Subcritical Experiments

The experiments were carried out in a 4-ft-dia by 5-ft-deep tank of water placed in the reactor room of the Hanford Thermal Test Heactor. The lattice frameworks were so placed in the tank as to provide effective infinite water reflection on all sides. iWo Or three 8-in.-long fuel elements were loaded into lAicite tubes and placed in the lattice framework. 

A pencil-size neutron source, 0.22 gm RaBe, was placed between the fuel rods at the center of the lattice during approach measurements. Three BF, counters were used to monitor the neutron multiplication. Two safety channels with control and trip circuits from the TTR were used in conjunction with lour safety and two control rods for each experiment. A fast water-moderator dump system was also provided. 

Loadings were taken to 96% of the critical mass as determined by inverse multiplication plots of the three BFj channels. 

The critical number of fuel rods, Nc, was obtained by a linear least-squares fit to the VN/CR-vs-N plot of the center counter. The fit was made to the region, for N between 85 and 96% of the critical number of rods. The poisonous effects of the center counter, source and holder, and safety rod guide tubes were evaluated during the critical approaches. 

The experimental results are given in the table together with the values of the minimum critical mass and maximum buckling for each rod size obtained by interpolation of 

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Many the lattices which were investigated at BNL as subcritical assemblies were also studied in critical assemblies at Westinghouse (Bettis). b some cases the techniques at the two laboratories were similar, and in other cases the techniques were different. There were also a few critical e]q >eriments at BNL. The subcritical experiments at BNL fall. into two classes miniature lattice measurements and exponential measurements. Values of e, p, and f were obtained in small assemblies (miniature lattices) which have a neutron multq>lication of about 3, and in which the central neutron spectrum is characteristic of a critical assembly. Values of M and B were measured in larger assemblies (exponential assemblies) in which the neutron multiplication is between 3 and 50. Thus, there are considerable data to compare reactor parameters as measured in similar lattices by subcritical and critical techniques. 

In the case of some measurements, it is possible to interpret the data in several different ways. For instance, measurements of migration area may be interpreted by age-diffusion, age, one group, or multigroup theory. Numerical values of M as computed by the different methods will vary coMiderably, although each will form a con-slstent set of reactor parameters in its own critical equation, hi addition, there appears to be agreement in values of M as measured by subcritical and by critical. methods. Thus, it is necessary to make any comparison between critical and subcritical experiments with a common theoretical interpretation. 

The scope of this subject is here limited to a comparison of the values of the ckling B and the excess infinite multiplication factor kx,- , which are variously derived from critical and subcritical experiments. These two Integral parameters can be measured more or less directly. They are about equally Important to the reactor physics design of low-enrichment graphite reactors, since the real question is the size of the assembly which gives a sufficient margin of excess effective k to allow the reactor to reach the planned operating conditions. 

The results of subcritical experiments and analytical correlations are described for PuOz-UOb rods moderated with H2O. The correlation is the first reported with this type of fuel, and the results are important as a check on the analytical techniques used in calculating such systems. 

Critical and subcritical experiments, in support of the Advanced ium Graphite Reactor fASGR) program, . have been performed to determine values of material buckling for graphite assemblies fueled with slightly-enriched (3.02 wt%), uranium carbide-fuel elements. These experiments provide the first data available for comparison with results of calculations. 

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. ... 

The inverse relaxation length, k, was obtained by static measurements in the Subcritical Experiment (SB) and combined with calculated values of M and the transverse buckling to produce a kgff (static). Pulsed experiments extracted the reactivity in dollars,, with (fozani, Garells-Russell, and Sjdstrand methods of analysis. 

Table I summarizes the results of these subcritical experiments. The systematic differences between static and pulsed determinations of k ff are probably due to the use of a calculated infinite-lattice migration area in k ff (static) and the presence of harmonic distortions in k f (pulsed). These differences decrease as the true critical condition is approached.

Subcritical Experiments with Mixed-Oxide Fueisof Plutonium and Uranium in Poisoned Lattices, C. L. Broum, L. C. Davenport, E. D. Clayton (PNL)... 

