Patent Number: 
Section: claims

1. A nuclear reactor system comprising:a fuel assembly including a fuel channel;fuel rods inside the fuel channel; anda temperature indicator inserted in between two or more of the fuel rods, the temperature indicator including,an indicating rod made of a first material;an outer housing having an upper opening, the outer housing being made of a second material and surrounding at least a portion of the indicating rod; anda rod holder attached to an inner surface of the outer housing, the rod holder being made of a third material and being configured to support the indicating rod such that a top surface of the indicating rod extends out of the upper opening of the outer housing, the third material having a lower melting point than the first and second materials,wherein,the rod holder extends in a first direction from a first portion of the inner surface to a second portion of the inner surface opposite the first portion,the indicating rod extends in a second direction from an upper surface of the rod holder out of the upper opening of the outer housing,a length of the rod holder in the first direction is greater than a thickness of the rod holder in the second direction,the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that the top surface of the indicating rod is not visible above the upper opening of the outer housing, andthe rod holder is configured to melt at a temperature exceeding a threshold temperature in the fuel channel. 2. The nuclear reactor system of claim 1, wherein the outer housing has a bottom surface that is spaced away from the upper opening and spaced away from a first position on the inner surface of the outer housing at which the rod holder is attached to the outer housing such that the first position is in between the upper opening and the bottom surface of the outer housing. 3. The nuclear reactor system of claim 2, wherein a first distance between an uppermost surface of the indicating rod and the upper opening of the outer housing is less than a second distance between a lowermost surface of the indicating rod and the bottom surface of the outer housing. 4. The nuclear reactor system of claim 2, wherein a region within the outer housing in between the first position and the bottom surface of the outer housing is filled with air. 5. The nuclear reactor system of claim 1, wherein the third material is aluminum. 6. The nuclear reactor system of claim 1, wherein the outer housing is cylindrical in shape and the rod holder is a circular disc attached to the inner surface of the outer housing. 7. The nuclear reactor system of claim 6, wherein the circular disc is attached to the outer housing such that the circular disc spans an inner diameter of the outer housing. 8. The nuclear reactor system of claim 3, wherein the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that the lowermost surface of the indicating rod contacts and rests upon the bottom surface of the outer housing. 9. A nuclear reactor system comprising:a fuel assembly including a fuel channel;fuel rods inside the fuel channel; anda temperature indicator inserted in between two or more of the fuel rods,the temperature indicator including,an indicating rod made of a first material,an outer housing having an upper opening, the outer housing being made of a second material and surrounding at least a portion of the indicating rod, anda rod holder attached to an inner surface of the outer housing, the rod holder being made of a third material and being configured to support the indicating rod such that a top surface of the indicating rod extends out of the upper opening of the outer housing, the third material having a lower melting point than the first and second materials,wherein,the rod holder extends in a first direction from a first portion of the inner surface to a second portion of the inner surface opposite the first portion,the indicating rod extends in a second direction from an upper surface of the rod holder out of the upper opening of the outer housing,a length of the rod holder in the first direction is greater than a thickness of the rod holder in the second direction,the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that the top surface of the indicating rod is not visible above the upper opening of the outer housing, andthe rod holder is configured to melt at a temperature exceeding a threshold temperature in the fuel channel. 10. The nuclear reactor system of claim 9, wherein the outer housing has a bottom surface that is spaced away from the upper opening and spaced away from a first position on the inner surface of the outer housing at which the rod holder is attached to the outer housing such that the first position is in between the upper opening and the bottom surface of the outer housing. 11. The nuclear reactor system of claim 10, wherein a first distance between an uppermost surface of the indicating rod and the upper opening of the outer housing is less than a second distance between a lowermost surface of the indicating rod and the bottom surface of the outer housing. 12. The nuclear reactor system of claim 10, wherein a region within the outer housing in between the first position and the bottom surface of the outer housing is filled with air. 13. The nuclear reactor system of claim 9, wherein the third material is aluminum. 14. The nuclear reactor system of claim 9, wherein the outer housing is cylindrical in shape and the rod holder is a circular disc attached to the inner surface of the outer housing. 15. The nuclear reactor system of claim 14, wherein the circular disc is attached to the outer housing such that the circular disc spans an inner diameter of the outer housing. 16. The nuclear reactor system of claim 11, wherein the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that the lowermost surface of the indicating rod contacts and rests upon the bottom surface of the outer housing. 17. The temperature indicator of claim 1, wherein the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that an uppermost surface of the indicating rod changes from being outside the outer housing and visible above the upper opening of the outer housing before the rod holder melts to being inside the outer housing and not being visible above the upper opening of the outer housing after the rod holder melts. 18. The nuclear reactor system of claim 9, wherein the indicating rod, the outer housing and the rod holder are each configured such that if the rod holder melts, the indicating rod will fall within the outer housing such that an uppermost surface of the indicating rod changes from being outside the outer housing and visible above the upper opening of the outer housing before the rod holder melts to being inside the outer housing and not being visible above the upper opening of the outer housing after the rod holder melts. 19. A nuclear reactor system comprising:a fuel assembly including a fuel channel;fuel rods inside the fuel channel; anda temperature indicator inserted in between two or more of the fuel rods, the temperature indicator including,a tubular housing having an upper opening at one end and a bottom surface at the other end, the tubular housing being made of a first material and having a first length,an indicating rod made of a second material and having a second length, the first length of the tubular housing being greater than the second length of the indicating rod, anda rod holder, having a disc shape, attached to an inner surface of the tubular housing and spaced between the upper opening and bottom surface of the tubular housing defining an intermediate surface within the tubular housing, the rod holder being made of a third material having a lower melting point than the first and second materials, and the intermediate surface being spaced from the bottom surface of the tubular housing by a first distance,wherein the intermediate surface defined by the rod holder supports the indicating rod within the tubular housing such that an upper end of the indicating rod extends above the upper opening of the tubular housing by a second distance, the first distance being greater than the second distance,wherein the bottom surface of the tubular housing supports the indicating rod when the rod holder melts such that the upper end of the indicating rod extends below the upper opening of the tubular housing, andthe rod holder is configured to melt at a temperature exceeding a threshold temperature in the fuel channel.