Axial power distribution

B17. Bertoletti, S., Gaspari, G. P., Lombardi, C., and Zavattarelli, R., Critical heat flux data for fully developed flow of steam and water mixtures in round vertical tubes with non-uniform axial power distribution, CISE-R.74 (1963). 

The worst operating condition in a common design practice consists of overly conservative assumptions on the hot-channel input. These assumptions must be realistically evaluated in a subchannel analysis by the help of in-core instrumentation measurements. In the early subchannel analysis codes, the core inlet flow conditions and the axial power distribution were preselected off-line, and the most conservative values were used as inputs to the code calculations. In more recent, improved codes, the operating margin is calculated on-line, and the hot-channel power distributions are calculated by using ex-core neutron detector signals for core control. Thus the state parameters (e.g., core power, core inlet temper-... 

In the case of uniform axial power distribution, Eq. (5-141) becomes... 

C-E Report, 1975, C-E Critical Heat Flux Correlation for C-E Fuel Assemblies with Standard Spacer Grids, Part I, Uniform Axial Power Distribution, CENPD-162, Combustion Engineering Co., Winsor, CT. (5)... 

Based on the comparison to MOX criticals shown in Fig. 5, it would appear that Surry BZ and TMI BZ are most consistent with the experimental measurements, and that only the Surry EF case would be considered as an outlying result. Sequoyah results are self-consistent and are relatively close to the results of the experimental cases, as is the North Anna result. Note that the Surry BZ case is closest to the fit to the zero-power MOX criticals, and that all full-power cases (Sequoyah BF, Sequoyah MF, and Surry EF) are more removed from the data. This trend indicates that perhaps aspects of full-power operation (e.g., nuclides present, temperature effects, axial power distributions, xenon distributions, etc.) are not being well represented in the KENO V.a models. 

The reflector is installed inside the reactor vessel and the heat generated in the reflector is cooled by sodium. The equivalent core diameter is 0.8m which satisfy negative void reactivity requirements. The reflector length is 1.5m and the reflector gradually moves up to control the reactivity leading to bum-up. The axial power distribution changes as shown in Fig. 3 according to the reflector position. 

Fig. 3 Axial Power Distribution as a Function of Reflector Position... 

Relative Axial Power Distribution for Equilibrium Cycle... 

Figure 4.2-11 shows the axial power distribution for an equilibrium cycle. This distribution indicates 65 percent of the power in the top zone, 25 percent in the middle zone, and 10 percent in the bottom zone. This distribution is expected to minimize peak fuel temperatures. The selection of the active core height of ten fuel elements was made to yield a maximum power rating while maintaining an axial power shape that remains stable with burnup and stable to axial xenon transients. 

RELATIVE AXIAL POWER DISTRIBUTION FOR EOUILIBRIUM CYCLE... 

Ultimately, bundles can be removed from the channel during on-power refueling and reshuffled, and reinserted in any order. This axial shuffling provides nearly unlimited capability for shaping the axial power distribution, if necessary. Adjuster rods are located interstitially between fuel channels, in the low-pressure moderator. They flatten the power distribution with NUE fuel, a function not required with enriched fuel, and provide xenon-override capability. With an enriched fuel, the adjuster rods can be easily replaced, if desired, or even eliminated, providing further flexibility in accommodating advanced fuel cycles. 

Each rod cluster control assembly consists of 24 absorber rods fastened at the top end to a common hub (or spider) assembly. The rod cluster control assemblies are used to make relatively rapid changes in reactivity and to control the axial power distribution. Figure 4.2-9 of Reference 6.1 shows a rod cluster control assembly, with Figure 4.2-10 giving detail of an individual absorber rod. 

A detailed treatment of the axial power distribution local heat transfer, two-phase mixture dynamics, and coupling with the rest of the reactor coolant system requires the use of complex computer models. Figure 3.2-1 compares the predictions based on Eq. (3.2-1) with code calculations for a Zion station blackout scenario compounded by failure of turbine-driven auxiliary feedwater (the so-called TMLB scenario). As indicated by the comparison, the exponentially decreasing function defined by Equations 3.2-1 and 3.3-2 is a reasonable approximation for the water level in the core region during this stage of the accident. 

In order to parametrically test fiie effect of different pre-acddoit conditions, in particular the reactor operation time prior to the acddent, calculations were performed for two diffarent axial power distributions in tiie core Namely, in addition to the power distribution obtained from the project sponsor, in which a typical end-of-life (higWy nonuniform, bottom-peaked) power disfribution was used, the APRIL.MOD3 code was also nm for a situation simulating the otitia end of possible al power profiles, in which a uniform axial decay heat generation rate was assumed. 

The MCST of the fuel channel groups range from about 390 to 650°C. The hot region with MCST greater than 570°C is relatively limited at BOC or MOC, but spreads to a greater part of the core toward the EOC. This is related to the gradual shift of the core axial power distribution from the bottom peak to the top peak due to control rod withdrawals. 

The bumup profile of the density reactivity coefficient of the equilibrium core is shown in Fig. 2.61 [9]. Although the calculation methods used in this chapter are not accurate enough to state the precise density coefficient values, the tendency of the density reactivity coefficient to decrease with the cycle bumup exposure can be seen. This decreasing trend is due to the increase in the core average density with the bumup from about 0.50 g/cm at the BOC to about 0.57 g/cm at the EOC. The gradual increase of the core average water density can be explained by the gradual shift of the axial core power distribution from the bottom peak to the top peak towards the EOC. As the axial power distribution shifts to the top peak, the axial... 

Due to the large coolant temperature rise in the core, a cosine distribution may not be the ideal axial power distribution for the Super LWR. From the viewpoint of reducing the fuel temperature and effectively cooling the fuel rods, a bottom peak distribution may be more suitable than the cosine distribution. A bottom peak power distribution can be attained by dividing the fuel into two axial enrichment zones as shown in Fig. 2.76. Compared with the middle peak design (for the cosine power... 

Nuclear hot factors are employed in the calculation to consider the effects of the power distribution in the core, including the radial nuclear hot assembly factor /, the axial nuclear hot assembly factorand the linear heat flux nuclear hot spot factor/p. The factoris defined as the ratio of the hot assembly power to the core average assembly power and is used to determine the power level of the hot assembly. The factor is defined as the ratio of the maximum planar power to the average planar power in the hot assembly and is used to describe the axial power distribution in the hot assembly. The factor fp is defined as the ratio of the maximum linear heat flux to the core average linear heat flux and is used to determine the power distribution of the hot channel and the power of the hot spot of the core. 

Two cases of the probability distribution are considered in the calculation. The normal distribution is used in case 1 and the uniform distribution is used in case 2 to consider the uncertainties of system parameters. In each case, different core power distributions at the typical bumups, BOC, MOC, and EOC, are considered by using the corresponding nuclear hot factors and the axial power distributions. 

The nominal values of the nuclear hot factors at different bumups, BOC, MOC, and EOC, are taken from the results of the three-dimensiraial design of the Super LWR and are listed in Table 2.12 [27], The axial power distributions in the hot assembly at BOC, MOC, and EOC under the nominal condition are shown in Fig. 2.89 [27]. 

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