Study on dynamic characteristics of fission products in 2 MW molten salt reactor

In this study,a numerical flow model of the fission products(FPs)in the primary loop system of a molten salt reactor(MSR)was established and solved using Mathematica 7.0.The simulation results were compared with those of the ORIGEN-S program in the static burnup mode,and the deviation was found to be less than 10%,which indicates that the results are in good agreement.Furthermore,the FPs distribution in the primary loop system under normal operating conditions of the 2 MW MSR was quantitatively analyzed.In addition,the distribution phenomenon of the FPs under different flow rate conditions was studied.At the end of life,the FPs activity in the core region(including active region,and upper and lower plenum regions)accounted for 77.3%,and that in the hot leg #1,main pump,hot leg #2,heat exchanger,and cold leg region accounted for 1.2%,16.15%,0.99%,2.5%,and 1.9%,respectively,of the total FPs in the primary loop under normal operating conditions.The proportion of FPs in the core decreased with the increase in flow rate in the range of 2.24-22,400 cm^3 s^-1.The established analytical method and conclusions of this study can provide an important basis for radiation safety design of the primary loop,radioactive source management design,thermal-hydraulic safety analysis,and radiochemical analysis of FPs of 2 MW MSRs.

Verification of neutron-induced fission product yields evaluated by a tensor decompsition model in transport-burnup simulations

Neutron-induced fission is an important research object in basic science.Moreover,its product yield data are an indispensable nuclear data basis in nuclear engineering and technology.The fission yield tensor decomposition(FYTD)model has been developed and used to evaluate the independent fission product yield.In general,fission yield data are verified by the direct comparison of experimental and evaluated data.However,such direct comparison cannot reflect the impact of the evaluated data on application scenarios,such as reactor transport-burnup simulation.Therefore,this study applies the evaluated fission yield data in transport-burnup simulation to verify their accuracy and possibility of application.Herein,the evaluated yield data of235U and239Pu are applied in the transport-burnup simulation of a pressurized water reactor(PWR)and sodium-cooled fast reactor(SFR)for verification.During the reactor operation stage,the errors in pin-cell reactivity caused by the evaluated fission yield do not exceed 500 and 200 pcm for the PWR and SFR,respectively.The errors in decay heat and135Xe and149Sm concentrations during the short-term shutdown of the PWR are all less than 1%;the errors in decay heat and activity of the spent fuel of the PWR and SFR during the temporary storage stage are all less than 2%.For the PWR,the errors in important nuclide concentrations in spent fuel,such as90Sr,137Cs,85Kr,and99Tc,are all less than 6%,and a larger error of 37%is observed on129I.For the SFR,the concentration errors of ten important nuclides in spent fuel are all less than 16%.A comparison of various aspects reveals that the transport-burnup simulation results using the FYTD model evaluation have little difference compared with the reference results using ENDF/B-Ⅷ.0 data.This proves that the evaluation of the FYTD model may have application value in reactor physical analysis.

Uncertainty and sensitivity analysis of in-vessel phenomena under severe accident mitigation strategy based on ISAA-SAUP program

The phenomenology involved in severe accidents in nuclear reactors is highly complex.Currently,integrated analysis programs used for severe accident analysis heavily rely on custom empirical parameters,which introduce considerable uncertainty.Therefore,in recent years,the field of severe accidents has shifted its focus toward applying uncertainty analysis methods to quantify uncertainty in safety assessment programs,known as“best estimate plus uncertainty(BEPU).”This approach aids in enhancing our comprehension of these programs and their further development and improvement.This study concentrates on a third-generation pressurized water reactor equipped with advanced active and passive mitigation strategies.Through an Integrated Severe Accident Analysis Program(ISAA),numerical modeling and uncertainty analysis were conducted on severe accidents resulting from large break loss of coolant accidents.Seventeen uncertainty parameters of the ISAA program were meticulously screened.Using Wilks'formula,the developed uncertainty program code,SAUP,was employed to carry out Latin hypercube sampling,while ISAA was employed to execute batch calculations.Statistical analysis was then conducted on two figures of merit,namely hydrogen generation and the release of fission products within the pressure vessel.Uncertainty calculations revealed that hydrogen production and the fraction of fission product released exhibited a normal distribution,ranging from 182.784 to 330.664 kg and from 15.6 to 84.3%,respectively.The ratio of hydrogen production to reactor thermal power fell within the range of 0.0578–0.105.A sensitivity analysis was performed for uncertain input parameters,revealing significant correlations between the failure temperature of the cladding oxide layer,maximum melt flow rate,size of the particulate debris,and porosity of the debris with both hydrogen generation and the release of fission products.

