Subcritical reactor | EPFL Graph Search

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion–fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor. Motivation A subcritical reactor can be used to destroy heavy isotopes contained in the used fuel from a conventional nuclear reactor, while at the same time producing electricity. The long-lived transuranic elements in nuclear waste can in principle be fissioned, releasing energy in the process and leaving behind the fission products which are shorter-lived. This would sho

Delayed Hydride Cracking in Irradiated and Unirradiated Zircaloy-2 Cladding

Aaron William Colldeweih

Zirconium alloys used in the nuclear industry are exposed to extreme conditions undergoing high levels of irradiation damage and corrosion. Zircaloy-2 is used as nuclear fuel cladding in boiling water reactors and for the encapsulation of the spallation target in the neutron source SINQ at the Paul Scherrer Institute. As a result of oxidation, hydrogen is produced and partially taken up into the zirconium matrix. As the hydrogen reaches the solubility limit, it precipitates in the form of a zirconium hydride. A number of deleterious effects of hydrides have been extensively studied, including the embrittlement of the material and a unique cracking phenomenon, namely delayed hydride cracking (DHC). DHC occurs as hydrogen in solid solution diffuses towards locations of higher tensile stress, where it subsequently precipitates as the local solvus limit is reached. Once the hydride grows to a critical size, it fractures, which can continue to propagate in repeated steps by the same mechanism.Several studies have been performed in order to understand properties of DHC based on parameters like, alloy type, cracking temperature, hydrogen content, and cracking direction. The most important mechanical properties include the fracture toughness and cracking velocity, which governs the material failure conditions. Crystallographic properties have been investigated in order to understand how complex hydride precipitation develops within the context of DHC conditions. In this project, radial DHC is explored in irradiated and unirradiated thin-walled Zircaloy-2 nuclear fuel cladding with and without an inner liner, in addition to spallation target rod material. A unique three-point bending test, applicable also for irradiated samples in the hotlab, was developed to induce a consistent radially propagating outside-in crack. Results have provided insight into the effects of the inner liner on cracking velocity, showing correlations with the hydrogen depletion. The combined DHC and creep mechanisms, which cannot be decoupled, is coined as âcreep-delayed hydride crackingâ. As the initiating conditions for DHC are of great technical importance, the threshold stress intensity factor has been investigated, under ideal DHC conditions, where a minimum value occurs around 6 MPaâm. Quantitative analysis of the hydrogen distribution around the tip of an arrested DHC crack tip has been performed with neutron radiography at the SINQ. Trends show that the hydrogen concentration around the crack tip increases with temperature resulting likely from the larger hydrogen diffusion, and that the liner reduces the hydrogen available for DHC. Crystallographic analysis has provided insight on the hydride phases responsible for DHC with synchrotron micro X-ray diffraction, performed at the Swiss Light Source at PSI. The results directly suggest that the Î³-hydride is a stable phase precipitated at low temperatures. Testing of irradiated material provided insight into the crack initiation of DHC as a result of the strongly irregular outer surface generated from the oxidation. The propagation pattern is highly irregular compared to the unirradiated material due to the embrittled nature of the cladding from irradiation damage.

EPFL2022

Standardisation des outils de calcul pour les ADS et leur application à différents scénarios de transmutation des déchets

Marco Cometto

The management of radioactive wastes from the nuclear fuel cycle has become an important issue in the development of future, more sustainable nuclear energy systems. Partitioning and transmutation (P&T) of actinides and some long-lived fission products could reduce the mass and radiotoxicity of highlevel wastes and possibly ease repository licensing requirements. Influenced by political and technological developments, an increasing number of countries employing nuclear power have become interested in P&T technology which involves the development of new types of critical and/or sub-critical reactors and very efficient fuel cycle strategies. The present thesis is closely connected to two P&T related projects of the OECD Nuclear Energy Agency in Paris, a numerical benchmark exercise for an accelerator-driven minor actinides burner and a calculational system study on the role of accelerator-driven systems (ADS) and advanced fast reactors (FR) in advanced nuclear fuel cycles. The author co-coordinated the benchmark exercise and performed the comparative analysis of the different "fuel cycle schemes" investigated in the system study. The benchmark exercise contributed to a greater understanding of the physical phenomena characteristic of the transmutation of actinides in a closed fuel cycle, and allowed to test and improve the tools used for modeling ADSs and corresponding advanced fuel cycles. This work thus effectively led to the development of an efficient and more complete calculational scheme, which has permitted an in-depth and reliable analysis of different transmutation scenarios to be carried out. To quantitatively assess the advantages and drawbacks of transmutation, we considered a limited number of "base" transmutation scenarios which constitute an envelope for various possible transmutation strategies employing fast spectrum systems. We then evaluated, for an equilibrium situation, the main aspects which characterise the different transmutation strategies, such as the nuclear park composition, the mass and toxicity of the waste, the need for natural uranium, the in-pile and out-pile mass inventories and, finally, the fuel cycle requirements. Thus, this study has allowed for the reliable comparison of performances of the three main options in the nuclear waste management, viz. open cycle, recycling of plutonium alone and recycling of all the actinides, as well as an evaluation of the advantages and disadvantages of ADSs in comparison to advanced critical reactors in the management of minor actinides or transuranics. The last part of the dissertation is dedicated to a phase-out study for two schemes employing either ADSs or critical reactors. It has been shown thereby that advanced systems, optimised for an equilibrium fuel composition, can indeed be used to reduce the inventory of transuranics. A comparison of ADS and FR performances has been made in this context, and the relative benefits of transmutation have been quantified in each case taking into consideration, more realistically, the residual mass inventory after the shutdown of all nuclear installations.

EPFL2003

Subcriticality measurements using an epithermal pulsed-neutron-source technique in pebble-bed HTR configurations

Rakesh Chawla

The Pulsed-Neutron-Source technique, commonly used for the measurement of the subcriticality of multiplying systems, suffers from the fact that the values of reactivity obtained are dependent upon calculated correction factors and/or kinetic parameters. Calculations have been carried out to quantify the energy dependence of these correction factors in graphite moderated systems, and these indicate that epithermal neutron distributions are much less affected by kinetic distortion. Measurements subsequently made at the PROTEUS facility of the Paul Scherrer Institute in Switzerland have confirmed this effect and show that even in strongly reflected systems the reactivity deduced from epithermal measurements is, within experimental uncertainties, spatially independent