DE3308619A1 - Nuclear fuel cluster having enrichment in axial zones - Google Patents

Abstract

A configuration of a nuclear fuel cluster (15) which permits the clearance for the cold, shutdown state to be preserved with minimum impairment of the operational efficiency. The cluster has a component which is made from fissile material and which is distributed over a substantial axial extent of the nuclear fuel cluster, the fuel having a relatively reduced enrichment in an axial zone (62) (the "zone of reduced enrichment") in an upper region, this zone corresponding at least partially to a section of the axial region in which the reactivity has a maximum in the cold shutdown state. The nuclear fuel cluster normally comprises a first subgroup of nuclear fuel rods (1) having a reduced enrichment in the zone of reduced enrichment as well as a second subgroup of nuclear fuel rods (2 to 8) having a uniform enrichment along its axial dimension. In order for the number of nuclear fuel rods which have to be characterised by an enrichment that is not axially uniform to be kept as low as possible, it is preferred for the first subgroup (1) to contain one of the rods having the highest enrichment of the nuclear fuel cluster. The resulting somewhat higher axial power peak towards the lower part of the nuclear fuel cluster can be compensated by arranging more burnable absorbers in a corresponding lower zone (67). A middle zone (70) is characterised by a higher enrichment and a lower gadolinium content, which leads to the section of the core having the highest reactivity. <IMAGE>

Description

description

Nuclear fuel bundle with axially zoned enrichment The present one Invention relates to nuclear reactors in general, and more particularly to one Fuel bundle arrangement for a boiling water reactor.

In a nuclear fuel reactor, one atom of a fissile absorbed Nuclear fuel, such as U235, a neutron in its core and suffers a nuclear fission, the on average two fission fragments of lower atomic weight with higher kinetic ones Energy and several neutrons also generated high energy.

In a typical boiling water reactor (English abbreviated BWR ") lies the nuclear fuel in the form of nuclear fuel rods, each of which has a multitude Contains sintered pellets enclosed in an elongated cladding tube are. Groups of such fuel rods are held by upper and lower retaining plates supported and form separately replaceable nuclear fuel bundles.

A sufficient number of such nuclear fuel bundles are in a matrix arranged, which is approximated to a right-angled circular cylinder to the Forming the core of the nuclear reactor, leading to a self-sustaining fission reaction be able to.

The kinetic energy of the fission products is used as heat in the nuclear fuel rods distributed. Further energy is generated by the neutrons, gamma rays and other radiation, which results from the fission process, in the fuel structure and deposited in the moderator. The entire core is immersed in a coolant, which can be e.g. water, and which removes the heat, which is then more useful for performance Work can be extracted. If the coolant is water then it also acts as a Neutron moderator who slows down the neutrons so that they start another fission reaction can set in motion.

The most commonly used nuclear fuel for water cooled and moderated nuclear reactors comprises uranium dioxide, of which about 0.7 to about 5% by weight are fissile U235 mixed with breeding U238. During the reactor operation part of the breeding U238 converted into fissile Pu239 and Pu241. The U238 is also fissile, but only for high-energy neutrons. The relationship with fissile Material that is produced, such as the said Pu239 and Pu241, to the fissile material, that is destroyed by the fission, such as U235, Pu239 and Pu241 is called the "conversion ratio" Are defined.

If the reactor is to be operated at a constant level of performance, then the number of neutrons formed by the fission must remain constant. This means that every fission reaction must produce a net neutron, the next one Initiates cleavage reaction so that the operation is self-sustaining. Of the Operation is characterized by an effective multiplication factor keff, the must be one for continuous operation. It should be noted that the effective Multiplication factor keff the neutron reproduction factor of the nuclear reactor as Whole is, and that this is to be distinguished from the local or infinite Multiplication factor cinema, which is a neutron reproduction of an infinitely large Defined system that has the same composition and the same properties like the local area of the reactor core in question.