Yield of long-lived fission product transmutation using proton-, deuteron-, and alpha particle-induced spallation

The transmutation of long-lived fission products through spallation induced by light nuclides was investi-gated for the purpose of determining the feasibility of this approach for long-lived fission products,in both economic and environmental terms.The cross-section data were obtained from the TALYS Evaluated Nuclear Data Library(TENDL).A thick target model was used to study the consumption of the target isotopes in the transmutation process.The transmutation yield was calculated using the highest beam intensity available with the China initiative accelerator-driven system.It was found that the light nuclide-induced spallation reaction can significantly reduce the radio toxicity of the investigated long-lived fission products.Using the transmutation target made of elemental LLFP and the proton beam with an intensity of 5 mA,the consumption of 90 Sr,93 Zr,107 Pd,or 137 Cs can reach approximately 500 g per year.

First-principles study on the diffusion behavior of Cs and I in Cr coating

Cs and I can migrate through fuel-cladding interfaces and accelerate the cladding corrosion process induced by the fuel-cladding chemical interaction.Cr coating has emerged as an important candidate for mitigating this chemical interaction.In this study,first-principles calculations were employed to investigate the diffusion behavior of Cs and I in the Cr bulk and grain boundaries to reveal the microscopic interaction mitigation mechanisms at the fuel-cladding interface.The interaction between these two fission products and the Cr coating were studied systematically,and the Cs and I temperature-dependent diffusion coefficients in Cr were obtained using Bocquet’s oversized solute-atom model and Le Claire’s nine-frequency model,respectively.The results showed that the Cs and I migration barriers were significantly lower than that of Cr,and the Cs and I diffusion coefficients were more than three orders of magnitude larger than the Cr self-diffusion coefficient within the temperature range of Generation-IV fast reactors(below 1000 K),demonstrating the strong penetration ability of Cs and I.Furthermore,Cs and I are more likely to diffuse along the grain boundary because of the generally low migration barriers,indicating that the grain boundary serves as a fast diffusion channel for Cs and I.

Analysis of Fission Fragments Contributors on Total Decay Heat of Thermal Neutron-Induced Fission of U-235

Calculation of the decay heat from the decay/buildup of radionuclides generated after nuclear fission is one of the highest priorities in the nuclear industry. These calculations become more important if they are made together with the analysis of the most important isotopes affecting the decay heat. They are useful in designing the necessary nuclear safety for spent fuels, and their importance cannot be overlooked in the designs of transporting fuel storage containers as well as in the management of the radioactive waste generated. In this paper, by using MATLAB, the decay heat after the thermal fission of a U-235 nucleus was numerically calculated by solving linear differential equations for all the buildups/decays of the fission products. Also, the most contribution of radioactive isotopes to the decay heat was analyzed by using Microsoft Excel. The most influential isotopes were deduced in two ways;either by calculating the most influential isotopes at specific times, or by determining the largest influences in a cumulative manner. All required nuclear data such as decay constants their branching ratios, independent fission yield, and average α-, β-, and γ-energies released per disintegration of any nuclide, have been extracted from the latest version of the Evaluated Nuclear Data Files (ENDF) database ENDF/B-VIII.0. The two different methods used showed a difference in the contributing isotopes, which is logical for the difference in the method of calculation. The first method is suitable for instantaneous data while the second method is more suitable when there is a need to know the cumulative calculations. In sum, we can say that both methods complement each other, and neither of them can be dispensed with in the accurate calculations related to transportation and storage of spent fuel.

ELECTRON MICROSCOPIC AUTORADIOGRAPHIC STUDY ON SUBCELLULAR LOCALIZATION OF FISSIONPRODUCT ^（147）Pm INTISSUE CELLS

The early risk of internal contaminated accumulation of 147Pm is in blood cells and endothelial cells, especially in red blood cells. Then 147Pm is selectively deposited in ultrastructure of liver cells, such as in nucleus, nucleolus, rough endoplasmic reticulum, mitochondria and microbodies. Dense tracks also appear in mitochondria and lysosome of pedal cells within renal corpuscle, and so does in nucleus as well as in mitochondria and microbodies of epicyte of kidney near-convoluted tubule. With the prolongation of observing time, 147Pm is selectively and steadily deposited in subcellular level of organic component for bone. Substantial amount of 147Pm is taken up into the nuclear fraction of osteoclasts and osteoblasts. Particularly, in organelles 147Pm is mainly accumulated in rough endoplasmic reticulum and in mitochondria.Autoradiographic tracks especially localize in combined point between Golgi complex and transitive vesicle of rough endoplasmic reticulum. In addition, numerous 147Pm deposited in collagenous fibre within interstitial of bone cells is hardly excreted.

