Gamma-Ray Spectra of Fission Products observed with Li-Drifted Germanium Detectors, (II)

The γ-ray spectra of short-lived fission products from thermal neutron irradiation of highly enriched U were observed with an encapsulated Li-drifted Ge γ-ray spectrometer. The spectra at various periods—10 min, 30 min, 1,2,5,10 and 20 hr—after irradiation were measured up to about 1 meV. The relative activities of fission products at various periods after irradiation (10 min–20 hr) were calculated and used for assigning photopeaks.     

Studies of Ceramic Fuels with the Use of Fission Gas Release Loop, (II)

The γ-ray spectra of fission gases released from UO₂-graphite pellets under neutron irradiation were measured. With and without separating fission gases into xenon and krypton, 25 kinds of γ-ray were observed and assigned to nine nuclides, ^85mKr, ⁸⁷Kr, ⁸⁸Kr(⁸⁸Rb), ¹³³Xe, ¹³⁵Xe, ^135mXe and ¹³⁸Xe (¹³⁸Cs). A value of 15 min is proposed for the half-life of ¹³⁸Xe, based on analysis of the decay curves. Discussion is given on problems related to determination of the release rate of each fission product through measurement of the height of each peak in the γ-ray spectrum.     

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.     

Comparison of Nuclear Waste Glass Leached in Natural Groundwater at 14°C with That in Synthesized Groundwater at 70°C

The influence of fast neutron irradiation was studied for CdTe radiation detectors in the range of fluence 1.0 × 10⁸-1.8 × 10¹¹ n/cm². The effect of γ-ray heavy irradiation toward the detector was also investigated for the comparison. Americium-241 and ¹³⁷Cs γ-ray spectra measured before and after irradiation were compared to examine the change of the detector performance. A pulse height and a rise time of a radiation signal were simultaneously measured in order to evaluate the transport property of both elections and holes. Peak shift toward lower energies was observed in the ²⁴¹Am γ-ray spectrum after fast neutron irradiation. Electron μτ product showed large decrease with increasing the neutron fluence, while the one for holes remained unchanged. For the γ-ray irradiation, the spectrum shape of ¹³⁷Cs changed due to the influence of carrier detrapping. Experimental results support that the electron-trapping center generated by fast neutrons degrades the detector performance of the CdTe detector.     

Novel methodology for the quantitative assay of fissile materials using temporal and spectral β-delayed γ-ray signatures

An analytical model for the generation of β-delayed γ-ray spectra following thermal-neutron-induced fission of mixed samples of ²³⁵U and ²³⁹Pu is presented. Using an energy-dependent figure-of-merit to designate the spectral regions employed in the assay, the unique temporal β-delayed γ-ray signatures are utilized to determine the fraction of ²³⁹Pu in a mixed U-Pu sample. By evaluating the β-delayed γ-ray temporal signatures of both ²³⁵U and ²³⁹Pu within a 3 keV energy bin, traditional sources of systematic uncertainty in quantitative assay using β-delayed γ-ray signals, such as self-attenuation of the sample and energy-dependent γ-ray detection efficiency, are significantly reduced. The effects of the time-dependent Compton-continuum and growth of longer-lived nuclides on the quantitative assessment are explored. This methodology represents a promising extension of the conventional means of analysis for quantitative assay of fissile materials using β-delayed γ-ray signatures.     

Measurement of ke V-Neutron Capture Cross Sections and Capture Gamma-Ray Spectra of 147,148,149,150,152,154Sm

The neutron capture cross sections and capture γ-ray spectra of ^{147,148,149,150,152,154}Sm were measured in the neutron energy region of 10 to 90 keV and at 550 keV. A neutron time-of-flight method was adopted with a 1.5-ns pulsed neutron source by the ⁷Li(p, n)⁷Be reaction and with a large anti-Compton NaI(Tl) γ-ray spectrometer. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to derive capture yields. The capture cross sections were obtained with the error of about 5% by using the standard capture cross sections of ¹⁹⁷Au. The present results were compared with the evaluated values of JENDL-3.2 and previous measurements. The capture γ-ray spectra were obtained by unfolding the observed capture γ-ray pulse-height spectra. An anomalous shoulder was clearly observed around 3 MeV in the γ-ray spectra of ^150,152,154Sm, and the energy position of the shoulder was consistent with the systematics obtained in our previous work.     

