Moderator/fuel ratio

The effectiveness of the moderators and the optimum moderator/fuel ratios previously presented have been based only on the use of as a fuel. On the basis of cross-sectional data for the fissile nuclides, it is to be expected that the optimum moderator/fuel ratio would be larger for Pu fuel and smaller for fuel. In addition, for heterogeneous, low-enrichment, or natural-uranium reactors it is desirable to achieve a large resonance escape probability, and, consequently, the optimum moderator/ fuel ratio is even larger than that previously indicated. Typical initial values for the C/U ratios in several gas-cooled reactors can be found in Table I. 

The value of k increased with increasing lattice pitch. Similar shapes have been observed for the rod cluster and tubular elements. The moderator/fuel ratio does not, however, appear to be a convenient parameter to employ in obtaining a correlation. 

The critical mass (and plot versus moderator-fuel ratio). 

Small moderator/fuel ratio means that neutrons are not optimally thermalized before interacting with adjacent fuel plates. This undermoderation results in even less thermalization as temperature increases negative reactivity negative temperature and void coefficients. Large moderator/fue1 ratio means that optimum thermalization occurs somewhere in the water gap. As temperature increases this flux peak shifts to adjacent fuel positive reactivity overmoderation positive temperature and void coefficients. 

Increased moderator/fuel ratio. The moderator/fuel ratio is substantially increased, mainly through the implementation of six water rods (Figure 4.7) and the reduction of the original fuel rod outer diameter to a smaller value. This leads to increased reactivity for this fuel design. 

Moderate fuel cost, high weight ratio ca. 14 /ktv-hr for entire, coll including compressed oxygen storage... 

The turbocompressor concept using self-sustaining compliant foil air bearings has demonstrated low power consumption and moderate pressure ratio at low flow rates in a compact lightweight package. Predicted power consumption, which includes the predicted effects of the VNT variable turbine nozzle can be further reduced if increased expander/turbine temperatures can be provided by the fuel cell system. The... 

Prismatic graphite-block fuel with traditional refueling. The LS-VHTR would be fueled with prismatic fiiel and refueled when shut down. This particular fuel geometry provides a large latitude for the reactor core designer in the choice of (1) fuel-moderator-coolant ratios and (2) core geometry. 

The neutronic effectiveness of a moderator material can be expressed in terms of the conversion potential of a mixture of fuel and moderators in an infinite medium. This approach has been used in a relative comparison of HjO, BeO, and C with fuel which has appeared in a progress report on BeO reactor studies (26). In this evaluation, the excess production of neutrons per neutron absorbed in the fuel has been calculated for homogeneous mixtures of fuel and moderator having different moderator/ atom ratios, i.e.,... 

It can be seen in Table V that the excess production of neutrons per neutron absorbed in fuel increases with the moderator/U ratio to some optimum value, if the calculation is carried sufficiently far. On the basis of this comparison, it appears that the optimum conversion potential for BeO is about 11 % greater than that for C, and the optimum for C is about 8 % greater than for HjO. Although the results for DjO have not been included in the reference, it would be expected to lie between C and BeO. However, in the case of D2O and H2O, pressure tubes or calandria, as well as insulation, would be required in a gas-cooled reactor to separate the coolant and the moderator, and the neutron losses in the tubes would reduce the excess neutrons significantly. 

The importance of the moderator cost to the power cost for a nuclear plant can be seen by the very approximate data indicated in Table VI. In this table, the cost contribution of the moderator material has been calculated for reactor conditions typical of some gas-cooled reactors. The capital cost of the moderator in dollars per installed kilowatt(e) is related to the unit cost of the moderator, the moderator/fuel weight ratio, the reactor specific power, and the plant efficiency by... 

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...

A series of thermal and epithermal critical eq )erl-ments was performed to demonstrate the reactivity holddown Of DiO in conventional light water lattices and study the nuclear properties of,lattices moderated by DiO-HiO mixtures. The fuel rods consisted of 4%-enriched UC swaged In stainless steel, 0.444 In. ID 0.475 in. OD and 66.7 In. active length. Each rod contained 56.61 gms U-235. The rod to moderator volume ratio in the lattice was 1.0 in all cases. The experiments were performed in a 60-in. diam aluminum tank so that a finite radial reflector existed. 

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. 

Pitch (cm) Water-to-Oxide Volume Ratio. Fuel-to- Cadmium Ratio Moderator to-Cadmium Ratio Bare-Moderator -to-Bare-Fuel Ratios t Thermos Number Density Thermos U-235 Fission B692/RP Flux... 

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. 

CriUcaiity calculations have been performed for various hexagonal lattices of rods with a plutonium enrichment of 2 wt% in UOt (natural), isotopic ratios of 8 to 26% at moderator/fuel volume ratios of 1.5 to 9.8. The calculations, utilizing the FORM-TEAM-TEMPEST LASER, and THERMOPILE programs have been compared with data obtained from subcritical experiments at the Battelle Northwest Laboratory. ... 

From the results of the calculations given in Table 1, it is seen that while LASER predicts criticality to within 1 to 2% for a wide range of lattice pitches, FORM-TEAM-TEMPEST calculates values of that increase with lattice pitch, varying from an underprediction of 1% at a moderator/fuel volume ratio of 1.5 to an overprediction of 4% at a moderator/fuel volume ratio of 9.8. 

The optimum moderation condition for uranium oxide pellets in water was determined for three different lump density systems. Optimum moderation for this study was defined as that combination of pellet diameter (POD) and water-to-fuel ratio (W/F) resulting in the maximum material buckling (Bm ), and hence the minimum critical dimensions. This point does not result in optimum reactivity, which is defined as the condition producing the greatest infinite multiplication (k, ). The data to determine these optimum moderation conditions were generated using the computer codes GAMTEC (Ref. 1) and THERMOS (Ref. 2). The results of these determinations are shown in Table I. 

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.

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 ... 

Now consider a moderator-fuel mixture in which the ratio Nf/Nm is so low that the fast fissions may be completely neglected, that is, in Eq. 

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). 

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