Flux flattening

The development of commercial plants using gamma radiation as the ionizing medium has centered around cobalt 60 because it is the cheapest source of gamma radiation power combined with a reasonable half-life of 5i years. In addition, it is virtually unaffected by radiation and can be used up to temperatures of nearly 1000°C. A low cost of production can sometimes be gained by using cobalt as a flux flattener, i.e., for controlling the reactivity of the core of a nuclear reactor. 

The cobalt would go into the control rods of Hydro s CANDU reactors, where it would serve as a flux flattener as it was irradiated. The shape of the LSA cobalt-60 employed in irradiators differed from the pellets used for HSA cobalt-60 teletherapy sources. It was formed into slugs, then loaded into pencils, which were bundled and then distributed along the length ofthe rods. The bundles could be removed durir regular maintenance shutdowns. Once CPD and Hydro worked out pricing and a production schedule, they signed a deal. 

How important is flux flattening Just prior to or immediately after "turnaround" ... 

The radial neutron flux flattening achieved was one of the best among operating reactors. The radial neutron flux irregularity coefficient, K, for both units was 1.28—1.30, while the design values were = 1.46 for Unit 1 and = 1.24 for Unit 2. 

Flux is independent of pressure when the process flux is much less than the water flux K Km)- If Pg > Pm > process is limited by the membrane water flux and flux would flatten out at low concentrations of soHds (see Fig. 6). 

The mass flux in the spray scales with liquid metal flow rate. Gas pressure tends to narrow the spray whereas melt superheat tends to flatten the spray)3] By changing the process parameters and/or manipulating the configuration and/or motion of the spray, the mass distribution profile can be tailored to the desired shape. For example, a linear atomizer produces a relatively uniform mass distribution in the spray. The mass flux distribution in the spray generated with a linear atomizer has been proposed to follow the elliptical form of the Gaussian distribution)178]... 

Figure 14.1 Flattening of a ruled surface h(x) by surface diffusion. The normal velocity is proportional to the accumulation of flux. The rate of vertical motion dh/dtt is related to the normal velocity vn by the local geometry of the surface.

Different combinations of spatial finite difference formulas were tried to determine the best set for our system of equations. The two point upwind formula was found to be best for the solids component molar fluxes. The low order formula was used because most of the gasifier reactions turn off abruptly when a component disappears and this creates sharp discontinuities. Higher order formulas tend to flatten out discontinuities, and in some cases, this causes material balances to be lost which then leads to numerical instability problems. Maintaining component material balance is an important aid to preserving numerical stability in the calculations. The low order formulas minimized these difficulties. 

Figure 13.2d. But near the interface flattening is less in material 1 and greater in material 2, so that the interface itself moves to the left with respect to the remote elements. In fact, rather obviously, the volume of space that the interface sweeps through equals the volume of material that crosses the interface by diffusion. The varying intensity of the diffusive flux must be somewhat as indicated in Figure 13.2e.

When residual stresses are considered, the stress distributions flatten considerably and become almost uniform at in situ length and pressure. Figure 57.10 shows the radial stress distributions computed for a vessel with /3 = 1 and /3 = 1.11. Takamizawa and Hayashi have even considered the case where the strain distribution is uniform in situ [9]. The physiologic imphcations are that vascular tissue is in a constant state of flux. New tissue is synthesized in a state of stress that allows it to redistribute the internal loads more uniformly. There probably is no stress-free reference state [7,8,17]. Continuous dissection of the tissue into smaller and smaller pieces would continue to relieve residual stresses and strains [ 10). 

Each zone shall be so defined as to provide information representative of the neutron flux levels sdong the total socleil direction of the reactor within the flattened portion of the reactor.. Figure 2, Division of Reactor Into Zones, shows an example of a two, four, nine and sixteen zone reactor. A single high level system may be used If It divides the reactor Into nine or more zones and has a response time of seconds or less with fixed or percentage level trips. 

Radial power variations across the core are undesirable, and devices such as flattening of the radial flux or variable orificing of coolant channels may be used to increase the total power output. 

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