Prompt fission neutron multiplicity and spectrum evaluations for n+238U reaction

The prompt fission neutron multiplicity and spectra for n+²³⁸U reaction are calculated using an improved Los Alamos model which includes the linear relation between the average prompt gamma ray energy and the prompt neutron multiplicity and also the average fission fragment kinetic energy dependence on the incident neutron energy. The coefficients describing the quadratic variation of the fission fragment kinetic energy versus the incident energy are obtained by extrapolation of the data and procedure used for n+²³⁵U reaction. The inverse process compound nucleus cross-section of the fissioning nucleus is calculated using the coupled channel method. In the incident energy range where only the first fission chance is involved the comparison of present spectrum evaluation with spectrum calculation using multi-modal model is made too. The calculated prompt neutron multiplicity and spectra of ²³⁸U neutron induced fission are in good agreement with the experimental data for the entire incident energy range required in evaluations, proving the validity of the used procedure.     

Feynman-α and Rossi-α analyses for a subcritical reactor system driven by a pulsed spallation neutron source in Kyoto University Critical Assembly

ABSTRACT

For a subcritical reactor system driven by a periodically pulsed spallation neutron source in Kyoto University Critical Assembly (KUCA), the Feynman-α and the Rossi-α neutron correlation analyses were carried out to determine the prompt-neutron decay constant and quantitatively to confirm a non-Poisson characteristics of the neutron source. In these correlation analyses, a non-negligible contribution of delayed neutrons and a non-Poisson character of the source were considered, and each pulse was assumed to be a delta function. When a neutron counter was placed closely to the reactor core, the prompt-neutron decay constant determined from the present Feynman-α analysis well agreed with that done from a previous analysis for the same subcritical system driven by an inherent neutron source. However, the decay constant determined from the present Rossi-α analysis was in poor agreement with that done from the above previous analysis. This disagreement originated from an inevitable excitation of a higher mode. In the Rossi-α counting probability distribution, the excitation deformed a sharp cusp arising from the delta function to a smooth convex shape. When the data around the convex top were masked for least-squares fitting of the present Rossi-α formula, the disagreement could be successfully resolved. Compared with the previous Feynman-α and Rossi-α analyses under the Poisson inherent source, the non-Poisson spallation source definitely enhanced the respective prompt-neutron correlation amplitudes. The enhancement rate increased with an increase in subcriticality. Moreover, the Degweker’s factor (m ₂-m ₁ ²)/m ₁ ² of 0.067 ± 0.011, which indicated a non-Poisson character of the present spallation source, could be determined from the present correlation analysis and the non-zero value of the factor convinced us that the present source had a different statistical distribution from the Poisson.     

Possibility of Building a Thermal Nuclear Reactor with Total Utilization of the Initial Uranium and Thorium

It is shown that total utilization of the natural stocks of uranium and thorium is possible in thermal nuclear reactors. The technology is based on eliminating parasitic neutron absorption in ¹³⁵Xe, using cyclic operation of the fuel elements in the reactor. In addition, neutron losses in the production chains of fissioning materials are smaller and fissioning materials, especially ²³³U, are put into active operation more quickly. The advantages of cyclic transposition can be fully realized by using the core superposition method. In this method, the excess neutrons from a part of the core where the reactivity is positive are directed into a zone with a high breeding ratio and negative reactivity.Fuel based on natural uranium permits using up to 18% of the initial material with recycling. Adding more than 20% ²³²Th to the initial material permits total utilization of ²³²Th and ²³⁸U in the recycling regime.     

Heterogeneous Transmutation of Am, Cm, and Np in a BREST Core

The effect of heterogeneous (in individual units) transmutation of Am, Cm, and Np on the radiation characteristics of fuel is examined. For heterogeneous transmutation of Am, Cm, and Np in individual fuel elements containing nitrides, the radiation characteristics and energy release increase substantially compared with fresh homogeneous fuel elements. Heterogeneous transmutation is dangerous from the standpoint of nonproliferation of fissioning materials because of the low critical mass of the main nuclides – ²³⁹Np and americium isotopes. 2 tables, 4 references.     

