Patent Number: 
Section: claims

1. An optical gamma thermometer, comprising:an outer tube;a metal mass disposed within the outer tube in such a way that the metal mass is thermally isolated from an ambient environment, the metal mass having a temperature proportional to a gamma flux within a core of a nuclear reactor; andan optical fiber cable at least partially disposed within the outer tube for measuring the temperature of the metal mass without using a thermocouple, the optical fiber cable having a fiber core extending along a long axis and a fiber cladding circumferentially covering the fiber core, wherein the fiber optical cable further includes a fiber Bragg grating having a periodic refractive index modulation profile along the long axis. 2. The optical gamma thermometer according to claim 1, further comprising a plurality of fiber Bragg gratings, wherein a spacing between each fiber Bragg grating is between about 2 millimeters to about 10 centimeters. 3. The optical gamma thermometer according to claim 2, further comprising a metalized coating layer having a thickness between about 10 micrometers to about 20 micrometers. 4. The optical gamma thermometer according to claim 1, wherein the fiber Bragg grating has a length along a long axis of the optical fiber cable of about 5 millimeters to about 20 millimeters. 5. The optical gamma thermometer according to claim 1, wherein the fiber core is made of pure silicon dioxide and the fiber cladding is fluorine doped to form a single clad fiber sensing cable. 6. The optical gamma thermometer according to claim 1, wherein the fiber core is F/GeO2 co-doped and the fiber cladding is F-doped to form a single clad fiber sensing cable. 7. The optical gamma thermometer according to claim 1, wherein the fiber cladding comprises a double clad structure having a first clad area that is lightly F-doped and a second clad area that is heavily F-doped. 8. The optical gamma thermometer according to claim 1, wherein the temperature is measured by using one of Raman, Brillouin and Rayleigh scattering techniques. 9. The optical gamma thermometer according to claim 1, wherein the optical gamma thermometer includes a second metal mass in direct contact with an ambient environment to provide a reference temperature. 10. A nuclear reactor comprising a core for containing a reaction within a reactor vessel, and a plurality of instrument guide tubes extending into the core at spaced apart locations, each instrument guide tube permitting insertion and removal of a local power range monitoring string, each string comprising a plurality of local power range monitors for measuring a thermal neutron flux at a predetermined location within the core and an optical gamma thermometer adjacent a corresponding local power range monitor, each optical gamma thermometer comprising an outer tube, a metal mass disposed in the outer tube in such a way that the metal mass is thermally isolated from an ambient environment, the metal mass having a temperature proportional to a gamma flux within the core, and an optical fiber cable at least partially disposed within the outer tube for measuring the temperature of the metal mass without using a thermocouple, the optical fiber cable having a fiber core extending along a long axis and a fiber cladding circumferentially covering the fiber core, wherein the fiber optical further includes a fiber Bragg grating having a periodic refractive index modulation profile along the long axis, and wherein the optical gamma thermometer is capable of calibrating the corresponding local power range monitor during steady state power operation of the reactor. 11. The reactor according to claim 10, wherein the optical gamma thermometer includes a second metal mass in intimate contact with an ambient environment for providing a reference temperature.