Conclusions of an IAEA–JRC research project on the safety significance of near-field seismic motions

In 2002–2005, the IAEA, with the support of the JRC, conducted a coordinated research project on the safety significance for nuclear installations of low–medium magnitude near-field input motions, with the objective of eliciting the relative low damaging capacity of this type of signal, not properly predicted by the nuclear industry practice. It was concluded that the near-field attribute does not pertain in understanding the phenomenon; what pertains is the frequency content of the input motion. As opposed to classical low-frequency inputs, high frequency inputs should not be regarded as force-controlled loads but rather as displacement-controlled loads. Consequently the ductile capacity of structures provides for high-frequency inputs substantial margins that are not available in case of low-frequency inputs. The IAEA drew conclusions for evolutions of the nuclear industry practice towards a more realistic estimate of the damaging capacity of seismic input motions.

► High-frequency seismic input motion are displacement-controlled loads, not force-controlled.
► Therefore, for a given PGA, near-field inputs are less damaging than far-field ones.
► Combination of high frequency input and structural ductile capacity is a source of margins.
► Modelling small non linearity enables to elicit origin of large margins in nuclear structures.
► Modelling small non-linearity is necessary to get safe floor response spectra.