Consistent application of "Design as you go"-principle is especially important during the design and construction of National Radioactive Waste Repository (Hungary). Due to the geo-logical and hydrogeological constraint discovered within the previously licensed disposal zone during the construction, the area available for safe final disposal became significantly smaller than originally planned. That reason and the requirement for developing economical solutions necessitate the continuous optimization of chamber design. The construction of first two disposal chambers was controlled by a comprehensive rock mechanical monitoring program. As a consequence of its results some modifications of design have been decided. The second two chambers are being built with modified pillar and chamber geometry, using optimized getting and supporting technologies. Their geotechnical consequences are measured with similar monitoring system. Comparison of data-sets gained during two construction phases is important both for controlling the efficiency and safeness of modifications and making possible further optimizations.

1 Introduction
1.1 National radioactive waste repository

The National Radioactive Waste Repository (NRWR) is being constructed in SW Hungary, at Bátaa-páti for disposing the Hungarian NPP-origin short-lived, low- and intermediate-level radioactive wastes (L/ILW). The work is going on under the control of Public Limited Company for Radioactive Waste Management (PURAM).

The repository level locates at 240–270 m below the ground surface and it was discovered by inclined shafts and a horizontal access tunnel system between 2005 and 2010, using drilling and blasting method. The first two disposal chambers were constructed in 2011. Since the end of 2012, when the first chamber put into operation, the tunnels and infrastructures have been divided into a nuclear controlled and a building zone. The chambers No 3 and 4 are being under construction.

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