Total Delayed Neutron Yield Research Articles

Development of a Reference Database for Beta-Delayed Neutron Emission

Beta-delayed neutron emission is important for nuclear structure and astrophysics as well as for reactor applications. Significant advances in nuclear experimental techniques in the past two decades have led to a wealth of new measurements that remain to be incorporated in the databases.We report on a coordinated effort to compile and evaluate all the available β-delayed neutron emission data. The different measurement techniques have been assessed and the data have been compared with semi-microscopic and microscopic-macroscopic models. The new microscopic database has been tested against aggregate total delayed neutron yields, time-dependent group parameters in 6-and 8-group re-presentation, and aggregate delayed neutron spectra. New recommendations of macroscopic delayed-neutron data for fissile materials of interest to applications are also presented. The new Reference Database for Beta-Delayed Neutron Emission Data is available online at: http://www-nds.iaea.org/beta-delayed-neutron/database.html.

A new reference database for beta-delayed neutrons

A new database containing all available experimental and evaluated β-delayed neutron data is presented in this paper. The database is the product of an international effort coordinated by the International Atomic Energy Agency. It comprises a microscopic section including all available experimental data on beta-decay half-lives, β-delayed neutron emission probabilities and spectra, as well as new systematics and global theoretical calculations for comparison. The beta-delayed neutron data for individual precursors have been benchmarked against available data on macroscopic properties such as total delayed-neutron yields and spectra, delayed-neutron decay curves and time-dependent group parameters using the summation method. All available measured macroscopic data have also been compiled in the macroscopic section of the database. The dedicated database is available online on the IAEA server (URL:http://www-nds.iaea.org/beta-delayed-neutron/database.html).

МОДЕЛИРОВАНИЕ ШУМОВЫХ ЭКСПЕРИМЕНТОВ ДЛЯ ВЕРИФИКАЦИИ ПАРАМЕТРОВ КИНЕТИКИ, ПОЛУЧЕННЫХ ПО ПРОГРАММЕ САПФИР_РФ

New multi-group neutron data libraries and a new scheme of fuel burnup have been developed on the basis of the modern ROSFOND evaluated nuclear data files. The upgraded version has a working title - the SAPFIR_RF code. Noise experiments for defining the ratio of total delayed neutron yield to neutron lifetime, βeff/l, were performed to verify the computational scheme of kinetics parameters implemented in the SAPFIR_RF. These experiments have been performed using the homogeneous assemblies with various isotopic composition of uranium-plutonium fuel. In order to analyze and interpret the experimental data correctly, numerical investigations simulating the experimental conditions and features of measurement results processing were performed; these investigations highlighted the need for systematic correction to be made for measurement results. Taking into account the correction, the calculation results are qualitatively and quantitatively in agreement with measurement data within the measurement accuracy of ±8 %. This paper draws on the modern data of Russian neutron data library, ROSFOND, as well as on G.R. Keepin’s kinetic parameters and evaluated nuclear data of the foreign libraries, ENDF/B-VI and ENDF/B-VII. Kinetic parameters obtained by G.R. Keepin and ENDF/B-VI library give two extreme evaluations of βeff/l ratio; the difference between them averages ~4,5 %. It should be noted that the calculation results received using Keepin's parameters are closer to experimental data in the most cases.

Uncertainty quantification of total delayed neutron yields and time-dependent delayed neutron emission rates in frame of summation calculations

To contribute to further improvements of the nuclear data related to delayed neutron emissions, uncertainty quantification calculations for total delayed neutron yields, ν¯d, and time-dependent delayed neutron emission rates after a burst fission have been carried out. Those are based on the summation calculations with fundamental nuclear data taken from JENDL/FPD-2011 and a partly-modified JENDL/FPY-2011. Sensitivities required for uncertainty propagation calculations are obtained efficiently by the help of the generalized perturbation theory for time-dependent problems.It is found that ν¯d and neutron emission rates after a burst fission obtained in frame of summation calculations generally agree with the JENDL-4.0 evaluations within 2σ of nuclear data-induced uncertainty. While further improvements of the fundamental nuclear data are crucial, application of the summation calculations to actual problems is now not unrealistic, and further efforts from the application side are helpful.

