RELAP5 SUMMARIZATION
1. Development History AB811148157BL. EM376772762CS
RELAP5 is a general system program for transient behavior analysis developed by Idaho national laboratory. The program has experienced for the following process: 1966 RELAPSE; 1968 RELAP2; 1971 RELAP3 and 1975 RELAP4. The two phase flow processing of these programs are all based on homogeneous model. The program’s final version is RELAP4/MOD7, which is promulgated by US National Energy Software Center in 1980. After that, the researchers used two-fluid, non-homogeneous and non-equilibrium two-phase model, and overcame the limitation of single fluid model to develop the RELAP5 program. By 1995, RELAP5/MOD2 version came out. On this basis, through improving and outspread, RELAP5/MOD3 program was developed successfully. RELAP5 has many special functions for PWR thermal hydraulic analysis. It is a program bases on one-dimensional transient, two fluid, six equation hydraulics, one-dimensional heat conduction and point pile kinetics model. Furthermore, it apply mass, momentum and energy equations for water and steam respectively. In addition to general component model of pump, catheter, injection pump, turbine, separator, and control system, RELAP5 also includes special process models of reflooding heat transfer, gas gap thermal conductivity, choked flow, noncondensible gas.
RELAP5 is a general transient analysis program for thermal hydraulic system, and its application scope includes: coolant loss accidents, running transient state, power supply lose, flow loss and sub-cooling transient state. After a certain development work, now RELAP5 has already been suitable for ordinary microcomputer completely, and had corresponding graphics post-processing software for supporting use. 2. RELAP5—3D
RELAP5-3D, the latest in the RELAP5 series of codes, is a highly generic code, that in addition to calculating the behavior of a reactor coolant system during a transient, can be used for simulating of a wide variety of hydraulic and thermal transients in both nuclear and nonnuclear systems involving mixtures of vapor, liquid, non-condensable gases, and nonvolatile solute[1].
The mission of the RELAP5-3D development program was to develop a code version suitable for the analysis of all transients and postulated accidents in LWR systems, including both large and small-break loss-of-coolant accidents (LOCAs) as well as the full range of operational transients.
The RELAP5-3D code contains several important enhancements over previous versions of the code. The most prominent attribute that distinguishes the RELAP5-3D code from the previous versions is the fully integrated, multi-dimensional thermal- hydraulic and kinetic modeling capability. This removes any restrictions on the applicability of the code to the full range of postulated reactor accidents.
Enhancements include a new matrix solver for 3D problems, new thermodynamic properties for water, and improved time advancement for greater robustness. The multi-dimensional component in RELAP5-3D was developed to allow the user to more accurately model the multi-dimensional flow behavior that can be exhibited in any component or region of a LWR system. Typically, this will be the lower plenum, core, upper plenum and downcomer regions of an LWR. However, the model is general, and is not restricted to use in the reactor vessel. The component defines a one, two, or three- dimensional array of volumes and the internal junctions connecting them. The geometry can be either Cartesian (x, y, z) or cylindrical (r, θ, z).
An orthogonal, three-dimensional grid is defined by mesh interval input data in each of the three coordinate directions. The multi-dimensional neutron kinetics model in RELAP5-3D is based on the NESTLE code, which solves the two or four group neutron diffusion equations in either Cartesian or hexagonal geometry using the nodal expansion method and the non-linear iteration technique. Three, two, or one-dimensional models may be used. Several different core symmetry options are available including quarter, half, and full core options for Cartesian geometry and 1/6, 1/3, and full core options for hexagonal geometry. Zero flux, non-reentrant current, reflective, and cyclic boundary conditions are available. The steady-state eigenvalue and time dependent neutron flux problems can be solved by the NESTLE code as implemented in RELAP5-3D.
Therefore, some scholars use RELAP5-3D and ANSYS to simulate a same heat exchanger[2], the results of temperature and power distribution ate basically same. The
components represented in RELAP5 match the performance predicted by ANSYS, and then it is possible to accurately represent component performance in a broader system analysis, which may consist of a combination of many components developed with multi-physics tools. This will permit system analysis to be performed using more appropriate transient system analysis tools with control volumes representative of the detail designs.
3. RELAP5 and Natural Circulation
RELAP5’s original design is used for calculation and analysis of PWR transient accidents, and then for assessment of facilities safety under normal, abnormal and accident conditions. But with the expansion of its application, the applicable field of RELAP5 has extended continuously.
Natural circulation phenomenon is very important for the safety and design of nuclear reactors. Advanced reactors have been designed using passive safety systems based on natural circulation. There are also some conceptual design using the natural circulation where the components and systems have been simplified by eliminating pumped recirculation systems and pumped emergency core cooling systems [3].
As a kind of circulation state, natural circulation exist some particularity. It relies on the density difference between its cold and heat source to achieve circulation flow without any external driving force. At the same time its influencing factors are quite complicated. But now many scholars have already utilized RELAP5 to analyze the natural circulation system, which confirmed its applicability for natural circulation. The RELAP5 program series for natural circulation system analysis and calculation have gained wide acceptance[4]. Reference[5] used AC-600 passive heat removal system to evaluate the applicability of RELAP5 in natural circulation system, and then compared calculated data by RELAP5 with experimental data. The results are basically the same. Reference[6] also made an experimental validation for RELAP5, which confirmed that RELAP5 code can be used to estimate the running characteristics of natural circulation system. Reference[7] applied RELAP5 code to make a evaluation and calculation for AP1000 passive heat removal system, and confirmed the feasibility of this program. At the same it also analyzed the influence of height difference between cold and heat source to natural circulation ability. Through