since a density of dysprosium is more than four times that of boron carbide. The increase in the rod weight raises the velocity of the rod insertion into the core by gravity drop in the emergency protection mode, thereby improving the reactor safety. However, the considerable length of the neutron absorber stack part occupied by the dysprosium-based material, which in the prior apparatus occupies more than 20% of the total stack length, impairs the efficiency of the rod as a whole, since this essentially decreases the quantity of boron carbide possessing a greater, as compared to dysprosium, absorption capacity, resulting in detrimental effect on the control mechanism characteristics. SUMMARY OF THE INVENTION It is an object of the present invention to provide a control mechanism having an increased physical weight and, at the same time, a reasonable efficiency (absorption capacity). The technical result of the present invention is the increase in the velocity of the control mechanism drop in the emergency protection mode at required neutron absorption characteristics. The object of the invention is accomplished in a control rod of a water-cooled vessel reactor, comprising a cladding having a neutron absorber stack disposed within the cladding, said stack being comprised of two parts along length L, one of the parts including a material having (n,α) reaction with neutrons, while the other part, inserted first into a core, includes a dysprosium-based material, wherein length 1 of the part of the neutron absorber stack including a dysprosium-based material is selected by the expression: where is a maximum possible value of l/L ratio, providing a required efficiency, is a calculated value at which efficiency of the control rod is equal to a minimum required efficiency (E 0 ) of a rod of length L, which is homogeneous along the length and completely filled with an absorber having (n,α) reaction with neutrons, E ad is an additional efficiency to provide a minimum required combined efficiency along a control mechanism length at increased length of an absorber having (n,γ) reaction with neutrons. Further, the material having (n,α) reaction with neutrons is boron carbide, while the material having (n,γ) reaction with neutrons is dysprosium monotitanate (Dy 2 O 3 ·TiO 2 ), and/or dysprosium dititanate (Dy 2 O 3 ·TiO 7 ), and/or dysprosium hafnate (Dy 2 O 3 ·HfO 2 ). Boron carbide and the dysprosium-based material may be utilized in the form of powder and/or pellets. When in the form of powder, boron carbide preferably has grains from 5 to 160µm in size, the powder being consolidated by vibration to a density no less than 1.7g/cm 3 . When in the form of powder, the dysprosium-based material has grains from 5 to 315µm in size, a density of dysprosium monotitanate or dititanate, upon consolidation by vibration, being from 4.9 to 7.0g/cm 3 . Powdered dysprosium hafnate is consolidated by vibration to a density of 7 or 9.0g/ cm 3 . A distinctive feature of the present invention is that length I of the part of the neutron absorber stack including a dysprosium-based material is selected by a particular relationship which is restricted by a maximum possible value of the combined weight of the rod and takes account of a required efficiency (absorption capacity) of the rod. Increase in length I of the part of the neutron absorption stack made of a dysprosium-based material, resulting in increased combined weight of the rod, and, hence, in increased velocity of it insertion (drop) into the core, is possible only to such a value at which l/R ratio does not exceed a maximum (threshold) value. A maximum l/L ratio value, on one hand, is restricted by a calculated value of l/R ratio at which the efficiency of the control mechanism is equal to a minimum required efficiency (E 0 ) of a rod of length L, which is homogeneous along the length and completely filled with an absorber having (n,α) reaction with neutrons, and, on the other hand, can be increased by the provision of an additional efficiency (E ad ). to provide a minimum required total efficiency along a control mechanism length at increased length of an absorber having (n,γ) reaction with neutrons. The additional efficiency can be provided, for example, by increasing the total number of control mechanisms, by enhancing absorption capacity of the rod parts, as well as by any conventional means. The aforementioned advantages and features of the present invention will become apparent upon a reading of the following detailed description of the preferable embodiment of the invention. Brief Description of the Drawings Fig. 1 shows a general view of a control rod of a water-cooled vessel reactor; Fig.2 shows an embodiment of a control rod, in which a material having (n,α) reaction with neutrons (boron carbide) is in the form of pellets, while a dysprosium-based material is in the form of a powder consolidated by vibration; Fig.3 shows an embodiment of the design in which a material having (n,α) reaction with neutrons (boron carbide) is in the form of a powder consolidated by vibration, while a dysprosium-based material is in the form of pellets; and Fig.4 shows an embodiment in which the entire neutron absorber stack is formed from different materials in the form of pellets. Detailed Description of Preferred Embodiment Referring now to the drawings, there is shown a control rod 1 of a nuclear reactor, including a cladding 2, in which a neutron absorber stack 3 of length L is located. One part 4 of the stack 3 includes a material having (n,α) reaction with neutrons, such as boron carbide. The other part 5 of length 1, which is inserted first into a core (not shown), comprises a material, such as a dysprosium-based compound, having (n,γ) reaction with neutrons. Height 1 of the part 5 is no less than 2% of height L of the stack 3. The cladding is sealed, e.g. by welding, using a lower end 6 and an upper end 7. A cavity 8 can be provided between the upper end 7 and the stack 3 to collect gases and accommodate retainers 9 of the stack 3. Since at radiation of the part 4 of the stack 3 by neutrons, gas emission from boron carbide is neglectful due to the presence of the part 5 occupied by dysprosium, a rod weighting member 10 may be placed into the cavity 8. The parts 4 and 5 of the stack 3 may be filled with powder 11 consolidated by vibration or assembled from pellets 12. The control mechanism in accordance