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The sight of wild horses is not uncommon in the Chernobyl exclusion zone.

A small herd of wild horses gallops across the grassland, fleeing from the loud noise of the Mi-8 helicopter flying above. A moment later the helicopter has the same effect on grazing elk.

What appears to be a national reserve is the Chernobyl* exclusion zone in the Ukraine. The area has been largely abandoned by its former human inhabitants and is now teeming with wildlife. It's June 2007 and the helicopter carries a group of experts who are in the midst of an on-site inspection exercise organized by the Preparatory Commission for the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO).

The CTBTO is mandated to develop the tools needed to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) once it enters into force. These tools, integrated in the CTBT verification regime, will ensure that no nuclear explosion goes unnoticed. On-site inspections are an essential part of the verification regime and represent the ultimate means to verify whether or not a nuclear explosion has occurred. Experts at the CTBTO develop methods and procedures to establish how best to go about such an on-site inspection.

The team consisted of 36 experts from 21 countries and the CTBTO.

Theory has to be tested in practice. Project manager Gregor Malich of the On-Site Inspection Division (OSI) of the CTBTO, along with a group of 36 experts from 21 countries and the CTBTO, did just that. "We wanted to bring together experts and equipment and let the experts apply methods and procedures for the implementation of Treaty-approved techniques in a realistic environment", he said.

A number of different techniques are applied during an on-site inspection. The exercise in the Ukraine focused on measuring radioactivity and identifying the relevant elements causing that radioactivity. The aim of the exercise was to determine best practices and operational requirements, including related logistics and infrastructure.