Patent Number: 060350113
Section: summary

TECHNICAL FIELD The present invention relates to a reactor core for a boiling water nuclear reactor. Such a reactor core comprises a plurality of fuel assemblies with a plurality of vertical fuel rods and possibly occasional vertical water-filled rods which are surrounded by a fuel channel of substantially square cross section. Each rod is included in two rows perpendicular to each other and each rod includes a stack of circular-cylindrical pellets of a nuclear fuel, which stack is arranged in a cladding tube. The fuel assemblies are arranged in groups of four, forming a so-called supercell. A control rod is arranged centrally in the supercell. BACKGROUND ART, PROBLEMS When an atom of a fissionable substance, especially uranium-235, absorbs a neutron in its core and undergoes disintegration, there are formed, on average, two fission fragments with lower atomic weight and greater kinetic energy as well as a plurality of high-energetic neutrons. In a reactor core a sufficient number of assemblies with nuclear fuel are arranged to make possible a self-supporting fission reaction. The kinetic energy of the fission products escape as heat from the fuel rods. The reactor core is immersed in a coolant, for example water, which discharges the heat for utilization thereof. When the coolant is in the form of water, it also serves as a neutron moderator which retards the neutrons so as to increase the probability of fission reactions. If the reactor is to operate at a constant power level, the amount of fission-generating neutrons must be constant. This means that each fission reaction must generate one neutron net, which in turn gives rise to a subsequent fission reaction such that the process becomes self-supporting. This is usually expressed such that the effective multiplication factor k.sub.eff must be equal to 1. The multiplication factor describes the ratio of the number of produced neutrons to the number of absorbed neutrons (or neutrons leaking out of the system). During operation, the fissionable material is depleted while at the same time some of the fission products themselves are neutron-absorbing. Considering this fact, the reactor is normally provided from the start with an operating cycle with an excess of nuclear fuel, which initially entails too high a reactivity. For this reason, a control system is required which is capable both of maintaining the effective multiplication factor k.sub.eff exactly at 1.0 during operation and of reducing it to below 1 when the reactor is to be shut down. An important part of this reactivity control is taken care of by neutron-absorbing material, which absorbs or captures neutrons without any fission taking place. At least part of the neutron-absorbing material is included in a plurality of the selectively operable control rods, which are pushed up from the bottom of the core to the necessary extent for control of the power level thereof and of the power distribution as well as for shutdown of the reactor. When the control rods are inserted into the core, the neutrons are absorbed which are a condition for the nuclear fission, the reactivity thus dropping. The higher neutron-absorbing effect the control rod has, the better is the so-called control rod effect thereof. Some of the fuel rods may contain burnable absorption material for reducing the need of mechanical control. Such burnable absorber is transformed by absorption of neutrons into a material with a lower neutron-absorbing capacity. A well-known such material is gadolinium, usually in the form of gadolinium oxide. The burnable absorbers which are available as construction material, however, have a non-negligible residual absorption capacity. When using, for example, gadolinium as burnable absorber, the isotopes which have a high neutron capture cross section will be consumed relatively fast, whereas a residual absorption capacity remains as a result of continued neutron capture of the other isotopes. When the need arises, the power production in the core must be capable of being rapidly interrupted, that is, the neutron supply and hence the power generation from nuclear fissions in the fuel be interrupted. There must always be sufficient shutdown margins such that the neutron supply does not unexpectedly start, resulting in powerful power generation, for example when the reactor vessel is opened and service work or refuelling is in progress. A typical requirement by the authorities for operational approval is that if any one of the control rods has stuck in its outer position, then the shutdown margin shall correspond to a reactivity reduction of at least 0.38% (k.sub.eff is to be less than 0.9962). To obtain additional safety, these values are in practice often changed to 1% and 0.99, respectively. It is known to improve the shutdown margin by incorporating some burnable neutron absorber in the fuel pellets, for example gadolinium. The burnable neutron absorber provides a reduction of the reactivity in both a cold and a hot state. Incorporating burnable absorbers in the fuel pellets is costly and, in addition, the burnable absorbers cannot be burnt up completely, which means that a certain percentage of neutron-absorbing material always remains, which reduces the reactivity in the hot operating state, which is not desirable. An additional problem is that burnable neutron absorbers such as gadolinium oxide reduce the thermal conductivity of the fuel rods. Fuel rods which contain gadolinium oxide will have a considerably lower relative power because of the absorber, which has an unfavourable influence on the local power distribution. The larger the number of rods with burnable absorber and the larger the concentrations of burnable absorber, the greater will be the negative effect on the local power distribution. To sum up, thus, the requirements imposed on the reactor