The fission-yield data sets in the most recent evaluations are listed in Table 7. Thermal (T) yields cover all measurements at 0.025 eV, or in well-moderated thermal reactor spectra. Fast (F) yields include all measurements in fast reactor spectra (mean neutron energies of 150-500 keV) and fission neutron spectra (mean energies around 1 MeV and above). H means neutron energies around 14-15 MeV, while S stands for spontaneous fission.

Measurements of the energy dependence of fission yields are too scarce to derive systematic trends and develop a semi-empirical model for obtaining reliable predictions; many more systematic measurements are required before a reliable model can be developed. Therefore, ‘thermal’, ‘fast’ and ‘high’ (around 14 MeV) yields continue to be evaluated for data files and are used in applied calculations.

(a) The US evaluation has increased from 10 to 60 yield sets, each yield set consisting of cumulative and independent yields (total of about 132,000 yield values and their uncertainties). Corrections have been applied, models have been used to estimate unmeasured yields, and adjustments have been made to all yield sets in ENDF-6 format (England and Rider, 1994).

(b) UKFY2 includes 39 yield sets (UKFY3 is in the process of being assembled, and also contains 39 sets of cumulative and independent yields). Both UKFY2 and UKFY3 are in ENDF-6 format (James et al, 1991a, 1991b and 1991c; Mills, 1995), and have been adopted as the fission yield files for different versions of the NEA — OECD Joint Evaluated File (NEA-OECD, 2000). A number of important short-lived fission products are absent from the decay data files (with greater than 10% of the yield in some mass chains of specific fissioning systems) — while assembling this library for JEF-2.2, correction terms were applied to adjust the independent yields for each mass, so that calculations gave the recommended chain yield values. An improved method of calculation of the correction terms has subsequently been developed for UKFY3, and decay-data evaluations have been undertaken for the most significant missing fission products in order to avoid this problem.

(c) The Chinese fission yield file was released in 1987 as part of the CENDL library (Wang Dao and Zhang Dongming, 1987); a new evaluation is in progress that will be converted into ENDF-6 format.

Fission yields adopted in other applications files have been taken from these sources (for example, the Japanese JENDL library and the French files have adopted US ENDF/B-VI fission yields).

The facilitating role of a recent IAEA-CRP on the compilation and evaluation of fission yield nuclear data should be noted and acknowledged (IAEA-CRP, 2000); specific facets of this work are described below. Co-operation has been established between internationally-respected fission yield experts, and resulted in considerable improvements to the evaluation process (e. g., cleanup of data bases, analyses of experimental data, model development, and evaluation procedures). A PC-based program has also been written and made available through the Internet to calculate mass yields, fractional independent and cumulative yields for 12 fission reactions [YCALC (Gromes, Kling and Denschlag), based on the studies of Wahl, 1988; see also IAEA-CRP, 2000]:

252Cf(sf), 229Th(T), 233U(T), 235U(T), 238Np(T), 239Pu(T), 241Pu(T), 242Am(T),

249Cf(T), 232Th(F), 238U(F) and 238U(H);

sf, T, F and H refer to spontaneous fission, thermal, fast and 14 MeV neutron — induced fission, respectively. Results can be displayed and down-loaded in graphic and tabulated forms via LINKS on the University of Mainz Website

http://www. kernchemie. uni-mainz. de

Empirical models are available for mass and charge distributions and for ternary fission yields that allow the reliable derivation of fission yields for neutron energies from thermal to 15 MeV. Computer programs have also been developed for the introduction of correlations and covariance matrices in future fission yield evaluations.