Energy Spectra Of Delayed Neutrons Research Articles

ФИЛЬТР КАЛЬМАНА В ЗАДАЧЕ ОЦЕНКИ 8-ГРУППОВЫХ СПЕКТРОВ ЗАПАЗДЫВАЮЩИХ НЕЙТРОНОВ ПРИ ДЕЛЕНИИ 235U ТЕПЛОВЫМИ НЕЙТРОНАМИ

In the present work the estimation of the 8-group delayed neutron energy spectra emitted in the fission of 235U by thermal neutrons have been done. Aggregate delayed neutron spectra that were measured with high resolution in the different time intervals after the thermal neutron irradiation of the 235U sample presented in the IAEA reference database for beta-delayed neutron emission have been used for the estimation process. Due to the lack of the uncertainties for the spectra presented in this database and the necessity of this data in the process of estimation of delayed neutron group spectra at first the estimation of uncertainties of these spectra have been done. In the estimation process the following components of uncertainty have been considered: statistical; the component due to the neutron background; component due to the distinctive features of neutron spectra restoration process measured using spectrometer filled by helium-3 and also the component appearing due to introduction of the correction on the distortion of the neutron spectra as a result of passing through the lead sheilding. Estimation of spectra have been made using the technique based on the Kalman filter that allows to take into account the uncertainties of the input data. Estimation results are shown in the numerical and graphic form.

Complete β -decay pattern for the high-priority decay-heat isotopes I137 and Xe137 determined using total absorption spectroscopy

Background: An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the $\ensuremath{\beta}$ decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called pandemonium effect and their $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ decay heat should be restudied using the total absorption technique. The fission products $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring $\ensuremath{\beta}$-delayed neutron emission probabilities is challenging and any new technique for measuring the $\ensuremath{\beta}$-neutron spectrum and the $\ensuremath{\beta}$-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques.Purpose: To obtain the complete $\ensuremath{\beta}$-decay pattern of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ and determine their consequences for reactor decay heat and ${\overline{\ensuremath{\nu}}}_{e}$ emission. Complete $\ensuremath{\beta}$-decay feeding includes ground state to ground state $\ensuremath{\beta}$ feeding with no associated $\ensuremath{\gamma}$ rays, ground state to excited states $\ensuremath{\beta}$ transitions followed by $\ensuremath{\gamma}$ transitions to the daughter nucleus ground state, and $\ensuremath{\beta}$-delayed neutron emission from the daughter nucleus in the case of $^{137}\mathrm{I}$.Method: We measured the complete $\ensuremath{\beta}$-decay intensities of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a technique for measuring the $\ensuremath{\beta}$-delayed neutron energy spectrum, which also provides a measurement of the $\ensuremath{\beta}$-neutron branching ratio, ${P}_{n}$.Results: We validate the current Evaluated Nuclear Structure Data File (ENSDF) evaluation of $^{137}\mathrm{Xe}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay. We find that major changes to the current ENSDF assessment of $^{137}\mathrm{I}\ensuremath{\beta}$-decay intensity are required. The average $\ensuremath{\gamma}$ energy per $\ensuremath{\beta}$ decay for $^{137}\mathrm{I}\ensuremath{\beta}$ decay ($\ensuremath{\gamma}$ decay heat) increases by 19%, from 1050--1250 keV, which increases the average $\ensuremath{\gamma}$ energy per $^{235}\mathrm{U}$ fission by $0.11%$. We measure a $\ensuremath{\beta}$-delayed neutron branching fraction for $^{137}\mathrm{I}\ensuremath{\beta}$ decay of $7.9\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{fit})\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{sys})%$ and we provide a $\ensuremath{\beta}$-neutron energy spectrum.Conclusions: The Modular Total Absorption Spectrometer measurements of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ demonstrate the importance of revisiting and remeasuring complex $\ensuremath{\beta}$-decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure $\ensuremath{\beta}$-delayed neutron energy spectra.

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.

