[en] This publication reports on the outcome of a technical meeting on high burnup fuel experience and economics, held in Buenos Aires, Argentina in 2013. The purpose of the meeting was to revisit and update the current operational experience and economic conditions associated with high burnup fuel. International experts with significant experience in experimental programmes on high burnup fuel discussed and evaluated physical limitations at pellet, cladding and structural component levels, with a wide focus including fabrication, core behaviour, transport and intermediate storage for most types of commercial nuclear power plants

[en] Due to the extension of the permanence time of the nuclear fuels within the reactor, physical and chemical modifications take place in the fuel material, especially in the external ring of the fuel pellet. The codes for simulating the rod behaviour during reactor operation need adequate models to describe these phenomena and be capable of making accurate predictions in the whole burnup range. A complex group of subroutines has been included in DIONISIO to represent the radial distribution in the pellet of the power density, burnup, porosity and concentration of diverse nuclides, particularly those capable of undergoing fissions, in terms of overall parameters like initial enrichment and average burnup. In this work we summarize the models recently developed, related to the high burnup scenario. On the one hand, empirical expressions representing the absorption and capture cross sections of several uranium and plutonium isotopes, as functions of the initial enrichment in "2"3"5U, average burnup and radial coordinate are presented. In addition, new models that give the distribution of porosity, fission gas retention and release in the pellet edge are described. Moreover, an empirical formula that relates the thermal conductivity of the fuel material with the burnup and the content of gadolinium, usually added as burnable poison, is presented. Subroutines corresponding to each of these models have been incorporated to the DIONISIO code. With these improvements the code was used to simulate data provided by the FUMEX I/II/III NEA data bank. The results presented here make evident the good agreement between experiments and simulations. (author)

[en] Slovenske elektrarne operate four units of VVER 440 V213 type. Nuclear fuel cycle is being continuously modified in order to satisfy current operational requirements as well as to optimize nuclear fuel cycle costs. These trends caused that fuel type has been modified practically every couple of years during the last decade, based on the fuel portfolio offered by the producer - TVEL. The latest fuel type, first time loaded into the reactor core in 2011, is designed to reach relatively high burnup values up to 72,5 MWd/ng for fuel rod or up to 65 MWd/ng for fuel assembly in six year fuel cycle. These values are substantially higher than previously reached values and are therefore subject to detail safety assessment during the licensing phase. In order to obtain license for the nominal design burnup values, Slovenske elektrarne had to launch a new initiative in order to summarise an adequate safety case. At the moment the licences are still limited for four years of operation only and utility is working on relevant documentation to be submitted to NRA for approval. The paper describes main technical issues and overall strategy selected in order to fulfill its licensing intent. (author)

[en] The nuclear power plants in Switzerland have followed since long the strategy to upgrade burnups, aiming fundamentally at front-end incentives to reduce the assembly production and intermediate storage costs. Some of them implemented a rather aggressive high burnup strategy, like the Gösgen nuclear power plangits core design and fuel management needed to be adapted and the safety of high burnup fuel had to be consequently demonstrated through long-term experimental programs. In particular, new zirconium-based alloys were tested and fuel behaviour during normal and transient conditions required extensive analytical and experimental validation. The paper shows a summary of the basic results of these programs for Gösgen, leading to the current licensed burnup limits that represent record commercial values for this reactor type (70, 75 and 82 GWd/tHM for maximum fuel-assembly average, maximum fuel rod average and maximum local burnups, respectively). Additionally to the technical and nuclear safety challenges that this effort required, the economic aspects of the introduction of high enrichment, high burnup core loadings on the front- and back-end fronts are briefly summarised. (author)

[en] If CNEA wants to handle the fuel elements technology for reactors of Generation III (high bum-up and cooling water at higher temperature and pH) must access to the technology of zirconium alloys low in Nb and free of tin. This paper present a roadmap to achieve, as semi-finished product, strips of Zr-le-O alloy of about 27 mm wide and 1 mm thick which must reach the standards for the yield stress and creep resistance of the commercially available French M5"T"M alloy or Russian E110 alloy. (author)

