Patent Number: 
Section: claims

1. A nuclear core component hold-down assembly that accommodates top mounted instrumentation systems comprising:a base plate sized to seat within a top nozzle of a nuclear reactor fuel assembly above and spaced from an adapter plate of the top nozzle, the base plate having a number of openings that align with an equal number of holes in the adapter plate through each of which a corresponding control rod guide thimble in the fuel assembly is accessed;a vertical, hollow sleeve elongated along a dimension of elongation, having an axis along said dimension of elongation, the sleeve extending through and below a central opening in the base plate to mate with an upper opening in an instrument thimble in the fuel assembly, the sleeve extending vertically above the base plate and sized to extend through an upper core plate in the reactor when installed in a core of the reactor;a hold-down bar slidably mounted on the sleeve and having an axial travel length that is restrained a given distance below the top of the sleeve so that the sleeve extends above the hold-down bar when the hold-down bar is fully extended in a direction away from the base plate; anda spring concentrically positioned around the sleeve and extending substantially between the hold-down bar and the base plate. 2. The hold-down assembly of claim 1 wherein the spring comprises two concentric springs. 3. The hold-down assembly of claim 1 wherein the vertical, hollow sleeve is sized to extend above the upper core plate in the reactor when installed in the core of the reactor. 4. The hold-down assembly of claim 1 wherein the sleeve has an axially extending slot that extends over the travel length of the hold-down bar and the hold-down bar has a radially, inwardly extending pin that respectively travels within the slot. 5. The hold-down assembly of claim 1 wherein an interior hollow cavity of the vertical sleeve has two different diameters along its axial length between the base plate and the hold-down bar. 6. The hold-down assembly of claim 5 wherein an upper section of the interior hollow cavity of the vertical sleeve has a larger inner diameter to receive an instrumentation shroud, than a lower portion of the interior hollow cavity of the vertical sleeve which guides a top mounted instrumentation through the top nozzle adapter plate into the instrumentation thimble in the fuel assembly. 7. The hold-down assembly of claim 6 wherein the upper section of the interior hollow cavity of the vertical sleeve has a sufficient cavity length to accommodate a differential thermal and irradiation growth between the fuel assembly and a reactor vessel in which the fuel assembly will be supported. 8. An elongated nuclear reactor fuel assembly having an axial dimension along its elongated length, the fuel assembly comprising:a top nozzle having an adapter plate;a plurality of control rod guide thimbles extending into corresponding openings in the adapter plate;an instrumentation thimble extending into a central opening in the adapter plate; anda hold-down assembly that accommodates top mounted instrumentation systems comprising;the elongated nuclear reactor fuel assembly above and spaced from the adapter plate of the top nozzle, the base plate having a number of openings that align with an equal number of holes in the adapter plate through each of which the corresponding control rod guide thimble in the fuel assembly is accessed;a vertical, hollow sleeve elongated along a dimension of elongation, having an axis along said dimension of elongation, the sleeve extending through and below a central opening in the base plate to mate with an upper opening in the instrumentation thimble in the fuel assembly, the sleeve extending vertically above the base plate and sized to extend through an upper core plate in a nuclear reactor when installed in a core of the reactor;a hold-down bar slidably mounted on the sleeve and having an axial travel length that is restrained a given distance below the top of the sleeve so that the sleeve extends above the hold-down bar when the hold-down bar is fully extended in a direction away from the base plate; anda spring concentrically positioned around the sleeve and extending substantially between the hold-down bar and the base plate. 9. The elongated nuclear reactor fuel assembly of claim 8 wherein the vertical, hollow sleeve is sized to extend above the upper core plate in the reactor when installed in the core of the reactor. 10. The elongated nuclear reactor fuel assembly of claim 8 wherein the sleeve has an axially extending slot that extends over the travel length of the hold-down bar and the hold-down bar has a radially, inwardly extending pin that respectively travels within the slot. 11. The elongated nuclear reactor fuel assembly of claim 8 wherein an interior hollow of the vertical sleeve has two different diameters along its axial length between the base plate and the hold-down bar. 12. The elongated nuclear reactor fuel assembly of claim 11 wherein an upper section of the interior hollow of the vertical sleeve has a larger inner diameter to receive an instrumentation shroud, than a lower portion of the interior hollow of the vertical sleeve which guides a top mounted instrumentation through the top nozzle adapter plate into the instrumentation thimble in the fuel assembly. 13. The elongated nuclear reactor fuel assembly of claim 12 wherein the upper section of the interior hollow cavity of the vertical sleeve has a sufficient cavity length to accommodate a differential thermal and irradiation growth between the fuel assembly and a reactor vessel in which the fuel assembly will be supported. 14. A nuclear reactor power generating system having a core comprising a number of fuel assemblies, at least some of the fuel assemblies comprising:a top nozzle having an adapter plate;a plurality of control rod guide thimbles extending into corresponding openings in the adapter plate;an instrumentation thimble extending into a central opening in the adapter plate; anda hold-down assembly that accommodates top mounted instrumentation systems comprising;a base plate sized to seat within the top nozzle of a nuclear reactor fuel assembly above and spaced from the adapter plate of the top nozzle, the base plate having a number of openings that align with an equal number of holes in the adapter plate through each of which the corresponding control rod guide thimble in the fuel assembly is accessed;a vertical, hollow sleeve elongated along a dimension of elongation, having an axis along said dimension of elongation, the sleeve extending through and below a central opening in the base plate to mate with an upper opening in the instrumentation thimble in the fuel assembly, the sleeve extending vertically above the base plate and sized to extend through an upper core plate in a nuclear reactor when installed in a core of the reactor;a hold-down bar slidably mounted on the sleeve and having an axial travel length that is restrained a given distance below the top of the sleeve so that the sleeve extends above the hold-down bar when the hold-down bar is fully extended in a direction away from the base plate; anda spring concentrically positioned around the sleeve and extending substantially between the hold-down bar and the base plate. 15. The nuclear reactor power generating system of claim 14 wherein the vertical, hollow sleeve is sized to extend above the upper core plate in the reactor when installed in the core of the reactor. 16. The nuclear reactor power generating system of claim 14 wherein the sleeve has an axially extending slot that extends over the travel length of the hold-down bar and the hold-down bar has a radially, inwardly extending pin that respectively travels within the slot. 17. The nuclear reactor power generating system of claim 14 wherein an interior hollow of the vertical sleeve has two different diameters along its axial length between the base plate and the hold-down bar. 18. The nuclear reactor power generating system of claim 17 wherein an upper section of the interior hollow of the vertical sleeve has a larger inner diameter to receive an instrumentation shroud, than a lower portion of the interior hollow of the vertical sleeve which guides a top mounted instrumentation through the top nozzle adapter plate into the instrumentation thimble in the fuel assembly. 19. The nuclear reactor power generating system of claim 18 wherein the upper section of the interior hollow cavity of the vertical sleeve has a sufficient cavity length to accommodate a differential thermal and irradiation growth between the fuel assembly and a reactor vessel in which the fuel assembly will be supported.