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.     

Relative yield of delayed neutrons and half-life of their precursor nuclei from <Superscript>238</Superscript>U fission by 14.2–17.9 MeV neutrons

The energy dependence of the relative yield of delayed neutrons and half-life of their precursor nuclei from ²³⁸U fission by 14.2–17.9 MeV neutrons is measured. In addition, the detector-blocking effect, which is due to the exposure of the detector to intense fluxes of high-energy neutrons, and the effect of the secondary neutron source D(d, n)³He, a reaction which occurs when a T(d, n)⁴He based neutron source is used, are investigated experimentally and taken into account in the data analysis. The data are analyzed in terms of the average half-life < T> of the precursor nuclei of the delayed neutrons. It is shown that the observed increase of <T> with increasing primary-neutron energy in the energy range studied is predominantly due to the opening of emissive nuclear fission channels. The energy dependence of the average half-life of the delayed-neutron precursors can be described on the basis of the model of emissive fission of three nuclei that fission in this energy range − ²³⁷U*, ²³⁸U*, and ²³⁹U*. __________ Translated from Atomnaya énergiya, Vol. 102, No. 2, pp. 124–132, February, 2007.     

Fission Product Yields of U, U, U and Pu in Fields of Thermal Neutrons, Fission Neutrons and 14.7-MeV Neutrons

The yields of more than fifteen fission products have been carefully measured using radiochemical techniques, for ²³⁵U(n,f), ²³⁹Pu(n,f) in a thermal spectrum, for ²³³U(n,f), ²³⁵U(n,f), and ²³⁹Pu(n,f) reactions in a fission neutron spectrum, and for ²³³U(n,f), ²³⁵U(n,f), ²³⁸U(n,f), and ²³⁹Pu(n,f) for 14.7 MeV monoenergetic neutrons. Irradiations were performed at the EL3 reactor, at the Caliban and Prospero critical assemblies, and at the Lancelot electrostatic accelerator in CEA-Valduc. Fissions were counted in thin deposits using fission ionization chambers. The number of fission products of each species were measured by gamma spectrometry of co-located thick deposits.     

Use of γ Radiation from Short-Lived Fission Products for Detecting <Superscript>235</Superscript>U and <Superscript>239</Superscript>Pu

It is suggested that γ radiation with E _γ > 4900 keV from short-lived fission products produced by thermal neutrons be used to detect ²³⁵U and ²³⁹Pu in samples. A time regime is substantiated: 120 sec irradiation, 60 sec holding time, and 120 sec measurement time. The contribution of the reaction (n, p) on fast neutrons is studied.__________Translated from Atomnaya Energiya, Vol. 98, No. 5, pp. 365–370, May 2005.     

Gamma-ray energy release in the decay of fission products

Total disintegration rate, gamma-ray energy release rate and energy spectrum of the fission products of ²³⁵U, ²³⁹Pu, ²⁴¹Pu, and ²³³U by thermal neutrons, and ²³⁵U, ²³⁸U, ²³⁹Pu and ²³²Th by fission spectrum neutrons have been reevaluated as a function of reactor operating times from 10² to 10⁸ sec after shutdown. Decay scheme data were taken mainly from the Table of Isotopes [1] and fission yields from Meek and Rider's 1972 recommendations [2]. Gamma energy releases do not depend strongly on the incident neutron energy, and those for ²³⁹Pu and ²⁴¹Pu are much different from that for ²³⁵U. Soon after fission, the present values for burst fission are lower than those of Perkins [3], but agree after 10⁴ sec and agree better with the experimental results of Sugarman et al. [4] and Borst [5]. The calculated data are tabulated in detail to facilitate interpolation.     

Prompt-Neutron Spectrum Formation Mechanism in 235U, 239Pu, and 252Cf Fission

The mechanism of the formation of the prompt-neutron spectrum in fissioning of ²³⁵U and ²³⁹Pu by thermal and fast neutrons and spontaneous fission of ²⁵²Cf is investigated. A method is proposed for the formation of the prompt-neutron spectrum in fissioning of nuclei as a superposition of three partial Weisskopf evaporation spectra with average neutron energy 0.4, 2.06, and 2.8 MeV.     

