Analyses for Criticality Safety

The accident evaluation assumed all the available space within the unit to be occupied by a homogeneous mixture of UO2 and water with the maximum enrichment of 4.0 wt% in U. A uranium density of 2.4 gU/cm was used because it resulted in the highest K-effective 

There are two factors that contribute to the variations in K-effective shown in Table L The first is the fact that KENO calculations are statistical in nature. Varying the number of neutrons per generation results in statistically independent calculations. This is the reason for the variations observed between the fourth, fifth, and sixth calculations. The other factor is the spatial distribution with which the first few generations of neutrons are started. Differences due to two slightly different starting distributions can be seen by comparing the first two calculations. 

These data show that the K-effective from any one calculation may vary from the actual value by a significant amount, b fact, another paper on a similar subject also illustrates how this pitfall can occur when the fission density distribution does not adequately account for the distribution of fissile material. The system discussed here illustrates a situation where the fission density distribution is strongly influenced by the presence of a lumped neutron poison. This can also contribute to uncertainties in the calculated K-effectives. The user can most easily recognize the potential for errors of this kind by comparing the relative fission densities in localized sections of the system with the fissile material distribution. The optimum condition would occur when the two distributions are the same, hi fact, this should be considered any time the fissile material is distributed in an unsymmetrlc or complex manner. This also must be considered any time there are lumped-neutron absorbers in the system. 

K-EFFECTIVE 0 ff 6ENERATI0NS0 h1pER GFMFRATinN START TYPF DROPPED (TNFRATIOMS KENO VERSION 

PETRIE and N. F. CROSS, KENO IV-An Improved Monte Carlo Criticality Program, ORNL  

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