Calculation of Delayed Neutron Yields for Various Libraries

This paper presents the comparison between the total delayed neutron yields (νd¯) calculated and the recommended values proposed by Tuttle, the experimental data of Waldo and those of Benedetti. These data are given for thermal, fast, and high energy fission ranges. The calculation of total delayed neutron yields is performed either by the NJOY nuclear data processing system or by the summation method. The decay data found in the various evaluations as the delayed neutron branching ratios (Pn) and the cumulative fission yields (CY) can also be validated by delayed neutron yield calculation using the summation method. In the first method, where the treatment is performed by the NJOY system, the general purpose evaluation files (JEFF-3, JEF-2, ENDF/B-VII.0 and ENDF/B-VI.4 were considered. In the summation calculation, the data used are the delayed neutron branching ratios (also called delayed neutron emission probabilities) and the cumulative fission yields that are given for thermal, fast, high energy fission and spontaneous fission. These data are found in the Radioactive Decay Data and Fission Yield Data files (File 8) of nuclear data evaluations. In this study, we also perform a benchmark calculation with various libraries: JEF-2.2, JEFF3.1.1, ENDF/B-VII.0, ENDF/B-VII.1 and JENDL/FP-2011.

Total delayed neutron yield from 232Th fission by 3.2–4.9 MeV neutrons

The results of measurements of the total yield of delayed neutrons from 232Th fission by 3.2–4.9 MeV neutrons in a setup in the ion guide of the KG-2.5 electrostatic accelerator at the Physics and Power Engineering Institute are presented. Two types of measurements are performed in the experiments. In the first type, the relative yield of delayed neutrons is measured. In the second type, the energy dependence of the total yield of delayed neutrons is studied. The measurements are compared with data obtained by other authors. Three types of data are used for the comparison: experimental, computed, and evaluated data from the ENDF/BVII, JENDL 3.3, and JEFF-3.1 data libraries. The following are obtained: the energy dependence of the total yield of delayed neutrons in the range 3.2–4.9 MeV and the average over the indicated energy range total yield of delayed neutrons per fission 0.0562 ± 0.0015, which can be recommended for use in reactor calculations.

Yield of delayed neutrons in the thermal-neutron-induced reaction 245Cm(n, f)

The yield of delayed neutrons, v d , from thermal-neutron-induced fission of 245Cm is measured. Experiments aimed at studying the properties of delayed neutrons from the fission of some reactor isotopes and initiated in 1997 were continued at the upgraded Isomer-M facility by a method according to which a periodic irradiation of a sample with a pulsed neutron beam from the IBR-2 reactor was accompanied by recording emitted neutrons in the intervals between the pulses. The accuracy of the resulting total delayed-neutron yield v d = (0.64 ± 0.02)% is two times higher than that in previous measurements. This work was performed at the Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research (JINR, Dubna).

Measurement of delayed-neutron yield from 237Np fission induced by thermal neutrons

The delayed-neutron yield from thermal-neutron-induced fission of the 237Np nucleus was measured using a sample periodically exposed to a pulsed neutron beam with subsequent detection of neutrons during the time intervals between pulses. The experiment was realized on an Isomer-M setup mounted in the IBR-2 pulsed reactor channel equipped with a mirror neutron guide. The setup and the experimental procedure are described, the background sources are thoroughly analyzed, and the experimental data are presented. The total delayed-neutron yield from 237Np fission induced by thermal neutrons is ν d = 0.0110 ± 0.0009. This study was performed at the Frank Laboratory of Neutron Physics (JINR, Dubna).

The 2Z–N correlation related to the total delayed neutron yield of 243Am

It was found that the experimental value for the total delayed neutron yield of 243Am agrees with the value predicted by the 2Z–N correlation.

Experimental studies of the absolute total delayed neutron yields from neutron induced fission of 238U in the energy range 1–5 MEV

Measurements of the total delayed neutron yield from fast neutron induced fission of 238U have been made using the experimental method based on the periodic irradiation of the fissionable sample by neutrons from a suitable nuclear reaction. The preliminary results on the energy dependence of the total delayed neutron yield are presented. According to the comparison of the experimental data with our prediction based on correlation properties of delayed neutron characteristics, it is concluded that the value of the total delayed neutron yield near the threshold of the ( n,f) reaction is not a constant.

Characteristics of delayed neutrons: Systematics and correlation properties

Newly found features of delayed neutron characteristics are used to evaluate total delayed neutron yields, V d and average half-lives, 〈 T〉, of delayed neutron precursors from fast neutron induced fission. On this basis different sets of time dependent parameters ( Y i, T i) and V d values obtained using the summation method are evaluated.

