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Calculations related to the temperature coefficient of reactivity of enriched gas-cooled reactors show the high sensitivity of this parameter to the proper description of thermalization effects in the moderator. Additionally, the calculation of the temperature dependence of the inelastic-scattering cross section with current ENDF/B formalisms correlates the errors of the cross sections as functions of the temperature. Neglecting this temperature correlation introduces unnecessary conservatism in the estimation of the error of the reactivity coefficient. These two facts force the efforts to characterize the present status of the inelastic cross section of graphite and to calculate its covariance file. The ENDF/B evaluation of the scattering matrix S({alpha},{beta},T) is still based on the approximations (incoherent component only) and phonon spectra of the early 1960s. Subsequent measurements showed that the structure observed in S({alpha},{beta},T) cannot be described using the incoherent approximation, and soon after the availability of highly intense neutron beams and large specimens of pyrolitic graphite have allowed the direct measurement of elastic constants of relevance for a better calculation of the phonon spectra. Calculations of the probability distributions of the moment and energy transfer, {alpha} and {beta}, in a Maxwellian spectrum allow us to define a range of {alpha} and {beta}more » for which comparison of experimental and theoretical data are of most interest for reactor analysis, and to point out regions of deficient resolution or excessive details in the present {alpha},{beta} mesh used in the ENDF/B files. Because the phonon spectrum defines S({alpha},{beta},T), mathematical formulas have been found that allow the calculation of the covariance matrix of S by propagating the errors of the phonon spectra.« less

The ORNL study of safety-related aspects of nuclear power plant control systems consists of two interrelated tasks: (1) failure mode and effects analysis (FMEA) that identified single and multiple component failures that might lead to significant plant upsets and (2) computer models that used these failures as initial conditions and traced the dynamic impact on the control system and remainder of the plant. This report describes the simulation of Oconee Unit 1, the first plant analyzed. A first-principles, best-estimate model was developed and implemented on a hybrid computer consisting of AD-4 analog and PDP-10 digital machines. Controls were placed primarily on the analog to use its interactive capability to simulate operator action. 48 refs., 138 figs., 15 tabs.

The ratio of $M$- to $L$-orbital electron capture in the decay of 35-day ${\mathrm{Ar}}^{37}$ has been measured in a single-wire proportional counter containing the gaseous source. This counter was operated with variable paralysis times, 2.0 and 3.8 msec being used in the present experiments to minimize the contribution from afterpulses. Recent investigations of the pulse-height distribution from proportional counters due to single-and multiple-electron events make it possible to measure single-electron spectra down to essentially zero energy with much greater accuracy than heretofore. The experiment was undertaken because of the considerable disagreement between experimental and theoretical $\frac{M}{L}$ capture ratios, particularly in the low-$Z$ region. A survey of all experimental work on $M$ capture is included for comparison with available theoretical results. In the decay of ${\mathrm{Ar}}^{37}$ the experimental value for the $\frac{M}{L}$ capture ratio is $\frac{{P}_{M}}{{P}_{L}}={{0.104}_{\ensuremath{-}0.003}}^{+0.006}$, where the error limits have been subjected to careful detailed analysis. From this result, a value of ${k}_{\ensuremath{\alpha}}=0.980\ifmmode\pm\else\textpm\fi{}0.015$ for chlorine was determined, where ${k}_{\ensuremath{\alpha}}$ is the fraction of ${K}_{\ensuremath{\alpha}}$ x rays in the $K$ series of chlorine. The theoretical predictions of Bahcall for $\frac{{P}_{M}}{{P}_{L}}$ lie about 23% above the experimental point, for a theory with exchange correction. The older theoretical value due to Wapstra et al. is in no better agreement with experiment, even if a correction for exhange is applied. An intensive effort is reported to measure the $\frac{L}{K}$ capture ratio in the decay of 53.5-day ${\mathrm{Be}}^{7}$, as gaseous ${({\mathrm{C}}_{2}{\mathrm{H}}_{5})}_{2}$${\mathrm{Be}}^{7}$ in the single-wire proportional counter, but owing to the low energy of the $K$-capture events (55 eV) and very low counting statistics, this measurement appears to be beyond the present state of the proportional-counter art.

