The convectivediffusion equation is the governing equation of many important transport phenomena in building physics. This work deals with this model for nuclear reactors with hexagonal. It consists of a set of secondorder partial differential equations over the spatial coordinates that are, both in the academia and in the industry, usually solved by discretizing the neutron leakage term using a structured grid. It is revealed that the diffusivity corresponds to the angular momentum operator in quantum theory. Pdf solution of fixed source neutron diffusion equation. Before attempting to solve the equation, it is useful to understand how the analytical. Unstructured grids and the multigroup neutron diffusion.
Solution of onegroup neutron diffusion equation for. Twogroup diffusion theory and the approximate representation of. With only a firstorder derivative in time, only one initial condition is needed, while the secondorder derivative in space leads to a demand for two boundary conditions. This notebook is an entirely selfcontained solution to a basic neutron diffision equation for a reactor rx made up of a single fuel rod. This problem contains no information about the spatial distribution of neutrons, because it is a point geometry problem. Solution of the transport equations using a moving coordinate system ole krogh jensen and bruce a. To satisfy this condition we seek for solutions in the form of an in nite series of. The neutron diffusion equation is often used to perform corelevel neutronic calculations. Secondorder parabolic partial differential equations nonhomogeneous heat diffusion equation 1. Nuclear scientists and engineers often need to know where neutrons are in an apparatus, what direction they are going, and how quickly they are moving. Application of the finite element method to the three. The diffusion equation to derive the homogeneous heatconduction equation we assume that there are no internal sources of heat along the bar, and that the heat can only enter the bar through its ends. Understand origin, limitations of neutron diffusion from. The nal piece of the puzzle requires the use of an empirical physical principle of heat ow.
It is not exact, but for most of this course it is the model that we will use to describe the behavior. In general, the substances of interest are mass, momentum. We now add a convection term \ \boldsymbolv\cdot\nabla u \ to the diffusion equation to obtain the wellknown convectiondiffusion equation. These equations are based ontheconceptoflocal neutron balance, which takes int 11. Pdf numerical techniques for the neutron diffusion equations in. This implies that the diffusion theory may show deviations from a more accurate solution of the transport equation in the proximity of external. A comparison of some numerical methods for the advectiondi. The main calculation method explored in this chapter is the neutrondiffusion equation. Pdf the spacetime neutron diffusion equations with multigroup of delayed neutrons are a couple of the stiff nonlinear partial differential.
The steady state and the diffusion equation the neutron field. Chapter 7 the diffusion equation the diffusionequation is a partial differentialequationwhich describes density. Need to solve the transport equation for an accurate estimate of local reaction rates, particularly near fuel rods and strong neutron. The diffusion equation parabolic d is the diffusion coefficient is such that we ask for what is the value of the field wave at a later time t knowing the field at an initial time t0 and subject to some specific boundary conditions at all times. The general solution is composed by sum of the general integral of the associated homogeneous equation and the particular solution. The results show that solutions of the diffusion equation with these well. This section was about conditions for a stable, selfsustained fission chain reaction and how to maintain such conditions. Equation 1 is known as a onedimensional diffusion equation, also often referred to as a heat equation.
The helmholtz equation is derived, and the limitations on diffusion equation as well as the boundary conditions used in. This is the energydependent neutron diffusion equation. The derivation of the diffusion equation will depend on ficks law, even though a direct derivation from the transport equation is also possible. To run this example from the base fipy directory, type. Diffusion mse 201 callister chapter 5 introduction to materials science for engineers, ch. Summary the neutron diffusion equation is a basic balance equation that describes the transport of neutrons in space, energy, and time. Derivation of the neutron diffusion equation introduction. The plots all use the same colour range, defined by vmin and vmax, so it doesnt matter which one we pass in the first argument to lorbar the state of the system is plotted as an image at four different stages of its evolution. Usa received 4 march 1979 a convectiondiffusion equation arises from the conservation equations in miscible and.
This work introduces the alternatives that unstructured grids can provide. The convectiondiffusion equation is a combination of the diffusion and convection equations, and describes physical phenomena where particles, energy, or other physical quantities are transferred inside a physical system due to two processes. Neutron transport is the study of the motions and interactions of neutrons with materials. The convectiondiffusion equation convectiondiffusion without a force term. We shall then derive the neutron balance equation with diffusion. Chapter 2 the diffusion equation and the steady state. A comparison of some numerical methods for the advection. The neutron diffusion equation describes the neutron population in a nuclear re actor core. Here, a finite difference method is used to discretize the neutron diffusion equation pazirandeh et al. Pdf averaging the neutron diffusion equation researchgate. Depending on context, the same equation can be called the advectiondiffusion equation, driftdiffusion equation, or. Nonlinear reaction diffusion equation with michaelis.
