| Abstract: | The SEURBNUK-2 code is now being developed jointly by AEE Winfrith and JRC Ispra for use in Fast Reactor Containment Studies. To meet the needs of such studies and the needs of the COVA program, a number of improvements and extensions of the code have been made. A selection of these changes and illustrations of their use are given in this paper.The structural capability of SEURBNUK-2 was originally limited to the treatment of thin shells and shell junctions. Although this facility proved surprisingly useful, it was realised that a more versatile and powerful means of calculating the deformation of more complicated structural geometries would be required. The finite element code EURDYN which employs convected coordinates was adapted for the purpose, so that axially symmetric elements of the quadrilateral, triangular and thin shell families could be used to model various parts of the reactor structure. The method of coupling this finite element code to the fluid motion is described and the use of this new version of the code is illustrated and the results compared with those obtained by the original code and the ALE code EURDYN 1M. This last exercise revealed small differences between the solutions which were subsequently resolved by a further investigation involving a spherical cap problem.A feature of many reactor designs which is being modelled in the later COVA experiments is the perforated plate or porous structure. For fixed perforated plates and porous structures, the additional pressure drop and inertia effects can be included in the momentum equations by addition of suitable terms and the original technique of solution is unaltered. Details of the finite difference equations are given in this paper together with the results of check calculations which were performed to ensure the correct functioning of the code.The extensive use of SEURBNUK-2, particularly in conjunction with COVA, has highlighted a number of code problems which have been successfully resolved. Many of these related to particular circumstances, and are therefore of limited interest, but a general and quite frequent problem is that of gross bubble distortion which, if untreated, leads to logic problems within the code and consequent failure. Although the basic cause of this distortion is understood, eliminating it is not straightforward. A successful palliative is to manually rezone the bubble interface since this then avoids the logical problems in SEURBNUK with little or no effect on the calculation results. A further technique is the damping of the incipient discontinuities by automatic smoothing of the particle velocities. Examples of the use of rezoning and smoothing techniques are given.It is generally recognised that the numerical processes in an Eulerian code such as SEURBNUK introduce spurious diffusion into the solution so that pressure profiles, for example, are smoother than in an equivalent Lagrangian calculation. To give guidance to the calculator on the input parameters which affect these diffusion terms, an analysis of the truncation errors involved in the derivation of the finite difference equation was made. The various diffusion like terms are listed in this paper and their relevance to the calculation is discussed. An example is given which illustrates the changing nature of the solution as the amount of diffusion is modified. |
