Search

This paper summarized the efforts performed to understand decay heat estimation from existing spent nuclear fuel (SNF), under the auspices of the Working Party on Nuclear Criticality Safety (WPNCS) of the OECD Nuclear Energy Agency. Needs for precise estimations are related to safety, cost, and optimization of SNF handling, storage, and repository. The physical origins of decay heat (a more correct denomination would be decay power) are then introduced, to identify its main contributors (fission products and actinides) and time-dependent evolution. Due to limited absolute prediction capabilities, experimental information is crucial; measurement facilities and methods are then presented, highlighting both their relevance and our need for maintaining the unique current full-scale facility and developing new ones. The third part of this report is dedicated to the computational aspect of the decay heat estimation: calculation methods, codes, and validation. Different approaches and implementations currently exist for these three aspects, directly impacting our capabilities to predict decay heat and to inform decision-makers. Finally, recommendations from the expert community are proposed, potentially guiding future experimental and computational developments. One of the most important outcomes of this work is the consensus among participants on the need to reduce biases and uncertainties for the estimated SNF decay heat. If it is agreed that uncertainties (being one standard deviation) are on average small (less than a few percent), they still substantially impact various applications when one needs to consider up to three standard deviations, thus covering more than 95% of cases. The second main finding is the need of new decay heat measurements and validation for cases corresponding to more modern fuel characteristics: higher initial enrichment, higher average burnup, as well as shorter and longer cooling time. Similar needs exist for fuel types without public experimental data, such as MOX, VVER, or CANDU fuels. A third outcome is related to SNF assemblies for which no direct validation can be performed, representing the vast majority of cases (due to the large number of SNF assemblies currently stored, or too short or too long cooling periods of interest). A few solutions are possible, depending on the application. For the final repository, systematic measurements of quantities related to decay heat can be performed, such as neutron or gamma emission. This would provide indications of the SNF decay heat at the time of encapsulation. For other applications (short- or long-term cooling), the community would benefit from applying consistent and accepted recommendations on calculation methods, for both decay heat and uncertainties. This would improve the understanding of the results and make comparisons easier.

Japanese Evaluated Nuclear Data Library version 5 (JENDL-5) was released in 2021. JENDL-5 is intended to extend its generality from JENDL-4.0 by covering a wide variety of nuclear data for applications not only to nuclear design and decommissioning, but also to other radiation-related fields. Overview of JENDL-5 and a plan for the next of JENDL-5 are presented.

JENDL group has developed evaluated fission yield and decay data. They are now planned to be upgraded in order to embody new experimental data and theoretical developments obtained after the last release. In this paper, we briefly describe a plan and present activity toward next JENDL fission yield and decay data.

A new database containing all available experimental and evaluated β-delayed neutron data is presented in this paper. The database is the product of an international effort coordinated by the International Atomic Energy Agency. It comprises a microscopic section including all available experimental data on beta-decay half-lives, β-delayed neutron emission probabilities and spectra, as well as new systematics and global theoretical calculations for comparison. The beta-delayed neutron data for individual precursors have been benchmarked against available data on macroscopic properties such as total delayed-neutron yields and spectra, delayed-neutron decay curves and time-dependent group parameters using the summation method. All available measured macroscopic data have also been compiled in the macroscopic section of the database. The dedicated database is available online on the IAEA server (URL:http://www-nds.iaea.org/beta-delayed-neutron/database.html).

Recent progress and future plan of the JENDL project are summarized. Two special purpose files were released recently. One is Photo-nuclear Data File 2016 (JENDL/PD-2016) which contains the data of photo-nuclear reaction cross sections covering a wide area of the nuclear chart. The other one is the JENDL Activation Cross Section File for Nuclear Decommissioning 2017 (JENDL/AD-2017) which provides production cross sections of radioactive nuclei by neutrons. A special purpose file JENDL/ImPACT-2018 dedicated to transmutation of long-lived fission products will be released in 2019. Regarding the general-purpose file, we are preparing to release the next version, which would be made available by 2022 as JENDL-5. Evaluation efforts on nuclides across wide range of nuclei form light to heavy have being made. The first test version JENDL-5α1 was created in 2018. The evaluated data will be updated on the basis of benchmark tests on reactor criticalities and neutron shieldings.

Status of the JENDL project after releasing the latest general purpose file JENDL-4.0 is described. By correcting errors and adding covariance data of JENDL-4.0, 38 files have been released as JENDL-4.0u. Development of next general purpose file JENDL-5 is also in progress. New evaluations have been performed for light nuclei, structural materials and fission products. Two special purpose files, JENDL-4.0/HE and JENDL/DDF-2015 were released in 2015. The former includes neutron induced reaction data for 130 nuclides in the energy region up to 200 MeV as well as proton induced reaction data for 133 nuclides. The latter is the decay data file of 3,237 nuclides from Z = 1 to 104 and neutron. Other two special purpose files of activation cross section and photonuclear data are under preparation and will be released soon.

Neutron-nucleus cross sections calculated by macroscopic potentials are compared with a microscopic one to study the performance for long-lived fission products. The macroscopic potentials show a good agreement with the microscopic one at higher energies, where neutron experimental data are scarce. Besides it, analyses of differential elastic cross sections at low energies also suggest that the macroscopic potentials are still effective and applicable enough for the long-lived fission products.

