Handling Of Spent Nuclear Fuel Research Articles

An introduction to Spent Nuclear Fuel decay heat for Light Water Reactors: a review from the NEA WPNCS

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.

Активність і залишкове енерговиділення ядерного палива під час експлуатації і зберігання

General information on the amount of accumulated spent nuclear fuel (SNF) at NPPs in the world is presented. An analysis of the rates of SNF accumulation was carried out, and forecasts of SNF accumulation in the world and in Ukraine for the next decade were made. Information is given on the technologies for SNF handling, as well as some characteristics of SNF storage systems of wet and dry types. Radiation characteristics of SNF — radioactivity and residual energy release of nuclear fuel — largely determine both possible accident scenarios and possible radiation consequences of accidents at nuclear facilities. The article presents the results of modeling in the SCALE program code of changes in radioactivity and residual energy release of VVER-1000 nuclear fuel after period of operation of the fuel assembly in the VVER-1000 core. The model of VVER-1000 fuel assembly with 4.4% enrichment, which is used in the mode of stationary refueling, was chosen for simulation. The simulation results were compared with the data given in the relevant reference books regarding the change in the radiation characteristics of nuclear fuel during its operation in the VVER-1000. The results of comparison of the radiation characteristics of VVER-1000 fuel assemblies with the same burnup, but with different fuel loading schedules in the last year of operation are presented for a four-year fuel campaign. A significant influence on the radiation characteristics (radioactivity and residual energy release) of the fuel assembly load mode has been demonstrated. The simulation results show that the specific radioactivity and, accordingly, the total radioactivity of nuclear fuel in the VVER-1000 cores at the Zaporizhzhia NPP after ~1 year of forced outage decreased by ~100 times. Therefore, the possible radiation consequences in case of damage of nuclear fuel located in the core or unloaded into the spent fuel pools will be much lower than in the case of an accident at an operating reactor. The same applies to comparisons of the consequences of a possible accident at the ZNPP with the consequences of the accident at the Chornobyl NPP in 1986, which occurred at a working reactor with the release of at least ~3–5 % of nuclear fuel.

STATISTICS OF FAILURES AND MONITORING OF THE STATE OF POOLS AND PROTECTIVE HERMETIC FENCES OF NUCLEAR POWER PLANTS

Assessment of the technical condition of the main systems and structures is one of the main stage tasks of technical diagnostics of NPP elements. One of the important elements of the hermetic volume of the protective shells of nuclear power plants is a storage pool (SP) of spent nuclear fuel (SNF). For provision of SNF storage the appropriate systems and structures are used: SNF handling system, SP cooling system and SP by itself. Pools are made of monolithic reinforced concrete with stainless steel lining intended to retain cooling water (boric acid solution) and protect the concrete from direct effect of aggressive environments. Emergency leaks may occur through the SP lining during operation of a nuclear power plant. Materials used for pool structures must be radiation resistant and resistant to a boric acid solution. The structure of the storage pool must be earthquake-proof, must allow monitoring of emergency leaks and maintain a controlled water level in case of leaks for the time period until the fuel will be discharged. Features of failures during operation, results of research and recommendations for extending the service life of storage pools at domestic nuclear power plants with VVER are considered. Assessment of the technical condition of the main structures of the SP provides for specialized research. Thus, the SP survey generally involves visual and instrumental examination of the bottom lining, internal lining, as well as concrete walls and bottom.

Structural integrity of a high-burnup spent fuel rod under drop impact considering pellet-clad interfacial bonding influence

Abstract Variation of several uncertain properties of an irradiated fuel assembly would result in changes in the buckling and bending resistance that could lead to fuel rod failure in a drop accident. Dynamic analysis of the fuel rod composite is a very complicated process due to limited knowledge on important parameters relevant to design safety, such as the material properties of the irradiated cladding and the interaction between fuel pellets and cladding. In order to better understand the behavior of a spent nuclear fuel (SNF) rod in a drop impact, a detailed finite-element (FE) model for a single fuel rod was developed considering post-irradiated fuel conditions. Free-drop analyses of two bounding cases were conducted using FE analysis, with ten assumed generalized interface bonding and de-bonding configurations to demonstrate the pellet-clad interfacial bonding influence on fuel rod behavior. It is revealed that the interfacial bonding between pellet and cladding shows a significant influence on the magnitude of the maximum principal plastic strain and its location, and on the overall structural resistance. The key feature of the proposed analysis procedure is seen in its capability to consider a range of parameters that can be included in the FE model, in addition to providing a more realistic and descriptive prediction of fuel rod behavior. The procedure and methodology developed for the numerical simulation in this work can be further extended to support the development of guidelines for safe handling of SNF when augmented by realistic data obtained experimentally.