During operation, the fissile fuel and some are depleted the fission products are even neutron absorbers or poisons. To compensate for this, the reactor is normally with an initial excess of nuclear fuel provided, which leads to an initial excess reactivity. This initial Excess reactivity requires a control system to determine the effective multiplication factor to keep it at one during reactor operation and to reduce it to below one, if necessary, shut down the reactor. The tax system usually uses neutron absorbing material that enables neutron production by absorption of Controls neutrons that do not result in fission.

At least part of the neutron absorbing material is put into a Multiple selectively actuatable control rods inserted from the bottom of the core from being introduced axially as needed to the performance level and distribution or shut down the core. Burnable absorbers can be used in some The nuclear fuel rods are introduced to the amount of mechanical required To keep control as low as possible. A burnable absorber is a neutron absorber, which is converted by neutron absorption into a material that is less capable is to absorb neutrons. A well-known burnable absorber is gadolinium, usually in the form of gadolinium oxide. The odd-numbered isotopes of gadolinium (Gd155 and Gd157) have very large capture cross-sections for thermal neutrons.

The available burnable absorbers exhibit undesirable neutron absorption towards the end of the fuel cycle, as they turn into neutron-absorbing isotopes convert that have small neutron absorption cross-sections.

Gadolinium is depleted when it is used as a burnable absorber, quickly to the isotopes Gd155 and Gd157, but there remains a residual absorption by the even-numbered gadolinium isotopes Gd154, Gd156 and Gd158 As is known, burnable absorbers work like gadolinium, in a self-shielded manner when in sufficient concentration available. This means that when you expose it to the neutron flux, the neutron absorption takes place essentially on the outer surface of the rod containing the absorber, so that the absorber volume shrinks radially at a rate that of the Concentration of the absorber depends. It is thus possible1 through a suitable selection the number of rods containing absorber and the absorber concentrations therein, a desired variation in the absorption value over one or more operating cycles of the reactor.

During the operation of the reactor, the percentage of vapor bubbles increases towards the upper part of the core. This leads to a reduced moderation in these upper areas of the core and thus to a power distribution that increases towards the lower areas of the core.

It is known to compensate for this by using the burnable Axially inhomogeneous distribution of absorber. It becomes a number of nuclear fuel rods provided with a burnable absorber, the distribution of which is such that it in the direction of the axial area with the reactivity maximum in the hot operating state increases towards. A typical configuration for this is described in US Pat. No. 3,799,839.

Another approach is shown in U.S. Patent 4,244,784, US Pat an axial power peak to some extent due to an increased U235 enrichment or compensates for a concentration of plutonium in the upper regions of the core is.

For the cold shutdown state, however, the situation is complete different. In the cold state is the upper part of an irradiated core of a boiling water reactor more reactive than the bottom part because of the greater plutonium production and the less destruction of the U235 during operation in the upper part (higher conversion ratio and lower burn-up in the upper part of the core). When switched off when cold the vapor bubbles in the upper part of the core are eliminated and thus make the upper part Part of the core more reactive than the bottom part. Typical standards require one Reactivity shutdown margin of 0.38% (keff less than 0.9962), where any Control rod is outside the core. To allow scope for forecast uncertainties usually a predicted switch-off margin of 1% (keff less than 0.99) delivered by the control rods and the burnable absorber is used as the basis for the design of the reactor core.

While the axial performance profile by providing larger quantities of a burnable absorber or a reduced accumulation in the lower sections of the reactor core can be set in the desired manner, results in the optimal Absorber and nuclear fuel distribution for an optimized axial performance profile but not to a reasonable margin for the cold shutdown state. To meet the requirements for the cold shutdown state, it is common necessary to take into account an excess of burnable absorber residues, which complicates the requirements for initial enrichment and uranium ore and increases the cost of the fuel cycle of the reactor.

Another problem is that gadolinium oxide has the thermal The conductivity of the fuel rods is reduced and the release of fission gas is increased. The rods containing gadolinia are therefore often the most restrictive Rods in the nuclear fuel bundle and therefore need to be in one area of the reactor be arranged, which has a lower level of performance, which is detrimental to affects the local distribution of power. The proportion of the reduction in performance, the amount required depends on the gadolinium oxide concentration away, however, becomes a serious problem in fuel bundles for extended burnout and / or for high energy cycles that require elevated levels of gadolinium oxide to allow adequate headroom for the cold shutdown state.