Method of Measuring Boron-10 Depletion in Control Blade

The neutron capture cross sections and capture γ-ray spectra of ^143,145,146Nd were measured in the neutron energy region of 10 to 90 keV and at 550 keV. A neutron time-of-flight method was adopted with a 1.5-ns pulsed neutron source by the ⁷Li(p, n)⁷Be reaction and with a large anti-Compton NaI(Tl) γ-ray spectrometer. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to derive capture yields. The capture cross sections were obtained with the error of about 5% by using the standard capture cross sections of ¹⁹⁷Au. The evaluated values of JENDL-3.2 and previous measurements were compared with the present results. The capture γ-ray spectra were obtained by unfolding the observed capture γ-ray pulse-height spectra. An anomalous shoulder was observed around 2 MeV in the γ-ray spectra of ^145,146Nd, and the energy position of the shoulder was consistent with the systematics obtained in our previous work.     

In-Situ Measurement of Fission Product Plateout in In-Pile Gas Loop Using Portable Ge(Li) Gamma-Ray Spectrometer

A nondestructive and quantitative method of measuring fission products deposited on inside surface of the primary duct has been developed. A portable Ge(Li) detector with a lead collimator was used for the external scanning of γ-rays emitted from the fission product plateout in the duct.

Upon termination of the first irradiation cycle for the coated particle fuels in the high temperature in-pile gas loop OGL-1 installed in JMTR (Japan Material Testing Reactor), in-situ measurements of the plateout were carried out by the above method at different points in the primary system.

Identified nuclides in the measured γ-ray spectra were ¹³¹I, ¹³³I, ⁹⁵Zr, ⁹⁵Nb, ¹³⁷Cs, ^110mAg, ⁹⁹Mo, ¹⁴⁰La, ⁶⁰Co, ⁵⁸Co, ⁵⁴Mn, ⁵¹Cr, ⁶⁵Zn, ⁵⁹Fe, ¹²²Sb, ¹²⁴Sb, ¹⁸²Ta, ¹⁸¹Hf, ²²Na and ⁴⁶Sc. The plateout densities of these nuclides were obtained using conversion coefficients determined by detector calibration and calculation, which were from 10^-5 to 10^-2 μCi/cm².

It was shown clearly by the experiment that the present measuring method using an in-situ γ-ray spectrometer is useful in the studies of fission product plateout in the primary cooling system of a high temperature gas cooled reactor.     

Measurement of keV-Neutron Capture Cross Sections and Capture Gamma-Ray Spectra of 167 Er

The neutron capture cross sections and capture γ-ray spectra of ¹⁶⁷Er were measured in the neutron energy region of 10 to 90keV and at 550 keV. Using a neutron time-of-flight method with a 1.5-ns pulsed neutron source by the ⁷Li(p, n)⁷Be reaction, the measurement was performed by detecting prompt γ rays from an enriched capture sample with a large anti-Compton Nal(Tl) spectrometer. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to extract capture yields. The capture cross sections were derived with the error of about 5% by using the standard capture cross sections of ¹⁹⁷Au. The present results were compared with the evaluated values of ENDF/B-VI and the previous measurement. The present measurement at 550 keV was the first one. The capture γ-ray spectra were obtained by unfolding the observed capture γ-ray pulse-height spectra. An anomalous shoulder was clearly observed around 3 MeV in the γ-ray spectra and the energy position of the shoulder was consistent with the systematics obtained in our previous work. The multiplicities of the observed γ rays were derived from the γ-ray spectra.     

Gamma-ray emission probabilities of the fission products 144Ce144Pr, 106Rh, 95Zr

The fission products ¹⁴⁴Ce¹⁴⁴Pr, ¹⁰⁶Rh and ⁹⁵Zr play an important role in the burn-up determination of fuel elements by means of γ-ray spectrometry. One of the crucial quantities in this field is the absolute γ-ray emission probability (intensity) for which values available in literature have not the desirable accuracy of 1–2%.By using Ge(Li)-spectrometers of known efficiency and sources calibrated by 4πβ-γ-coincidence methods, we determined absolute γ-ray emission probabilities of ¹⁴⁴Ce¹⁴⁴Pr, ¹⁰⁶Rh and ⁹⁵Zr with an uncertainty of ±1% for the main γ-rays.     

