Patent Number: 043671969
Section: summary

The present invention relates generally to neutronic reactors and in particular to heterogeneous neutronic reactors provided with neutron capturing control means. Heterogeneous reactors are those which employ neutron fissionable materials in the form of bodies, usually disposed within a moderator material, rather than homogeneously distributing the thermal neutron fissionable material throughout the moderator material. The first neutronic reactor to be successfully operated was a heterogeneous reactor. Most heterogeneous reactors employ uranium fuel with a U.sup.238 content, and such reactors are constructed with "lumped geometry", i.e., the active portion of the reactor contains a body of moderator material and thermal neutron fissionable material wherein the fissionable material is arranged in lumps spatially disposed in a lattice arrangement in the moderator material. Such construction of the active portion of the reactor leads to a higher resonance escape probability, and hence a higher neutron multiplication factor (K.sub..infin.) than is possible in homogeneous reactors with the same amounts of fuel and moderator. These and other advantages of heterogeneous reactors are discussed in "The Elements of Nuclear Reactor Theory", by Glasstone and Edlund, published by D. Van Nostrand Co., Inc., 1952, starting at Section 9.24 and extending through Section 9.46. Heretofore, it has been the practice to control nuclear reactors with a relatively small number of individual control elements inserted at different points into the reactor. The rate of a neutronic reaction is controlled by varying the extent of insertion of such control elements made of materials having very high neutron-absorbing characteristics, commonly referred to as "neutron capture cross sections", in the active portion of the reactor. These elements, using solid absorber materials such as boron or cadmium, are also known as control rods. The above described control elements, which are used generally at the present time, are usually adapted to occupy individual positions within the core of a reactor. The use of aforementioned control elements to achieve reactivity control has a disadvantage in that the resultant axial and radial neutron flux distributions deviates from the distribution that permit the maximum amount of heat to be removed from the reactor in the core of the reactor. The insertion of a control element into the core results in a local flux depression, as is shown in FIG. 11.10, Glasstone and Edlund, in "The Elements of Nuclear Reactor Theory", D. Van Nostrand Co., Inc., 1952, page 317. This uneven flux distribution may be the cause of abnormal local temperature rises commonly known as "hot spots" which cause thermal stresses in fuel elements and other reactor structures. As a result of the limitations imposed by these local disturbances of the neutron flux, maximum power output of the reactor may not be achieved. An improvement over the aforementioned individually spaced control elements is described in the copending patent application of Untermyer and Hutter, Ser. No. 459,219, filed Sept. 29, 1954, now U.S. Pat. No. 2,898,281, wherein a plurality of rods, each rod having alternate neutron-absorbing and non-absorbing portions distributed along its length, are grouped together in a common housing and are maintained completely within the reactor during all phases of operation. But the reactor using this type of control element has to have a substantial amount of excess reactivity built thereinto because the rods comprising the control element remain at all times within the reactor proper. Furthermore, the range of this type of control is substantially limited. The control elements, as described above, generally utilize material such as boron or cadmium for absorbing a certain portion of the neutrons released in the active portion of a reactor. This type of control is uneconomical inasmuch as the absorbed neutrons are not utilized in any manner. Even though the abnormalities introduced by the use of the individual control rods are eliminated, the power output of the reactor is still limited by the maximum permissible temperature of the fuel elements. In some cases it is possible to increase a power output by decreasing the ratio P.sub.max /P.sub.av, that is the average power output of the reactor can be increased by a more uniform distribution of power density. The method of achieving this uniform distribution of power density through the reactor structure is commonly referred to as "flattening" of the power distribution, or neutron flux, inasmuch as the flux is directly proportional to the power. An operating reactor will generally have a horizontal flux distribution resembling a portion of a cosine curve, i.e., the flux is a maximum in the centrum of the active portion of the reactor and decreases toward the sides of said active portion. A flux distribution in a horizontal direction in an upright cylinder type reactor is called radial flux distribution and in the vertical direction is called axial distribution. The power output of a reactor having a cosine flux distribution can be increased by arranging fuel and moderator in the reactor in a particular manner to increase the flux near the sides of the active portion of the reactor to achieve a flattened effect on flux distribution. Although the power output of a reactor can be increased or improved by flattening the power distribution or the flux in the reactor in the horizontal direction, the inventors have found that additional improvement in extraction of power from the reactor can be achieved by modifying the flux distribution in the vertical (axial) disposition of the reactor. Since a coolant is introduced into a reactor usually from a single direction, the inventors have found that, by increasing the flux (power) in the region adjacent to the entry of the coolant into the reactor, more heat (power) can be removed by the cold coolant as it enters the reactor. The magnitude of the flux density is maximum near the point of entry of the coolant into the reactor, said magnitude decreasing along the path of flow in the reactor towards the exit of said coolant from the reactor. This method of increasing power output of a reactor is termed "rooftopping". The principal object of the invention is to provide an arrangement of reactivity control means to achieve optimum flux distribution and maximum power output in a reactor. Another object of the invention is to provide means comprising various neutron-absorbing materials for controlling reactivity wherein the absorbed neutrons are utilized gainfully. Another object of the invention is to provide means for controlling neutronic reactivity in a reactor over a wide range in small steps. A further object of the invention is to provide an arrangement of control elements for controlling reactivity in a nuclear reactor wherein the control elements are arranged in a particular pattern and each element has groups of control rods which are moved individually or in groups to provide flexibility in control. Other objects and advantages of neutronic reactors constructed according to the teachings of the invention will become readily apparent from a study of the following description of the invention, together with the illustrative embodiment enclosed herein. In accordance with the teachings of this invention, there is provided a neutronic reactor having a plurality of regions of control in which a plurality of control elements are disposed. Each control element contains a group of full-length and short-length control rods made of materials possessing different neutron-capture cross section characteristics. The control elements are arranged in concentric rings or gangs and occupy particular positions in a lattice arrangement of fuel elements in the core of the reactor. Some of the control rods may function only to absorb thermal neutrons per se while others absorb thermal neutrons to produce radioactive isotopes. The short-length control rods which are substantially shorter than the fuel rods, are adapted to be placed in a particular portion of the core to modify the overall flux distribution. An improvement in the power output of the reactor is obtained by improving the radial (horizontal) flux distribution of the reactor by using the control rods grouped in concentric rings to give a flattening of the radial neutron flux distribution and by rooftopping the flux distribution in the axial (vertical) direction of the reactor by introducing short-length control rods in the region of the core which is most remote from the entry of the coolant flowing therethrough. The positioning of the short-length control rods in a down flow relationship to the coolant has the effect of modifying the axial flux distribution in such a way as to present a maximum flux density available near the point of entry of the coolant, where the temperature differential between the coolant and the fuel elements is the greatest.