Adjustment of Total Delayed Neutron Yields of 235U, 238U and 239Pu in JENDL-3.2 Using Benchmark Experiments on Effective Delayed Neutron Fraction βeff

The cross section adjustment method was applied to total delayed neutron yields of 235U, 238U and 239Pu of the JENDL-3.2 file by using experimental results of effective delayed neutron fraction βeff at six cores built in two fast critical facilities of the MASURCA and FCA and a thermal critical facility of the TCA to improve these yields. The adjustment was carried out on the yields given at several incident neutron energy points in the file. After the adjustment, the yield of 238U was almost uniformly decreased by about 3% below 7 MeV. The yield of 239Pu was increased by 2.6% and that of 235U was decreased by 0.9% at the thermal energy point, while the change of yield was less than 0.3% at the other energy points for these nuclides. By using these adjusted yields, the uncertainty of calculated βeff was reduced and the agreement of βeff between experiment and calculation was improved.

Adjustment of Total Delayed Neutron Yields of 235U, 238U and 239Pu in JENDL-3.2 Using Benchmark Experiments on Effective Delayed Neutron Fraction .BETA.eff.

Adjustment of Total Delayed Neutron Yields of 235U, 238U and 239Pu in JENDL-3.2 Using Benchmark Experiments on Effective Delayed Neutron Fraction .BETA.eff.

Measurements of Delayed Neutron Decay Constants and Fission Yields from 235U, 237Np, 241Am, and 243Am

Delayed neutron yields and decay constants for 235U, 237Np, 241Am, and 243Am were measured at the Texas A&M University TRIG A reactor using a fast pneumatic transfer system. The detection system consisted of an array of BF3 proportional counters embedded in a polyethylene cylinder. The measured values of the total delayed neutron yield per 100 fissions from thermal neutron-induced fission of 235U, 237Np, 241Am, and 243Am were determined to be 1.59 ± 0.04, 1.29 ± 0.04, 0.49 ± 0.02, and 0.84 ± 0.04, respectively. The newly measured values of delayed neutron group parameters and total yields were compared with other values recommended by Keepin; Waldo, Karam, and Meyer; and Tuttle. Very good agreement was obtained, especially for 235U.

96/00365 The importance of delayed neutrons in nuclear research — A review

Abstract This paper presents a comprehensive review of the uses of β-delayed fission neutron information in nuclear research with special emphasis on the energy spectra and reactor applications. A short introduction is followed by a discussion of the applications of delayed neutron properties in nuclear structure and astrophysics. Section 3 gives the delayed neutron requirements for reactor physics purposes and points out the difference between the physical and the mathematical representations of the delayed neutron data. In Sections 4 and 5, respectively, there are discussions of the total delayed neutron yield and the decay constants. Section 6 highlights the importance of and the need for an exact knowledge of delayed neutron energy spectra in high-accuracy criticality calculations and in precise evaluations of reactor kinetic characteristics, particularly the fast breeders. Section 6 also gives a brief description of the principal methods that are commonly used for determining the delayed energy spectra. Section 7 gives a chronological account of the developments in the measurement of delayed neutron energy spectra, both aggregate (composite) and from individual fission product isotopes, and compares the spectra, wherever possible, with the ENDF/B evaluations. A comparison is made of the energy spectra of a few precursors measured at different laboratories. There are discussions on the major spectrometry techniques employed in the measurements of delayed neutron energy spectra as well as on the methods of performing spectral analysis. This covers the response function, the efficiency and the sensitivity of the spectrometers, their merits and demerits, and their applicability. Calculations of delayed neutron energy spectra from precursor data, and decomposition of composite spectra into six-group delayed spectra using summation and/or fitting procedures are described in Section 8. Sensitivity studies of fast reactor kinetic behaviour to delayed neutron energy spectra are reviewed in Section 9. Both direct and adjoint methods are discussed. Section 10 gives a summary of the paper which concludes in Section 11 with a number of recommendations.

