Cecilia Martín-del-Campo

A practical optimization procedure for radial BWR fuel lattice design using tabu search with a multiobjective function

Abstract An optimization procedure based on the tabu search (TS) method was developed for the design of radial enrichment and gadolinia distributions for boiling water reactor (BWR) fuel lattices. The procedure was coded in a computing system in which the optimization code uses the tabu search method to select potential solutions and the HELIOS code to evaluate them. The goal of the procedure is to search for an optimal fuel utilization, looking for a lattice with minimum average enrichment, with minimum deviation of reactivity targets and with a local power peaking factor (PPF) lower than a limit value. Time-dependent-depletion (TDD) effects were considered in the optimization process. The additive utility function method was used to convert the multiobjective optimization problem into a single objective problem. A strategy to reduce the computing time employed by the optimization was developed and is explained in this paper. An example is presented for a 10×10 fuel lattice with 10 different fuel compositions. The main contribution of this study is the development of a practical TDD optimization procedure for BWR fuel lattice design, using TS with a multiobjective function, and a strategy to economize computing time.

Boiling Water Reactor Fuel Assembly Axial Design Optimization Using Tabu Search

Abstract In this paper the implementation of the tabu search (TS) optimization method to a boiling water reactor’s (BWR’s) fuel assembly (FA) axial design is described. The objective of this implementation is to test the TS method for the search of optimal FA axial designs. This implementation has been linked to the reactor core simulator CM-PRESTO in order to evaluate each design proposed in a reactor cycle operation. The evaluation of the proposed fuel designs takes into account the most important safety limits included in a BWR in-core analysis based on the Haling principle. Results obtained show that TS is a promising method for solving the axial design problem. However, it merits further study in order to find better adaptation of the TS method for the specific problem.

Boiling Water Reactor Fuel Lattice Enrichment Distribution Optimization Using Tabu Search and Fuzzy Logic

Construction of a fuzzy inference system for describing the objective function in a fuel lattice composition optimization problem is proposed. This technique allows for incorporating human expertise in searching for the best radial fuel enrichment and gadolinia distributions in a typical boiling water reactor fuel lattice. The optimization procedure adopted is a modified tabu search algorithm. Evaluation parameters included in the objective function are obtained by the neutronic lattice simulator HELIOS. The performance of the new objective function is compared to the objective function consisting in a single sum of individual objectives pondered by weighting factors. Results show that the fuzzy inference system performs very well for modeling the objective function in order to qualify the investigated solutions in a fuel composition lattice optimization process based on tabu search. The best solution found is a lattice with the desired neutronic characteristic.

Development of a scatter search optimization algorithm for boiling water reactor fuel lattice design

Abstract The development of a basic scatter search (SS) algorithm for the optimization of radial enrichment and gadolinia distributions for boiling water reactor (BWR) fuel lattices is presented in this paper. Scatter search is considered an evolutionary algorithm that constructs solutions by combining others. The goal of this methodology is to enable the implementation of solution procedures that can derive new solutions from combined elements. The main mechanism for combining solutions is such that a new solution is created from the strategic combination of other solutions to explore the solutions’ space. Thus, an algorithm based on SS to design a 10 × 10 fuel pin array with two water zones and diagonal symmetry was developed. The lattice performance is evaluated using a global objective function, in which the multiobjective optimization problem is converted into a single-objective problem using weighting factors to attach decision-maker preferences to each objective. The objective function is evaluated using values obtained from the HELIOS code. The results show that the main design variables (average lattice enrichment and power peaking factor) are improved, related to the reference lattice, while the reactivity requirement is satisfied. Results also demonstrate that the SS method is an efficient optimization algorithm when it is applied to the BWR design and optimization problem. Its main features are based on the use of heuristic rules since the beginning of the process, which allows directing the optimization process to the solution, and the use of the diversity mechanism in the combination operator, which allows covering the search space in an efficient way.

Mexico’s Transition to a Net-Zero Emissions Energy System: Near Term Implications of Long Term Stringent Climate Targets

Mexico has positioned itself as a leader among emerging countries for its efforts to mitigate climate change through ambitious climate policies aimed at reducing greenhouse gas (GHG) emissions. However, the Energy Reform bill approved in 2014 promotes the production of hydrocarbons to develop the economy of this sector, as well as the use of natural gas for electricity generation in order to reduce electricity prices in the short term. In 2016, nearly 80% of Mexico’s total electricity was generated by thermal power plants. While natural gas prices stay low, there might be a limited role for natural gas to act as a fuel bridge in this sector if the government is to pursue deep decarbonisation targets to 2050. There is a risk that over-investing in gas infrastructure may delay a transition to lower carbon sources, potentially leading to less cost-efficient pathways towards decarbonisation. This analysis is based on three decarbonisation scenarios that have been modelled using an energy system optimisation model soft-linked to a power systems model. Our results suggest that a deep decarbonisation of the power system is techno-economically feasible and cost-optimal through renewables (mainly solar PV and wind); also, that decarbonisation paths post-2030 are largely dependent on the investment decisions made in the 2020s. It is therefore essential that Mexico’s energy planning decision-makers avoid a natural gas “lock-in” that would either cause carbon targets to be missed or risk leaving some natural gas infrastructure stranded.