The ones for the hot operating state and those that are switched off when the cold State required leeway therefore require competitive restrictions in the design of the reactor core and therefore have to achieve an optimal Core configuration prevented.

The present invention relates to a configuration of a Nuclear fuel bundle, which allowed the leeway for the cold shutdown Condition, with minimal detriment to operational effectiveness in Purchase are to be taken. The invention can be used with little or no need to to use special fuel pellets or the like.

Generally, a nuclear fuel bundle in accordance with the present invention a component of fissile material that has a substantial axial extent of the fuel bundle is distributed, the fuel being a relatively diminished Enrichment in a crxial zone (the "zone of reduced enrichment") in one having upper region, this zone at least a portion of the axial Corresponds to the range in which the reactivity in the cold switched-off state a Has maximum value.

The nuclear fuel bundle typically comprises a first subgroup of nuclear fuel rods with decreased enrichment in the zone of decreased Enrichment as well as a second subgroup of nuclear fuel rods with uniform Enrichment along its axial extent. To get the number of nuclear fuel rods, by a non-uniform axial enrichment characterized must be to keep as low as possible, it is preferred that the first subgroup one of the most highly enriched nuclear fuel rods in the nuclear fuel bundle contains0 The enrichment in the zone of reduced enrichment preferably corresponds to the enrichment of some of the evenly enriched nuclear fuel rods. on In this way, the nuclear fuel rods of the first subgroup can be made from fuel pellets which have the enrichment in stock.

The zone of reduced enrichment leads to a slightly higher axial one Power peak towards the lower part of the fuel bundle, however this can be achieved through a greater concentration of burnable absorber in a corresponding one lower zone to be balanced. A middle zone is characterized by the higher enrichment and a lower gadolinium content and so gives the section the core with the highest reactivity.

The zone of reduced enrichment has the effect of reducing the reactivity in the cold switched-off state is reduced, while the excess reactivity is influenced very little in the hot operating state. In this way, therefore a more efficient use of the fuel is made, so that each cycle can provide more energy from the same amount of U235 that is in the nuclear fuel bundle is introduced. The greater control deviation created by the present invention allows a significant unplanned variation in Energy of each cycle allowing flexibility in operating the reactor independently of the originally planned energy of the cycle.

In the following the invention with reference to the drawing explained. Show in detail: Fig. 1 is a schematic representation a water-cooled and moderated nuclear reactor, FIG. 2 a schematic View of the general layout for the core of the nuclear reactor, Fig. 3 a simplified, partially cut away isometric view of one of the fuel cells in the core, Figures 4A and 4B are schematic views of the transverse distribution of the nuclear fuel rods in nuclear fuel bundles, Figure 4C is a schematic view of the composition different types of fuel rods in the bundles of FIGS. 4A and 4B in longitudinal and in the axial direction, respectively, Figs. 5A and 5B show the cross-average composition the nuclear fuel bundle of Figures 4A and 4B in the longitudinal direction, Figures 6A, 6B, 6C show Curves of the relative power in the hot operating state at the beginning, in the middle and at the end of the cycle, Figures 7A, 7B, 7C show curves of relative performance im cold switched-off state at the beginning, in the middle and at the end of the cycle, Fig. 8 shows curves of the effective multiplication factor keff over an operating period for both the hot operating and the cold switched-off states and Fig. 9A to 9L show other compositions for a nuclear fuel bundle.

