Thermal Neutron Induced Fission Of 235U Research Articles

Prompt-fission observable and fission yield calculations for actinides by TALYS

The nuclear reaction code TALYS adopts the Hauser-Feshbach statistical decay theory, to de-excite fission fragments. This involves for instance the evaporation of prompt fission neutrons and γ-rays. TALYS incorporates databases of primary fission fragment distribution which consists of primary fission fragment yield and data for excitation energy distribution of fission fragments. We conducted a sensitivity study on three parameters in TALYS and fitted them in order to reproduce experimental and evaluated data, in thermal neutron-induced fission of 235U. Moreover, we demonstrate a large-scale calculation of average prompt neutron and γ-ray multiplicities for 243 selected actinide isotopes.

Fission yield covariance matrices for the main neutron-induced fissioning systems contained in the JEFF-3.1.1 library

Nuclear data improvement, validation and completion are one of the major concerns for ensuring safety-by-design standards and economic optimization for modern nuclear facilities. Uncertainty and sensitivity analyses must be carried out to estimate the nuclear data influence on integral reactor parameters, highlighting margins of improvement needed to get full benefit from modern advanced modeling. The quality of any uncertainty quantification analysis strongly depends on the covariance matrices made available in the evaluated nuclear data libraries. In modern nuclear data libraries (e.g. JEFF, ENDF/B or JENDL) no correlations for fission yields (FY) are provided, so in the present work we propose a covariance generation methodology which yielded reasonable results for application-relevant thermal and fast neutron-induced fission product yields. The main goal was not to perform a new evaluation but to reproduce coherently the JEFF-3.1.1 fission yield library and add covariance information. We did so using the Generalized Least Square Method (GLSM) and the Marginalization techniques implemented in the CONRAD code, developed at CEA-Cadarache. Results on the thermal neutron-induced fission of 235U,239Pu,241Pu and on the fast-neutron induced fission of 238U,239Pu and 240Pu are presented and tested on elementary fission decay heat uncertainty quantifications.

Study on Prompt Fission Neutron Spectra and Associated Covariances for 235U(nth,f) and 239Pu(nth,f)

Prompt Fission Neutron Spectra (PFNS) are very important nuclear data for reactor neutronic calculation tools. Most of the international evaluated nuclear data libraries lie on the Madland-Nix model, which is a based on evaporation theory of fission fragments. But very scarce data can be found regarding the PFNS covariance matrix associated to these evaluations. As an illustration of the impact of the PFNS on neutronic calculations, we will show a Monte-Carlo calculation of the neutron flux received by a PWR vessel, using different PFNS evaluations. The neutrons that have the highest probability to contribute to the vessel flux are those that are emitted at the highest energies; however most of the fission neutrons are emitted around 2 MeV. These results show the necessity to have very precise PFNS evaluations, and a proper estimation of associated covariances. The estimation of the PFNS covariance matrix associated to a model, after adjustment of model parameters, will be shown. This is performed by the CONRAD code, developed at CEA Cadarache. The final goal of the study is to adjust the parameters involved in fission fragments de-excitation in the FIFRELIN Monte-Carlo code, also developed at CEA Cadarache, which computes the PFNS among other fission quantities, and to provide the associated PFNS covariance matrix. However for the moment we focused the study on three historically widely used PFNS models: Maxwellian, Watt and Madland-Nix models. The covariance matrix on the adjusted spectrum comes mainly from the systematic uncertainty on some experimental parameters — namely the normalization, background, detection efficiency, etc. In order to propagate this type of uncertainties properly, the marginalization technique is used. A close knowledge of the conditions in which a particular experimental PFNS has been measured is required, in order to have a correct estimation of the PFNS uncertainties after adjustment. In this work, we propagated the uncertainty on normalization of experimental spectra, and the uncertainty on the energy-dependent neutron detection efficiency. We show the resulting PFNS and associated covariance matrix in the case of thermal neutron-induced fission of 235U and 239Pu.

The effect of chemical etching on LA–ICP-MS analysis in determining uranium concentration for fission-track chronometry

Abstract LA–ICP-MS (laser ablation–inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235 U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238 U, has upon LA–ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples – three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238 U concentrations measured on the etched and unetched mineral surfaces. The 238 U concentrations determined by the LA–ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA–ICP-MS apatite FT results, promotes the use of the LA–ICP-MS for FT chronometry.

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of 232Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of 232Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of 235U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.

First observation of isomeric transition from 148m Pr and parity assignment for excited levels in 148Pr

Internal conversion electrons from the decay of 148Ce and 148mPr have been measured with a Si(Li) detector. The radioactive sources were produced by the thermal neutron-induced fission of 235U, followed by an on-line mass separation. K- and L-conversion electrons of the isomeric transition from 148mPr were newly observed at 34.6 and 70.8 keV, respectively. The M3 multipolarity was assigned to this transition from the experimental conversion coefficients, and, consequently, the 76.8 keV isomeric level was found to be a 4- state. Parities of some excited levels in 148Pr were also deduced.

Miss Piggy, a californium-252 fission fragment source as a generator of short-lived radionuclides

Carrier-free short-lived nuclides are employed in many different fields of modern nuclear chemistry. The two main production strategies are either thermal neutron-induced fission of 235U or 239Pu at nuclear reactors or spallation neutron sources or charged particle-induced nuclear reactions at accelerator facilities. An alternative method is to use a spontaneously fissioning nuclide. A facility applying this technique (“Miss Piggy”) was built at the University of Berne (Switzerland). Californium-252 ( 252Cf), which has a 3% fission branch and a half-life of 2.645 a, is used for the production of short-lived fission products that are stopped in an adjacent recoil chamber. Short-lived nuclides are transported out of the recoil chamber using the well-known gas-jet technique. Over 100 nuclides have been identified so far and used in different applications. Since such a device does not require any large facility and is easy to operate it serves well the needs of typical university laboratories.