Calculation of Gamma-Ray Production Cross Sections at Neutron Energies of 1–20 MeV

In order to prepare the p-ray data library requested in a design of fission and fusion reactors, γ-ray production cross sections and spectra of Al, Si, Ca, Fe, Ni, Cu, Nb, Ta, Au and Pb have been obtained at the neutron energies of 1–20 MeV, using a spin-dependent multi-step evaporation model. Calculations include dipole and quadrupole transition without the distinction between electric and magnetic process, and take explicit account of the role of yrast levels. The effects of the yrast levels and γ-ray strength function upon γ-ray production are also investigated in relation to particle emission. At the incident neutron energies where (n,n^γ ) and/or (n,2^nγ ) reactions are dominant, the present model is shown to be able to predict the production of secondary γ-rays (<9.0 MeV) from medium-heavy to heavy nuclei with reasonable accuracy.     

Fission Density in Plutonium-Uranium Mixed Oxide Fuel Irradiated by Pulsed Power in NSRR Experiments

Fuel behavior during a reactivity initiated accident condition is recognized to be predo minantly related to energy deposition in the fuel. The first stage of NSRR in-pile experiments addressing the behavior of PuO₂-UO₂ mixed oxide fuels evaluated the energy deposition per unit integrated reactor power by γ-ray spectrometry. Solid samples were used to measure the γ-rays because the facility is permitted to handle solid plutonium only. Determination of the penetration ratios of γ-rays from the actinides contained in the fuels allowed correction for the self-attenuation of γ-rays in the solid samples. Evaluation of the effect of epithermal neutron fissions was also necessary since the fissile nuclides of ²⁴¹Pu and ²³⁹Pu have high resonance cross sections in the epithermal energy region. For this evaluation, the fission density was first calculated for the fission products as a function of the contribution ratio of the fissions of epithermal neutrons. Accurate fission density was then determined using the contribution ratio which minimized the deviation of the calculated values for the fission density. The fission densities determined by this simplified method agreed well with the values calculated using the computer codes CITATION and GGC-4.     

Compression and Decompression Waves in Steam-Water System

The ratio of the γactivities per fission from fission products of ²³⁹Pu and ²³⁵U, and its time dependence were measured by double fission chamber technique. The γ-activity from the fission products of ²³⁹Pu fission was lower than the corresponding activity relevant to ²³⁵U fissions. The ratio varied with the cooling time allowed after irradition.

This ratio was applied to power distribution measurements by γ-scanning method in multi-region cores composed of PuO₂-UO₂ and UO₂ fuels. To obtain the relative power, the measured γ-activities from the fission products in the fuel rods were corrected for the difference between the γ-activities per fission from the fission products.     

Correction Factor P (†) for Fast Fission Effect Measurements

The correction factor P (t) used in determining the fast fission effect of ²³⁸U was measured in the range of cooling time t=30~200 min for irradiations of 30 min, 2 and 5 hr, respectively. This factor depends quite sensitively on the bias level in γ-ray counting as well as on the irradiation time and cooling time after irradiation. It is emphasized that neglect of the dependence of P (t) on irradiation time and bias level results in a large error in measurements of fast fission effect of ²³⁸U.     

Evaluation of Neutron Capture Gamma-Ray Spectra in Hafnium and Tantalum

A method has been developed for evaluation of neutron capture γ-ray spectrum. It couples measured intensities of primary and secondary discrete—-ray with a γ-ray cascade model to calculate the unresolved part of the capture spectrum, and adds the discrete part and the unresolved part to obtain the whole spectrum. The cascade model uses the level density formula proposed by Gilbert & Cameron and the Brink & Axel form of El γ-ray profile function with a modification. This method was applied to thermal neutron capture spectra in six hafnium isotopes and ¹⁸¹Ta and was extended also to non-thermal capture spectra in ¹⁸¹Ta for 0.25 and 0.5 MeV neutrons with empirical assumptions. The calculated results were compared with experiments and agreement was good not only in terms of the gross structure, but also in terms of the fine structure which appears at high and low γ-ray energies.     

