TACIS Nuclear Safety Project Re-evaluation

In 1996, during the TACIS Project "Chernobyl - Unit 4 Short and Long Term Measures" coordinated by Trischler and Partner, a subtask was devoted to the revision of seismic input, as the value of 0.18 g of PGA seemed high for the ChNPP site on the basis of other studies carried out on other Russian sites. The available material used by different studies for assessing ChNPP seismic input (ALLIANCE, 1995; NDIBC, 1994; VNIPIET,1991/92) was analysed by the Contractor.

It appeared necessary to revalue seismic hazard at ChNPP site according to a well defined and widely accepted methodology as that of the IAEA Safety Guide. In particular, the following issues were identified:

• A unique seismotectonic zoning, comprising far, intermediate and near site sources should be identified.
• Around Chernobyl some single seismic station were operating since 1985. Those data should be fully exploited to obtain at least preliminary indication of microearthquake activity in the vicinity of ChNPP, and thus provide indication about the feasibility (or meaningfulness) of installing a microearthquake network. Microearthquake indication should also be included in the seismotectonic model.
• Historical data should be treated in more detail, and probabilistic estimate of site return period or the different intensities should have more emphasis.
• More care should be devoted in preparing an attenuation relationship taking into account only data coming from stable continental region and recorded in site condition similar to those of ChNPP, and in particular considering the difference between crustal and deep earthquakes like the Vrancea ones.
• No collection of equivalent strong motion recording was available for near-field sources.

The project aimed at re-evaluating the seismic input for the re-evaluation of the stability of structures and equipment at the ChNPP site. The specific objectives were:

• Reinterpret the available seismic data with the use of commonly agreed methodologies
• Implement an additional seismic survey to monitor the possible source of near field earthquakes
• Survey the local amplification using Nakamura's technique
• Evaluate a new seismic input for the site, validated on both International and Ukrainian norms, through comparison with an independent analysis carried out by a Local Ukrainian Subcontractor.

The project was organised as in the following:

• The Contractor evaluated the seismic spectra for the site according to the methodology proposed by the IAEA in the relevant Safety Guide. It included the following main steps:
  • Analysis of the regional and near-regional geology
  • Analysis of the site vicinity and site area geology
  • Analysis of the seismic activit
  • Development of the seismotectonic model
  • Evaluation of the site amplification
  • Development of the free-field design spectrum and time histories at the site
• The Ukrainian subcontractor (Subbotin Insititute of Geophisics, Kiev) carried out an independent study, according to the current Ukrainian standards.

The Ukrainian study concluded that the previous estimate of the seismic hazard for ChNPP was conservative, as the peak acceleration value for both sources did not exceed 0.1 g, also in line with the former USSR standards.

The Contractor independent analysis concluded the following:

• The seismotectonic studies identified one near- and one far-field seismic zone.
• Real accelerogram occurring in the same focal, magnitude, distance and soil condition were selected.
• SL-1 was determined by probabilistic analysis, SL-2 was taken as the envelope of real far- and near-field average smoothed response spectra.
• Time histories were developed through a convolution analysis.

After the comparison of the two studies, that gave similar results, a discussion took place to reach a consensus statement about the response spectra to be adopted. The agreed spectrum is shown in the attached figure (84%), where the independently evaluated spectra are compared with the linearized spectrum, chosen as conservative estimate of the maximum possible horizontal ground motion at Chernobyl NPP.

The maximum PGA value, even at most conservative estimate of 84.1 % should not exceed 0.12 g. In the range of frequency of interest for the Chernobyl Sarcophagus (1-3 Hz): with respect to previous estimates the reduction is of a factor 3.

Comments

Quality of the results, lesson learned, recommendations for follow-up

All the involved experts agreed that the two studies fully exploited the available data, and benefited also from investigation campaigns especially undertaken for this project, including a microtremor measurement campaign (Nakamura's technique) and the installation of a temporary digital seismic station.

The only kind of data that in the future may lead to a possible reduction of the acceleration values results presented in the final report are detailed microseismic investigation and an update of the historical seismic catalogue. The first objective may be met with the installation of a permanent high gain microseismic network, that may help to decide if the assumption of a moderately active seismic zone in the vicinity of the site is conservative or not, and even eliminate this assumption. The second objective may be met through a revision of the seismic catalogues, aimed at eliminating the following sources of uncertainty:

• Duplicate events
• Observations of events originated in the Vrancea source (Romania), considered as local ones while actually due to effects from far-field sources.

The project met the objectives stated in the TORs.

The management of the project was very sound and full agreement was reached between the Contractor and the Local Institution who carried out the independent analysis.

Further Information

Further information on the project results could be sought from the beneficiary organizations.

The Project Final report is available at the EC/JRC-IE-TSSTP archive.