Nuclear data, uncertainties

J. J. Sdimidt, Influence of nuclear data uncertainties on the theoretical prediction of Doppler coefficients in fast and intermediate reactors. EANDC(E)-46 L (1963). 

At present, IAEA SAL estimates at 0.4% the total uncertainty (Is) of its determinations of Pu abundance in safeguards samples by a combination of alpha and mass spectrometry taking into account the contributions of short and long term measurement reproducibility, calibration, and nuclear data uncertainties. 

Comparisons of experiments and calculations of the conventional cores with ENDF/B-IV data generally agreed with such comparisons for smaller cores, in spite of an increase in sensitivity of some parameters to nuclear data uncertainties. The csdculated keff for ZPPR-9 of 0.985 was essentially the same, power distributions were predicted to within 2% in core zones, and sodfum-void calculated-to-experimental ratios were similar to values observed before. Large control rod interactions were observed up to 50% for siz outer-ring rods compared with 20% in the smaller cores, but were calculated to within a few percent. Sensitivity analyses were made for 350-, 700-, and 1250-MW(e) core sizes. This has ted to a quantitative understanding of Ae way in which bias factors and uncertainties obtained from... 

All of the data within a nuclide library - energies, emission probabilities and half-lives - have an uncertainty and, ideally, it should be possible to incorporate all of those uncertainties into the library. This is not always the case and it may then be necessary to account for nuclear-data uncertainties by increasing the uncertainty on the final result by an appropriate amount. 

Uncertainties in reactor parameters caused by nuclear data uncertainties. 

Smith, D. L., Probability, Statistics, and Data Uncertainties in Nuclear Science and Technology, OECD/NEA, Vol. 4, American Nuclear Society, La Grange Park, Illinois, 1991. 

Uncertainties in physical constants may or may not be significant, depending on the measurement. Half-lives in particular tend to be well known and usually contribute only negligible uncertainty to the result of a radioanalytical measurement. Errors in radiation emission probabilities may contribute additional uncertainty to some measurements. Uncertainties for both categories are available from tables of isotopes and the public Web site of the National Nuclear Data Center at Brookhaven National Laboratory. 

Solving Eq. (3) for the reference critical configuration seldom produces exactly Aq = 1 even though the reactor is known to be critical. This is due to different approximations made in the physical and numerical models, as well as to uncertainties in the nuclear data used for the calculations. For this reason, the static reactivity is often defined, in practice, as = Aq — A. The corresponding perturbation expression is... 

The configuration and other important characteristics of a fast reactor core design are strongly influenced by the nuclear parameters of the fuel and other core materials. It is important, therefore, to have some assessment of the effect of uncertainties in the basic nuclear data. 

An extensive survey was carried out to identify those uncertainties in nuclear data which were expected to be of significance from the point of view of fast reactor core design. This work was presented in papers by Greebler et al. 8-10) in this study, the discussion is restricted to the effects of only those changes in nuclear parameters which appear to be of greatest importance, and for which strong experimental evidence exists. 

The significance of these predictions is shown in Table XV. If the nuclear data are adjusted in line with this experimental information, the design calculations show that at least 50% of the BeO, which was included in the core to soften the neutron spectrum and to increase the Doppler coefficient, can be removed and still meet the Doppler and sodium loss criteria of the reference core. The fissile inventory required decreases by about 3%, and the breeding ratio increases from 1.18 to about 1.25. This results in a decrease in the fuel cycle cost of about 0.1 mill/kW-hr. If one assumes a favorable combination of nuclear data within the limits of uncertainties reported by Greebler (5) and, furthermore, if the safety criteria are relaxed to allow a calculated Doppler effect (T dkjdT) of — 0.004 (with sodium out) and a positive total sodium loss reactivity effect between 1 and 2, all of the BeO can be removed... 

The recoUless fraction decreases with increasing y energy (note that in the recoilless case Ey = Eq except for the energy uncertainty expressed by O Eq. (25.2)). This means that isomeric transitions with Eq > 200 keV are not practical for the purpose of Mossbauer spectroscopy, since the/ factor would be too small. (The Mossbauer effect has been observed with over 100 transitions of near 50 elements shown inO Eig. 25.4. The nuclear data for the Mossbauer transitions are summarized in O Sect. 36.1 of the Appendix of this Volume.)... 

Tbe ttU-range Monte Carlo program RECAP has been used in a consistent analysis of physics parameters measured in seven HtO-moderated uranium lattices. The motivation has been to test input nuclear data, which cmistitute the major source of uncertainty. The fi owing parameters are considered epithermal-to-thermal ratio for U capture (p ) and for tJ fission (6 ), ratio of -fission-to- fission and eigmivalne... 

Although some experiments can be used as benchmarks for both areas of application, in general the requirements of the two categories are quite different. Nuclear data benchmarks consist of simple-geometry, simple-configuration experiments dependent on as few components (cross sections) as possible. Such experiments are often designed to accentuate sensitivities to cross-section features of interest. Their accuracy requirements are very high, since to provide usefUl information, the experimental uncertainties must be lower than the combined uncertainties of the nuclear data and methods of analysis. 

The value of the analysis result ultimately depends upon the value of the gamma-ray emission probability. Reputable nuclear data tables will provide you with an uncertainty on these values. 

The assessment of the uncertainties in the prediction of reactor properties is an important requirement, both for economic and safety reasons. Allowances must be made in the design and operation of reactors to cover uncertainties by introducing suitable margins. A very high level of confidence in the safety aspects of reactors must be achieved. Uncertainties in nuclear data contribute to the overall uncertainty. Consequently the uncertainties in the data and the sensitivity of calculated reactor parameters to these uncertainties must be estimated. The estimation and representation of uncertainties in evaluated differential cross-sections is a complex problem. However, since the required information is the imcertainty in reactor neutron spectrum averaged values of cross-sections, or in ratios of such averages, the required... 

The International Reactor Dosimetry File, IRDF, produced by a cooperative effort organised by the IAEA Nuclear Data Section, contains a set of recommended cross-sections and associated uncertainty data. The current version is described in IAEA-NDS-141 Rev. 2 (1993). There is also a dosimetry file produced at Obninsk. 

The imcertainties of calculated reactor parameters calculated may be divided into two components, the first resulting from uncertainties in the nuclear data, and the second from approximations made in the nuclear data processing and the reactor calculations. 

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