The results of the transmutation of fission fragments in the spectrum of neutrons of thermal and fast reactors

Radioactivity of spent nuclear fuel (SNF) discharged from nuclear reactor core is determined during the first 100 years by fission fragments (FF), after that the main contribution in the SNF activity is made by actinides. Existing scenarios of SNF handling are based on the transmutation of minor actinides (MA) into fission fragments accomplished in fast reactors. Scenarios of transmutation of fission fragments in thermal and fast neutron spectra and time-dependent radiation characteristics are examined in the present study. Nuclide composition of fission fragments is taken from the results of simulation of burnup of 439GT fuel assembly (TVSA type) for VVER-1000 nuclear reactor during 3 years performed using MCU-5 software complex. The obtained data were used for determining starting nuclide composition for different cooling-down times prior to the beginning of transmutation (irradiation in neutron fluxes) to be input in the ORIGEN2 code.The following three options of irradiation of fission fragments are presented: transmutation without cooling down, cooling down fission fragments during 4 years prior to irradiation, cooling down fission fragments during 30 prior to irradiation. Duration of irradiation was selected to be equal to 3 and 15 years. Efficiency of transmutation was determined using time-dependent “transmutation factor” equal to the ratio of radioactivity of nuclides in the process of transmutation and after its completion to their radioactivity without transmutation.The calculated values of transmutation factors proved to be noticeable only during irradiation in reactor core: these values reached 5–10 and were dependent only on the duration of fission fragment cooling down prior to the beginning of transmutation. After removal of fission fragments from neutron flux transmutation factor decreased to unity within several years. After one hundred more years after irradiation in neutron spectrum of thermal reactor transmutation factor reduces to 0.8–0.5 depending on the duration of the transmutation process. Slight growth of transmutation factor to the values of 1.2–1.8 was observed after irradiation in fast reactor spectrum within the time interval of 200–1000 years and after 1000 years following this its reduction to the value of 0.9–0.7 was noted.The main conclusion is that purposeful incineration of fission fragments is senseless because only insignificant gain in radioactivity (a little less than by the factor two) is achieved after 1000 years.The indifference of fission fragments with regard to transmutation can be partially explained by the fraction of stable nuclides which increases with extension of the period of fission fragments cooling down. Upon completion of the cycle of fuel use it contains approximately 15% of stable nuclides among fission fragments, and after 30 years of cooling down the fraction of stable isotopes reaches 85%.

Current state of nuclear fuel cycles in nuclear engineering and trends in their development according to the environmental safety requirements

The state and trends in the development of nuclear fuel cycles in nuclear engineering, taking into account the ecological aspects of using nuclear power plants, are considered. An analysis of advantages and disadvantages of nuclear engineering, compared with thermal engineering based on organic fuel types, was carried out. Spent nuclear fuel (SNF) reprocessing is an important task in the nuclear industry, since fuel unloaded from modern reactors of any type contains a large amount of radioactive elements that are harmful to the environment. On the other hand, the newly generated isotopes of uranium and plutonium should be reused to fabricate new nuclear fuel. The spent nuclear fuel also includes other types of fission products. Conditions for SNF handling are determined by ecological and economic factors. When choosing a certain handling method, one should assess these factors at all stages of its implementation. There are two main methods of SNF handling: open nuclear fuel cycle, with spent nuclear fuel assemblies (NFAs) that are held in storage facilities with their consequent disposal, and closed nuclear fuel cycle, with separation of uranium and plutonium, their purification from fission products, and use for producing new fuel batches. The development of effective closed fuel cycles using mixed uranium–plutonium fuel can provide a successful development of the nuclear industry only under the conditions of implementation of novel effective technological treatment processes that meet strict requirements of environmental safety and reliability of process equipment being applied. The diversity of technological processes is determined by different types of NFA devices and construction materials being used, as well as by the composition that depends on nuclear fuel components and operational conditions for assemblies in the nuclear power reactor. This work provides an overview of technological processes of SNF treatment and methods of handling of nuclear fuel assemblies. Based on analysis of modern engineering solutions on SNF regeneration, it has been concluded that new reprocessing technologies should meet the ecological safety requirements, provide a more extensive use of the resource base of nuclear engineering, allow the production of valuable and trace elements on an industrial scale, and decrease radioactive waste release.

Development of the RAW and SNF handling system in the Federal Atomic Energy Agency and in the Russian Federation

The experience the Russian Federation has gained over the period longer than half a century in handling radioactive wastes (RAW), including their burial in geological formations, is quite unique. These guidelines state the necessity of establishing an integrated state RAW and spent nuclear fuel (SNF) handling system, ensuring safe, effective, efficient, and socially-acceptable development of using nuclear power to solve issues of national security in all its aspects and utilising secondary recovered and re-used raw materials. This system is increasing safety of handling radioactive materials at all stages of their life cycle.