Interference effect of neutron capture cross section between the compound and direct processes is investigated. The compound process is calculated by resonance parameters and the direct process by the potential model. The interference effect is tested for neutron-rich 82Ge and 134Sn nuclei relevant to r-process and light nucleus 13C which is neutron poison in the s-process and produces long-lived radioactive nucleus 14C (T1/2 = 5700 y). The interference effects in those nuclei are significant around resonances, and low energy region if s-wave neutron direct capture is possible. Maxwellian averaged cross sections at kT = 30 and 300 keV are also calculated, and the interference effect changes the Maxwellian averaged capture cross section largely depending on resonance position.

Some fission yields data of JENDL FP Fission Yields Data File 2011 (JENDL/FPY-2011) revealed inadequacies when applied to delayed neutron related subjects. The sensitivity analyses of decay heat summation calculations also showed some problems. From these results the fission yields of JENDL/FPY-2011 have been revised. The present report describes the revision of the yield data by emphasizing the sensitivity analyses.

Nuclear β-decay and delayed neutron (DN) emission is important for the r-process nucleosynthesis after the freeze-out, and stable and safe operation of nuclear reactors. Even though radioactive beam facilities have enabled us to measure β-decay and branching ratio of neutron-rich nuclei apart from the stability line in the nuclear chart, there are still a lot of nuclei which one cannot investigate experimentally. In particular, information on DN is rather scarce than that of T1/2. To predict T1/2 and the branching ratios of DN for next JENDL decay data, we have developed a method which comprises the quasiparticle-random-phase-approximation (QRPA) and the Hauser-Feshbach statistical model (HFSM). In this work, we calculate fission fragments with T1/2 ≤ 50 sec. We obtain the rms deviation from experimental half-life of 3:71. Although the result is still worse than GT2 which has been adopted in JENDL decay data, DN spectra are newly calculated. We also discuss further subjects to be done in future for improving the present approach and making next generation of JENDL decay data.

Recently, great interest has arisen concerning 177Lu as one of the most important therapeutic radionuclides for treating neuroendocrine tumors. This has driven demand for 177Lu with high radionuclide purity produced in accelerators, by using highly enriched 176Yb samples to develop a variety of 177Lu-labelled radiopharmaceuticals. In this paper, a method is presented to estimate isotopic compositions of enriched 176Yb samples required for achieving large-scale production of 177Lu having high radionuclide purity by the 176Yb(d,x)177Lu reaction. These isotopic compositions were estimated using the latest measured excitation functions of the natYb(d,x)Lu reaction that were confirmed in this study by measuring the integral yields of the reaction, and a particle transport simulation code. The estimated radionuclide purity of 177Lu for commercially available enriched 176Yb sample is over 99% at a deuteron energy of 20 MeV. The method plays an important role in determining isotopic compositions of enriched samples for producing high purity medical radionuclides in accelerators. [ABSTRACT FROM AUTHOR]

We measured the yields of radionuclides after neutron irradiation on an enriched 68ZnO sample covered with polyethylene blocks. The neutrons were generated from the 9Be(d,n) reaction by 40 and 50 MeV deuterons. For 50 MeV deuterons we observed anomalously enhanced yields of 67Ga, 66Ga, 69mZn, and 64Cu, more than 20-times larger than the yields for a 68ZnO sample without the polyethylene blocks. At the same time, the yields of the same radioisotopes produced from an enriched metallic 68Zn sample were almost insensitive to the presence of polyethylene. On the other hand, for 40 MeV deuterons the enhanced production was not observed. This finding would provide a unique capability of accelerator neutrons to simultaneously produce large amounts of several radioisotopes via neutron-induced reactions on a single sample. The experimental data are compared with the yields calculated by the simulation code Particle and Heavy Ion Transport code System (PHITS). The calculated yields of these radioisotopes produced from the 68ZnO sample covered with the polyethylene blocks considerably disagree with the measured ones, while for the 68ZnO sample without the blocks they agree with each other. [ABSTRACT FROM AUTHOR]

β-decay half-lives around 78Ni and 132Sn are studied with proton-neutron Quasiparticle Random Phase Approximation on the basis of Skyrme-Hartree-Fock (SHF) method. We take into account tensor force correlation in calculating the ground and excited states and find it improves the half-life of 132Sn. However, the Q value is still underestimated. We also discuss that half-lives do not strongly depend on type of particle-particle interaction in isospin T = 0 channel. [ABSTRACT FROM AUTHOR]

Abstract: We estimated statistical uncertainties of a nondestructive assay system using nuclear resonance fluorescence (NRF) for spent nuclear fuel including low-concentrations of actinide nuclei with an intense, mono-energetic photon beam. Background counts from radioactive materials inside the spent fuel were calculated with the ORIGEN2.2-UPJ burn-up computer code. Coherent scattering contribution associated with Rayleigh, nuclear Thomson, and Delbrück scattering was also considered. The energy of the coherent scattering overlaps with that of NRF transitions to the ground state. Here, we propose to measure NRF transitions to the first excited state to avoid the coherent scattering contribution. Assuming that the total NRF cross-sections are in the range of 3–100eVb at excitation energies of 2.25, 3.5, and 5MeV, statistical uncertainties of the NRF measurement were estimated. We concluded that it is possible to assay 1% actinide content in the spent fuel with 2.2–3.2% statistical precision during 4000s measurement time for the total integrated cross-section of 30eVb at excitation energies of 3.5–5MeV by using a photon beam with an intensity of 106 photons/s/eV. We also examined both the experimental and theoretical NRF cross-sections for actinide nuclei. The calculation based on the quasi-particle random phase approximation suggests the existence of strong magnetic dipole resonances at excitation energies ranging from 2 to 6MeV with the scattering cross-sections of tens eVb around 5MeV in 238U. [Copyright &y& Elsevier]