Measurement of neutron capture and fission cross sections of <Superscript>236</Superscript>U

The cross sections for radiative capture and fission for ²³⁶U in the neutron energy ranges 1–2000 eV and 1–1000 eV, respectively, have been measured. The measurements were performed on a flight baseline 5.2 m of the FAKEL linear electron accelerator at the National Research Center Kurchatov Institute using a method based on measuring the distribution of the number of particles emitted by an excited nucleus with simultaneous measurement of the energy of each particle. The spectrum of the neutrons incident on a sample was measured by detecting the reaction ¹⁰B(n, α)⁷Li. The cross sections were normalized with respect to the resonance in ²³⁶U at 5.45 eV. The data obtained are compared with the results of other experiments and evaluated data.     

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.     

MEASUREMENT OF PROMPT NEUTRON SPECTRA OF ^238U FISSION INDUCED BY 10.17 AND 12.12 MeV NEUTRONS

Experimental method to measure the prompt neutron spectra of 238U fission induced by fast neutrons has been developed at HI-13 Tandem Van de Graaff Accelerator Laboratory of CIAE.These techniques employ a multi-segment fission chamber and two liquid scintillator neutron detectors.TOF(time of flight)techniques are used for primary neutrons to select the fission events induced by monoenergetic neutron from 2H(d,n) reactions instead of breakup neutrons from 2H(d,np) reactions.The fission neutron TOF spectra are measured in coincidence with the fission fragments to distinguish fission neutrons from other secondary neutrons.The method permits measurements to a fairly good accuracy under large neutron and gamma ray background.The techniques are described and experimental spectra are presented.     

Fission cross-sections of U and Pu averaged over Cf neutron spectrum

A series of measurements have been carried out to derive values for the spectrum-averaged fission cross-section of ²³⁵U and ²³⁹Pu for ²⁵²Cf fission neutrons. Two nearly identical target foils were mounted on either side of a Cf source (10⁷ neutron/sec) in a compensated beam geometry. Fission fragments passing through limited solid angle apertures were recorded from each foil by solid-state track-etch techniques. The Cf neutron source strength was calibrated in manganese bath relative to the standard source NBS-II. Values of 1.215 ± 0.022 barn for ²³⁵U and 1.790 ± 0.041 barn for ²³⁹Pu were obtained for the fission cross-sections, corresponding to a ratio value of 1.473 ± 0.041.     

Summation calculations of fission-product decay heat, their uncertainties and their applications to a fast breeder reactor

Prediction of unmeasured fission parameters is described concerning mass yields of fission products, average total kinetic energies of fission fragments and average numbers of prompt neutrons in neutron-induced fission of several nuclides related to nuclear reactors. Reliability of predicting unmeasured mass yields of the products from measured mass yields of the primary fragments is discussed. The predicted values of the mass yields of the products are presented for the fissions of ^233,235U, ^239,241Pu by thermal neutrons, of ²³²Th by 1.38-MeV neutrons and of ²³¹Pa, ²³⁸U and ²³⁷Np by fission-spectrum neutrons.A semi-empirical method based on the statistical theory of nuclear fission is extended to unmeasured distributions of the fragments. The mass yields of the products, the average total kinetic energies of the fragments and the average numbers of prompt neutrons are predicted for the fissions of ²³²U and ²³⁸Pu by thermal neutrons, and of ^234,236U, ²⁴⁰Pu and ²⁴²Pu by 2-MeV neutrons. Errors in the predicted values are also discussed.     

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.     

Absolute measurements of U and Pu fission cross-sections with photoneutron sources

the fission cross-sections of ²³⁵U and ²³⁹Pu for Na---Be, La---Be, Na---D and Ga---D photoneutrons have been measured absolutely. The neutron source strength was measured using a manganese bath to compare the photoneutron yield from the sources with the standard source NBS-II. Fission counts were accumulated with the source positioned symmetrically between two identical fission foils in an experiment package suspended in a low-albedo laboratory. Fission fragments passing through limited solid angle apertures were recorded on polyester track-etch films. The masses of the foil deposits were determined by microbalance weighings and confirmed by thermal neutron fission and alpha counting. After making a correction for the calculated energy distribution of the source neutrons, values of 1.471 ± 0.029, 1.274 ± 0.026, 1.162 ± 0.025 and 1.195 ± 0.026 barns were obtained for the ²³⁵U fission cross-section at the source median energies of 140, 265, 770 and 964 keV, respectively. Corresponding values of 1.469 ± 0.045, 1.515 ± 0.038, 1.670 ± 0.039 and 1.643 ± 0.038 barns were determined for ²³⁹Pu.     

