Patent Number: 039649695
Section: summary

Liquid metal cooled, fast breeder reactors (LMFBR) designed for commercial power generation or as large test reactors have several unique characteristics associated with the reactor core. As a result, special attention must be given to the structure that supports and retains the core components. For example, one problem is the material swelling and creep induced by the high fluence (neutron flux .times. time) associated with economically acceptable fuel burnup. This swelling and creep results in the deformation of fuel assemblies and the introduction of restraint loads necessary to maintain fuel assembly lateral positioning. The positioning is very important since a characteristic of large fast reactor cores is the sensitivity of nuclear reactivity to small geometric changes in core configuration (typically $4 reactivity increase for a 1 inch radius reduction). This results in the necessity for tight geometric control of core configuration and yet be able to accommodate irradiation induced swelling in addition to the normal thermal expansion. Several basic concepts can be envisioned that could perform this function. In general, these concepts involve constraint mechanisms and some form of compliance or flexibility to accommodate normal distortions without excessive restraint on the components. One of these concepts consists of a fixed external boundary located at several elevations near the core and the use of an internal tightening device, located at a number of the core flow channels, that removes the clearance necessary in the refueling process. Compliance can be provided by spring devices located on the periphery of the core and blanket assemblies and within the fixed boundaries. Alternately, compliance can be provided locally on each of the tightener assemblies located throughout the core and blanket. The use of internal tightener compliance has an advantage in that the accidental release of possible core disarrangement caused by ratcheting of the channels at the clamping planes will not cause a decrease in core radius and thus will not introduce positive reactivity. This is a potential safety feature in the use of internal compliance. SUMMARY OF THE INVENTION The present invention is directed to an apparatus that provides the basic function of internal tightening and provides for local compliance. The primary features of the apparatus include large contact areas to transmit loads during reactor operation, actuation cam surfaces loaded only during clamping and unclamping operations, and preloaded pads with compliant travel at each face of the hexagonal assembly at each clamping plane to accommodate thermal expansion and irradiation induced swelling. Therefore, it is an object of the invention to provide a device that accomplishes the basic function of internal tightening and provides for local compliance in the core of a liquid metal cooled, fast breeder reactor. A further object of the invention is to provide an internal core tightener for an LMFBR which is a linear actuated expanding device using a minimum of moving parts to perform the lateral tightening function. Another object of the invention is to provide an internal core tightener for an LMFBR which utilizes large contact areas to transmit loads, actuation cam surfaces loading only during certain operations, and preloaded pads with compliant travel to accommodate thermal expansion and irradiation induced swelling.