Patent Number: 055307280
Section: claims

1. A method of measuring the linear dimension of a spent nuclear fuel rod, said method comprising the steps of obtaining a first optical image, said first optical image being of a support surface, digitizing 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 said spent nuclear fuel rod to be measured when placed on said support surface, digitizing the second optical image and storing then digitized second optical image in the form of grey level values, and processing the two stored digitized images to obtain a difference in grey level values between the first and second digitized images so as to determine the required linear dimension of the spent nuclear fuel rod, wherein the linear dimension to be measured is the length of said spent nuclear fuel rod. 2. A method according to claim 1, wherein the support surface forms part of a tray for receiving the spent nuclear fuel rod to be measured. 3. A method according to claim 2, comprising the steps of storing the first and second digitized images as a plurality of columns, each column containing a plurality of pixels having a grey level value, and determining the difference in grey level values of corresponding columns in the first and second optical images as a root mean square value. 4. A method according to claim 3, comprising the step of selecting a root mean square value 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 square values below the threshold value are indicative of the support surface. 5. A method according to claim 4, wherein the two stored digitized images are processed using an algorithm which moves across the columns and processes each column in turn, the number of columns having a root mean square value above said threshold value being a measure of the length of the fuel rod. 6. A method according to claim 5, wherein 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. 7. A method according to claim 6, wherein the algorithm is adapted to perform the averaging test on said first column and the next nine succeeding columns. 8. A method according to claim 6, wherein the algorithm is 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. 9. A method according to claim 8, wherein the algorithm is adapted to perform the averaging test on said subsequent column and the next nine succeeding columns.