Chernobyl Nuclear Power Station - Ukraine

Written 5 November 2016 – published 9 November 2017.

There is now less than four weeks to go before the Arch structure, which forms the New Safe Confinement (NSC), is finally slid into position over the Object Shelter (OS), containing the destroyed reactor 4 at Chernobyl nuclear power plant. A process that once underway, will be entered into the record books as the heaviest structure ever moved across land – a distance of over three hundred (300) meters.

Apart the general excitement and anticipation presently on-site, the Arch sliding activity comes with a certain level of risk, and also a level of uncertainty following atmospheric containment once the sealing membranes are installed, and basement areas within the OS and the destroyed unit 4 dry out.

Below is a none technical summary of some of the more interesting risks with potential radiological consequences that we need to consider accolated with Arch sliding, and the later atmospheric containment of the OS by the NSC. That said, much more research needs to be undertaken on uncertainties, as some of the exact mechanisms are yet not fully understood.

1. Ground displacement:

The first risk relates to ground displacement as the mass of the Arch is slid over the OS initiating a possible failure and/or collapse of elements, on or within the aging legacy structures, resulting in the possible release of activated dust into the atmosphere.

The ground under the Arch transfer system has a high water-table and comprises mainly of medium-fine riverbed sand which may displace or even liquefy under specific conditions. The foundation systems of both the Arch assembly area and Arch final position over the OS, are piled to adequately support the structure. Between these two points, the transfer rails of the Arch pass over 100 meters (approximately) of a raft foundation system. Failure of any legacy structural component of the OS as a result of ground displacement may lead to a release of activated dust initiated due to impact. Current calculations and soil sampling suggest any displacement will be no more than 2mm to 6mm in the localised area.

Risk reduction methodology include extensive monitoring using a number of additional accelerometers and laser sensors for displacement distance and position which are to be placed at specific locations within the Arch, the legacy structures and ground positions. In addition there will be specific monitoring for any structural vibrations/harmonics and period motion of the Arch during sliding, and use of a mobile foam dust suppression system.

2. Deformation of Arch structure:

The second risk within the Arch sliding activity is related to displacement and deformation of the Arch structure during the sliding process, leading to misalignment of installed components possibly due to uneven loading of the Arch structure during transfer, and/or settlement of the structure. This will result in an increased radiological exposure for installation engineers working over the OS within the Arch Ceiling, Annular Space and Roof.

As the Arch sliding activity has been brought forward in the Programme (Schedule of Work), this movement in the original work plan resulted in a number of important activities being rescheduled (moved), either side of the Arch sliding milestone. One such activity has been the installation and alignment of the Main Crane System (MCS) rails, which has been undertaken over the last twelve (12) months.

There is a high probability that once the Arch moves, the work already completed in aligning the Crane rails will be lost, which will then require undertaking again, extending the Programme. Extra work will then be required with the Arch whilst positioned over the OS and Unit 4 with increased radiological consequences for installation engineers.

This will require installation of additional mobile shielding – dependent on areas effected, some installations may have a level of difficulty that would require additional engineering, further compounding time at risk.

3. Radiological uncertainties following Arch sliding.

3a. Activated dust particulate (Alpha & Beta):

Corium and highly irradiated uranium fuel may have the property of spontaneous dust generation – by the alpha decay of isotopes degrading the material, releasing submicron particles from their surface. In addition to the Corium and highly irradiated fuel dust, there is a large amount of activated dust-like material throughout the OS and unit 4.

There is general concern that once the structures and building dry out following the sealing of the NSC to the outside atmosphere, radioactive particulate may migrate through buildings, increasing the risk of ingestion of Alpha and Beta particles to decommissioning personnel.

As a part of environmental/atmospheric monitoring of this issue, additional Air Monitoring Units (AMU’s) will be installed at specific locations throughout the buildings.

3b. Elevated radiation streaming (Gamma/Neutrons):

Radiation detector observations over the past nineteen years (since 1997), have infrequently  recorded an elevation in radiation ‘spiking’, streaming from locations under reactor 4 at Chernobyl. These areas have included room 012/7, 012/15, 210/7, 210/6, and 304/3 – all rooms of which, are situated under, or close to, reactor 4 base plate, room 305/2.

There are three types of molten corium which settled within the unit 4 building under the reactor. These were previously labelled and identified as black, brown and ceramic, each has varying inclusions of other materials and underwent different mechanisms during the hardening phase – particularly at levels +2.2m and 0.0m where corium penetrated into the water tank rooms and immersed in water.

Current observations suggest these elevations in radiation ‘spiking’ have a tendency to occur during particularly dry periods, and have been recorded with a peak increase at around 20 to 50 percent before burning out and returning to a lower level of detection.

As both gamma and neutrons are captured/slowed by water, it is considered that levels of water/water-saturated sand or other materials are moderating particles emitted by Fuel Containing Materials (FCM’s)/corium within the lower part of the reactor building under room 305/2.

Currently, rainwater that seeps through the OS retains levels of water within certain areas of the building. There is general concern, that once the OS structures and unit 4 building dry out following the sealing of the NSC to the outside atmosphere, there will be a prolonged elevation in gamma and neutron particles emitted those areas within the building under reactor 4.

[edited] Both uncertainties [3b, and particularly 3a above] are still somewhat theoretical. They will require continual monitoring as the structures dry out, and may also require future intervention.

Furthermore, although there is some data regarding the 3b uncertainty, both 3a and 3b, may require additional funding to further model currently unknown parameters.


Reference

1. Chernobyl Projects – Abbreviations and Terms.
2. SNRIU7/734-20/10/2016.
3. NVK-Project-Arch-riskbook-Sep16.
Nuclear Safety Projects & Radiation Science | Particle Physics | Fluid & Thermodynamics | Climate Variability & Environmental Science | Other areas of research: British (Royal) Etiquette | Social & Behavioural Psychology.