Approach for the fission fragment total kinetic energy TKE(A) calculation: Application to prompt neutron emission models

A simple approach for the calculation of the fission fragment total kinetic energy, TKE(A), based on the electrostatic repulsion between the fragments connected by a neck in the pre-scission configuration is described. The calculated TKE(A) is obtained in good agreement with the experimental data for many fissioning systems, such as ^233,235U(n_th, f), ²³⁹Pu(n_th, f), ²³⁷Np(n, f), ²⁴²Pu(SF), with minor adjustment of only one parameter. Due to the fact that the present approach can provide with enough trust TKE(A) distributions for fissioning systems for which experimental TKE(A) data do not exist, the possibilities to use the refined Point by Point model of prompt neutron emission can be considerably extended.     

Calculation of the effective delayed-neutron fraction and prompt-neutron lifetime in IBR-2M

The main parameters of IBR-2M are presented: the effective delayed-neutron fraction β_eff and the promptneutron lifetime τ, calculated using the DORT two-dimensional multigroup neutron-transport compute code and the SCALE4 code with a system of multigroup nuclear constants. For a regular IBR-2M regime β_eff = 0.00216 ± 0.00007, τ = (6.5 ± 0.5)·10^–8 sec, the delay-neutron value γ = 0.980, the prompt-neutron decay constant in the critical state α = 3.5·104 sec^–1. The calculations showed that the effective delayed-neutron fraction for IBR-2M is identical, within the error limits, to the measured value for IBR-2, the prompt-neutron lifetime is approximately 5% longer (β_eff = 0.00216, τ = (6.2 ± 0.2)·10^–8 sec). It is shown that β_eff and τ increase somewhat as the IBR-2 core size increases in the radial direction.     

Energy Dependence of Plutonium Fission-Product Yields

A method is developed for interpolating between and/or extrapolating from two pre-neutron-emission first-chance mass-asymmetric fission-product yield curves. Measured ²⁴⁰Pu spontaneous fission and thermal-neutron-induced fission of ²³⁹Pu fission-product yields (FPY) are extrapolated to give predictions for the energy dependence of the n + ²³⁹Pu FPY for incident neutron energies from 0 to 16 MeV. After the inclusion of corrections associated with mass-symmetric fission, prompt-neutron emission, and multi-chance fission, model calculated FPY are compared to data and the ENDF/B-VII.1 evaluation. The ability of the model to reproduce the energy dependence of the ENDF/B-VII.1 evaluation suggests that plutonium fission mass distributions are not locked in near the fission barrier region, but are instead determined by the temperature and nuclear potential-energy surface at larger deformation.     

The relation between ternary fission and cluster decay

It was found that the main ternary fission products, such as ⁴He, ¹⁴C, ²⁰O, ²⁴Ne and ²⁸Mg, are also the main clusters in cluster radioactivity decay. This similarity suggests that ternary fission can be described as a cluster decay of the fissioning nucleus in the last phase of the scission process.     

γ-Spectrometric Monitoring of Large Plutonium Samples

The -ray spectra of the samples of spontaneously fissioning ²⁵²Cf in Pu with different isotopic composition were measured. Peaks belonging to definite fission products were found in the spectrum. It was concluded that the mass and isotopic composition of large plutonium samples can be monitored on the basis of the results of only spectrometric measurements.     

Relative Yield and Period of Individual Groups of Delayed Neutrons in 233U, 235U, and 239Pu Fission by Epithermal Neutrons

The relative yield of delayed neutrons and the half-life of their precursor nuclei in fissioning of ²³³U, ²³⁵U, and ²³⁹Pu by epithermal neutrons are measured on a setup which is based on a KG-2.5 electrostatic accelerator. The experimental samples are irradiated in a polyethylene cube, one face of which is irradiated by a neutron beam consisting of T(p, n)³He neutrons. A procedure is developed for averaging the group parameters of delayed neutrons from different series of measurements, taking account of their correlation. A comparative analysis of the data of this work and in ENDF/BVI in terms of the average half-life of the precursor nuclei of the delayed neutrons and using the dependence of the reactivity on the asymptotic period of the reactor is performed.It is shown that the data of this work agree, to within the uncertainty limits, with the recommended experimental data and differ substantially from the ENDF/BVI data.     

