Moderator to Fuel Ratio

Geometrical Buckling vs. Degree of Clustering for the Moderator to Fuel Ratio of a Uniform 3/4-in.-Triangular Lattice. 

Fig. 1. Natural Uranium Metal Tubes in D>0 Material Buckling vs Moderator-to-Fuel Ratio...

The diameter of the assembly was increased from 27 to60 in. The height was 48 In. The 848 natural uranium fuel elements were clad with 0.060-in. aluminum and aligned in a square array to give a moderator to fuel ratio of 6.46 1. The entire assembly was at a temperature of 180 1 2 F. 

Fig. 1. Effect of Moderator to Fuel Ratio on Differences Between Exponential and Critical Buckllngs for Natural U Rods in DgO...

The new experiments (Fig. 1) include an assembly stacked on a 30-in.-square base with a 6-in.-square void centered on the vertiotl axis of symmetry and extending the full 2Q.6 t 0.2 in. critical height of the stack. This assembly has 8-mil oralloy (93.2% U-235) foils arranged in pianes separated 1-in. blocks of BeO to give an effective BeO. density of 2.86 gms/cm and a moderator-to-fuel ratio, BeO U-235 - 247. 

A series of experiments with hydrogen-moderated plutonium systems has been performed in the Physical Constants Testing Reactor (PCTR). The boron concentration required to reduce the infinite multiplication factor, k , to unity was measured for various Pu-240 concentrations, fuel plate thicknesses, and moderator-to-fuel ratios. The measured values of the boron concentra-Uon were compared to theoretical predictions to check the accural of calculational methods and parameters. The principal components for these experiments were Pu-Al (M wt% Pu), pure polyethylene, and borated lyetlqrlene (0.90 wt% natural boron). All three materials were in the form of disks, 1.960-in. in diam and 0.020-in. thick. 

H Reactor la a graphite-moderated water-cooled theroal reactor of the nfoz d type It differs somewhat from the earlier reactors principally in moderator-to-fuel ratio enrichment and operating conditions. The physics discussion ill he limited to the Phase I operatloia (plutonium production only) of N Reactor ... 

If one now examines the resonance escape finds that there is a decrease in the resonance ty as the moderator to fuel ratio decreases. The not serve well as a moderator and the neutrons time to slow down. They will lose very little ollision. It will be impossible for them to absorption peaks, so they will be absorbed and ssion chain reaction. However, as more and more are added relative to the amount of fuel it becomes easy for neutrons to slow down to ithout encountering a resonance absorber while at ergy level. The probability of resonance escape... 

The fast fission factor tends to be greater as the moderator to fuel ratio is reduced in a reactor. However, it does not exhibit a significant variation with the %20/ fuel 2is is shown in Figure 6.3(a). 

If one now considers the product of all the six factors comprising K ff and plots K fp versus moderator to fuel ratio, Figure 6.3(b) results. At low moderator to fuel ratios, is small principally because P is small. At... 

The operating range of the moderator to fuel ratio is determined by the design of the reactor core. From Figure 6.3(b), it can be seen that at 70 F, an increase in temperature results in positive reactivity addition which means that the moderator temperature coefficient is positive. 

How does vary with moderator to fuel ratio ... 

Overmoderation means that the moderator to fuel ratio is greater than optimum for proper moderation and interaction between fuel rods. Primarily because of neutron absorption in water and a decrease in f, Kgn is less than maximum. An increase in moderator temperature or voids inserts positive reactivity. 

Keff varies with moderator to fuel ratio as shown in (a) At operating temperature of 100 F, would the... 

Lower coolant volume fraction Neutron lifetime 1ms kgff increases with higher moderator to fuel ratio (undermoderated in design region)... 

Also, IRIS core designs capable of 8-year refuelling period have been developed using UO2 or MOX fuel with fissile content increased to 7-10% [11-15]. This is facilitated by the variable moderation approach , whereas the moderator-to-fuel ratio is increased with the increased fissile content, to achieve adequate neutron thermalization. 

When the MZFR core was about to be designed, small fuel rod diameters were chosen, because, for a given moderator to fuel ratio, they will result In a high power density and a small core. Today, the limitations on vessel size are no longer so severe, but, it is the capital investment, particularly due to the heavy water Inventory, as well as to some extent to the price of the reactor vessel and the fuel which favour a core of high specific power. The most effective means to achieve this are 37-rod bundle clusters and fuel rods of small diameter. 

Moderator and coolant Number of coolant channels Lattice pltchj hexagonal Moderator to fuel ratio Coolant to fuel ratio Heat rating... 

Selection of Moderator-to-Fuel Ratio and Uranium Enrichment... 

Figures D-2 and D-3. Furthermore, the relative temperature expansions have another interesting effect. Assuming the fuel has an average expansion coefficient of aluminum (2o x 10 K ), while aluminum, stainless steel, or a molybdenum alloy cladding has average thennal expansion coefficients of 26 x, 6 x, and 5 X 10 K respectively, then as the temperature rises, the axial linear fuel density varies differently inside each of the three cladding materials. Correspondingly, the axial linear moderator density also varies in each of the three clad materials. Both of these linear density changes affect the moderator-to-fuel ratio and, hence, the degree of reactivity control that can be obtained from the moderator temperature coefficient.

The numerical value of p, which is always less than one, tends to increase with fuel enrichment, fuel lumping, and moderator-to-fuel ratio. Enrichment increases p by reducing the amount of Because uranium... 

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