In Fig. G is a partially cut-away side view schematically a water cooled and moderated boiling water type nuclear reactor system 10 shown. The system includes a pressure vessel 11 in which a reactor core is located 12 is located in a coolant moderator, such as light water , is immersed0 The core 12 comprises a plurality of fuel cells 13, the are surrounded by an annular envelope 14 each fuel row closes four nuclear fuel bundles 15 and a tax ## from 16 a The fuel cells are by an upper Kergitter 18 and a lower core plate 19 at a distance from each other held and supported on their respective base parts by suitable brackets 20. The control rods can be selectively inserted between the nuclear fuel bundles, to control the feactivity of the core. Associated with every Swenerstab is a Guide tube 21 which guides the control rod when it is pulled out from below the core wird0, Fig0 2 gives a schematic representation of the manner in which the fuel cells 13 are arranged within the core 12. A typical core would be of the order of magnitude from 300 to 900 nuclear fuel bundles contain0 The part of the pressure vessel 11 below of the core 112 delimits a coolant supply chamber 22, while the part of the Pressure vessel 11 above the core, an arrangement 25 for separating and drying of the steam it contains. During operation, a coolant circulation pump 27 sets this coolant located in the supply chamber 22 under pressure and pushes it through the core 12 upwards. The coolant absorbs the heat generated by the inside of the core occurring cleavage reaction is generated, and thereby becomes part of the cooling medium converted into steam, which is passed through the arrangement 25 to a utilization device 2 such as A turbine 30 will pass a condenser 322 which is in line with the turbine 30 is 2 condenses the steam emitted by the turbine, and this is carried out Condensate as feed water by means of a condensate return pump 35 to the inlet side of the coolant circulation pump 27 back In Fig0 3, the structure is one of the Fuel cells 13 can be seen The control rod 16 has a cross-shaped cross section Profile and forms control blades 40, each of which is between two adjacent nuclear fuel bundles lies Each nuclear fuel bundle 15 comprises a plurality of long ones Nuclear fuel rods 42 sandwiched between upper and lower support plates 45 and 46 are supported and the total number of nuclear fuel rods is held by one surrounding tubular flow channel 48 rectangular cross-section. The lower section of the nuclear fuel bundle is provided with a nose 50 with openings 52 through which enters the coolant water and within the flow channel 48 along the Nuclear fuel rods 42 flowing upward. The nose 50 is designed so that they into a correspondingly designed, not-shown recess in the carriers 20 fits the nuclear fuel bundle.

Each nuclear fuel rod may consist of a cylindrical cladding tube consist of a variety of sintered pellets made from enriched uranium and / or Contains plutonia fuel. The enrichment varies within a nuclear fuel bundle from rod to rod generally within a range of about 0.7 to 5 weight percent fissile material to provide an average of about 1.5 to 3.5 wt%, wherein natural uranium has an enrichment of 0.7% by weight.

The nuclear fuel rods can have a diameter of about 1.25 cm and are about 3.60 to about 4.50 meters in length.

4A and 4B show schematic views of the horizontal Distribution of fuel; ofçståbe into two types of nuclear fuel bundles according to the present invention, which are intended for reloading. These two types differ mainly by the number of nuclear fuel rods containing gadolinium oxide and a typical core will contain both types of nuclear fuel bundles. the Fuel rods 42 are arranged in an 8 x 8 matrix, with two of the central ones Places for fuel rods of water channels 55 are occupied. Any nuclear fuel rod is provided as a circle with a code number from 1 to 8 inside.

Figure 4C illustrates the longitudinal composition of the eight Types of fuel rods. Each rod has a cover made of natural uranium inside an uppermost, approximately 15 cm thick zone 60, but this cover is used in the following Explanation not mentioned further. The rod types 2 to 8 are axially uniform Enrichment (with the exception of the aforementioned cover) over a length of approximately 3.60 m provided, while the rod type 1 has an axially zone-shaped enrichment. In particular, the enrichment of the rod number 1 is in an upper, approximately 0.90 m thick zone 62 (the "zone of reduced enrichment") diminished with reference on the enrichment in the lower approximately 2.70 m thick zone 65. The rod types 7 and 8 contain burnable absorbers in the form of gadolinium oxide. The concentration is axially zoned and it is larger in a lower one, about 1.5 m thick zone 67 (the "zone with increased gadolinium oxide content") with respect to the in an upper, approximately 2.10 m thick zone 68. One can therefore consider the core as consisting of three consider existing axial zones, namely zone 62 with reduced enrichment, a central zone 70 in which zones 65 and 68 overlap and the zone 67 with he increased gadolinium oxide content.