Energy Dependence of the Relative Yield and Period of Individual Groups of Delayed Neutrons in 239Pu Fission in Primary-Neutron Energy Range 0.37–4.97 MeV

The KG-2.5 cascade generator at the State Science Center of the Russian Federation – Physics and Power Engineering Institute is used as the basis to measure the relative yield and period of groups of delayed neutrons in fissioning of ²³⁹Pu in the incident-neutron energy range 0.37–4.97 MeV. A comparative analysis of the data obtained in the present work and in works by other authors is made in terms of the average half-life of the delayed-neutron precursor nuclei. This approach made it possible to show for the first time the character and scale of variation in the values of the group parameters of delayed neutrons as a function of the excitation energy of the fissioning compound nucleus.A substantial energy dependence of group parameters in fissioning of ²³⁹Pu by neutrons is found. It is expressed as a decrease in the average half-life of precursor nuclei by 10% in the primary-neutron energy range 2.85 eV–5 MeV.It is established that the relative change / in the reactivity is approximately equal to the relative change in the average half-life of the delayed-neutron precursor nuclei <T>/<T> (provided that in both cases the same sets of group parameters are used).     

A new method for uranium enrichment

A new method for uranium enrichment is presented. This method is based on exciting ²³⁸U to its isomeric levels at 2557.6 and 2557.6 + x keV. These isomeric levels have SF decay modes. Enrichment is thus obtained by fissioning only the ²³⁸U isotopes. Three ways for exciting ²³⁸U to ^238mU are discussed.     

Possibility of Activation γ-Spectrometric Measurements for Determining the Masses of Nuclear Materials in Samples

Measurements of -ray spectra performed on uranium and plutonium samples irradiated with ²⁵²Cf source neutrons are described. Peaks due to ¹³⁸Cs and other short-lived fission products are observed in the spectra. The intensity of the peaks characterizes the content of fissioning isotopes in the samples.     

Fission-enhanced diffusion in dispersion fuels

Irradiation-induced diffusion in finite, two-phase systems is analyzed and applied to the geometry of dispersion fuels of the type used in research and test reactors. A fissioning sphere irradiates the surrounding medium in which fission also takes place. In place of the fission rate used in conventional irradiation-induced diffusion coefficient (D^∗) correlations, the energy deposition rates due to electronic and nuclear stopping of the fission fragments are separated. The former is used to drive the point-defect contribution to D^∗ and the latter is the source of the thermal-spike component. This separation accounts for the preponderance of electronic energy loss early in the track of a fission-fragment and the dominance of nuclear stopping near the end of the range. This distinction accounts for the difference in the relative intensities of these two energy loss modes in a fission-fragment that exits one phase and deposits energy in an adjacent medium in which D^∗ is to be determined. Fission-fragment stopping powers and projected ranges are obtained from SRIM software, thereby permitting extraction of the two types of energy deposition rates from the fission rate. As expected, the ratio of nuclear stopping to electronic stopping in the medium surrounding a fissioning inclusion increases with distance from the interface. The effective irradiation-enhanced diffusivity for use in the diffusion equation depends upon two parameters: the fraction of D^∗ in an infinite, homogeneous solid attributable to nuclear stopping and the ratio of the volumetric fission rates in the dispersed and continuous phases.     