Study on the temperature distributions in fuel assemblies of lead-cooled fast reactors

The aim of this paper is to make a comparative study of two concepts of Lead-Cooled Fast Reactor (LFR) fuel assemblies, from a point of view of the thermofluids performance. The sub-channel analysis approach was applied to determine the temperature distribution in the fuel, in the cladding and in the lead-coolant. The mathematical model is fully transient and takes into account the heat transfer in an annular fuel pellet design. The thermofluid is modelled with a mass, energy and momentum balance with thermal expansion effects. The neutronic processes are modelled with point kinetic equations for power generation with feedback fuel temperature and expansion effects. The numerical experiments consider steady-state and transient behaviours. The numerical comparison shows that a hexagonal assembly is an option to compact the size of the LFR core design. This option leads to higher temperature in the fuel and the cladding than in the case of a rectangular assembly design. Results show the LFR with square array is more sensitive to power changes than the hexagonal array at the same nominal power and with the same transient conditions.

Study of the Tritium Breeding Blanket in an Hybrid Fusion–Fission Transmutation System

The reactor studied in this work is the hybrid fusion–fission transmutation system (FFTS), which is a fusion–fission hybrid reactor with a central compact fusion neutron source (CFNS). It is based on the Tokamak concept, and it is surrounded by a zone made of transuranic elements obtained from reprocessing and recycling of spent fuel of light water reactors. High-energy neutrons, of fourteen MeV, are generated in the CFNS; they are produced by the deuterium-tritium reaction. In this study, the MCNPX Monte Carlo code was used to build up a model of the FFTS for studying the tritium breeding capability of the system. Tritium is produced from neutron capture in lithium, which is located in blankets specifically designed for this purpose. The tritium breeding ratio (TBR) is defined as the average number of tritium atoms bred per tritium atom burnt in the deuterium-tritium reaction. We must have TBR>1, for a self-sustained fusion economy. In the first step of this work, the location of the lithium blankets was defined. Afterwards, different blanket materials were tested: natural lithium, enriched lithium in 6Li, different lithium alloys with neutron multipliers like lead and beryllium [Li4SiO4, LiTiO3, FLiNaBe, FLiBe, Pb-15.8Li (Li-6 at 90 %)]. Finally, a study was carried out to determine the relationship between the width of the blanket and the tritium breeding. Concerning the blanket locations, we defined four: one in the central column of the FFTS, one in the upper and one in the bottom part of the fusion region of the system, and the last one in the external part of the fission region. This means that the first three blankets use high-energy neutrons from the deuterium-tritium reaction, and the fourth blanket uses neutron leaking from the fission reactions. The principal results show that the best option is the blanket with Pb-15.8Li with lithium enriched at 90 % in lithium-6 with TBR = 1.09. It was found that the blanket at the external part of the fission region has the higher tritium breeding capability. Regarding the blanket width, it was observed that most of the tritium breeding is carried out in the first 5 cm of the blanket, and beyond this width breeding is minimal; therefore, for the blanket it is more important to have a high view factor to neutrons (i.e., a big surface exposed to neutrons) than a deep region. Finally, it is important to mention that the FFTS was critical during 1000 days that were simulated with MCNPX.

Transmutation Analysis of Nuclear Waste in a Gas Fast Reactor

It is a given that fast reactors are sustainable nuclear energy sources, for both utilization of fissile material and minimization of nuclear waste, due to the hard neutron spectrum and the strategies for recycling the nuclear fuel materials. The goal of the gas-cooled fast reactor (GFR) is to convert it into an economic electricity generator, with good sustainability and safety characteristics, but also capable of minimizing nuclear waste via transmutation of minor actinides. This work presents a contribution to the neutronic analysis of the GFR as a transmutation facility of minor actinides. In this study, the fuel assembly is a hexagonal lattice of fuel pins. The materials used are mixes of uranium and plutonium carbide or oxide as fuel in pins, silicon carbide as cladding, and helium gas as coolant. The Monte Carlo code SERPENT was used to perform the criticality calculations during the fuel depletion. Two different fuel mixes of uranium, plutonium and minor actinides in the pins of the assembly were compared during a burnup of 1200 days of irradiation (equivalent to 50 GWd/t). The evolution of the atomic densities and the mass inventory, that of consumption versus production, and that of different fissile, minor actinides, fission products and transuranic nuclides in the fuel, as well as the k-effective multiplication factor during the irradiation time, were tracked. The results confirmed that the radiotoxicity of the nuclear waste of LWRs can be reduced using GFRs. One of the fuel mixes studied came from nuclear fuel discharged of a typical PWR with a burnup of 48 GWd/t and five years of cooling post-discharge. This mix was compared with another resulting from a second recycling. Results for several nuclides are presented and an assessment in terms of advantages for breeding and/or transmutation capabilities of each mix is discussed in the paper.

Minimal global regret analysis for electricity generation expansion

ABSTRACT A comparative assessment of alternative expansion plans for the Mexican electricity generation system was made by applying the Position Vector of Minimum Regret Analysis as a decision tool. The expansion plans were ranked according to seven decision criteria which consider: internal cost, risk, diversity, external cost, foreign capital fraction, carbon-free fraction, and severe accidents. Electricity expansions were optimized by using the WASP-IV model; internal costs and externalities over a long-term planning horizon were simultaneously minimized when the external cost was added into the variable component of the operation and maintenance cost. Special attention was paid to studying the convenience of including nuclear power in the electricity expansion. The new decision analysis tool ranked the plans in terms of the minimum global regret, and results showed that the plans which added nuclear power plants were in general relatively more attractive than the plans that did not.