Intercomparison of LMFR seismic analysis codes:

One of the primary requirements for nuclear power plants and facilities is to ensure safety and the absence of damage under strong external dynamic loading from, for example, earthquakes.

The use of seismic isolation for structures has been gaining worldwide acceptance as an approach to aseismic design. Seismic isolation of important buildings such as nuclear power plants would result in reducing in seismic induced load and, hence lead to economical structural design. Fast reactors operate at high temperature which induces high thermal stresses during transients. Hence the thickness of the structures needs to be minimized to limit the thermal stresses, which contradicts the conventional requirements. This design approach was pursued by adopting seismic isolation as is studied for the ALMR (Fig. 2) and other LMFRs.

FIG. 2. ALMR: nuclear steam supply system [9].

Therefore, the IAEA through its advanced reactor technology development programme supports the activities of Member States to apply seismic isolation technology to LMFRs. The application of this technology to LMFRs, and other nuclear plants and related facilities would offer the advantage that standard designs may be safely used in areas with a seismic risk. The technology may also provide a means of seismically upgrading nuclear facilities. Design analyses applied to such critical structures need to be firmly established, and the CRP provided a valuable tool in assessing their reliability.

The IAEA has sponsored two Coordinated Research Projects (CRPs) aimed at establishing the reliability of analytical methods and computer codes applied to predicting the behaviour of the reactor core and base-isolated reactor block structures to earthquakes. The studies under the first CRP were useful for the verification and improvement of the reactor core seismic analysis methodologies on the basis of comparison of experimental and numerical results [13]. The second CRP was set up following the good performance of base-isolated buildings [14].

High damping rubber bearings (HDRBs) provide a simple and economical isolation system.

They possess the low horizontal stiffness needed and are capable of safely withstanding the large horizontal displacements imposed on them during an earthquake. The need for additional dampers is avoided. In the HDRBs damping is incorporated into the rubber compound. Figure 3 shows a diagrammatic section of an HDRB.

FIG. 3. Sketch of the 1:8 scale prototype of the ALMR high damping isolation bearing;

HDRB deformed under a compression and shear strain [14].

Numerical simulation of rubber bearings by code ABAQUS gives satisfactory results as far as material properties are evaluated properly and a suitable strain energy density function is selected. Rubber material can be characterized by using ABAQUS by two important forms of strain energy density functions: polynomial and Ogden. For the cyclic loading tests, numerical simulation using a strain function of polynomial formulation gives better agreement with the HDRB test. However, for the ruptures test, where the displacement is larger than the cyclic loading test, simulation using Ogden’s formulation gives better results.

The achieved results confirmed that finite element (FE) methods are useful tools for both the detailed analyses of elastomeric bearings and their design for the better, they allow for a considerable reduction of the number of tests to be performed. The FE analysis of the whole of the rubber bearing carried by participant shoved that the analysis of a single layer of the bearing can be used to predict the horizontal deformation of the bearing by scaling up the results. This is significant in the sense that it reduces the computational time greatly. Further this model can be used effectively to validate the material behaviour. However, more detailed three-dimensional FEM is necessary to analyze the stress distribution of the isolator or to evaluate the behaviour of the bearing at very large deformation.

When the same input data were used, all codes provided predictions of the horizontal force-deformation characteristics of all isolators consistent with the test data. All predictions of vertical behaviour in the absence of horizontal displacement were consistent with the test data. A significant deviation of the numerical prediction of vertical displacement when compression loading is combined with shear was found for some types of rubber bearings.

This deviation is attributable to the modelling of rubber, as almost incompressible, in the constitutive equations.

The modelling of lead proved to be a problem for all teams as the material is deformed in shear within the isolators, while the codes require as input data in tension. Continuing research is needed on the accurate prediction of isolator hysteresis.

The main recommendation derived from the results of the CRP is that the study of isolated nuclear structures should be continued and extended to non-seismic extreme load conditions.

The refinement of the characterization of hyper-elastic behavior of the elastomer is needed to predict multi-directional response under combined loading. The modelling of the flexibility of the reinforced plates and connecting plates should be improved. Investigation of the impact on material characteristics of the special environmental conditions of nuclear facilities is needed.

Investigation of the finite element prediction of isolator failure mechanisms is needed.

Simple, accurate, reliable models for the isolator response over a wide range of multi-directional deformation is essential for accurately predicting floor response spectra and other dynamic design quantities [14].

Future research work should also look at the development of alternative seismic protective technologies such as passive, semi-active and active control for the seismic protection of nuclear facilities and components. The dynamic method to test large scale structures has been validated for base-isolated civil structures and should be extended to isolated nuclear facilities. The influence of vertical ground input on the response of all internal components of an isolated nuclear structure should be investigated.

Fifteen Institutes from India, Italy, Japan, the Republic of Korea, Russian Federation, U.K., and the USA cooperated in this CRP.