Patent Number: 055307280
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the measurement of objects using an optical measuring system. More particularly, the invention relates to a method of measuring the lengths of spent nuclear fuel rods so that the mass of the rods can be determined. 2. Discussion of Prior Art In the reprocessing of spent nuclear fuel it is important for inventory and accounting purposes that the amount of spent fuel to be reprocessed is calculated accurately. This requires a system for obtaining the mass of the spent fuel rods to be reprocessed. In one particular application, measurement of the mass of the fuel rods takes place in a decanning cave in which a metal cladding is stripped from fuel elements to provide fuel rods for subsequent reprocessing. Because of the hostile environment existing within the cave, systems for direct weighing of the fuel rods have many disadvantages. For example, the components of such a system are subjected to adverse conditions of radiation, heat, mechanical shock and water existing within the cave. Thus, these systems tend to have a short operational life and require frequent servicing and maintenance. Because of the inaccessibility and the danger of radiation exposure associated with handling the weighing system components in the cave, direct weighing systems are unsatisfactory. One known type of direct weighing system has a weighing device in which a weighing platform is supported on solid state load cells located in the decanning cave. Although such a system can produce accurate results, it has not proved satisfactory because the load cells are unable to withstand the mechanical shock loads and the high radiation fields existing within the cave and so tend to fail after a short period of time. SUMMARY OF THE INVENTION According to the present invention there is provided a method of measuring the linear dimension of an object, said method comprising the steps of obtaining a first optical image, said first optical image being of a support surface, digitising the first optical image and storing the first optical image in the form of grey level values, obtaining a second optical image, said second optical image being of an object to be measured when placed on said support surface, digitising the second optical image and storing the digitised second optical image in the form of grey level values, and processing the two stored digitised images to obtain a difference in grey level values between the first and second digitised images so as to determine the required linear dimension of said object. To overcome the above-mentioned disadvantages of the prior art we have devised a non-intrusive measuring system as defined above which optically measures the lengths of the objects such as fuel rods. Having obtained the length of a fuel rod the system is able to calculate the mass of the fuel rod using a known value of mass per meter for the fuel material. An advantage of the present invention is that it provides a non-intrusive, indirect weighing system for spent nuclear fuel rods and that none of the components of the system is located within the hostile environment of the decanning cave. As a result, the measuring system has a longer operational life than direct weighing systems and requires only a minimum amount of servicing and maintenance. A further advantage of the present invention is that once the system is set up and calibrated it does not require any further input from an operator. In a preferred embodiment the linear dimension is the length of a spent nuclear fuel rod. Preferably the support surface forms part of a tray for receiving spent nuclear fuel rods. The first and second digitised optical images are each stored as a plurality of columns, each column containing a plurality of pixels having a grey level value, the difference in grey level values of corresponding columns in the first and second optical images being determined as a root mean square value. Preferably a root mean square value is selected as a threshold value, said threshold value being selected so that root mean square values above the threshold value are indicative of the presence of a fuel rod, and root mean squares values below the threshold value are indicative of the support surface. In a preferred embodiment the two stored digitised images are processed using an algorithm which moves across the columns and processes each column in turn, the number of columns having a root means square value above said threshold value being a measure of the length of the fuel rod. Preferably the algorithm is adapted to perform an averaging test on the first of said columns which indicates a root mean square value above the threshold value and on a plurality of columns next succeeding said first column, the result of said test indicating whether or not said root mean square value of said first column represents one end of the fuel rod. The averaging test may be performed on the said first column and the next nine succeeding columns. Preferably the algorithm is also adapted after establishing said one end of the fuel rod to perform an averaging test on a subsequent column indicating a root mean square value below said threshold value and on a plurality of next succeeding columns, the result of said test indicating whether or not said root mean square value of the said subsequent column represents the other end of the fuel rod. The averaging test may be performed on the said subsequent column and the next nine succeeding columns.