基于中子活化分析的129I嬗变率测量方法实验研究

基于中子活化分析（NAA）,建立一种简便可靠的¹²⁹I嬗变率测量方法。采用¹²⁷I制作模拟靶件,利用1个参比靶件与两个嬗变模拟靶件同时照射,测量¹²⁸I特征γ射线强度之比,并根据比值计算嬗变率。实验结果验证了方法的可行性,为¹²⁹I的高注量率反应堆嬗变率测定提供了技术支持。     

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the 129I(n, γ) 130I Reaction

To obtain fundamental data for research on the transmutation of nuclear wastes, the thermal neutron cross section and the resonance integral of the ¹²⁹I(n, γ)¹³⁰I reaction have been measured using an activation method. The neutron cross sections for the formation of the ground (5⁺) state and the isomeric (2⁺) state of ¹³⁰I were measured separately.

Six ¹²⁹I targets were irradiated for 10 min with thermal reactor neutrons; three of them containing 2.55- 2.61 kBq of ¹²⁹I were irradiated within a Cd capsule, and the other three targets containing 259–261 Bq of ¹²⁹I were irradiated without it. The Co/Al and Au/Al alloy wires were used to monitor the neutron flux and the fraction of the epithermal part (Westcott's epithermal index). The gamma-ray spectra from the irradiated samples were measured with a Ge detector.

The thermal neutron capture cross section (the 2,200 m/s neutron cross section) and the resonance integral of the ¹²⁹I(n, 7)¹³⁰I reaction were determined to be 12.5±0.5b and 15.6±0.7b for the formation of the ground state ^130gI(5+), 17.8±0.7b and 18.2±0.8b for the formation of the isomeric state ^130mI(2⁺), and 30.3±1.2b and 33.8±1.4b for the formation of ¹³⁰I (the sum of the 2⁺ and the 5⁺ states), respectively. The sum of the thermal neutron capture cross sections forming the 2⁺ and the 5⁺ states was 12% larger than the evaluated values of JENDL-3.2 and ENDF/B-VI and that reported by Roy et al. This discrepancy is explained by the population of the isomeric level.     

Increasing 129I Transmutation Efficiency

The special features of ¹²⁹I transmutation are discussed. The changes in content and equilibrium concentration of ¹²⁷I in a target under recycling are calculated, and the conditions for decreasing the required radiochemical capacity in reprocessing of iodine are found. It is shown on the basis of an analysis of the neutron consumption in targets and estimates of the pressure of daughter xenon under the casing that the gaseous products of transmutation must be removed during the irradiation process along a loop channel. To increase the effectiveness and safety of ¹²⁹I burnup, it is recommended that ¹²⁹I be included in the volume of the gas permeable porous nuclear-inert targets by permeation with melts of iodine compounds.     

A consideration to reduce the potential hazard of I-129 in radioactive wastes

Transmutation of ¹²⁹I to ¹³⁰I by (n, γ) reaction and isotope dilution with stable iodine were discussed. The transmutation in LWR is calculated by supposing that targets irradiated for 25 years are substituted with new targets. The result showed that initial amount of ¹²⁹I will be reduced to 11.5%. In order to lower dose equivalent of general public than 0.1mSv/y, 400 times isotope dilution of ¹²⁹I is required. The feasibility of the isotope dilution was discussed at dissolution process of spent fuel, conditioning process of the waste and disposal conditions.     

Vertical distributions of Iodine-129 and iodide in the Chukchi Sea and Bering Sea

ABSTRACT

Currently, most Iodine-129 (¹²⁹I) present in the environment is anthropogenic; it has been released from European nuclear reprocessing plants and transported into the northern part of the Arctic Ocean via ocean circulation. This study investigates the ¹²⁹I distribution in the Chukchi Sea and Bering Sea by considering water mass structure. The range of ¹²⁹I concentration is 0.89–5.03 × 10⁷ atoms L^?1 in the studied area, which is considered to be the fallout level. ¹²⁹I was found to be distributed almost uniformly. An increase in ¹²⁹I concentration levels caused by high ¹²⁹I water inflow from the Atlantic Ocean was not observed. Additionally, we revealed the vertical distribution of iodide, one chemical form of iodine, from the Bering Shelf area to the Chukchi Sea for the first time. The range of iodide concentrations was found to be 10–157 nM. Moreover, an increasing tendency of iodide near sea bottom was observed, suggesting that iodide is released from sediments at the bottom of the sea.     

