Neutronics of Advanced Nuclear Systems

Neutron transport theory is the research basis for neutronics and focuses on the description of neutron motion in media and the corresponding laws. There are two types of methods for neutron transport calculation: the Monte Carlo method (also called the probabilistic method or the stochastic method) and the deterministic method. The Monte Carlo method is a numerical method based on probability and statistical theories, and can explicitly describe the characteristics of randomly moving particles and the process of physical experiments. In contrast, in the deterministic method, a group of mathematical–physical equations are first built up to explain the physical characteristics of the target system. Then, by discretizing the variables in these equations, including direction, energy, space, and time, an approximate solution can be obtained with numerical calculation. There are complicated features for advanced nuclear systems, such as the complex neutron spectrum and angular distribution, complicated material composition, large spatial span, complex geometry, etc. The Monte Carlo method uses the continuous-energy cross section, and can be used to address any complex geometry, with prominent advantages for neutron transport simulations for advanced nuclear systems. However, some challenges, such as the slow convergence rate and difficulty in addressing problems of deep penetration, still exist. The deterministic method is faster, but falls short in addressing advanced nuclear systems with complex geometries, strong anisotropy of neutron scattering, and complicated energy spectrums. In recent years, the method of characteristics (MOC) and the discrete ordinates method with unstructured meshes have been developed with improved geometry processing abilities. However, problems, such as the ray effects and the high cost of large-scale systems, still need to be solved. The Monte Carlo–deterministic coupling method, which combines the advantages of both the Monte Carlo and deterministic methods, is one of the most efficient and accurate methods for solving transport problems in advanced nuclear systems.

Neutron kinetics is dedicated to studies of the time-dependent neutron behavior of nuclear systems over a short time range (typically from microseconds to seconds) and the factors that affect this behavior [1]. Neutron kinetics provides neutronics support not only for exploring the conditions of the stable operation of various nuclear systems but also for the prediction and analysis of potential accidents and their consequences. Therefore, the study of neutron kinetics is of great significance for the safe operation of nuclear systems. Compared with traditional nuclear systems, advanced nuclear systems show new characteristics including a complex neutron spectrum, strong coupling effects with an external neutron source, and delayed neutron precursor movement, which will be associated with the new features of neutron kinetics, and pose challenges to traditional neuron kinetics theories and methods [2–6]. This chapter mainly introduces the characteristics of the neutron kinetics, the corresponding theories, and computational methods for advanced nuclear systems.

Transmutation is defined as the conversion of one nuclide into another, along with the corresponding change in the number of neutrons or protons in the resulting nucleus. Transmutation can be realized via nuclear reactions or radioactive decays. In nuclear systems, neutron-induced transmutation includes the processes of burnup, nuclear waste transmutation, nuclear fuel breeding, and material activation. Research on burnup mainly focused on studying the consumption of nuclear fuel isotopes and its impact on nuclear system performance. During the process of fuel burnup, a large amount of long-lived nuclear wastes with high level of radiation will be produced, the improper handling of which will cause substantial radiological hazards. In advanced nuclear systems, especially hybrid nuclear systems, long-lived high-level radionuclides in nuclear wastes can be converted into short-lived nuclides or stable nuclides via nuclear waste transmutation. The fissile fuel can only be used by existing nuclear systems for decades as estimated by the International Energy Agency (IEA) [1], while tritium, which is used by fusion systems, is virtually nonexistent in nature. Nuclear fuel breeding refers to the conversion of fissionable nuclides into fissile nuclides or the conversion of other nuclides into tritium via nuclear reactions to satisfy the nuclear fuel demands for the long-term stable application of nuclear energy. During the operation of nuclear systems, neutrons react with nuclei to produce radionuclides, and the materials are activated. The radionuclides that are produced typically decay and emit α, β, and ɣ rays, which poses potential radioactive hazards to nuclear systems, workers, and the environment.

Radiation of nuclear systems mainly comes from neutrons, photons, charged particles, and radionuclides produced by nuclear reactions between neutrons and materials. Radionuclides can migrate from the reactor to the environment, resulting in radiation risks to workers, the public, and the environment. Comparing with the traditional nuclear system, new radionuclides will be produced in advanced nuclear systems because of the differences of coolants between both nuclear systems. This chapter focuses on the radiation effects of these nuclides. Fundamental safety principles need to be followed, and safety requirements need to be met to control radiation risks arising from nuclear systems and to ensure the protection of public health and the natural environment from the harmful effects of ionizing radiation. For workers and the public, it is necessary to estimate the external exposure from neutrons and photons and to evaluate both internal and external exposures to radionuclides. The radioactive source term, nuclide migration, and radiation dosimetry calculations and biological effects of radiation will be described in this chapter.

Material neutron irradiation damage refers to the microstructural change and performance degradation of material induced by neutron irradiation [1]. These changes caused by collision displacement, transmutation, and ionization effects can significantly affect the macroscopic physical and mechanical properties of a material. In advanced nuclear systems, the service environment of core components is quite different from that of traditional nuclear systems, and the corresponding materials need to withstand much higher temperatures and higher flux neutron irradiation. Taking the fusion system as an example, 14.06 MeV fusion neutrons will generate more serious irradiation damage than the thermal neutron of the traditional nuclear system. Meanwhile, transmutation reactions, such as (n, p) and (n, α), are more easily generated by high-energy neutrons. In addition, the transmutation gases will interact with the irradiation defects and aggravate the irradiation damage of materials. The degradation of material properties induced by neutron irradiation poses a serious threat to the structural integrity of components, which is a key issue in the development of an advanced nuclear system. Thus, irradiation damage research of materials under harsh environment, such as high operation temperature, high-energy neutron and high-flux neutron irradiation, etc., is an urgent need.

