Sensitivity studies of sfr unprotected transients with global neutronic feedback coefficients

International audienceImprovements on SFR design are expected to meet the safety goals of GEN IV reactors. One main objective is to enhance the core behaviour during unprotected transients to increase the level of prevention of severe accidents. However, performing a detailed safety analysis for all initiators requires many multiphysical analyses and is a rather lengthy process when designers need to assess safety trends quickly. To compare some core design options from a safety point of view, simplified modelling using the global neutronic feedback coefficients is able to estimate the core behaviour during unprotected transients. The paper explains how to use these coefficients to provide some trends for these transients like loss of flow (ULOF) or loss of heat sink (ULOHS). Main parameters to optimize the inherent safety of SFR cores are discussed to show for example that the primary pumps halving time is not always the key for improving the ULOF behaviour. The paper shows that the ULOF inherent behaviour of a core can be driven by one single coefficient. The paper gives also some validation insights of this methodology and an analytical comparison of some French SFR cores

Evaluation of the neutronic Superphenix start-up commissioning tests with TRIPOLI4

International audienceThe Superphenix reactor was a Sodium-cooled Fast Reactor (SFR) developed under a European framework that intended to provide a general design of a commercial power plant. During the start-up of the reactor, six different batches containing dummy assemblies allowed to verify the main safety criteria at zero-power conditions tests at 180DC. The present paper presents a stochastic analysis of two start-up cores by comparing the experimental results with the performed calculations with TRIPOLI-4 and the JEFF 3.1.1 nuclear data evaluations. The main and back-up control rod evaluations, the critical mass, as well as the handling error experiment are examined. In addition, two particular irradiation tests are studied to determine the flux distribution of the core and finally, the assessment of a subcriticality analysis is performed to verify the validity of the former MSM factors. In general, the obtained results show good agreement with the experiment, having a general reactivity bias with the performed calculations, the flux distribution is also in great agreement with the tests with small discrepancies and the MSM evaluation permitted to observe better consistency with experimental data. The obtained results allowed to validate our model and will serve as reference calculation for further research

Analysis of the feedback coefficients of the Superphenix start-up core with APOLLO3

International audienceThe development of Sodium-cooled Fast Reactors (SFRs) is promoted by the Generation IV International Forum (GIF) since its design and operation is competitive against other technologies and projects. In France, considerable experience has been acquired with three experimental facilities: Rapsodie, Phénix and Superphénix. The Superphénix was a large-size SFR that remains as a unique source of data for these cores. Particularly, during the start-up a set of tests were performed to check the safety criteria of the core in which the feedback coefficients of the core were assessed. These feedback coefficients are the K (1°C variation at the inlet of the core temperature), G (1°C variation in the sodium temperature elevation through the core) and H (1% variation of the nominal power). In this paper, the evaluation of these coefficients is performed using two neutronic platforms: ERANOS and the new APOLLO3. By using these two codes the elementary feedback coefficients are calculated and allow evaluating the global feedback coefficients with a simplified model. The results show the same trend than measurements for the K and H coefficients while for the G coefficient a significant discrepancy is observed at 80% nominal power. The use of a thermal-hydraulic system code such as CATHARE-3 (eventual-ly complemented by CFD calculations) is envisaged to better understand the sources of this bias, which might be due to the simplistic thermal hydraulic models unable to tackle some physical effects of the core operation

Some considerations on the design of a small versatile fast reactor

International audienceIAEA compiled a list of around 56 Small Modular Reactors (SMR) currently under consideration across the world in the 2018 Supplement to its Advanced Reactors Information System (ARIS). Several of the reactors described in this reference are currently under construction or under licensing, while the majority of them are currently being designed. Out of these 56 SMR concepts, only 9 fast reactors designs are under consideration, with 7 of these designs considering the use of lead or leadbismuth eutectic as a coolant. None of these reactors concepts are under construction as of now

