Analysis of a Nuclear Reactor Boilure Closure Unit Through Development of a 3D Parallel Finite Element Code

Abstract

Three dimensional (3D) finite element analysis (FEA) allows the mechanical integrity of complex structures
to be estimated with some confidence. This research is concerned with extending an existing parallel
FEA code. This code has been run on up to 16 processors on Durham University’s high performance computing
(HPC) cluster and two different parallel linear solvers have been compared. A key feature of the
work has been to develop tools for structural analyses. An automatic mesh refinement program has been
written, the Zienkiewicz and Zhu error estimator has been coded for 3D hexahedral meshes and post
processing techniques have been used to calculate and visualise principal stress data and peak stress
criteria. This project also reports on three peak stress envelopes used to assess the condition of a concrete
sub-structure.

The development of this parallel code has enabled the deformation behaviour of a key component of
a nuclear rector vessel to be determined. The BCU is a prestressed cylindrical concrete vessel (depth
of 1.73m and diameter of 3.37m) sealing the top of a boilers housed within the walls of the reactor. In
recent years possible problems have been identified at the Hartlepool and Heysham I Advance Gas-Cooled
nuclear reactors (AGR) with respect to the structural condition of the BCU (in particular the condition
of the prestressed circumferential wires designed to maintain the BCU in a state of compression). This
problem provides an interesting case study for this project. Four different BCU meshes have been used
containing either 40201 or 321608 elements (the elements are either 8 or 20-noded hexahedral elements).
Three different load cases have been used to model the BCU. The results of the analyses confirm that the
circumferential pre-stressing is vital in order to keep the BCU is a state of compression and operating
under safe working conditions. These results have been confirmed using principal stress plots and three
different peak stress envelopes. The results show that when the pre-stressing is released approximately
one quarter of the elements contain stresses at Gauss points which exceed the peak strength of the
concrete. This suggests that under these extreme conditions the BCU’s structural integrity has been
severely compromised, concrete rupture is possible and the nuclear reactor is no-longer safe to operate.