By denoting the external boundary of a domain r by aer, eq. Solution of the neutron diffusion equation by the finite element method in the general multigroup formalism, the neutron diffusion equation is represented by a coupled system of differential equations on the scalar flux 4, where the notation is conventional. Then we shall discuss the common boundary conditions that are used to solve problems. Numerical solution of the neutron diffusion equation has been done by many numerical methods such as the finite difference, finite element and boundary element. We let cx,y,z,t be the density mass per unit volume of a diffusing substance x, and let e be any small subregion of the region where diffusion is occurring. Diffusion equation laboratory for reactor physics and systems behaviour neutronics comments 1 domain of application of the diffusion equation, very wide describes behaviour of the scalar flux not just the attenuation of a beam equation mathematically similar to. Approximation of the neutron diffusion equation on. Chapter 2 the diffusion equation and the steady state weshallnowstudy the equations which govern the neutron field in a reactor. Define i in zzo, 5 xzo, sz zol, n abk, and 5, a cu,kq, where q is the source strength cm sl. In previous section we dealt with the multiplication system and we defined the infinite and finite multiplication factor. Solutions of diffusion equation 2461 to a great extent, this lack of consistency is due to the limited information that can be derived from the infiniteseries solutions. The helmholtz equation is derived, and the limitations on diffusion equation as well as the boundary conditions used in its application to realistic. Solution of the transport equations using a moving.
To show the validity of these diffusion boundary conditions, we solve a sample problem. Steadystate diffusion when the concentration field is independent of time and d is independent of c, ficks second law is reduced to laplaces equation, 2c 0 for simple geometries, such as permeation through a thin membrane, laplaces equation can be solved by integration. Neutron diffusion 90 if we insert the diffusion approximation 23 into our balance equation 4, we obtain. Lou odette american international group aig october 17, 2006 1 nonlinear drift in the continuum limit the pdf. Fundamental concepts and language diffusion mechanisms vacancy diffusion interstitial diffusion impurities. Solution of diffusion equation for a groundlevel area source the first step in the solution is a transformation of equation 3.
Analytical solutions are derived for simple neutrondiffusion problems in one neutron energy group in systems of simple geometry. Different stages of the example should be displayed, along with prompting messages in. We discuss the diffusion equation as an asymptotic limit of the neutron transport equation for large scattering cross sections. In the present study, the imaginary time is incorporated into the diffusion equation for understanding of the collision problem between two micro particles. Multigroup diffusion 6 this work is detailed in garland1975 but for the present discussion, the main point to note is the inadequacy of the onegroup model or even the twogroup model since the appropriate cross sections are not explicitly available and. Lecture 12 neutron diffusion equation objectives in this. Nonlinear reaction diffusion equation with michaelismenten kinetics and adomian decomposition method.
The diffusion equation can, therefore, not be exact or valid at places with strongly differing diffusion coefficients or in strongly absorbing media. The paper deals in its first part with the general formulation of the convectivediffusion equation and with the numerical solution of this equation by means of. Derivation of diffusion equation the diffusion equation 5. The parameter \\alpha\ must be given and is referred to as the diffusion coefficient. It is commonly used to determine the behavior of nuclear reactor cores and experimental or industrial neutron beams.
A new analytical solution for the 2d advectiondispersion. Pdf this paper presents a general theoretical analysis of the neutron motion problem in a nuclear reactor, where large variations on neutron. An approximate analytical solution to the diffusion. Diffusion equation linear diffusion equation eqworld. Equation 19 is a nonhomogeneous ordinary differential equation that can be solved by the application of classical methods. For the love of physics walter lewin may 16, 2011 duration. With the closedform solutions of the diffusion equation established in the present study, we also derived the mathematical conditions under which the diffusion and surface. Derivation of the neutron diffusion equation nuclear.970 178 1013 977 1019 424 1027 1356 78 1135 424 254 1471 33 1226 596 729 1240 497 1235 94 1399 1330 313 1187 377 190 276 1264 824 694 282 1085