The $\frac{M}{L}$ orbital-electron-capture ratio in $^{71}\mathrm{Ge}$ decay has been measured in a wall-less multiwire proportional counter containing the gaseous radioactive compound $^{71}\mathrm{Ge}$${\mathrm{H}}_{4}$. The counter was filled to 6 atm with argon containing 15 cm Hg propane. Recent advances in proportional-counter spectrometry below 500 eV have extended the sensitivity for precision measurement to the detection of single- and few-electron events down to essentially zero energy, even in the presence of high intensities of more energetic radiation. These improvements are the result not only of the sophistication of the low-noise electronic system, but also of the present investigation of the degradation spectrum (low-energy tails and afterpulses) from spectral peaks, and of the resolution from each other of single- and few-electron spectra by using computer analysis. A fully transistorized electronic system was developed which includes a special dc-coupled logic unit with zero-time recovery (updating) to perform the following operations: (i) to ensure proper anticoincidence functioning of the ring counter; (ii) to provide paralysis pulses necessary to block possible afterpulses following an ionizing event in either the ring or central counter; and (iii) to eliminate the phenomenon of front- and backedge clipping of large pulses into small ones by the action of the anticoincidence gate. Measurements were made of the $K$, $L$, and $M$ peaks, having mean energies of 10.37 keV, 1298, and 158 eV, respectively, and the resulting orbital-electron-capture ratios were determined to be $\frac{{P}_{L}}{{P}_{K}}=0.117\ifmmode\pm\else\textpm\fi{}0.001$ and $\frac{{P}_{M}}{{P}_{L}}=0.162\ifmmode\pm\else\textpm\fi{}0.003$. These ratios are compared with theoretical results. Whereas the present $\frac{L}{K}$ capture ratio is in closer agreement with theory corrected for exchange according to Bahcall, the theoretical prediction of the $\frac{M}{L}$ ratio by Bahcall with correction for exchange lies approximately 7% above the present value, which agrees well with a recent theoretical calculation by Vatai. The half-life of $^{71}\mathrm{Ge}$ was found by least-squares fitting to be 11.15 \ifmmode\pm\else\textpm\fi{} 0.15 days, by following the decay of $K$ x rays from a solid source with a Ge(Li) x-ray detector.

(1969). A Method for Calculating Space-Dependent Reactor Source Transfer Functions with Feedback. Nuclear Science and Engineering: Vol. 36, No. 1, pp. 122-125.

This report summarizes the results of a study to assess the predictive capabilities of three radionuclide inventory/depletion computer codes, ORIGEN2, ORIGEN-S, and CINDER-2. The task was accomplished through a series of comparisons of their output for several light-water reactor (LWR) models (i.e., verification). Of the five cases chosen, two modeled typical boiling-water reactors (BWR) at burnups of 27.5 and 40 GWd/MTU and two represented typical pressurized-water reactors (PWR) at burnups of 33 and 50 GWd/MTU. In the fifth case, identical input data were used for each of the codes to examine the results of decay only and to show differences in nuclear decay constants and decay heat rates. Comparisons were made for several different characteristics (mass, radioactivity, and decay heat rate) for 52 radionuclides and for nine decay periods ranging from 30 d to 10,000 years. Only fission products and actinides were considered. The results are presented in comparative-ratio tables for each of the characteristics, decay periods, and cases. A brief summary description of each of the codes has been included. Of the more than 21,000 individual comparisons made for the three codes (taken two at a time), nearly half (45%) agreed to within 1%, and an additional 17% fellmore » within the range of 1 to 5%. Approximately 8% of the comparison results disagreed by more than 30%. However, relatively good agreement was obtained for most of the radionuclides that are expected to contribute the greatest impact to waste disposal. Even though some defects have been noted, each of the codes in the comparison appears to produce respectable results. 12 figs., 12 tabs.« less