Nuclear decay database in fission product mass region

Accurate and reliable nuclear decay databases are essential for fundamental and applied nuclear research studies.However,decay data are not usually as accurate as expected and need improvement.Hence,a new Chinese nuclear decay database in the fission product mass region(A=66−172)based on several major national evaluated data libraries has been developed under joint efforts in the CNDC working group.A total of 2358 nuclides have been included in this decay database.Two main data formats,namely ENSDF and ENDF,have been adopted.For the total meanβandγenergies,available data from total absorption gamma ray spectroscopy measurements have been adopted.For some nuclides without experimental measurements,theoretically calculated values have been added.

Segregation behavior and embrittling effect of lanthanide La, Ce, Pr,and Nd at Σ3(111) tilt symmetric grain boundary in α-Fe

The migration of lanthanide fission products to cladding materials is recognized as one of the key causes of fuel–cladding chemical interaction(FCCI) in metallic fuels during operation. We have performed first-principles density functional theory calculations to investigate the segregation behavior of lanthanide fission products(La, Ce, Pr, and Nd) and their effects on the intergranular embrittlement at Σ3(111) tilt symmetric grain boundary(GB) in α-Fe. It is found that La and Ce atoms tend to reside at the first layer near the GB with segregation energies of-2.55 eV and-1.60 eV, respectively,while Pr and Nd atoms prefer to the core mirror plane of the GB with respective segregation energies of-1.41 eV and-1.50 eV. Our calculations also show that La, Ce, Pr, and Nd atoms all act as strong embrittlers with positive strengthening energies of 2.05 eV, 1.52 eV, 1.50 eV, and 1.64 eV, respectively, when located at their most stable sites. The embrittlement capability of four lanthanide elements can be determined by the atomic size and their magnetism characters. The present calculations are helpful for understanding the behavior of fission products La, Ce, Pr, and Nd in α-Fe.

Resource Evaluation of Heavy Rare Earth Derived from the Spent Gd203 Burnable Poison in LWRs

A burnup calculation has been performed to evaluate heavy rare earth （terbium--Tb, dysprosium--Dy） production in spent gadolinium oxide （Gd203） installed as a BP （burnable poison）. Each amount of Tb and Dy production derived from the BP has been about 30-40 times larger than those created in FP （fission products）. Required cooling time to achieve exemption level on radioactivity concentration produced Tb and Dy derived from the BP are much shorter （BP-Tb： 7.9y, BP-Dy： 〈 0.1y） than those created in FP （FP-Tb： 3,616y, FP-Dy： 6.9y）. However, the BP is mixed homogeneously with UO2 pellet in current nuclear fuel system of LWRs （light water reactors）, and hence mixing of FP cannot be not avoided. In such a mixture case, the required cooling time of recovered Tb will become 2,653y and that of recovered Dy be 4.8y. For this reason, recovered Tb is unlikely to be resource for utilization, while recovered Dy must be the resource provided the precise separation from the other FP.

Construction of high-resolution energy-time-of-flight spectrometer for determination of fission fragmentmass distributions

Purpose With the aim of determining mass yield distributions of primary fission products,a one-arm spectrometer was developed based on kinetic energy and time-of-flight correlation measurement technique.Methods An axial grid ionization chamber(GIC)was designed for energy detecting.In order to minimize energy losses and straggling,a thin silicon nitride film with a thickness of 100 nm was performed as the entrance window of the GIC.The energy resolution is 0.38%for 80 MeV ^(63)Cu particles.Two-timing detectors based on the detection of secondary emission electrons by microchannel plates(MCPs)constitute the time pick-off system,and the time-of-flight resolution is better than 200 ps(FWHM)measured with a ^(241)Amαsource.With a flight path length of 47.6 cm,the path length resolution is 0.21%.Results and conclusion The first result of mass distribution from ^(252)Cf spontaneous fission was reported.Energy losses of fragments in dead layers of the spectrometer were corrected event-by-event depend on the Monte Carlo calculation.The mass resolution for light fission fragments peak A107 amu is 1.3 aum.

Corrosion Behavior of GH3535 Alloy Induced by Selenium

Corrosion behavior of a Ni-16Mo-7Cr base superalloy was systematically investigated under a selenium(Se)atmosphere at 700°C.It shows that Se can react with the alloy elements such as Ni,Mo,Cr et al.to form a reaction layer at the surface of alloy,which is mainly composed of Ni_(2)Se_(3),MoSe_(2),and Cr_(3)Se_(4).The tensile properties of the alloy are not greatly affected by changes of Se content.The slight decrease in the tensile strength is attributed to the formation of the reaction layer,which leads to the decrease in the effective undertaking area of the alloy.No intergranular diffusion characteristic of Se elements was observed,and the Se effect of embrittlement on grain boundaries is weaker than that of tellurium(Te).

