Large-diameter fuel elements

The fuel elements while in storage on the walls of the charging area can be represented if flooded as isolated slabs, approximately one fuel element diameter thick and five fuel element diameters wide. Ihe geometrical buckling for ouch a slab is so large that it is a certainty that a critical mass is precluded as long as the fuel remains in this... 

The core consisted of 37 fuel elements in a hexagonal array with an effective diameter of 1.08 m. This array was surrounded by 30 prismatic graphite columns of the inner reflector, machined on one side to match the profile of the adjacent fuel element and on the other to form a circle of 1.5 m diameter. The 24 control rods operated in holes in the inner reflector. The relatively small size of the DRAGON core meant that its neutron efficiency was not very high the hot core, just critical, with control rods withdrawn, lost 32% of the neutrons by leakage, whereas it was calculated that in large power reactors of about 1,000 MW (Th), the leakage would be around 2%. ... 

The control element assemblies consist of an assembly of 4. 8, or 12 fingers approximately 0.8-inch (2-centimeter) outside diameter and arranged as shown in Fig. 14. The use of cruciform control rods, as in boiling water and early pressurized water reactors, necessitates large water gaps between the fuel assemblies to ensure that the control rods will scram (prompt shutdown) satisfactorily. These gaps cause peaking of the power in fuel rods adjacent to the water channel compared to fuel rods some distance from the channel. 

The fuels were pellets and wood cylinders (pine). The pellets were made of compressed sawdust and had a diameter of 8 mm. The wood cylinders had three diameters, 8, 12 and 34 mm. The proximate analyses and elemental composition of the fuels were almost identical, as seen in Table 1. The density and the thermal conductivity of the pellets are about twice those of the wood. The pellets were burned both in the large and in the small rig, while the wood cylinders were burned only in the small one. 

The control system for the reactor incorporates the rod cluster control (RCC) concept, which has replaced the cruciform control elements used on the earlier PWRs. The control rod poison is distributed uniformly in the form of small-diameter rods which are inserted into the sheaths located within the fuel clusters. The 24-rod assembly is coupled to a drive shaft which is actuated by a drive mechanism mounted on the reactor vessel head. Compared with the large cruciform assemblies, the RCC system gives an increased reactivity worth per unit weight of absorber, coupled with a more uniform power distribution and a greatly reduced flux perturbation effect due to the water gaps which are created by movement of the control rods out of the core. The absorber material in the control rods is silver-indium-cadmium in the form of extruded rods which are sealed within stainless steel tubes. 

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