[en] Atucha I is a pressurized heavy water reactor, cooled and moderated with heavy water by two separate systems. In the year 1985, several natural uranium fuel elements failed owing to pellet-cladding interaction (PCI), after a cycle of power. The aim of this work is to assess TRANSURANUS Code capabilities regarding PCI failure prediction for Atucha-type fuels by using the available information from the above mentioned incident. Input preparation required the revision of old site data, recount on historical fuel rod design changes, and the execution of new neutronic simulations. A base case was calculated with as-delivered code executable and using recommended input parameters. Afterwards, a very detailed sensitivity study was conducted. The purpose of it was to assess the influence on results of model parameters and especially of fuel material properties. The SPAKOR PCI model on TRANSURANUS code was also studied and some model parameters were updated for Atucha type fuels, in particular Young modulus of Zr-4 and yield and rupture stresses for burst tests, provided by the fuel vendor (CONUAR). Overall results were compared and finally an Atucha-suited re-compiled code executable was selected for oncoming PCI calculations of Atucha-type fuels. Furthermore, uncertainties in power history data were identified and therefore it was concluded that boundary conditions provided in the performed calculations highly influenced failure/non failure code predictions. In light of this, it was concluded that results had to be analyzed on a global basis for each of the calculated sensitivity cases. The performed assessment contributed to gaining confidence on TRANSURANUS Code predictions when simulating natural uranium fuels of low burnup, for which phenomena have not been deeply investigated lately, as this type of fuels is rather unusual among the fuel designs currently found in the nuclear industry. (author)

[en] The MIR-LOCA/ 60 experiment with VVER high burnup fuel was performed in the MIR research reactor (SSC- RIAR) in 2010. Experimental fuel assembly with 16-th fresh and three refabricated VVER-1000 type test fuel rods with burnup of 58.1 - 58.6 MWd/ng was tested.The purpose of MIR-LOCA/60 experiment was the study of thermomechanical behaviour of VVER high burnup fuel at typical conditions for LOCA design accidents. The maximum indications of fuel thermal couple (TC) didn’t exceed the level of ∼860°C, TC registration for fresh fuel rods cladding was about 820°C. The duration of cladding temperature holding at the level of more than 700 °C was about one minute. As a result of PIE of the tested FA deformations and corrosion of claddings and structure of fresh (unirradiated) and spent fuel pellets were defined. In experiment there was a rupture of four fresh fuel rods. Refabricated fuel rods were remained intact. Fracture and relocation of fuel pellets of refabricated fuel rods weren't observed. Post-test neutron, thermohydraulic and thermomechanical calculations were carried out. As a result the estimations of temperature regime for refabricated fuel rods were received. The assessment of the maximum temperature of refabricated fuel rod claddings is in the range of ∼700 - 800°C. (author)

[en] Fragmentation and relocation of the high burnup fuel pellets was observed in Halden LOCA tests and in Studsvik hot cell experiments. A simple model has been developed for the analysis of the mechanical behaviour of the fuel pellet to explain the fragmentation mechanism. High pore pressure and porosity can cause such stresses in high burnup fuel pellet that can lead to fragmentation of UO_2. (author)

[en] The version 2.0 of the DIONISIO code has been recently developed with the purpose of improving the accuracy of the simulation of the whole fuel rod. To this end, the rod is divided into a number of axial segments. The local values of linear power and coolant temperature are given as input data to DIONISIO 1.0 which is executed in each segment obtaining as outputs the local values of temperature, stress, strain, among other physical variables. Then, the general rod parameters (internal rod pressure, amount of fission gas released, pellet stack elongation, etc.) are evaluated at the end of every time step, conveniently combining the results of all the axial segments. The new code architecture allows taking into account the axial variation of the linear power and, consequently, evaluating the dependence of all the significant rod parameters with the longitudinal coordinate. Moreover, new calculation tools designed to extend the application range of the code to high burn up have also been incorporated to DIONISIO 2.0 in recent times and are the subject of other presentation. With these improvements, the code results are compared with some experiments published in the IAEA data base, covering more than 380 fuel rods irradiated up to average burnup levels of 40-60 MWd/ng. The results of these comparisons, which are presented here, reveal the good quality of the simulations. (author)

[en] The formation of the RIM- or High Burnup Structure (HBS) is possibly the most significant example of the restructuring processes affecting commercial nuclear fuel in-pile. The HBS forms at the relatively cold outer rim of the fuel pellet, where the local burnup is 2-3 times higher than the average pellet burnup, under the combined effects of irradiation and thermo-mechanical conditions determined by the power regime and the fuel rod configuration. The main features of the transformation are the subdivision of the original fuel grains into new sub-micron grains, the relocation of the fission gas into newly formed intergranular pores, and the absence of large concentrations of extended defects in the fuel matrix inside the subdivided grains. The characterization of the newly formed structure and its impact on thermo-physical or mechanical properties is a key requirement to ensure that high burnup fuel operates within the safety margins. This paper presents a synthesis of the main findings from extensive studies performed at JRC-ITU during the last 25 years to determine properties and behaviour of the HBS. In particular, microstructural features, thermal transport, fission gas behaviour, and thermo-mechanical properties of the HBS will be discussed. The main conclusion of the experimental campaigns is that the HBS does not compromise the safety of nuclear fuel during normal operations. (author)