The State Nuclear Regulatory Inspectorate of Ukraine has extended by six years the licence relating to dismantling the most unstable parts of the shelter hastily constructed over unit 4 of the Chornobyl NPP (ChNPP) in the 206 days following the 1986 explosion 1986 accident that destroyed the reactor. Work to dismantle and stabilise the sarcophagus is currently conducted inside the New Safe Confinement (NSC) built to enclose the damaged facility.

The April 1986 explosion blew the roof completely off the reactor, which suffered a core meltdown. Radioactive materials and fuel was ejected into the surrounding environment. To prevent the continued release of radioactivity, a cover constructed under extremely difficult conditions and at great cost to those involved was completed in November 1986. The dangers of working in such a contaminated environment meant that structural elements for the roof could not be fastened to supports, but were simply laid on top of them. The design evolved as work progressed. Designers suggested that it would last 30 years. However, holes in the roof structure remained in sections that workers could not reach while runoff from water and snow that collect in lower reactor compartments was monitored and pumped out.

The main source of radiation is from the melted core comprising 200 tonnes of nuclear materials with a total radioactivity of about 20 MCi. The uranium fuel rods, their zirconium cladding, graphite control rods, and sand dumped by helicopter on the core to try to extinguish the fire melted together to form a lava, which flowed into the reactor hall’s basement rooms. There it hardened into fuel-containing materials (FCMs), estimated to contain 170 tonne s of irradiated uranium.

In 1997, an international panel of experts, including drew up the Shelter Implementation Plan (SIP) to stabilise the containment and make the site environmentally safe including construction of a monolithic shelter. To finance the project a donor country pledging conference took place in 1997 in New York and a specially established Chornobyl Shelter Fund was set up administered by the European Bank for Reconstruction & Development. Practical site activities started in 1998 involving repair of concrete beams supporting the roof and stabilisation of the vent chimney stack. In 2001, the roof was repaired.

From 2004-2006, a Russian-Ukrainian consortium including Atomstroyexport and Rovno NPP Construction Administration installed a tower of structural scaffolding to the west of the sarcophagus to support the roof and reduce the risk of collapse. By 2008, 80% of the roof load had been shifted from the reactor walls to the external support structure. Other work included patching the roof, installing structural supports inside the de-aerator, installing an integrated monitoring system of fuel-containing materials, a radiological monitoring system including neutron flux, a structural monitoring system, dust suppression system, fire protection system, and a seismic monitoring system.

Meanwhile, the area immediately west of unit 4 was cleared and prepared to become the NSC building site. In 2007 a consortium led by French firms Vinci Construction Grands Projects and Bouygues Travaux Publics won the tender to build the NSC. The new facility, with a 100-year design life, was constructed away from the sarcophagus to reduce radiation exposure to workers. It was moved into place encasing the sarcophagus in November 2016.

The original licence issued by SNRIU for decommissioning the sarcophagus was issued in 2019 following more than a year of pilot operation and testing of the NSC building. It envisaged completion by October 2023. The amended licence now extends of service life of the sarcophagus for six years to 2029 including shelter facility structures “whose collapse probability is very high”. A deadline of 2025 has been set for development of a new design for unstable structure dismantling. Completion of unstable structure dismantling is set for 31 October 2029. ChNPP says the licence extension was required for “a number of external reasons”. These include unavailable of timely, full and stable funding for the work; COVID-19; and the conflict with Russia.

Image: Scaffolding holding up the western wall of unit 4