Experimental or laboratory tevetopment of tee criticality concept is rather complete in the graduate courses using computer simulation, subcritical experiments, or crittcal mass experiments, in undergraduate laboratories, the subcritical experiment is most frequently used (some 60%) with computer simulation used next often. 

TRIGA Subcritical Experiments in Support of the ko8 Meter, H. Ocampo, S. H. Levine. M. A. Schultz (Penn State Univ)... 

It has been proposed that we measure the k of a spent fuel assembly by using a subcritical assembly wherein a neutron source multiplication measureihent, made with the fuel assembly of an unknown k , is compared with the same measurement performed with a fuel asisembly of known k . The dwice in which this measurement is performed is called a k >meter, and it requires that the keff of the initial suhcriti-cal assembly be measured. The purpose of this paper is to report the results of subcritical experiment performed with the Pennsylvania State University Breazeale TRIGA reactor to determine the keff of a subcritical assembly using the asymmetric source method (ASM). Similar experiments would be performed with the k -meter to establish the keff of the initial subcritical assembly. 

Systematic and detailed investigations of electrochemistiy in autoclaves started in the 1970s. Piraieering work has been done by the groups of D. D. Macdonald [205—222] and of A. J. Bard [223—231]. Subcritical experiments in water [213—215, 217, 222, 225, 235] as well as in non-aqueous solvents are not different from ordinary electrochemical studies in open cells. 

Experiments were conducted during the Metallurgical Project, centered at the University of Chicago, and led by Enrico Fermi. Subcritical assembhes of uranium and graphite were built to learn about neutron multiphcation. In these exponential piles the neutron number density decreased exponentially from a neutron source along the length of a column of materials. There was excellent agreement between theory and experiment. 

Destruction of the explosives RDX, HMX, and TNT has been examined using subcritical water in both laboratory- and pilot-scale experiments. In contaminated soils at 150°C, considerable amounts of TNT remained in the soil after reaction for 5 h, and of HMX for 2.5 h. In the pilot-scale experiments, heating at 275°C for 1 h accomplished complete destruction of RDX and TNT, and ca. 98% destruction of HMX (Hawthorne et al. 2000). 

Seeley (S21) discounted this mechanism on the basis of flow visualization studies. However, the experiments were at Re < Re given by Eq. (10-44), and thus appear to be in near-subcritical flow. 

A widely used method for assessing supercritical fluid phenomena consists of comparing physical and chemical behavior above the critical point with corresponding behavior in the subcritical liquid. Because this approach (unrealistically) seeks to observe discontinuous behavior between states, the results of such experiments are often ambiguous. In the present study, we have compared the photoisomerization of trans-stilbene in subcritical and supercritical C02 and, to model liquid behavior, we have also carried out these isomer-izations in cyclohexane. In all three systems, the effects of temperature and concentration on the cis/trans ratio were compared and, for the C02 systems, the effect of pressure on this photostationary ratio was also probed. The results from these experiments are shown in Tables I and II and are plotted in Figures 4 through 6. 

Table 4 compares the end-of-run coke laydown, BET surface area, and pore volume for identical runs with the 0.71 mm cylindrical extrudates (as received) and size-reduced catalyst particles. For the subcritical density experiment, the end-of-run coke laydown had increased 17%, surface area had decreased 13% and pore volume had decreased 9% for the smaller... 

The simulated and experimental variations of the end-of-run (i.e., 8 hr.) isomerization rates with density are compared in Figure 1. Details of the experiments are provided elsewhere [2, 3]. At subcritical densities, the extraction of coke precursors is insignificant. Hence, an increase in the concentration of the hexene and coke precursors (i.e., oligomers) leads to lower isomerization rates. At near-critical densities, the extraction of coke precursors becomes significant. Hence, the isomerization rate increases. Both the experimental and simulated rates show a decreasing trend when the density is increased from near-critical to supercritical values. This is attributed to pore-diffusion limitations as the fluid changes from gas-like to liquid-like. Above 2.0 pc, the isomerization rate increases with density as the ability of the reaction mixture to extract the coke precursors increases. 

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