Critical Loading Experiment

Describe primary and secondary neutron sources, critical loading experiment, and addition of a neutron source to a critical reactor. 

In the critical loading experiment, fuel elements are added around a neutron source. As fuel is added, K increases from zero and multiplication of neutrons occurs in the mass being assembled. Fission chamber counts increase as successive fuel elements are added. Inverse multiplication, 1/M, decreases toward zero, At the point, i/M=0, the reactor is critical. 

During a critical loading experiment, the output of two detectors produce contradictory results. The inverse multiplication plot for detector l slopes downward while the plot for detector 2 balloons outward. Why are the two not consistent and why is neither a straight line ... 

As noted from general experience, except for specific large inductive loads such as of furnace or rectifiers, the fundamental content of the load current is high compared to the individual harmonic contents. In all such cases, it is not necessary to provide a filter-circuit for each harmonic unless the current is required to be as close to a sinusoidal waveform as possible, to cater to certain critical loads or instruments and devices or protective schemes operating in the system, where a small amount of harmonics may lead to malfunctioning of such loads and devices. Otherwise only the p.f. needs be improved to the desired level. Also to eliminate a parallel resonance with the... 

Fig. 5. Results for G c calculated from different thermomechanical loading experiments in comparison with the critical value of G c,mm = 0.17 N/mm and with an indication of the cooling rates...

Under the action of multiaxial loads, crack precursors wifi develop in a manner that reflects their particular loading experiences. It may be assumed that a crack precursor is present at every point, and that the crack precursor may occur in any orientation. By computing the rate at which each possible precursor will develop, it can be determined (Mars, 2002) which particular precursor(s) will develop the fastest. The orientation of this precursor identifies the critical plane on which cracks will develop, which is needed in order to accurately estimate fatigue life. 

Eigure 7.14 shows that, at ultimate failure, the buckling load of the noncooled specimen approached the applied load of 145 kN after 43 min, which represents an underestimation of the measured time-to-failure (49 min) of 12%. The buckling load of the water-cooled specimens approached a value of 1007 kN, as shown in Figure 7.15, which represents the buckling load of a specimen that completely lost one face sheet. At the end of the longer experiment (120 min), however, this critical load was still exceeded by almost 70%. 

Calculations of core cell bum-up in the N2 PWR were performed by the spectral code CETERA [16]. The fission product and actinide activities were estimated using the RECOL [17] library data base, which was generated on the bases of the latest versions of the evaluated nuclear data files, ENDF/B-V, with corrections based on the results of critical experiments [18]. The criticality problem was solved for a realistic 3-D geometry model of a TFC by Monte-Carlo with RECOL and checked with MCNP [19] for fresh fuel load. One-group cross-sections were prepared for bum-up calculation of critical loads of both fresh and spent fuel and input to ORIGEN-2 [20] for detailed radionuclide content calculations. 

A comparison summary of the experimental results is shown in Table I. The corrected critical loadings differed by about 12%, which ostensibly can be attributed mainly to the partially nonhomogeneous distribution of fuel in the mockup experiment. A rough statistical analysis of the nuclear path-length variations in the mockup with respect to the optimum conditions in the gas configuration show that a 5 to 10% difference in interaction rate can be expected. 

A two-dimensional diffusion theory code in four enef . groups" was used to predict the critical loadings. These critical predictions were documented previous to the experiment. When the critical loading was achieved it was with the predicted loading of 7 fixed fuel elements,... 

What is the effect of geometric changes in the core shape in a critical experiment on the predicted critical loading Illustrate with data from this experiment. 

The excess measure of extra fuel lo needed to be critical in experiment insertions, f defect during operation, fission product poison, perpetual equilibrium be compared to the ten day samarium-149 is not cons the core s excess multip core fuel loading. 

The applicability of the elastic critical load, as in Eq. 4, to pile buckling failure is an important factor. Experiments show that the actual failure load of a slender column is much lower than that predicted by Eq. 4. Rankine (1866) recognized that the actual failure involves an interaction between elastic and plastic modes of failure. Lateral loads and inevitable geometrical imperfection lead to creatifMi of bending moments in addition to axial loads. Bending moments have to be accompanied by stress resultants that diminish the cross-sectimial area available for carrying the axial load thus the actual failure load is likely to be less than the elastic critical load,... 

This means that for a critically loaded PA 6 chain segment with L/Lq = 1.1 a temperature increase of 10 K can be compensated by a decrease of strain of only 0.23%. The result of this calculation agrees fairly well with the average value of 0.3%/10 K derivable from the experiments of Johnsen and Klinkenberg [11]. For preoriented PA 6 fibers they observed that at —60 °C a sample strain of 14.6% led to the same number of (10 ) free radicals as a strain of 12.2% at +20 °C. 

In the preparation of this edition, we are indebted for much help to many of our colleagues, and in particular to Dr. P. Sykes, Dr. F. B. Kipping, Dr. P. Alaitland, Dr. J. Harley-Mason and Dr. R. E. D. Clark. We have maintained the standard which was self-imposed when this book was first written, namely, that all the experiments in the book had been critically examined, and then performed either by the authors, or under their super vision. The heavy load of work which this has involved would have been impossible without the willing, patient, and very considerable help of Mr. F. C. Baker and Mr. F. E. G. Smith. 

To answer questions regarding dislocation multiplication in Mg-doped LiF single crystals, Vorthman and Duvall [19] describe soft-recovery experiments on <100)-oriented crystals shock loaded above the critical shear stress necessary for rapid precursor decay. Postshock analysis of the samples indicate that the dislocation density in recovered samples is not significantly greater than the preshock value. The predicted dislocation density (using precursor-decay analysis) is not observed. It is found, however, that the critical shear stress, above which the precursor amplitude decays rapidly, corresponds to the shear stress required to disturb grown-in dislocations which make up subgrain boundaries. 

It can be seen from equation 13.136 that the critical speed of a centrifuge will depend on the mass of the bowl and the magnitude of the restoring force it will also depend on the dimensions of the machine and the length of the spindle. The critical speed of a simple system can be calculated, but for a complex system, such as loaded centrifuges, the critical speed must be determined by experiment. It can be shown that the critical speed of a rotating system corresponds with the natural frequency of vibration of the system. 

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