Thermal neutron cross section determination of short-to-medium lived nuclides using a 20 Ci Am–Be neutron source

While there are growing demands for the nuclear data at higher energy regions than keV for up-to-date scientific and technological development, accurate capture cross sections at thermal energy are still needed. The thermal neutron capture cross sections for the reactions ¹²⁷I(n,γ)¹²⁸I, ¹⁵²Sm(n,γ)¹⁵³Sm,¹⁵⁴Sm(n,γ)¹⁵⁵Sm, and ²³⁸U(n,γ)²³⁹U were determined by the method of foil activation using ⁵⁵Mn(n,γ)⁵⁶Mn as a reference reaction. The experimental samples with and without a Cd cover were irradiated in an isotropic neutron field of a 20 Ci ²⁴¹Am–Be neutron source facility. A high purity Ge detector was used to measure the induced gamma-rays from the samples and the monitor. The thermal neutron capture cross sections of the reactions ¹²⁷I(n,γ)¹²⁸I, ¹⁵²Sm(n,γ)¹⁵³Sm, ¹⁵⁴Sm(n,γ)¹⁵⁵Sm, and ²³⁸U(n,γ)²³⁹U were deduced from the analysis of obtained gamma-ray spectra. The thermal neutron capture cross section values for ¹²⁷I(n,γ)¹²⁸I, ¹⁵²Sm(n,γ)¹⁵³Sm, ¹⁵⁴Sm(n,γ)¹⁵⁵Sm, and ²³⁸U(n,γ)²³⁹U reactions are (5.93 ± 0.52), (207.3 ± 9.4), (7.7 ± 0.3), and (2.79 ± 0.09) barns respectively. The obtained results have been discussed and compared with the available experimental data and were found to be in agreement with each other.     

Study on the LLFPs transmutation in a super-critical water-cooled fast reactor

The performance of the super-critical water-cooled fast reactor (Super FR) for the transmutation treatment of long-lived fission products (LLFPs) was evaluated. Two regions with the soft neutron spectrum, which is of great benefit to the LLFPs transmutation, can be utilized in the Super FR. First region is in the blanket assembly due to the ZrH_1.7 layer which was utilized to slow down the fast neutrons to achieve a negative void reactivity. Second region is in the reflector region of core like other metal-cooled fast reactors. The LLFPs selected in the transmutation analysis include ⁹⁹Tc, ¹²⁹I and ¹³⁵Cs discharged from LWR or fast reactor. Their isotopes, such as ¹²⁷I, ¹³³Cs, ¹³⁴Cs and ¹³⁷Cs were also considered to avoid the separation. By loading the isotopes (⁹⁹Tc or ¹²⁷I and ¹²⁹I) in the blanket assembly and the reflector region simultaneously, the transmutation rates of 5.36%/GWe year and 2.79%/GWe year can be obtained for ⁹⁹Tc and ¹²⁹I, respectively. The transmuted amounts of ⁹⁹Tc and ¹²⁹I are equal to the yields from 11.8 and 6.2 1000 MWe-class PWRs. Because of the very low capture cross section of ¹³⁵Cs and the effect of other cesium isotopes, ¹³⁵Cs was loaded with three rings of assemblies in the reflector region to make the transmuted amount be larger than the yields of two 1000 MWe-class PWRs.Based on these results, ⁹⁹Tc and ¹²⁹I can be transmuted conveniently and higher transmutation performance can be obtained in the Super FR. However, the transmutation of ¹³⁵Cs is very difficult and the transmuted amount is less than that produced by the Super FR. It turns out that the transmutation of ¹³⁵Cs is a challenge not only for the Super FR but also for other commercial fast reactors.     

AMS analysis of iodine-129 in aerosols from Austria

Atmospheric concentrations of many elements have been significantly increased by human activities. The quantification of these changes and their effect on the terrestrial and aquatic ecosystems is important because of their potentially adverse effects. The human nuclear activities, especially releases from the spent nuclear fuel reprocessing plants, are presently the main source of ¹²⁹I in the environment.In this work, the concentration of ¹²⁹I and the ratios of ¹²⁹I/¹²⁷I in aerosols weekly collected in Vienna, Austria (202 m a.s.l) during the year 2001 are presented. Iodine was extracted from the aerosol filters using a strong basic solution and separated from the matrix elements by anion exchange. The chemical yield of the procedure, determined by ICP-MS, ranges from 70% to 95%. The AMS results indicated that the ¹²⁹I/¹²⁷I isotopic ratios were of the order 10⁻⁸ to 10⁻⁷. The ¹²⁹I originated from gaseous emissions from the Sellafield reprocessing plant. The measured ¹²⁹I concentrations were compared with those of ⁷Be, a cosmogenic radionuclide. Although, both radionuclides exhibit nearly the same distribution pattern (higher levels in summer and lower in winter) their different origins can clearly be deduced from short term variations.     

