Validation with Applicable Critical Experiments

Fortunately, there are now criteria for acceptance of a model given in the ANSI Standard N16.9 The principal requirement for validation is the correlation of results from the proposed model with the results of criticality experiments, or with the results of another validated method, to establish a bias for the model. The bias serves to normalize the model over the applicable range of its parameters so criticality will be predicted within the ev uated uncertainty of the bias.. Following validation, (he parameters arc adjusted, or limited such that applications of the model produce specifications for systems known to. result in subcriticality. The.procedure contributes to bases for standards in the field of nuclear criticality safety. 

Numerical accuracy and efficiency are critical issues for the DML model to be accepted in engineering applications. FFT-based algorithm has been introduced to speed up the computations, and attempts have been made to validate the model by comparing the results from the present DML model with those from other sources of numerical analyses and experiments. 

In like manner, toe program with U(5) rods of several dianmters has been extmided, beyond toe arrays made critical with water and with dilute boron solution as moderator and. reflector, to arrays critical in dilute (about 300 g U/llter) aqueous iiraiqrl fluoride solution, to submerged arrays irdOected by lead on four sides, and to submerged arrays reflected by depleted uranium on one side. Some of these lattices also contained a solid neutron absorber, either stainless steel, Boral, or cadmium. The latter experiments have applicability to shipping casks for reactor foel elements for which validated calculational methods are needed. 

Conclusions Quantitative analysis, including orientation measurements, has been shown to be straightforward when the formalism based on Harrick s weak absorber approximation is applied. For thin adsorbed layers, such as the DPPA monolayer under discussion, the results are fairly good. Application to bulk materials may introduce systematic errors as discussed above. If the weak absorber approximation is still to be applied, one should take care to work with an angle of incidence which is at least 15° larger than the critical angle, in order to avoid significant band distortions. In many cases it is possible to use quantitative data from transmission experiments to check the validity of the formalism applied to ATR data. 

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