Improved Los Alamos model applied to the neutron induced fission of 239Pu and 240Pu and to the spontaneous fission of Pu isotopes

The Los Alamos model with multiple fission chances upgraded with (a) the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity and (b) the dependence of the average fission fragment kinetic energy on the incident neutron energy, is used for the n+²³⁹Pu and n+²⁴⁰Pu reactions, and also for the spontaneous fission of ^237–241Pu isotopes. In the case of ²⁴⁰Pu fissioning nucleus the variation of the average energy released versus the incident neutron energy is also taken into account. The calculated prompt fission neutron spectra and average prompt neutron multiplicity well represent the experimental data, proving a better predictive power of the improved Los Alamos model.     

Monitoring the Fast-Neutron Flux Density and Fluence in a RBMK Core using a Threshold Fission Chamber in a Screen-Absorber

A new method for measuring the flux density and fluence of fast neutrons in a RBMK core, using a threshold ionization fission chamber with a ²³⁸U radiator, and a computational-experimental method based on threshold chambers for fissioning of ²³⁸U with enrichment at least 99.999% are proposed. A method is proposed for taking account of the correction factors for the change in sensitivity of threshold fission chambers with and without screens on the basis of RBMK-1000 fast-neutron spectra determined experimentally by the activation method.__________Translated from Atomnaya Energiya, Vol. 98, No. 4, pp. 260–267, April, 2005.     

Improved Los Alamos model applied to the neutron induced fission of 235U and 237Np

The Los Alamos model, with multiple fission chances, was upgraded to include the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity. A global, model-free parameterization of this relation versus the charge and mass number of fissioning nuclei is obtained. The average fission fragment kinetic energy dependence on the incident neutron energy is also taken into account. Employing this model for the n+²³⁵U and n+²³⁷Np reactions, the prompt fission neutron spectra, the average prompt neutron multiplicity and the average prompt gamma ray energy are obtained in very good agreement with the experimental data, proving a better predictive power.     

244Cm Transmutation in Accelerator-Driven System

Mixed oxide (MOX) fueled LWRs are characterized by enhanced accumulation of curium isotopes compared to uranium fueled LWRs. Toxicity of relatively short-lived isotope ²⁴⁴Cm (T ₁/2=18 yr) appears to be dominant in transuranics discharge. The present paper deals with analysis of the scientific feasibility to transmute ²⁴⁴Cm. Liquid bismuth-curium core operated in a subcritical mode with neutron support from accelerator is proposed to provide transmutation rate through fissioning that exceeds the rate of natural decay by one order of magnitude. Beam current in the range 30–40 mA is required to drive the core. One accelerator-driven transmutor could solve the problem of curium accumulation in a large-scale nuclear energy system based on MOX fueled LWRs.     

Measurement of the effective delayed-neutron dose by the α-Rossi method in water-containing media

A modified method of determining the effective delayed-neutron dose is proposed. The method is based on a combination of determining the pseudoreactivity of the ²⁵²Cf source and measuring the α-Rossi time distribution. The advantage of the method proposed here is that there is no need to measure the absolute rate of fissioning of the nuclei in the medium. It is proposed that the latter be expressed in terms of the intensity of a known source of spontaneous fissions of ²⁵²Cf nuclei. The measurements are performed on an assembly with a hydrogen-containing moderator. The influence of the positions of the detectors (³He counters) on the measured effective delayed-neutron fraction is studied. __________ Translated from Atomnaya énergiya, Vol. 100, No. 5, pp. 393–399, May, 2006.     

Expanded 233U Breeding in the Heavy-Water Blanket of an Electronuclear System

Comparative investigations are performed of the neutron-physical characteristics and their variations during burnup for two types of fuel Th–U and U–Pu with heavy- and light-water lattices. Analysis of the results gives the basic parameters of the electronuclear system operating in a thorium–uranium fuel cycle. It is shown that an external neutron source (accelerator + target) makes it possible to switch to the thorium–uranium fuel cycle with expanded ²³³U breeding without using fissioning uranium or plutonium isotopes. There are grounds for believing that with a higher accelerator current it is possible to achieve a regime where the heavy-water blanket can be replenished only with thorium with no ²³³U.