B. Vezzoni

Preliminary Evaluation of a Nuclear Scenario Involving Innovative Gas Cooled Reactors

In order to guarantee a sustainable supply of future energy demand without compromising the environment, some actions for a substantial reduction of CO2 emissions are nowadays deeply analysed. One of them is the improvement of the nuclear energy use. In this framework, innovative gas-cooled reactors (both thermal and fast) seem to be very attractive from the electricity production point of view and for the potential industrial use along the high temperature processes (e.g., H2 production by steam reforming or I-S process). This work focuses on a preliminary (and conservative) evaluation of possible advantages that a symbiotic cycle (EPR-PBMR-GCFR) could entail, with special regard to the reduction of the HLW inventory and the optimization of the exploitation of the fuel resources. The comparison between the symbiotic cycle chosen and the reference one (once-through scenario, i.e., EPR-SNF directly disposed) shows a reduction of the time needed to reach a fixed reference level from ∼170000 years to ∼1550 years (comparable with typical human times and for this reason more acceptable by the public opinion). In addition, this cycle enables to have a more efficient use of resources involved: the total electric energy produced becomes equal to ∼630 TWh/year (instead of only ∼530 TWh/year using only EPR) without consuming additional raw materials.

Optimization of Safety Parameters and Accident Mitigation Measures for Innovative Fast Reactor Concepts

Traditionally the analysis of the evolution of severe core disruptive accidents (CDA) is broken down into different phases. This is mainly done for a better focussing on the key phenomena of the accident phase and also allows the application of specific codes for the analysis. In the current paper we mainly deal with the initiating phase and the transition phase of an accident as the ULOF (unprotected loss of flow). The key phenomenon of the initiating phase is the start of boiling and the development of voiding; key phenomena of the transition phase are the progression of core melting and the occurence of recriticalities by fuel compaction. The first level of optimizing safety is oriented to the initiating phase by reducing the positive void worth in order to avoid that a ULOF accident would enter a severe development. If accident prevention is not achieved the transition phase, characterized by a progressive core degradation leading to the occurrence of recriticalities, can be mitigated by dedicated features that enhance and guarantee a sufficient and timely fuel discharge – e.g. by a controlled material relocation (CMR) - and influence and ‘brake’; the recriticality path. In the paper both phases are analyzed. The results presented are in agreement with the activities performed within the European Collaborative CP-ESFR project.

Analysis of a Hypothetical Italian Fuel Cycle: Transition to Fast Reactors

In order to investigate the impact of nuclear energy introduction in a country with a fossil fuel based energy mix, several scenarios have been compared in terms of fuel cycle needs (resources and infrastructure) and wastes produced. As case study, the Italian situation (represented by ca. 300 TWhe-y of electricity needs in 2007 and by no nuclear energy production at present) has been selected. However, the obtained results could be extrapolated to other countries by means of scale factors. For the reference scenario, the introduction of Gen.III+ Light Water Reactors and once-through fuel cycle has been considered. Under the hypothesis that only the plutonium produced in the country will be available and used for a possible transition to a fast fleet, the introduction of different types of fast reactors (a 600 MWe lead-cooled and two 1500 MWe sodium-cooled systems with different breeding characteristics) and of a more sustainable fuel cycle (closed or partially closed) have been compared. The adoption of fast systems enables to reduce of 50% the uranium consumption and to favorably impact the cycle back-end by reducing the Pu inventory in the cycle, and by reducing the long term waste radiotoxicity and heat load in a repository. A parametric study has been carried out in order to deal with the systematic uncertainties connected to scenario investigations.

Wastes Management Through Transmutation in an ADS Reactor

The main challenge in nuclear fuel cycle closure is the reduction of the potential radiotoxicity, or of the time in which that possible hazard really exists. Probably, the transmutation of minor actinides with fast fission processes is the most effective answer. This work, performed in SCK⋅CEN (Belgium) and DIMNP Pisa University, is focused on preliminary evaluation of industrial scale ADS (400 MWth, 2.5 mA) burning capability. An inert matrix fuel of minor actinides, 50% vol. MgO and 50% vol. (Pu,Np,Am,Cm)O1.88, core content, with 150 GWd/ton discharge burn up, is used. The calculations were performed using ALEPH-1.1.2, MCNPX-2.5.0, and ORIGEN2.2. codes.