Radiative capture cross-sections of isotopes of Gd,Sm and V between 1 and 3 MeV

Fast-neutron radiative capture cross-sections have been measured relative to the ¹²⁷I(n, γ)¹²⁸I reaction cross-section using the comparative activation technique for ¹⁶⁰Gd, ¹⁵⁴Sm and ⁵¹V (using enriched isotopes and Specpure samples) at five different neutron energies between 1 and 3 MeV. The cross-section for ¹⁶⁰Gd is reported for the first time in the above energy region. Experimental values are compared with the statistical-model calculations.     

Studies on LLFP transmutation in a pressurized water reactor

A systematic study on the long-lived fission product (LLFP) transmutation in a pressurized water reactor (PWR) is performed, aiming at an optimal transmutation strategy for present nuclear energy development. The LLFPs selected in the analysis include ⁹⁹Tc and ¹²⁹I discharged from light water reactors (LWRs). The isotope ¹²⁷I is also considered to avoid the difficulties in isotopes separation. To minimize the negative impacts of LLFPs on the core performance and safety parameters, metallic technetium or MgI₂ target pins mixed with ZrH₂ are designed and investigated. Through the numerical analysis on equilibrium cycles, the transmuted amounts of ⁹⁹Tc and ¹²⁹I equal to the yields from 1.94 and 4.22 PWRs with a power of 1000 MWe, respectively. Numerical results indicate that both ⁹⁹Tc and ¹²⁹I can be transmuted conveniently in present PWRs in the form of target pins.     

Determination of 125I impurities in [123I]labelled radiopharmaceuticals,by liquid scintillation counting: sensitivity of the method

Iodine-125 is a radioisotopic impurity “always” present in iodine-123, produced by nuclear reactions induced either on natural or “highly” enriched targets. Liquid scintillation counting is a very sensitive tool to determine low level impurities of both low energy electrons and photons in aqueous and organic solutions of radiopharmaceutical compounds. With this technique it was possible to determine, on commercial samples, that the content of ¹²⁵I was of the order of not less than 0.1% for ¹²³I produced via ¹²⁷I(p,5n) reactions and not less than 0.01% for ¹²³I produced via “highly” enriched ¹²⁴Xe(p,X) nuclear reactions.     

Precise Intensity Measurements in the 14N(n, γ)15N Reaction as a γ-Ray Intensity Standard up to 11MeV

The main γ-ray intensities in the ¹⁴N(n, γ)¹⁵N reaction were determined at an accuracy of 0.3–1.0% based on the intensity balance principle. Measurements were performed at the supermirror neutron guide tube at the Kyoto University reactor (KUR). A liquid nitrogen target and a deuterated melamine (C₃D₆N₆) one were used. We prepared a full-energy-peak efficiency curve by combining the measured and calculated efficiency curves. The previous values widely used as intensity standards agreed with the present results within 4–5% in the 2–11 MeV region, but the ratio to the present values showed a monotonic decrease with the increase in _-ray energy. The results reported by Belgya in 2006 agreed with the present one within 2–3% in the 2–8MeV region; nevertheless, the ratio showed a small oscillation versusγ-ray energy.     

Trapping and Measuring Radioiodine (Iodine-129) in Cartridge Filters

To understand the distribution of ¹²⁹I trapped in AgS (silver-impregnated silica gel) adsorbent and to determine a calibration curve for measurement of this ¹²⁹I, cartridges containing 10 in-line (arranged in series) filter elements, each packed with a 10-mm thickness of adsorbent, were fed from 20Bq (3.3 μg) to 4.6 MBq (768mg) of ¹²⁹I at 150°C at a gas velocity of 22cm/s. The ¹²⁹I in each filter element was measured after the adsorbent had been mixed homogeneously until the counting rates at the front and back surfaces of the filter element became equal. The amount of input ¹²⁹I was allocated to each filter in proportion to the counting rates thus obtained.