core during operation and during shutdown often act in opposite directions, which has made the design of a core with an optimum configuration difficult. Some of the known configurations, in which an improved shutdown has been aimed at, will be described below. U.S. Pat. No. 4,863,680 discloses a fuel assembly in which an increased shutdown margin is achieved by arranging in the fuel assembly a number of small units with a small number of fuel rods in each unit. The units are arranged in a specific spaced relationship to each other. Centrally among the small units, a water rod is arranged. The shutdown margin can be ensured by varying the distances between the units in a suitable way. U.S. Pat. No. 4,968,479 discloses a fuel assembly with a number of partial-length rods arranged around a centrally located water rod. The water rod has an upper part with a larger diameter and a lower part with a smaller diameter, where the smaller diameter substantially corresponds to the diameter of the fuel rods. Some of the rods are provided with intermediate zones of non-fissile material. These zones are arranged around the upper part of the water rod such that the effective multiplication factor, k.sub.eff, in hot state can be effectively increased and in cold state be effectively reduced, whereby an improved shutdown margin is obtained. This is due to the fact that there is an excess of water around the water rod at the intermediate zones such that the water rod or the region around this rod is overmoderated in cold state, the neutron multiplication factor thus decreasing and the shutdown margin increasing. During the hot state of the reactor, especially when steam bubbles appear at the outer periphery of the water rod, the excessive water will disappear and the multiplication factor will recover. U.S. Pat. No. 5,128,097 shows a fuel assembly which comprises central fuel rods arranged in a square lattice with a larger diameter than peripheral fuel rods arranged in an triangular lattice. The peripheral triangular lattice pattern makes it possible to increase the cooling region at the periphery, whereby the shutdown margin is increased. The amount of coolant at the centre of the fuel assembly is increased by the introduction of two water rods with an enlarged diameter in relation to the fuel rods. SE 454 822 discloses a fuel assembly which comprises four sub-assemblies each provided with a reduced corner portion, wherein the reduced corner portions are facing each other forming an enlarged centre in the fuel assembly. The sub-assemblies are separated by a cruciform support means, the cruciform centre of which has been enlarged when being adapted to the reduced corner portions. The enlarged centre of the channel-formed support means contributes to the possibility of containing more non-boiling water in the centre part of the fuel assembly. The shutdown margin is thus improved in a cold reactor by containing the larger amount of water in the central part of the fuel assembly. SE 423 760 discloses another fuel assembly with reduced corner portions. The fuel assembly comprises four subassemblies, of which at least one is provided with four reduced corner portions. The reason for the corner reduction in this design is a desire to accommodate a further fuel rod in a limited space. To achieve this, the rods are arranged in a partially triangular pattern instead of in a square pattern. It is then natural to adapt the corner portions of the sub-assembly to the triangular rod configuration. This fuel assembly gives no improved shutdown margin. Admittedly, more water is let into the core but it provides no reduction of the reactivity in cold state since the ratio of water to uranium is not changed. SUMMARY OF THE INVENTION, ADVANTAGES The object of the invention is to provide a reactor core with an improved shutdown margin in a cold state by increasing the amount of water in the core. To achieve an additionally improved shutdown margin in a cold state, the control rod effect can be improved. A fuel assembly which results in such a core is provided with at least one outer, reduced corner portion facing a gap, at least one fuel rod being removed from the corner portion. The reduced corner portion (or portions) entails (or entail) an increase of the cross section area in the moderator regions between the supercells in the reactor core. Each corner reduction entails an increase of the moderator region, which increase corresponds to at least the cross section area of a fuel rod. More water in the core means that the reactivity in cold state decreases since the density of the water is then high and the diffusion length of the neutrons small. Also the water as such has a neutron-absorbing capacity. Taken together, this results in an improved shutdown margin in cold state. Improved control rod effect means that the control rod is allowed to reach, that is absorb, more neutrons, This is brought about by reducing the mean distance of the fissile material, that is, of the neutron generators, to the control rod and hence also the distance of the neutrons to the control rod. This, in turn, is achieved by removing at least one fuel rod in one or a few of the outer corners of the fuel assembly, facing away from the centre of the control rod, the remaining fissile material thus being arranged closer to the control rod. To obtain an improved shutdown margin by both admitting more water into the core and by improved control rod effect, at least that outer corner which is arranged at the largest distance from the control rod is reduced. By the corner reduction, more non-boiling water can be admitted into the reactor core. The change of the ratio of the volume of boiling water to the volume of non-boiling water provides an increase of the reactivity in hot state and a greater reduction of the reactivity in cold state relative to the increase in hot