An advanced partitioning process for key elements separation from high level liquid waste

To separate MA （Am, Cm） and some fission product elements （FPs） such as Tc, Pd, Cs and Sr from high level liquid waste （HLLW） systematically, we have been studying an advanced aqueous partitioning process, which uses selective adsorption as the separation method. For this process, we prepared several novel adsorbents which were immobilized in a porous sili- c^polymer composite support （SiO2-P）. Adsorption and separation behavior of various elements was studied experimentally in detail. Small scale separation tests using simulated HLLW solutions were carried out. Pd（II） was strongly adsorbed by the AR-01 anion exchanger and effectively eluted off by using thiourea. Successful separation of Pd（ll） from simulated HLLW was achieved. Te（VII） also exhibited strong adsorption on AR-01 and could be eluted off by using U（IV） as a reductive eluent. Am（Ⅲ） presented significantly high adsorbability and selectivity onto R-BTP/SiOz-P adsorbents over various FPs including Ln（Ⅲ）. The R-BTP adsorbents were fairly stable in 3 M HNO3, but instable against 7-irradiation-3M HNO3. An advanced par- titioning process consisting of three separation columns for the target elements separation from HLLW was proposed and the obtained experiment results indicated that the proposed process is essentially feasible.

Challenges to develop single-column MA(Ⅲ) separation from HLLW using R-BTP type adsorbents

In order to directly separate trivalent minor actinides （MA： Am, Cm） from fission products （FP） containing rare earths （RE） in high level radioactive liquid waste （HLLW）, the authors have challenged to develop a simplified MA separation process by extraction chromatography using a single column. Attention has been paid to a new type of nitrogen-donor ligands, R-BTP （2,6-bis（5,6-dialkyl-1,2,4-triazin-3-yl） pyridine, R： alkyl group） as an extractant because it shows high extraction selectivity for Am（Ⅲ） over RE（Ⅲ）. It is known that the R-BTP ligands show different properties such as adsorbability and stability by hav- ing different alkyl groups. Therefore, some novel adsorbents were prepared by impregnating different types of R-BTP ligands （isohexyl-, isoheptyl- and cyheptyl-BTP） and a similar ligand to the R-BTP, ATP （2,6-bis（l-aryl-lH-tetrazol-5-yl）pyridines）, into the porous silica/polymer support （SiOrP particles）. This work deals with comparison in adsorption and desorption prop- erties of Am and some FP in HNO3 solution onto such R-BTP type adsorbents, as well as chemical and radiolytic stability of the adsorbents. Then the possibility of a single-column separation of MA from main FP was pursued by evaluating the results of column experiments using the most promising adsorbent （isohexyl-BTP/SiO2-P） under temperature control. In addition, elu- tion behaviors of U and Pd were also estimated.

Effect of ligand initial conformation and counteranion on complexation behaviors of R-BTBP toward Pd(Ⅱ)contained in highly active liquid waste

Soft N-donor bis-triazin bipyridines derives(R-BTBP)are a type of very promising extratant for extraction and complexation with long-lived trivalent minor actinides over lanthanides from highly active liquid waste(HLW).In addition to minor actinides,R-BTBP also holds very strong complexation ability toward fission palladium.However,few studies have been focused on the separation and complexation with the fission product Pd(Ⅱ)by R-BTBP.Herein,the complexation behaviors of Pd(Ⅱ)with four typical R-BTBP ligands were systematically studied by single crystal X-ray diffraction,1H NMR titration and theoretical calculation.The effects of R-BTBP initial conformation and nitrate anions on the complexation behaviors of R-BTBP with Pd(Ⅱ)were thoughtfully analyzed.Both the 1:1 and 2:1 binuclear complexes could be formed between Pd(Ⅱ)and R-BTBP with initialⅡconformation in the presence of nitrate anions,while only one 1:1 type Pd(Ⅱ)complex could be formed for those with initial OO conformation.Without nitrate anion,only one 1:1 type complex was formed in solution.The structure of the 1:1 Pd(Ⅱ)/R-BTBP complex was firstly characterized by single crystal crystallography.DFT calculation results showed that a significant large rotational energy barrier(21.8~22.6 kcal/mol)must be overcome to form theⅡtype 2:1Pd(Ⅱ)complex for those OO type R-BTBP ligands,however which would not prevent them from forming the 1:1 type complex.