The first 7 of the 10 filter elements completely confined 4.6MBq of ¹²⁹I. Therefore, the width of the adsorption zone was 7cm. Although each filter element in the cartridge could theoretically adsorb 2.92 MBq (485mg) of ¹²⁹I, the first element captured only 1.42 MBq, which was 49% of its saturation value. Its DF had dropped to a level of 2.21 for its adsorption of only 17.4% of the saturated amount. As ¹²⁹I continued to be deposited, the DF dropped to 1.45. The high gas velocity causes this phenomenon. Plots of counting rates vs. adsorption quantity provide a calibration curve covering a range of 20Bq to 2MBq of ¹²⁹I with deviation of ± 13%. This curve is expressed by

1n y = ?0.57 + 0.961n x

where y is the counting rate (counts/300s) and x the quantity of ¹²⁹I in Bq.     

Measurement of thermal neutron cross-section and resonance integral for the Ho(n,γ)Ho reaction using electron linac-based neutron source

The thermal neutron cross-section and the resonance integral of the ¹⁶⁵Ho(n,γ)^166gHo reaction have been measured by the activation method using a ¹⁹⁷Au(n,γ)¹⁹⁸Au monitor reaction as a single comparator. The high-purity natural Ho and Au foils with and without a cadmium shield case of 0.5 mm thickness were irradiated in a neutron field of the Pohang neutron facility. The induced activities in the activated foils were measured with a calibrated p-type high-purity Ge detector. The correction factors for the γ-ray attenuation (F_g), the thermal neutron self-shielding (G_th), the resonance neutron self-shielding (G_epi) effects, and the epithermal neutron spectrum shape factor (α) were taken into account. The thermal neutron cross-section for the ¹⁶⁵Ho(n,γ)^166gHo reaction has been determined to be 59.7 ± 2.5 barn, relative to the reference value of 98.65 ± 0.09 barn for the ¹⁹⁷Au(n,γ)¹⁹⁸Au reaction. By assuming the cadmium cut-off energy of 0.55 eV, the resonance integral for the ¹⁶⁵Ho(n,γ)^166gHo reaction is 671 ± 47 barn, which is determined relative to the reference value of 1550 ± 28 barn for the ¹⁹⁷Au(n,γ)¹⁹⁸Au reaction. The present results are, in general, good agreement with most of the previously reported data within uncertainty limits.     

Comparison of three chemical extraction methods for I-129 determinations

We report here results comparing the efficiency of three iodine extraction methods for the determination of ¹²⁹I/I ratios in small samples. The direct organic extraction had the highest average yield. ¹²⁹I/I ratios determined from targets prepared by direct organic extraction and silver extraction are comparable to each other within the error limits. The smallest sample from which a reliable ¹²⁹I/I ratio was measured, started with an iodine input of 0.2 mg iodine and contained 0.14 mg AgI in the target.     

129I AMS at 0.5 MV tandem accelerator

The ¹²⁹I measurement program has been established at the 0.5 MV ‘Tandy’ accelerator of the PSI/ETH Zürich AMS facility. This development was made possible by using a SiN window instead of Mylar one in a gas ionization detector. The setting up of the ¹²⁹I measurement at Tandy is simple, the acquired performance is stable and reliable, and the quality of results is equal to or better than at our larger EN-tandem. With this setup, high sample throughput, which is required in many ¹²⁹I studies, can be easily achieved. The measurements are performed in the +3 charge state. At this charge state the major difficulty in the ¹²⁹I⁺³ identification is caused by a highly abundant 43⁺¹ (m = 43, q = +1) molecule interference. This is a positive molecular ion, because its intensity reduces exponentially with an increase in gas stripper pressure. We conclude that this molecule is ²⁷Al¹⁶O⁺ (m/q = 43/1 = 129/3) and comes from the break-up of (Al₂O₃ + Al)^? (m = 129) precursor at the terminal: (Al₂O₃ + Al)^? → ²⁷Al¹⁶O⁺. The expected isobaric interferences ⁴³Ca⁺¹ and ⁸⁶Sr⁺², which also originate from the break-up of molecules in the stripper, were found to be low and do not disturb the ¹²⁹I⁺³ measurements. The best repeatable performance with our standard sample material was achieved at 0.14 μg/cm² Ar gas stripper pressure with machine blanks showing ～6 × 10^?14 normalized ¹²⁹I/I ratio and 9% transmission through the accelerator. However, high ²⁷Al¹⁶O⁺ molecular rates were observed from the user samples, and in order to destroy these molecules we had to increase the stripper pressure to ～0.22 μg/cm². This increase in the stripper pressure degraded the machine blank values to ～9 × 10^?14 and reduced transmission to 8%. Nevertheless, the achieved measurement conditions are sufficient for measurement of nearly all ¹²⁹I samples that have been submitted to PSI/ETH over the last few years.     

