Patent Number: 055531091
Section: summary

TECHNICAL FIELD The present invention relates to apparatus and methods for simulating a nuclear fuel rod bundle transient and particularly relates to such apparatus and methods for simulating the effect of the change in both integrated bundle power and the axial power or flux shape as a function of time during a transient, for example, in a boiling water reactor. BACKGROUND There are many different types of nuclear reactor transient events. For example, the load on a turbine driven by steam from a nuclear power plant may be removed from the turbine by any one of a number of events, causing a short in the electric power transmission lines. Should such transient event occur, typically the stop valves to the turbine are closed, shutting down the delivery of steam to the turbine. The nuclear reactor, however, is still producing full power. To control the reactor in view of the transient, the control rods are driven into the reactor and valves are opened to bypass steam to a condenser. Safety considerations, however, require preparation for failures in the system, including, for example, those where the condenser bypass valve cannot be closed and the reactor continues to produce full steam power. In that event, and in a boiling water reactor (BWR), a pressure spike occurs, the position of the boiling boundary between the single and two-phase regions within the reactor vessel shifts upwardly, the average void distribution of the fuel changes downwardly and the overall power output increases. This changes the axial power or flux shape of the fuel rod bundle. The axial power or flux shape is the power associated with each axial location in the fuel bundle. In a typical situation, and early in the life of the fuel, the power distribution is at a peak adjacent the bottom of the fuel bundle and the instantaneous power along the fuel bundle upwardly from the peak falls off. When a transient occurs, the boiling boundary moves upwardly along the fuel bundle and displaces the peak of the heat flux curve upwardly along the bundle. The value of the peak of the heat flux curve also changes in response to movement of the boiling boundary. Thus, as the transient progresses, the peak of the heat flux curve is not only displaced upwardly along the fuel bundle but also changes in value so that a greater outlet peak power distribution momentarily occurs. As the effect of control rod insertion and increased reactor voiding decrease neutron flux levels, the peak reactor surface heat flux will be in approximately two seconds. Present-day nuclear fuel bundle simulators employ fuel rod simulators in a closed vessel containing a coolant. Such fuel bundle simulators, however, are limited in their capability. For example, present testing matches total bundle fuel rod surface heat flux with anticipated nuclear fuel bundle surface heat flux and it is the result of these tests which are used to quantify transient computer codes for application to reactor transient analyses. More particularly, today all test facilities employ heating elements in which the only control over each heating element is the magnitude of the power input to the heating elements. While more or less power could be supplied to the heating elements, the simulated axial power or flux shape is fixed. Thus, while the power supply to the heating elements could be varied over time to obtain the correct total bundle power as a function of time, the change in the axial power or flux shape could not be simulated. DISCLOSURE OF THE INVENTION According to the present invention, it has been recognized that during a transient event in a nuclear reactor core not only does the fuel bundle total power change but also the axial power shape varies with time. To my knowledge, this effect has not been studied in an out of reactor test using fuel rod simulators. Obtaining this type of experimental data is highly desirable in order to be able to check the ability of computer codes which predict the transient performance of nuclear reactor fuel to evaluate this more realistic simulation of postulated reactor transient events. The power of the fuel bundle can be represented mathematically as a function of axial position and time, i.e.: EQU P=f(x,t); Equation (1) where x and t represent axial position upwardly along the heated length and time from the beginning of the transient event, respectively. Mathematically, a function f(x,t) can be approximated as used in Equation (1) by EQU f(x,t).apprxeq.f.sub.1 (x,t)+f.sub.2 (x,t); Equation (2). To accomplish the foregoing, first, each fuel rod simulator (FRS) will have similar performance characteristics, i.e., power versus axial position and time, as the other FRS's in the bundle. Each FRS will have two separate, independent heating elements with identical power versus length characteristics. Each of the two heating element groups will be connected to two separate independent power supplies and each group will also have its own axial power shape. In this way by varying each group of heating elements separately with time, the functions f.sub.1 (x,t) and f.sub.2 (x,t) can be generated during a simulated nuclear reactor transient event, thus providing time varying axial power shapes typical of reactor transient events. Various designs of the heating elements can be provided. For example, a double helix heating member may be provided comprised of an internal double helix formed of the two heating elements separated from an outer tubular metallic cladding by suitable electrically insulating material. The heating element can be fabricated from a uniform wall thickness tube using a numerically controlled machine tool. In this way, two continuous helices are generated with a width versus length variation using the same table which represents the desired power versus length relationship for each of the length terms in Equation (2). One end of each of the two helices is connected to a common ground and the other ends to the two independently varied power supplies. In another form of the invention, the two heating elements may comprise two coaxial heating members separated by electrical insulating material. The outer element may also serve as the cladding for the fuel rod simulator. These elements can be either solid like the direct heater presently in use, or of the helix (either single or double helix) type. If the elements are solid, the axial power profile can be realized by either using tapered wall tubes of one material or uniform wall thickness tubes of s more than one material, with different coefficients of electrical resistivity. The method of FRS removal from the test vessels is also important to their design. In single-ended heaters, the heating element simulative of the fuel rod exits the pressure vessel at only one end while the opposite end remains in the vessel. In double-ended heaters, the heating elements exit the pressure vessel at both the top and bottom ends and, accordingly, pass straight through the vessel. Either heater style (single or double-ended) may be of the helix or coaxial type, or a combination of both types. In a preferred embodiment according to the present invention, there is provided apparatus for simulating a nuclear fuel rod bundle transient comprising a vessel for containing a coolant, a pair of heating elements disposed in the vessel for disposition in the coolant, a power supply for supplying power over time to each of the heating elements and means for independently controlling and thereby varying the supply of power over time to each heating element whereby an approximation of the variation in power and axial flux shape in a nuclear fuel bundle as a function of time can be obtained. In a further preferred embodiment according to the present invention, there is provided apparatus for simulating a nuclear fuel rod bundle transient comprising a vessel for containing a coolant, a plurality of nuclear fuel rod simulating members disposed in the vessel for disposition in the coolant and forming a simulated nuclear fuel rod bundle, each member including a pair of heating elements, a power supply for supplying power over time to each of the heating elements and means for independently controlling and thereby varying the supply of power over time to each heating element of the plurality of nuclear fuel rod simulating members whereby an approximation of the variation in power and axial flux shape in a nuclear fuel bundle as a function of time can be obtained. In a still further preferred embodiment according to the present invention, there is provided a method for simulating a nuclear fuel rod bundle transient comprising the steps of disposing a pair of heating elements simulative of a nuclear fuel rod in a vessel containing a coolant, independently supplying power to the heating elements and controlling the supply of power to the heating elements independently to vary the power supplied over time to simulate the variation over time of the power output and axial flux shape in a nuclear fuel bundle. Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for simulating an out-of-pile nuclear reactor transient event to approximate the variation over time of the power and axial flux shape in a nuclear fuel bundle during a transient event.