Westinghouse AP1000 internals heating rate distribution calculation using a 3D deterministic transport method

Abstract

Cost reduction and reliability increase are systematically pursued systems and components; this requires, amongst other, the availability of sophisticated computer programs and detailed analysis models. As an example, the core shroud, the structure having the function to maintain the core centered on its axis, is being designed in the Westinghouse AP1000, differently from previous plants, as a highly heterogeneous structure. Its thermal-mechanical sizing must take into due account accuarately determined internal heat generation rates. The latter, if determined by combining 2D and 1D neutron and γ-ray calculations which imply the separation of spatial variables and are mainly applicable for fluxes in the reactor beltline region, may include overly conservative margins. On the other hand, Monte Carlo methods do not allow an easy quantification of the uncertainties related to overall calculation. Three-dimensional deterministic models, based on the discrete ordinate transport theory, have the potential to provide accuarate design data; they can be also effective provided that the well-known difficulty to create and tune a complex geometrical model in a reasonable time is overcome and adequate computer resources are available to perform the calculation (until few years ago [Botta et al., 1996. Three-Dimensional Reactor Pressure Vessel Fast Neutron Fluence Calculations for the AP600 Using TORT, 3-D Deterministic Radiation Transport Computer Programs: Features, Applications and Perspectives, NEA/NSC/DOC, OECD/NEA. Paris, France], massive parallel computers (i.e. Cray Computers) available only to large national laboratories and selected industrieshad to be used). ANSALDO is acting as Westinghouse subcontractor and it supported Westinghouse in all AP1000 Licensing Process to NRC from 1999 to nowdays for the internal heating rate generation rate and RPV fluence calculations. As computer power growths up ANSALDO refined its calculation methodology in order to improve the design itself. The recent development, by ENEA Bologna, of a pre-post processor based on the combinatorial geometry (the computer program BOT3P, [ORSI, 2002. BOT3P: Bologna Transport Analysis Pre-Post-processors Version 1.0, Nuclear Science and Engineering, no. 142. American Nuclear Society, USA, pp. 349–354; ORSI, 2004. BOT3P: Bologna Transport Analysis Pre-Post-processors Version 3.0, Nuclear Science and Engineering, no. 146. American Nuclear Society, USA. pp. 248–255; ORSI, 2005. BOT3P: Bologna Radiation Transport Analysis Pre-Post-Processors Version 4.0, Nuclear Science and Engineering, no. 150. American Nuclear Society, USA, pp. 368–373; ORSI, 2003. BOT3P: A Mesh Generation Software Package for the Transport Analysis Codes DORT, TORT, TWODANT, THREEDANT and MCNP, International Conference on Supercomputing in Nuclear Applications SNA’2003. Paris, France; ORSI, 2004. NEA-1678/05 BOT3P4.1. BOT3P4.1 3D Mesh Generator and Graphical Display of Geometry for Radiation Transport Codes, Display of Results”, OECD/NEA Data Bank. Issy-les-Moulineaux, France]) and the ongoing increment of computer power make the three-dimensional deterministic program TORT (part of the DOORS 3.2 Package, [RSICC Computer Code Collection DOORS-3.2, 1998, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA]) an attractive choice to perform the determination of the core shroud heating rate distribution as a 3D continue functions. This paper illustrates the computational approach followed by ANSALDO Nucleare to determine the heating rate distribution into the W AP1000 Core Shroud (see Fig. 1). The calculation has been performed at 67 energy groups (47 neutrons plus 20 gammas) in P3 or in P5 Legendre approximation and 2,000,000 meshes have been used. The computational power required is massive: 2 GB of RAM and up to 50 GB of swap files peak storage for each run with an S8 full-symmetric angular quadrature and 120 GB of swap files peak storage with an S16 full-symmetric angular quadrature. By using a latest generation workstation based on an INTEL P-IV 3.2 GHz processor under LINUX operating system (both SUSE 9.2 or Red Hat 9.1 and better are suitable), each run requires less than a week of full computer time with the P3 Legendre approximation with an S8 full-symmetric angular quadrature. The code BOT3P has been used both for the preparation of the geometrical model and as post processor of the TORT program outputs. BOT3P permitted to cut the model development time by a significant factor and to retrieve efficiently large amount of data from the TORT binary files, allowing a graphical review of the input and output data and, finally, resulting in a unique tool for Quality Control for complex 3D models. This computational experience can be considered a good working example for the 3D deterministic transport method revival by considering that this method is widely accepted by licensing authorities.