Fig. 5A and SB show the cross section of all fuel rods for the axial distribution of the enrichment and the gadolinium oxide content for the Nuclear furnace bundles of Figures 4A and 4B. It can be seen that these embodiments are characterized by an enrichment reduction in zone 62 by about 0.2 wt%, with a preferred range for such reduced build-up which is between about 0.15 and 0.30 weight percent. This is an area that is suitable in the context of a nuclear fuel bundle with an average enrichment of about 3 wt .-%, the rods enrichments generally in the range of about 1.2 to 4% by weight. For the embodiments shown in Figures 4A and 4D lies the Enrichment in the bars in the range from 1.70 to 3.95 % By weight (with the exception of the natural uranium cover).

The enrichment reduction therefore represents a proportional one Reduction of about 5 to 10% and nuclear fuel bundles with a higher enrichment would be characterized by a comparatively proportional reduction and thus in absolute terms, through a correspondingly greater reduction in enrichment in zone 62.

To obtain the desired degree of axial zoning while at the same time only a minimum number of axial zones having nuclear fuel rods is required, it is preferred that the nuclear fuel rods with the highest used Enrichment are axially zoned. To avoid pellets with a special Needing enrichment also corresponds to the decreased enrichment in the Zone 62 preferably of the uniform enrichment of the other fuel rods in the nuclear fuel bundle q For the enrichments shown, the 3.3 corresponds percent enrichment in zone 62 of bars of type number 1 of the uniform Enrichment of rods of type number 2.

Figures 6A through 6C show relative performance curves 80a through 80c as a function of the axial position at the beginning of the cycle (BOC), in the middle of the Cycle (MOC) and end of cycle (EOC) for a reactor with a core that Includes nuclear fuel bundles of the type shown in Figures 4A and 4B, and that with operated with all control rods pulled out. The curves are on an average Power unit normalized. For comparison purposes, Figures 6A through 6C also show corresponding dashed curves 82a to 82c of the axial power distribution for a core, which is characterized by a uniform enrichment. At the beginning of the cycle it can be seen that the concentration of the absorber (gadolinium oxide) in zone 67 cancels the influence of the reduced enrichment in zone 62, so the present invention is generally the same Operating characteristics creates how the corresponding core with axially uniform enrichment. In the In the middle of the cycle there is a noticeable improvement because of the axial power distribution is considerably flattened compared to the core with uniform enrichment.

At the end of the cycle, the present invention again yields about that same axial power distribution as the core with uniform enrichment.

7A to 7C show corresponding curves 85a to 85c of the relative Power as a function of the axial position at the beginning of the cycle (BOC), in the middle of the cycle (MOC) and at the end of the cycle (EOC) for a reactor core in its cold switched off state with all control rods pushed in (CAR). Also here are the curves normalized to an average unit power. Also Fig.

7A to 7C show corresponding curves 87a to 87c with dashed lines for the power distributions in a core that is axially evenly enriched. It should be noted that the absolute neutron fluxes for FIGS. 7A to 7C are generally about 7 orders of magnitude less than for the hot operating conditions 6A to 6C (106 neutrons / cm2 / sec. in the cold, switched-off state 1013 neutrons / cm2 / sec. in the hot operating state. The effect of the axially zonal Enrichment manifests itself as a dramatic decrease in performance in zone 62 for the cold shutdown state during the entire cycle.

Since the curves 85a - c and 87a - c refer to the average power unit are normalized, it is impossible to get information regarding the absolute reduction derive the neutron flux through the axially zonal enrichments from it. However, this effect can be determined by looking at FIG. 8, in which the Curves 90a-90c show the effective multiplication factor keff over an operating cycle demonstrate (with corresponding curves 92a-c dashed for a core with axially more uniform Enrichment).