with the invention operates in the following manner. Depending on operation conditions and a required power level to be maintained, the rod 1 may be located in different positions relative to the core. When the rod is positioned above the core or partially inserted into the core, the part 4 of the neutron absorber stack does not have a considerable irregularity of burnup and is little subjected to detrimental impact of neutrons, causing its swelling and gas emission therefrom, which is provided by provision of the part 5 comprising dysprosium. On arrival of an emergency protection signal, the rod 1 is fully inserted into the core by gravity drop which is promoted by the weighting member 10. However, the increase in the rod weight due to the weighting member 10 is restricted by dimensions of the space available for its accommodation. Further, the increase in the rod weight is feasible by increasing length l of the rod part 5 filled with the dysprosium-based material having a greater density than that of boron carbide. Increase in length l and, respectively, in l/L ratio, results in an increased total weight of the rod, and hence, in increased velocity of the rod drop into the core. The maximum value of length I is, however, restricted by a combined rod efficiency value. Therefore, a further increase in l value, reducing the total efficiency of the rod, must be compensated for by methods and means which improve the composite rod efficiency in terms of its use in control mechanisms of a nuclear reactor. Therefore, in designing and constructing a new rod, at a given velocity of the rod drop provided by a given time of the rod insertion into the core, account must be taken of the change in its efficiency (absorption capacity) which may be compensated for by a number of rods, their diameter and by other conventional means and methods. In any case, a control rod of a vessel reactor, comprising two parts, of which one part, inserted first into the core, is made a dysprosium-based material, and the second part is made of boron carbide, should not have length 1 of the part 5 at which l/L ratio exceeds a maximum possible value of this parameter determined by the expression (1). The rod elements can be fabricated by any conventional method. The rod can be used independently and have an individual displacement drive. A set of rods can be assembled to a cluster with a common driving mechanism. The rods can be mounted in the fuel assembly instead of fuel elements. Different use of the rods is also accomplished in a conventional manner. Industrial Applicability A control rod of a water-cooled nuclear reactor in accordance with the invention exhibits improved stability of parameters owing to combination of absorbing elements at a maximum possible weight which has a beneficial affect on reactor reactivity and may be employed in acting nuclear reactors and in newly constructed ones. The invention takes account of the change in absorption capacity of the rod as a whole, resulting from the increase in the specific share of a dysprosium-based material in the total length of the neutron absorber stack.

Claims: A control rod of a water-cooled nuclear reactor, comprising a cladding having a neutron absorber stack disposed within the cladding, said neutron absorber being comprised of two parts along length L, one of the parts including a material having (n,α) reaction with neutrons, while the other part, inserted first into the core, includes a dysprosium-based material, wherein length l of the part of the neutron absorber stack made of a dysprosium-based material is selected by the expression: where is a maximum possible value of 1/L ratio, providing a required efficiency, is a calculated value at which efficiency of a control mechanism is equal to a minimum required efficiency (E 0 ) of a rod of length L which is homogeneous along the length and completely filled with an absorber having (n,α) reaction with neutrons, E ad is an additional efficiency to provide a minimum required combined efficiency along a control mechanism length at increased length of an absorber having (n,γ) reaction with neutrons. A control rod for a water-cooled vessel reactor as set forth in claim 1, wherein the material having (n,α) reaction with neutrons is boron carbide. A control rod for a water-cooled vessel reactor as set forth in claim 2, wherein boron carbide is in the form of a powder with grains from 5 to 160µm in size, the powder being consolidated by vibration to a density no less that 1.7g/cm 3 . A control rod for a water-cooled vessel reactor as set forth in claim 1, or 2, or 3, wherein the material having (n,γ) reaction with neutrons is dysprosium monotitanate (Dy 2 O 3 ·TiO 2 ), and/or dysprosium dititanate (Dy 2 O 3 ·TiO 7 ), and/or dysprosium hafnate (Dy 2 O 3 ·HfO 2 ). A control rod for a water-cooled vessel reactor as set forth in claim 4, wherein dysprosium monotitanate or dysprosium dititanate is in the form of a powder with grains from 5 to 315µm in size, said powder being consolidated by vibration to a density from 4.9 to 7.0 g/cm 3 . A control rod for a water-cooled vessel reactor as set forth in claim 4 or 5, wherein dysprosium hafnate is in the form of a powder with grains from 5 to 315µm in size. A control rod for a water-cooled vessel reactor as set forth in claim 6, wherein dysprosium hafnate is consolidated by vibration to a density of 7.0 g/cm 3 . A control rod for a water-cooled vessel reactor as set forth in claim 6, wherein dysprosium hafnate is consolidated by vibration to a density of 9.0 g/cm 3 . A control rod for a water-cooled vessel reactor as set forth in claim 1, or 2, or 4, wherein the material having (n,α) reaction with neutrons and/or the dysprosium-based material is in the form of pellets.

Inventors: CHERNYSHOV VLADIMIR MIKHAILOVI, LUNIN GLEB LEONIDOVICH, MAKOVSKY VLADIMIR DANILOVICH, OSADCHY ALEXANDR IVANOVICH, PONOMARENKO VIKTOR BORISOVICH, POSLAVSKY ALEXANDR OLEGOVICH, RYAKHOVSKIKH VIKTOR IVANOVICH, VASILCHENKO IVAN NIKITOVICH, CHERNYSHOV, VLADIMIR MIKHAILOVICH, LUNIN, GLEB LEONIDOVICH, MAKOVSKY, VLADIMIR DANILOVICH, OSADCHY, ALEXANDR IVANOVICH, PONOMARENKO, VIKTOR BORISOVICH, POSLAVSKY, ALEXANDR OLEGOVICH, RYAKHOVSKIKH, VIKTOR IVANOVICH, VASILCHENKO, IVAN NIKITOVICH
Applicants: G PREDPR MO ZD POLIMETALLOV, GOSUDARSTVENNOE PREDPRIYATIE MOSKOVSKY ZAVOD POLIMETALLOV
Application year: 1997
Main classification: G21C7
IPC codes: G21C7