Experimental prompt fission neutron “sawtooth” data described by the “point by point” model

The experimental data concerning the prompt fission neutron number as function of the fission fragment mass (currently named “sawtooth” data) when they exist, allow a more refined verification of the “point by point” model and in the same time the validation of the methods used to determine the model parameters corresponding to the fission fragment pairs covering the entire fission fragment range.The prompt neutron multiplicities of each fission fragment pair provided by the “point by point” model are compared with the experimental data concerning the sum of neutrons emitted by the light and heavy fragments forming a pair obtained from experimental sawtooth data.The available fission fragment pair multiplicity and sawtooth experimental data for the ^233,235,238U(n, f) and ²³⁷Np(n, f) reactions as well as for the spontaneous fission of ²⁵²Cf are well described by the “point by point” model results, proving again that this treatment is a powerful tool for prompt fission neutron multiplicity and spectrum evaluation purposes.     

Measurement of Double-Differential Neutron Emission Spectra from Uranium-238

We have performed the measurement of neutron emission spectra from ²³⁸U using a time-of-flight technique, and deduced the following data; (1) the prompt fission neutron spectra for 2 MeV incident neutrons at two emission angles of 90° and 135°, (2) the double-differential neutron emission cross sections at the incident energies of 1.2, 2.0, 4.2, 6.1 and 14.1 MeV. The emission spectra and the cross sections for scattering process were also deduced by subtracting the fission neutrons from the experimental spectra. The experimental results were compared with other experiments and the evaluations of JENDL-3 and ENDF/B-IV.

From the fission spectrum data ranging from 2 to 12 MeV, we have derived the best fit parameters for the Maxwellian and Watt type distribution functions. The experimental spectra are described with the Maxwellian spectrum with temperature of 1.24–1.26 MeV and are softer than both evaluations.

The spectra and cross sections for inelastic-scattering showed substantial disagreement with the evaluations concerning the discrete levels between 0.5 and 1.2 MeV, and continuum neutrons due to evaporation and pre-equilibrium processes. The secondary neutron angular distributions at 14 MeV incident energy were reproduced fairly well with the systematics.     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.     

Calculation of Beta-Ray Spectra from Individual and Aggregate Fission Products

The β-ray spectra of individual fission products were calculated by using the β-decay data assuming every β-decay to be allowed transition. For the nuclides without measured decay data the β-feeding function was evaluated with the gross theory of β-decay and the β-ray spectrum was calculated from the function. The measured decay data were also supplemented with the data calculated by the gross theory for the excitation energy range above the highest measured excitation energy level. The β-ray spectra from aggregate fission products after a burst fission were calculated by using the β-ray spectrum and the atom number of each fission product nuclide and they were compared with the ones measured for thermal neutron induced fission of ²³⁵U, ²³⁹Pu and ²⁴¹Pu at Oak Ridge National Laboratory. The spectrum calculations showed excellent agreement with the measured data at shorter cooling times than 10s when many short-lived nuclides without measured decay data contributed considerably to the spectrum.     

Cobalt Deposition and Release on Magnetite Powder in High Temperature Water

By making use of miniature ²³⁹Pu and ²³⁵U fission chambers and an accelerator-based neutron source, the ²³⁹Pu/²³⁵U neutron fission rate ratio profile has been measured in the region of a plane interface between a tank containing ordinary water and a tank containing an aqueous solution of boric acid. The reported preliminary results indicate an approximately 8 mm shift of the central point of the fission rate ratio profile towards the poisoned side of the system and an accompanying larger shift in the ²³⁵U Cd ratio profile towards the unpoisoned side.