Application of Minor Actinides as Burnable Poisons in Sodium Fast Reactors

International audienceThe reduction of the initial excess reactivity in fast reactor cores would enhance their inherent safety level as it would reduce the impact of a control rod withdrawal accident and lower the requirements on the absorption ability of control rods design. Compensation for burn-up reactivity loss is considered as a possible solution to limit initial excess reactivity. Minor actinides challenge the long-term nuclear waste management. Minor actinides can be transmuted from absorber isotopes to fissile isotopes, which show the possibility of their application as burnable poisons.Two loading modes of minor actinides as burnable poisons are considered in this paper the first one, denominated homogenous mode, mixes minor actinides with all the fuel and the second one, denominated hybrid mode, packages minor actinides in independent pins in the fuel assemblies. The content of americium or neptunium in these two designs is considered with regards to current technological feasibility, including burn-up, cladding stress, decay heat and the neutron source of the assemblies considered here. Both these two modes are able to compensate for the reactivity loss of an industrial power core and thus reduce excess reactivity at the beginning of cycle. The application of these designs in the cores with higher reactivity loss will be considered in next step work.The impact of minor actinides loading on the core characteristics, including power distribution, material balance and feedback coefficient, are considered from the assembly level to the core level. The hybrid mode shows better management feasibility while the use of neptunium exhibits lower impacts on the current fuel recycling

Design Directions of Optimized Reactivity Control Systems in Sodium Fast Reactors

International audienceIn sodium fast reactors, the control rods, a movable cluster of open pins with boron carbide as absorber, are almost the only approach to control reactivity. Boron-10 has a good absorption ability in fast spectrum and its enrichment can be adjusted to satisfy various requirements. However, boron carbide behavior under irradiation and the liability coming from the high initial core excess reactivity justify the optimization of reactivity control in Generation-IV fast reactors. This paper discusses the relationship between boron-10 enrichment in the control rods and reactivity management using a series of representative cores. Then, various alternatives designs to improve or supplement classical control rods are discussed and their feasibility is investigated. The results show that large cores with small power density have usually small reactivity loss and thus the effective control system design is limited by the shutdown function, e.g. the need to keep enough negative reactivity stored in the rods to stop the chain reaction at any time. In these cores, alternative absorbers can be considered for boron carbide substitution, among which hafnium hydride based materials are good candidates. Moreover, the limited introduction of moderating materials is also a potential solution to optimize the control rods in such fast reactors.The main constraints on the reactivity control system will be found for the cores with high reactivity loss, which are usually small modular or prototype cores. In these cores, control rods with high boron-10 enrichment are required for reactivity loss compensation. At the same time, the significant excess reactivity in the core will worsen the core behavior in case of inadvertent rod withdrawal for instance. The coupling of absorbing material and large quantities of moderating material enables the loading of burnable poisons in the core. Burnable poisons are able to share the reactivity loss compensation function with the control rods and thus enhance core inherent safety

ASTRID an innovative control rod system to manage reactivity

International audienceAn innovative control rod system which may be applied to the French project ASTRID is presented. All the rods participate to power management and safety shut-down. Comparatively to traditional systems, the gains of this architecture in terms of safety margins (after shutdown, in case of incident during refueling operation and consequences of control rod withdrawal) are noticeable and allows reducing the overall number of control rods.A possible way of implementing control rods for automatic power regulation with a constant efficiency during cycle is also described

A three-dimensional neutronics - thermohydraulics Unprotected Loss Of Flow simulation in Sodium-cooled Fast Reactor with mitigation devices

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Optimization of reactivity control in a small modular sodium-cooled fast reactor

International audienceThe small modular sodium-cooled fast reactor (SMSFR) is an important component of Generation-IV reactors. The objective of this work is to improve the reactivity control in SMSFR by using innovative systems, including burnable poisons and optimized control rods. SMSFR with MOX fuel usually exhibits high burnup reactivity loss that leads to high excess reactivity and potential fuel melting in control rod withdrawal (CRW) accidents, which becomes an important constraint on the safety and economic efficiency of SMSFR. This work applies two types of burnable poisons in a SMSFR to reduce the excess reactivity. The first one homogenously loads minor actinides in the fuel. The second one combines absorber and moderators in specific assemblies. The influence of burnable poisons on the core characteristics is discussed and integrated into the analysis of CRW accidents. The results show that burnable poisons improve the safety performance of the core in a significant way. Burnable poisons also lessen the demand for the number, absorption ability, and insertion depth of control rods. Two optimized control rod designs with rare earth oxides (Eu2O3 and Gd2O3) and moderators are compared to the conventional design with natural boron carbide (B4C). The optimized designs show improved neutronic and safety performance.Available via license: CC BY-NC-ND 4.

Optimized control rod designs for Generation-IV fast reactors using alternative absorbers and moderators

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