The importance of delayed neutrons in nuclear research—A review

This paper presents a comprehensive review of the uses of β-delayed fission neutron information in nuclear research with special emphasis on the energy spectra and reactor applications. A short introduction is followed by a discussion of the applications of delayed neutron properties in nuclear structure and astrophysics. Section 3 gives the delayed neutron requirements for reactor physics purposes and points out the difference between the physical and the mathematical representations of the delayed neutron data. In Sections 4 and 5, respectively, there are discussions of the total delayed neutron yield and the decay constants. Section 6 highlights the importance of and the need for an exact knowledge of delayed neutron energy spectra in high-accuracy criticality calculations and in precise evaluations of reactor kinetic characteristics, particularly the fast breeders. Section 6 also gives a brief description of the principal methods that are commonly used for determining the delayed energy spectra. Section 7 gives a chronological account of the developments in the measurement of delayed neutron energy spectra, both aggregate (composite) and from individual fission product isotopes, and compares the spectra, wherever possible, with the ENDF/B evaluations. A comparison is made of the energy spectra of a few precursors measured at different laboratories. There are discussions on the major spectrometry techniques employed in the measurements of delayed neutron energy spectra as well as on the methods of performing spectral analysis. This covers the response function, the efficiency and the sensitivity of the spectrometers, their merits and demerits, and their applicability. Calculations of delayed neutron energy spectra from precursor data, and decomposition of composite spectra into six-group delayed spectra using summation and/or fitting procedures are described in Section 8. Sensitivity studies of fast reactor kinetic behaviour to delayed neutron energy spectra are reviewed in Section 9. Both direct and adjoint methods are discussed. Section 10 gives a summary of the paper which concludes in Section 11 with a number of recommendations.

Distribution of Delayed Neutron Yields versus Proton, Neutron, and Mass Numbers: Application to Proton Pairing in Fission Yields

Evaluated delayed neutron emission probabilities for 74 precursors and estimated probabilities for another 174 precursors have been combined with ENDF/B-V fission yields to map out delayed neutron yields as a function of mass number, proton number, and neutron number. For all fissioning systems, more than 90% of the delayed neutron yield comes from precursors with measured P /SUB n/ values. Most of the remaining yield comes from the precursors /sup 88/As, /sup 100 -103/Y, and /sup 137/Sb. High priority should be given to P /SUB n/ measurements for these precursors. Periodic peaks about five mass numbers apart are seen when delayed neutron yields are plotted versus mass number. This behavior is explained by the very strong enhancement of yields from odd Z precursors. Even Z precursors contribute <10% of the total delayed neutron yield. Plots of delayed neutron yields versus neutron number show dramatic decreases at the closed shell numbers 50 and 82 and smaller effects at subshell numbers 40 and 56. A procedure based on calculated and total delayed neutron yields is proposed for determining proton pairing factors in the fission yield model.

Calculated Time-Dependent Delayed Neutron Group Yields for Thermal-Neutron Fission of Uranium-235

The time-dependent unseparated six delayed neutron group yields are calculated for thermal-neutron fission of 235U. These calculations are based on a recent reference set of delayed neutron precursors, available fission data, and fission systematics. The unseparated calculated time-dependent group yields are compared with separated group yields (groups based on half-life values) and the experimental unseparated group yields. The comparisons indicate the magnitude of the error involved when separated group yields are used for evaluating the accuracy of delayed neutron data. A total delayed neutron yield of 160.62 ± 11.52 per 104 fissions was obtained for thermal-neutron fission of 235U.

Delayed Neutron Yield Calculations for the Neutron-Induced Fission of Uranium-235 as a Function of the Incident Neutron Energy

The total delayed neutron yield from 235U was calculated as a function of the energy of the neutron inducing fission. The calculations (based on fission data and fission systematics) extend from thermal to 15-MeV neutron energies. The energy-dependent delayed neutron calculations are generally consistent with experimental results from thermal to 14-MeV neutron energies. Delayed neutron yields per 104 fissions of 168.7 ± 16.7, 178.2 ± 17.2, and 88.9 ± 10.0 were obtained at thermal, fission-spectrum, and 15-MeV neutron energies, respectively. The energy dependence of the odd-even effect in the fission charge distribution was found to partially account for the near constant yield observed below 5-MeV neutron energy, as well as the rapid decrease in yield observed at the second-chance fission threshold.

Measurement of mass- and time-dependence of delayed fission neutrons by an on-line mass separator

Delayed neutron emission for thermal neutron fission of235U is investigated by means of a helium-filled mass separator. At the focus of the separator, the neutron activity is detected in coincidence with theΒ decay preceding the neutron emission. Measurements of the activity build-up give the total delayed neutron yield and the yields for different times after fission. Measurements of the activity decay give the half-life distribution.