Conversion electron measurement in the β--decay of 151Pr

The β--decay of 151Pr produced by the thermal neutron-induced fission of 235U has been studied using an on-line isotope separator. From an internal-conversion electron measurement with a Si(Li) detector, K-conversion coefficients were obtained for 20 γ-transitions. Spins and parities of 6 excited levels in 151Nd were newly determined from the deduced multipolarities: even parities for the 543 and 627 keV levels, odd parities for 250 and 599 keV, (3/2, 5/2)+ for 685 keV, and (1/2, 3/2)+ for 880 keV. The level structure was compared with the rotation-vibration coupling Nilsson model.

Decay Studies on Neutron-Rich Isotope 151Ce

The β− decay of 151Ce produced by the thermal neutron-induced fission of 235U has been studied using an isotope separator on-line. From γ-ray singles, γ-γ coincidence and internal conversion electron measurements, the decay scheme of 151Ce with a half-life of 1.72(5) s was proposed for the first time.

Possible exit channel effect on isomer yield ratios

Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of235U are compared with those calculated from the simple statistical model byMadland andEngland. The method of calculation has been extended to the isotopes having more than one isomeric state. The results may be explained according to the multi-exit-channel model of fission.

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.

Determination of pre-scission kinetic energy in low energy fission using isotonic charge distribution model

The parameters of Isotonic Charge Distribution Model have been calculated for several isotones of three low energy fissioning systems. The global charge even-odd effects are determined as 0.27±0.04 for thermal neutron induced fission of233U(U233T), 0.27±0.05 for U235T and 0.13±0.05 for PU239T. The pre-scission kinetic energies for U235T and PU239T are estimated and found to be ∼16 MeV.

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in235U(n th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.

Evaluation of the excitation function for the232Th(n, 2n)231Th reaction from threshold to 21 MeV

Experimental results for the232Th(n, 2n) reaction have been collected from the literature and evaluated. Updating of the measured cross section has been carried out with recent nuclear data. The recommended excitation function is described by the Pade approximation. Cross sections averaged over the spectra of neutrons from thermal neutron induced fission of235U and spontaneous fission of252Cf are also given.

The pre-Fermi natural reactor: A note on the periodic mode of operation

The isotopic compositions of xenon released from the Oklo reactor at temperatures below 1000°C are such that the abundances of131Xe,132Xe and134Xe relative to136Xe are markedly enhanced when compared to the relative fission yields from the thermal neutron-induced fission of235U. These anomalies can be attributed to the fact that131Xe,132Xe and134Xe have fairly long-lived precursors: 8.04-day131I, 78.2-hour132Te and 42-minute134Te, respectively. It is possible to determine the duration of the time when the reactor was turned off from the ratios of excess132Xe to excess134Xe in these anomalous xenon fractions released from the Oklo reactor. Calculations based on the available xenon isotope data that the time period during which the reactor was turned off was approximately 2 to 3 hours.

Beta-decay of148,150,152Pr into levels of148,150,152Nd

Theβ −-decay of148,150,152Pr into levels of148,150,152Nd has been studied byγ-singles,γγ-coincidence and for148Pr also byγγ-angular correlation measurements. The praseodymium activities have been obtained by thermal neutron-induced fission of235U followed by on-line mass separation with the helium-jet coupled mass separator HELIOS. Decay schemes for these heaviest praseodymium nuclides are proposed. From the angular correlation measurements spin values for the seven lowest levels of148Nd are deduced. Spin and parity assignments for the lower levels in150,152Nd have been extrapolated from level systematics.

Independent yield of88Y from thermal neutron induced fission of235U

The independent yield of88Y from thermal neutron-induced fission of235U was determined relatively to the well-known yield of91Y. An upper limit obtained was ≤1.53×10−8%.

Angular distribution of polar light particles in the fission of 235-U

Abstract The angular distribution of polar light charged particles in thermal neutron-induced fission of 235-U has been measured. The angular distribution of protons is found to be very narrow compared to alpha particles and tritons.

Correlation between fractional independent yields and neutron-to-proton ratio of fission products in low energy fission

Fractional independent yields of fission products in the thermal neutron-induced fission of233U,235U,239Pu,241Pu and in the spontaneous fission of252Cf have been correlated with the neutron-to-proton ratio of the fission products. The yields of the products from a fissioning system, when plotted as a function of neutron-to-proton (N/Z) ratio of fission preducts, fall on two Gaussian distribution corresponding to light and heavy fission products. The centroids of the distribution or the most probable value of neutron-to-proton ratio is found to be very close to theN/Z of the fissioning nucleus. From the most probable value ofN/Z the various parameters of charge distribution e.g. most probable massAp, most probable chargeZp, the mass dispersionσAand the charge dispersionσZhave been obtained and are in good agreement with the experimental values ofApandZp.

Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of233U,235U and239Pu

Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of235U,235U and239Pu have been determined using a fast radiochemical separation technique followed by gamma spectrometry. The cumulative yields of107Ru and103Ru in233U (nth, f) and107Ru and109Ru in239Pu (nth, f) are determined for the first time. The measured cumulative yields are converted to chain yields assuming normal charge distribution systematics for comparison with the literature data on chain yields.