The neutronics simulation of nuclear systems relies on the availability of nuclear data to provide accurate numerical representation of the underlying physical processes. Essential nuclear data include energy-dependent reaction cross sections, the energy, and angular distributions of reaction products for various combinations of incident particles and targets, and the atomic and nuclear properties of excited states as well as their radioactive decay data. Nuclear data are the foundation of both nuclear science and technology research and nuclear engineering design. Advanced nuclear systems have the characteristics of complex neutron spectra and angular distribution, complex material composition, and extreme multi-physics coupling. These issues place higher requirements on the accuracy, the width of the energy region, and the species of nuclides for nuclear data.

Comprehensive neutronics simulations are coupled simulations of neutron and multiple neutron-related physical effects for the entire space and life cycle of nuclear systems. The key goal of comprehensive neutronics simulations is to solve neutron transport problems characterized by strong anisotropy, which exist in the whole-life design and safety operations of advanced nuclear systems. In this chapter, the framework of comprehensive simulation systems is presented. Then, the modeling, calculation, and visual analysis are discussed. Finally, several typical simulation systems are briefly introduced.

Advanced fission systems mainly refer to fourth-generation nuclear systems represented by six typical reactor types: lead-based fast reactors (LFRs), very-high-temperature reactors (VHTRs), sodium-cooled fast reactors (SFRs), supercritical water-cooled reactors (SCWRs), molten salt reactors (MSRs), gas-cooled fast reactors (GFRs), and other unconventional fission systems. These systems are usually designed with characteristics of sustainability, safety, economic efficiency, and effective prevention of nuclear proliferation.

A fusion system is a facility that could utilize the energy released via a fusion reaction in a controllable and peaceful way. There are many facilities based on different methods to achieve the fusion reaction, including magnetic confinement fusion (MCF), inertial confinement fusion (ICF), etc. Among these facilities, the tokamak is the well-developed candidate for a commercial fusion system. In this chapter, the tokamak is used as a representative system to introduce the neutronics design of a fusion system. In a fusion system, the neutron carries most of the fusion energy (the kinetic energy of the neutron accounts for ~80% of the energy released by the D-T fusion reaction), is the key to attain the tritium self-sufficiency, and is also the source of radioactivity in the system. Thus, the neutronics design is a crucial step in fusion system design and is concerned with the feasibility, safety, economy, and environmental friendliness of the system. The neutronics design for the fusion system is focused on the blanket, tokamak machine, and corresponding buildings, related to the whole lifecycle of systems, including the procedures of design, licensing, operation, and decommission. Compared to fission energy systems, fusion systems have a more complex geometry and a harsher service environment for in-vessel components, which creates great challenges for neutronics design and analysis. In this chapter, our discussion on fusion systems will be as follows: (1) the principles, features, and typical conceptual designs of fusion systems; (2) the neutronics design principles, requirements, and methods; and (3) taking the Dual-cooled Lead Lithium (DLL) blanket adopted in FDS-II and the tokamak machine and buildings in the ITER as examples to illustrate neutronics design for the blanket, the tokamak machine, and the corresponding buildings, respectively.

Hybrid nuclear systems, rather than a self-sustained chain reaction, are subcritical nuclear systems using additional neutrons from an external neutron source to sustain a chain reaction. Once the external neutron source supply stops, the fission reaction in the subcritical reactor/blanket soon stops. Hybrid nuclear systems include accelerator-driven subcritical system (ADSs) and fusion-driven subcritical system (FDSs). These systems can not only produce energy but also offer an effective approach to the settlement of accumulated of nuclear waste and the shortage of fissile fuel. In particular, FDS is considered as a potential path to the early application of fusion energy [1].

Neutronics experiments are important parts of neutronics research for nuclear systems. Neutronics experimental facilities contain accelerator-based neutron sources and nuclear systems. Neutronics experiments include nuclear data measurement and validation, validation of methods and software, validation of reactor core physics design of nuclear systems, etc. To carry out various types of neutronics experiments, it is necessary to build neutronics experimental facilities and develop neutron control and measurement technologies.

Neutronics experiments can be classified into three main categories: measurement and validation experiments for nuclear data, validation experiments for neutronics methods and codes, and validation experiments for nuclear systems and component designs. Measurement and validation experiments for nuclear data are performed to obtain nuclear data or validate the applicability of a nuclear data library for specific conditions. Validation experiments for neutronics methods and codes are employed mainly to validate the applicability of neutronics methods and codes in specific conditions. Validation experiments for nuclear systems and component designs are utilized to confirm whether the systems, component designs, equipment, and nuclear materials satisfy the design requirements (Wu in Fusion neutronics. Springer Nature Singapore Pte. Ltd., [1, Fusion Sci Technol 74(4):321–329 ,2]).

Since the 1950 s, with the development of fusion systems, many fusion neutron source facilities have been built around the world. A series of experiments have been performed to support the development of fusion systems with these facilities that aim to validate the reliability of neutronics design, the precision of nuclear data and the correctness of computational methods and tools [1]. In this chapter, a general introduction of fusion neutronics experiments is first presented. Then, certain typical experiments such as activation experiments, tritium breeding experiments, irradiation experiments, and shielding experiments are introduced. Last, the prospects for the development of fusion neutronics experiments are discussed.

Since the concepts of hybrid nuclear systems were proposed in the 1950s, many designs and simulations of hybrid nuclear systems have been performed. Various neutronics experiments of hybrid nuclear systems have been carried out to verify the effectiveness of experimental methods, the validity of simulation methods, and the reliability of evaluated nuclear data. This chapter is organized as an overview, experimental methods for the measurement of subcriticality, and transmutation nuclear data, typical experiments and prospects for future experiments.