Heterogeneous fuel-moderator systems

Fig. 1. Effects of homogeneous versus heterogeneous UOj-HjO system moderation bn the minimum criticality size of the system. Points for heterogeneous system correspond to the moderator/fuel volume ratio and fuel rod diameter for maximum material buckling at various fuel lump densities.

The advanced thermal reactor ATR now under development in Japan is highly heterogeneous in core structure a cluster-type fuel is contained in a pressure tube with coolant, which is separated from 0,0 moderator by a calandria tube, hi such a system, neutron behavior is complicated and then very sensitive to the slight change of structure or material of the core. Therefore, a large amount of experimental data on both microscopic and macroscopic point of view must be accumulated especially for the lattices close to that of the ATR. Then, accuracies of calculatlonal codes which are used for the core design should be confirmed by using those data in reference to the lattices of the ATR. 

One factor which is extremely sensitive to these differences in flux distribution is the thermal utilization. The important trends can be readily displayed with the aid of the unit-cell model. Let denote the volume of such a cell and Vy and Vm the volumes of the fuel and moderator regions, respectively, in this cell. Then if 0,(r) denotes the thermal flux in region i of the cell, we can define the thermal utilization for the heterogeneous system as... 

Some appreciation of the significance of this result may be had from a comparison of this heterogeneous system to an equivalent homogeneous system. We define such a system as one which has the same volume as the unit cell and moreover contains the same total mass of fuel and moderator as the heterogeneous system, but now distributed uniformly throughout V n, Then Feeii = Vy + VMy and if we call the nuclear density of material i distributed uniformly over V au, then clearly... 

These advantages stem partly from the fluid nature of the fuel and partly from the homogeneous mixture of the fuel and moderator i.e., an aqueous homogeneous reactor combines the attributes of liquid-fuel heterogeneous reactors with those of water-moderated heterogeneous reactors. If practical methods for handling a radioactive aqueous fuel system are developed, the inherent simplicity of this type of reactor should result in considerable economic gains in the production of nuclear power and fissionable material. 

In an Oak Ridge study [61] various types of fluidized-bed reactors were compared. Systems investigated were (a) a sodium-cooled fast reactor, (b) a gas-cooled system, (c) an organic-moderated reactor, (d) a heavy-water-moderated reactor, and (e) a light-water system. A detailed study of this latter system was carried out to compare its characteristics and performance with solid-fuel heterogeneous pressurized-water reactors. The results indicated that both the light-water-moderated and organicmoderated fluidized reactors showed promise, while the gas-cooled, the D20-cooled, and the fast (unmoderated) reactors were found to be less satisfactory for application of the fluidized-bed technique. 

The liciuid-metal system that has received the greatest emphasis to date is of the heterogeneous, circulating fuel type. This reactor, known as the Liquid iMetal Fuel Reactor (LMFR), has as its fuel a dilute solution of enriched uranium in liquid bismuth, and graphite is u.scd as both moderator and reflector. With as the fuel and Th as the fertile material, the reactor can be designed as a thermal breeder. Consideration is restricted here to this reactor type but, wherever possible, information of a general nature is included. 

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