state. Since the reactivity in hot state is predetermined, an increased hot reactivity can be compensated for by reducing the medium enrichment content in the fuel. By the corner reduction, the invention allows access to a larger volume of non-boiling water than when replacing fuel rods by water rods, which is described under the background art, since access is also provided to the volume available outside the rod. It is important for the invention that the space for the non-boiling water has a certain size in order to obtain a considerably improved moderation in hot state and a considerably reduced reactivity in cold state. Fuel assemblies in asymmetrical core lattices, that is, core lattices in which the water gaps between the fuel assemblies in a supercell, referred to as control rod gaps, are wider than the water gaps between the supercells, referred to as narrow gaps, usually have an uneven enrichment distribution. This is due to the fact that when dimensioning asymmetrical core lattices, an inferior power, caused by inferior moderating conditions at the outer part of the supercell, is compensated by a higher enrichment content at that part. The corner reduction according to the invention is particularly advantageous with this type of lattice since it makes it possible to reduce the enrichment content in those rods which are arranged nearest the reduced corner portion (or portions) because of the increased amount of non-boiling water which has a better moderating ability than boiling water. A corresponding increase of the enrichment content is made in the opposite part of the assembly (opposite in relation to the reduced corner) for maintaining the medium enrichment, which results in an equalization of the enrichment distribution in asymmetrical core lattices. The changed enrichment distribution, in both symmetrical and asymmetrical core lattices, results in fissile material being moved nearer the control rod centre such that the control rod effect can be improved, the reactivity in cold state thus being reduced. According to an advantageous embodiment of the invention, the enrichment content in those rods which are arranged nearest to a reduced corner portion, located at a maximum distance from the control rod, with a removed fuel rod, is determined according to the following empirical relationship: EQU B=A.multidot.F.sub.k (F.sub.s (a/b-1)+1) where B=enrichment content in a rod arranged near a reduced corner portion with one fuel rod removed, PA1 A=enrichment content in the corresponding rod in a non-reduced corner portion in the fuel assembly which is opposite to the reduced corner portion, PA1 The factor F.sub.k describes how the ratio B/A is influenced by a reduced corner portion in a lattice with symmetrical water gaps, PA1 The factor F.sub.s indicates a symmetry factor which describes how the ratio B/A is dependent on the ratio between the control rod gaps and the narrow gaps for a lattice with asymmetrical water gaps PA1 a=gap width, control rod gap PA1 b=gap width, narrow gap. When the whole core is provided with the type of fuel assembly described above, the reactivity in cold state is further reduced by cooperation between a plurality of reduced corners, whereby considerably enlarged moderator regions are obtained. In comparison with known technical solutions regarding an improved shutdown margin, the invention has a number of considerable advantages. The main advantage is that the solution entails a considerable simplification of the shape of the fuel assembly compared with previous solutions, while at the same time the reduction of the total amount of fissile material is limited and the safety requirements for shutdown are met more than satisfactorily. Further, it is an advantage that more non-boiling water, by the corner reduction, can be led into the core. The enlarged moderator region entails an advantage since a large coherent volume absorbs more neutrons than several smaller moderator regions with the same total volume. This is due to the diffusion length of the neutrons becoming shorter since, because of the increased quantity of water, they are retarded and absorbed before reaching the fissionable fuel. In cold state, the moderator region will thus to a certain extent serve as a neutron trap. Further, the enrichment content at a reduced corner portion can be reduced and at the corner opposite to the reduced corner portion be increased. This is a considerable advantage especially in asymmetrical lattices since in this way an equalization of the enrichment contents is obtained. In those cases where at least that corner which is located at the greatest distance from the control rod is reduced, the amount of burnable absorbers, such as gadolinium, in the corner can be reduced, which reduces the negative influence exerted by these absorbers on the reactivity in hot state. This is due partly to more water being led into the core because of the corner reduction, and partly to fissile material being moved nearer the control rod. The reduced need of burnable absorbers is also cost-saving. An improved control rod effect and hence a better shutdown magin are obtained by removing fuel rods such that the mean distance of the fissile material to the control rod is reduced. Also the reduction of the enrichment contents in fuel rods arranged adjacent reduced corners and the corresponding increase of the enrichment contents in corresponding rods at opposite corners, such that higher enrichment contents are arranged nearer the control rod (i.e. fissile material nearer the control rod), contribute to improve the control rod effect. A further advantage is that the fuel assembly according to the invention can be used in cores of existing reactors, which is of particular value. The invention will be explained in greater detail by description of several embodiments with reference to the accompanying drawings.