Effects of local radiolysis and geometric parameters on intergranular attack caused by crevice corrosion

Effects of γ-ray irradiation upon crevice corrosion (CC) of type 316L stainless steel (316L SS) as an initiation site of stress corrosion cracking in a boiling water reactor environment have been studied using a material corrosion test loop which could be irradiated with a ⁶⁰Co γ-ray source during testing. The CC tests were conducted using crevice specimens with various sizes of crevice gaps. Many of the examined specimen surfaces exhibited a selective grain boundary dissolution; that is, intergranular attack (IGA) as a result of the CC when the crevice gap size was lower than a certain value. The IGA initiation time was shortened by the γ-ray irradiation. The IGA occurred mostly near the crevice mouth at a distance of less than 2 mm from the mouth edge. When γ-ray exposure had occurred, it was found that the number of IGA sites deeper in the crevice increased compared with the IGA site distribution under the no-irradiation condition. Since the electrochemical corrosion potential inside crevice specimens must be low under the conditions for which IGA could occur, it was assumed that γ-ray irradiation accelerated the corrosion rate of 316L SS by decreasing the Fe²⁺ surface activity inside the crevice or increasing the cathodic current of radiolytic oxidants on the crevice surface. It was concluded that γ-ray irradiation affects the IGA occurrence not only temporally but also spatially.     

Changes to Plutonium Extraction Behavior of TBP and Alkylamines through Irradiation

Changes to plutonium extraction properties brought about through irradiation of 30v/o TBP (Tri-n-butyl phosphate)/kerosene and 5v/o alkylamines/kerosene modified with lauryl alcohol were studied in such respects as extraction, scrubbing and stripping characteristics in nitrate media.

The alkylamines examined were tri-n-octyl amine (TOA), N-cyclohexyl dilauryl amine (N-CHDLA) and Amberlite LA-2 (LA-2). These extractante were irradiated to the extent of 10⁸R by ⁶⁰Co γ-ray, prior to extraction.

Marked changes with irradiation were observed in plutonium extraction performance with the TBP system tending to bring losses of plutonium, but very little change was found to occur with amine systems.

Calculation of the single stage decontamination factors of fission products from plutonium proved that separation deteriorated with irradiation in the TBP system but hardly changed in the amine systems. From these findings, tertiary amines, especially N-CHDLA, can be judged superior to TBP for the direct recovery of plutonium from irradiated fuels, a process that has to be carried out under high irradiation.

The phase separation time after mixing in the extraction stage varied very little with irradiation, and no formation of emulsion occurred in the extraction of tracer plutonium with either irradiated TBP or amines.     

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.     

Gamma-ray Irradiation Effects on Corrosion Rates of Stainless Steel in Boiling Nitric Acid Containing Ionic Additives

Irradiation effects of γ-rays on corrosion rates of type 304ULC stainless steel in 9×10^?3 mol/m³ boiling nitric acid containing an ionic additive of multivalence elements of Ce(IV), Cr(VI) or Ru(III) were studied by measuring weight losses of specimens immersed under the ⁶⁰Co γ-ray irradiation of 1 kC/(kg·h) (4MR/h). Tests without irradiation were carried out as well to obtain reference data. All the coexisting ionic species enhanced the corrosion in comparison with those in pure nitric acid, and the γ-ray irradiation moderated the enhancement. The valence analyses of these additives and redox potential measurements before and after the immersion batch under the irradiation suggested that the reduction of Ce(IV) and Cr(VI) induced by the irradiation have moderated the corrosion environment.     

Proposal for selective isotope transmutation of long-lived fission products using quasi-monochromatic γ-ray beams

We have proposed a new selective isotope transmutation method using photonuclear reactions with quasi-monochromatic γ-ray beams. This method is based on the fact that the particle threshold of a long-lived fission product (LLFP) such as ⁹³Zr, ¹⁰⁷Pd, or ⁷⁹Se is lower than those of stable isotopes of the same chemical element. Therefore, this method has the excellent advantage that LLFPs cannot, in principle, be produced newly even if the target materials include stable isotopes in addition to LLFPs. Furthermore, this method is effective for ¹²⁶Sn, ^{135, 137}Cs, ⁹⁰Sr, and ³H. The nuclear data involved and suitable γ-ray sources are discussed. Laser Compton scattering γ-ray sources and neutron capture γ-rays in nuclear reactors are candidates for this method.