Six-Group Decomposition of Composite Delayed Neutron Spectra from235U Fission

In this paper, a constrained least-squares method is developed for deducing six-group delayed neutron energy spectra from composite spectra measured at six or more delay time intervals following fission. The constraining condition is chosen to yield stable solutions that also provide good fits to the measured spectra. The method is applied to previously measured composite spectra of {sup 235}U to obtain six-group delayed neutron energy spectra. The solutions are unique for a large range of constraint spectra. The dependence of the solutions on the choice of six-group parameters {beta} {lambda} is also examined.

The analysis of delayed neutron energy spectra recorded with 3He ionization chambers

Details of the neutron detection efficiency of 3He ionization chambers have been clarified. The usefulness of pulse rise-time analysis has been explored for the example of the 17N delayed neutron spectrum. Because of obvious disadvantages, this method has been rejected, however, in favour of a complete response correction that removes all others than 3He(n,p) 3H events with great precision without disturbing the efficiency function. Peak fitting analysis is applied to structured delayed neutron spectra, and its physical significance is discussed.

Composite Delayed Neutron Energy Spectra for Thermal Fission of235U

Composite delayed neutron energy spectra from the thermal neutron fission of /sup 235/U have been measured for eight delay-time intervals between 0.17 and 85.5 s. This experimental technique combines a helium-jet and tape transfer system with a beta-neutron time-of-flight spectrometer. The neutron energy range of 0.01 to 2.0 MeV is spanned with /sup 6/Li-glass, plastic, and liquid scintillators. Spectra are compared to ENDF/B-V as well as to individual precursors' data and average energies are tabulated for the present and previous compilations. An equilibrium spectrum is also calculated and compared to ENDF/B-V and individual precursor measurements.

The implication of sensitivity analysis on the safety and delayed-neutron parameters for fast breeder reactors

The delayed-neutron energy spectra for LMFBRs are not as well known as those for LWRs. These spectra are necessary for kinetics calculations which play an important role in safety and accident analyses. A sensitivity analysis was performed to study the response of the reactor power and power density to uncertainties in the delayed-neutron spectra during a rod-ejection accident. The accidents studied were central control-rod ejections with ejection times of 2, 10 and 30s. A two-energy group and two-precursor group model was formulated for the International Nuclear Fuel Cycle Evaluation (INFCE) reference design MOX-fueled LMFBR. The sensitivity analysis is based on the use of adjoints so that it is not necessary to repeatedly solve the governing (kinetics) equations to obtain the sensitivity derivatives. This is of particular importance when large systems of equations are used. The power and power-density responses were found to be most sensitive to uncertainties in the spectrum of the second delayed-neutron precursor group, resulting from the fission of 238U, producing neutrons in the first energy group. It was found, for example, that for a rod-ejection time of 30 s, an uncertainty of 7.2% in the fast components of the spectra resulted in a 24% uncertainty in the predicted power and power density. These responses were recalculated by repeatedly solving the kinetics equations. The maximum discrepancy between the recalculated and the sensitivity analysis response was only 1.6%. The results of the sensitivity analysis indicate the need for improved delayed-neutron spectral data in order to reduce the uncertainties in accident analyses.

Delayed Neutron Measurements with a 3MV Dynamitron Accelerator

As part of a programme in neutron physics, the Birmingham Dynamitron accelerator is being used with deuterated or tritiated targets to produce neutrons of different energies which then stimulate fission in heavy elements. This paper describes energy measurements of delayed neutrons emitted after bursts of fissions in a 235U sample; a 3He spectrometer with a resolution (f.w.h.m.) of about 30keV for fast neutrons is being used to measure neutron energies. Electrostatic deflection in the top terminal of the accelerator gives 0.8s pulses of protons or deuterons every 1.8s; delayed neutrons are detected after a delay of 0.1s to allow the electronics to recover from each burst and the primary neutrons to die away. Simultaneous recording of the rise time and amplitude of the detector pulses gives a clear separation of events caused by 3He(n,p) reactions from others. Examples of delayed-neutron energy spectra are given for fission produced by primary neutrons of different energies up to 6MeV.