The curves 90a and 92a apply to the hot operating state, at all control rods are pulled out of the core (HARO). Curves 90c and 92c apply to the cold switched-off state in which all control rods are in the Core are inserted (CARI). Curves 90b and 92b apply to the cold shutdown Condition in which a control rod is pulled out of the core. The curves can it can easily be seen that the core provided with axial zones of enrichment According to the present invention a further margin for the cold switched off State results (a smaller keff), and this both with all retracted into the core Control rods, as well as with a control rod pulled out of the core.

In summary, it can be stated that the present invention creates a configuration for a nuclear fuel bundle that has performance in the cold switched-off state reduced with minimal impairment of excess reactivity in hot operating condition, so that more efficient use of the nuclear fuel can be made.

Deviations may be made from the embodiments shown without departing from the scope of the present invention. So can instead of the approximately 15 cm thick cover made of natural uranium, other thicknesses were used will. Also, this cover could be in the lower part of the core instead of the upper part or at both ends (see Figures 9A to 9L).

In particular, the values for the enrichment and the absorber concentration are only exemplary. Other possible core configurations are shown in Figures 9A-9 9L, showing possible other arrangements in addition to the enlarged Tax area according to the present invention.

These other embodiments are characterized by an upper one Limit of enrichment of 3.95% by weight, but the present invention can also can be used in cores with even higher enrichment.

Claims (9)

Patent claims C nuclear fuel bundle for the core of a boiling water reactor, its operation is characterized by a considerable proportion of vapor bubbles with correspondingly reduced moderation towards the upper part of the core, this reduced moderation translating into slower burn-up in the upper core area leads, so that the cold switched-off state with the relatively increased moderation in this upper core area is accompanied by a reactivity profile that in this upper core area has a point, d u r c h e k e n fl 2 e i c h n e t that the core fuel bundle is a component of fissile material includes, which is distributed over a considerable axial part of the nuclear fuel bundle is, this fissile material has an axial distribution that characterizes is due to a reduced enrichment in an axial zone, which is at least partially corresponds to a section of the axial region in which the reactivity in the cold a switched state has a peak value. 2. Nuclear fuel bundle according to claim 1, d a d u r c h g e k e n n notes that there is further a component of a neutron absorbing Contains material, the distribution of which is characterized by a concentration in an axial zone which is at least partially a portion of the axial area corresponds, in which the reactivity in the hot operating state has a maximum. 3. Xern fuel bundle according to claim 1, d a d u r c h g e k e n n indicate that there are a plurality generally parallel and spaced from one another arranged nuclear fuel rods, of which a first sub-group is axial Has distributions that are characterized by a reduced enrichment of fissile nuclear fuel in said axial zone, while a second Subgroup of nuclear fuel rods has axial distributions that characterize are through a uniform enrichment of fissile nuclear fuel. 4. Nuclear fuel bundle according to claim 3, d a d u r c h g e k e n n I would like to point out that characterizes the nuclear fuel rods of the first subgroup are due to an enrichment outside the named zone, which is higher than that Enrichment of the second subgroup nuclear fuel rods. 5. Nuclear fuel bundle according to claim 4, d a d u r c h g e k e n n notes that the nuclear fuel rods of the first subgroup are an enrichment tion in the zone mentioned, the enrichment of at least one nuclear fuel rod corresponds to the second subgroup. 6. Nuclear fuel bundle according to claim 1, d a d u notices that the component is made of fissile material axial extension of about 3.00 to about 4.50 m and the zone of the reduced Enrichment has an axial extension of about 0.60 to about 1.20 m. 7. Nuclear fuel bundle according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the enrichment in said zone is proportional to 5 to 10% less than the enrichment outside this zone. 8. Nuclear fuel bundle according to claim 1, d a d u r c h g e k e n n it is noted that the enrichment in said zone is about 0.15 to 0.30 % By weight is less than the enrichment outside this zone. 9. Nuclear fuel bundle according to claim 1, d a d u r c h g e k e n n shows that the enrichment in said zone is axially uniform.