Flux-trap racks

The re-racking of a pressurized water reactor (PWR) SFP includes installation of high-density storage racks and removal of the existing low-density storage racks. Two types of racks must be used in the PWR SFP flux-trap racks and non-flux-trap racks. The flux-trap racks are qualified for storage of fresh fuel and fuel which has not been fully irradiated in the reactor, whereas the non-flux-trap racks are qualified for storage of fully-irradiated fuel and therefore are used to store the majority of the fuel in the SFP. 

The flux-trap racks are composed of stainless steel boxes separated by a gap, with fixed neutron absorber panels centered on each side. The steel walls define the storage cells, and stainless steel sheathing supports the neutron absorber panel and defines the boundary of the flux-trap water gap used to augment reactivity control. Stainless steel channels connect the storage cells in a rigid structure and define the flux-trap between the neutron absorber panels. 

The non-flux-trap racks are composed of stainless steel boxes with a single fixed neutron absorber panel, attached by stainless steel sheathing centered on each side. The stainless steel boxes are arranged in an alternating pattern such that the connection of the box corners form storage cells between the stainless steel boxes. Neutron absorber panels are installed on one side between neighboring non-flux-trap racks. 

The PWR flux-trap style racks are designed to accommodate tresh or spent tuel assemblies with a maximum initial enrichment ot 5.0 wt%. No burnup requirement is dictated tor the flux-trap racks. These racks are present in the SFP to accommodate new tuel to be loaded into the reactor during the retueling outage or to accommodate the entire core in the event ot a shutdown and subsequent detueling ot the entire core. 

The PWR non-flux-trap style racks, as shown in Figure 13.3, are designed to accommodate tuel assemblies with a maximum initial enrichment ot 5.0 wt% which have accumulated a given minimum burnup or tuel ot initial enrichment and burnup combinations within the acceptable domain. A representative example ot the burnup versus enrichment curve tor a typical PWR non-flux-trap rack is provided in Figure 13.4. 

Additionally, accident and abnormal conditions are considered such as misplacement ot a tuel assembly outside the rack, misleading ot a tresh, unburned assembly into the center ot the non-flux trap racks, variation ot water temperature, drop ot an assembly onto the top ot the rack, and lateral rack movement during a seismic event. For accident conditions, soluble boron in the SFP may be credited to ensure that the reactivity does not exceed 0.95. 

Neutron absorber panels are installed on all exterior walls facing all other racks. Figure 13.2 shows an elevation view of a typical flux-trap type rack used in PWR storage racks. 

Elevation view of a PWR flux-trap style spent fuel storage rack. 

Typical buinup versus enrichment curve for a non-flux trap PWR storage rack. 

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