Power flattening

The core is divided into two radial zones of equal volumes. The same fissile enrichment is used in both zones, but the BeO content is varied from 10.5 v/o in the central zone to 3.6 v/o in the outer zone. The resulting power-flattening is further enhanced by a BeO radial reflector that surrounds the core. A radial blanket outside the reflector was dropped as uneconomical. 

Fig. 7. Influence of power flattening on explosive energy release.

Fig. 11. Effect of power flattening on total energy release.

Other GEDANKEN cores are now being stadied in particular, a two-zoned-core simulation of a power-flattened reactor in infinite slab geometry. A cylindrical 2D core with radial plane plate structure identical to GEDANKEN 1 is being calculated to study the effect of leakage parallel to the plates. 

The control rods perform dual fimctions of power distribution shaping and reactivity control. Power distribution in the core is controlled during operation of the reactor by manipulation of selected patterns of rods. The rods, which enter from the bottom of the near-cylindrical reactor, are positioned in such a manner to counterbalance steam voids in the top of the core and affect significant power flattening. These groups of control elements, used for power flattening, experience a somewhat higher duty cycle and neutron exposure than the other rods. 

The Indian HWR is so designed that during the major part of its life, there is some power flattening in the central part of the core, achieved through a differential bumup distribution. In a newly started reactor, where the entire core is at zero burnup, the reactor will have to be derated unless some other measures can be taken to obtain a similar effect. Advantage was taken of this fact to use thoria bundles for power flattening in the initial core of new reactors. 

The firsf regular use of thoria fuel in a power reactor in India was at the Kakrapar Atomic Power Station. About 500 kg (35 bundles) of fhorium fuel was used for power flattening in the initial core of Unifl, which was conunissioned in 1993. The core has already seen 200 full-power days of operation, and all thoria bundles behaved well. The same scheme of power flattening has been employed in the second unit as well. Thus 500 kg of thoria are also present in Kakrapar Unit-2, which attained criticality in December 1994. All earlier HWRs in the world used DU in the initial core. 

The Rajasthan Unit-2 has been restarted after retubing. In this reactor, about 250 kg of thorium was loaded for initial power flattening. The thoria bundle distribution is different here from that of Kakrapar Unifs 1 and 2. This is because Kakrapar uses shutoff rods for reactor scram, and so the thoria bundle locations have to be chosen in such a manner as... 

Neutron-physical characteristics (temperature and coolant density reactivity effects, void reactivity effect and bum-up reactivity swing,), power flattening (peaking factors, approaches to reduce them)... 

Fuel type and enrichment Fuel assembly type and number Type of structural materials Fuel bum-up and cycle length Core dimensions reflectors, if any Approach to power flattening Average power density Major reactivity effects Breeding ratio, if applicable Decay heat removal systems Other characteristics suggested by the designer... 

Power flattening 3-zones of different material content and density. 

Power flattening Three-zone radial enrichment zoning and internal blankets... 

Approach to power flattening Three radial sub-zones in the core... 

Adequacy of geometrical coverage and flexibility to permit attaining adequate power-flattening efficiencies and satisfactory control over sj tlal power cycling. 

Power flattening approach Use of burnable absorbers Selection of fuel loading pattern ... 

In the RUTA-70 design, the following mechanisms of reactivity control and power flattening are applied ... 

Neutron-physical characteristics Temperature reactivity coefficient - < -6 10 Ak/k/°C Coolant void density coefficients - Negligible Excess reactivity - 17.7 %Ak/k Maximum axial peaking factor -1.7 Maximum horizontal peaking factor - 1.4 Power flattening by control rods and adequately arranging burnable poisons. The burnable poison consists of B4C/C compacts, stored in a hole under the dowel pin arranged at the top of three comers in a block. 

Small variation of power peaking factors is stipulated by the chosen approach to power flattening by the amount of fuel (use of three core zones with different fuel rod diameters and the same Pu content), and by ensuring the value of the active core breeding ratio close to 1. 

The currently preferred method for achieving a 9-cycle mode of operation Is to group the channels Into 3x3 supercell arrays repeated over the reactor core. Power flattening Is achieved by dividing the reactor Into two zones and feeding the outer zone with a higher feed enrichment. 

A thin thermal insulation of Zirconia is provided between the water rods and fuel coolant channels. Gadolinia is used for compensating bum-up reactivity and axial power flattening. The control rods are the cluster rod type. The control elements are inserted in the guide tubes that are located in the central water rods. 

In spite of the relatively low power density of the FDR core (33.8 kW/1) much attention was paid to obtaining a good radial power flattening, since in a self-pressurized reactor like the FDR all channels which have an enthalpy rise above the core average value produce vapor voids. These in turn give rise to reactivity changes when the core is accelerated vertically, which increase with the void content as may be seen from Table V. 

The power flattening is achieved mainly by dividing the core into four radial zones with different enrichments, the highest enrichment occurring at the core boundary. Figure 7 shows the radial power distribution for the beginning of the core life (BOL) when the central group of four control rods is somewhat more than half-way inserted into the core, and also for the end of the core life (EOL), after 500 days of full power... 

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