Neutron-capture cross-sections of 244Cm and 246Cm measured with an array of large germanium detectors in the ANNRI at J-PARC/MLF

The neutron neutron-capture cross cross-sections of ²⁴⁴Cm and ²⁴⁶Cm were measured by the time-of-flight method in the energy range of 1–300 300 eV with an array of large germanium detectors in the Accurate Neutron-Nucleus Reaction measurement InstrumentANNRI at Material and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research ComplexJ-PARC/MLF. The ²⁴⁴Cm resonances at around 7.7 and 16.8 8 eV and the ²⁴⁶Cm resonances at around 4.3 and 15.3 3 eV were observed in the capture reactions for the first time. The uncertainties of the obtained cross cross-sections are 5.8% at the top of the first resonance of ²⁴⁴Cm and 6.6% at that of ²⁴⁶Cm. The rResonance analyses were performed for low-energy ones using the code SAMMY. The prompt γ-ray spectra of ²⁴⁴Cm and ²⁴⁶Cm were also obtained. Eight and five new prompt γ-ray emissions were observed in the ²⁴⁴Cm(n, γ) and ²⁴⁶Cm(n, γ) reactions, respectively.     

Measuring the Photoneutrons Originating from D(γ, n)H Reaction after the Shutdown of an Operational BWR

The purpose of this study is to measure the photoneutron emission rate after the shutdown of an operational BWR. The photoneutrons originate from the D(γ, n)H reaction in the moderator region, while the high-energy gamma rays are generated from ¹⁴⁰Ba-¹⁴⁰La transient equilibrium of fission products in irradiated fuel. The photoneutron emission rate is measured by means of the photoneutron signal ratio of the Start-up Range Neutron Monitor (SRNM). The ratio is defined as a ratio of the photoneutrons to neutrons originating from spontaneous fission and the oxygen (γ, n) reaction of actinides (²⁴²Cm, ²⁴⁴Cm, etc.) in irradiated fuel. The principle of the measurement of the photoneutron signal ratio is the large difference between the decay constants of the ¹⁴⁰Ba-¹⁴⁰La transient equilibrium and those of the actinides. The measurement of the SRNM signal was continuously carried out over several months, and the photoneutron signal ratio was evaluated by using the least-squares method to fit a theoretical model to SRNM signal data. The measurements were performed in the middle of the cycle at three BWR cores. Comparisons of the measured photoneutron signal ratio and the calculated one showed reasonable agreement. This demonstrates the validity and usefulness of the measurement. The absolute value of photoneutrons in the SRNM signal ranged from approximately 1 to 35 counts per second during a five-day cooling period after shutdown. Converting the absolute value to the relative fraction of photoneutrons in the SRNM signal results in a range from approximately 2 to 50%.     

Reaction-yield dependence of the (γ, γ′) reaction of 238U on the target thickness

The dependence of the nuclear resonance fluorescence (NRF) yield on the target thickness was studied. To this end, an NRF experiment was performed on ²³⁸U using a laser Compton back-scattering (LCS) γ-ray beam at the High Intensity γ-ray Source facility at Duke University. Various thicknesses of depleted uranium targets were irradiated by an LCS γ-ray beam with an incident beam energy of ～2.475 MeV. The scattering NRF γ-rays were measured using an High-purity Germanium (HPGe) detector array positioned at scattering angles of 90° relative to the incident γ-beam. An analytical model for the NRF reaction yield (NRF RY model) is introduced to interpret the experimental data. Additionally, a Monte Carlo simulation using GEANT4 was performed to simulate the NRF interaction for a wide range of target thicknesses of the ²³⁸U. The measured NRF yield shows the saturation behavior. The results of both of the simulation and the analytical model can reproduce the saturation curve of the scattering NRF yield of ²³⁸U against the target thickness. In addition, we propose a method to deduce the precise integral cross section of the NRF reaction by fitting the NRF yield dependency on the target thickness without any absolute measurements.