Twenty-five years on from the Chernobyl accident on April 26^th 1986, the impacts of the devastating explosion in the number 4 reactor at the Chernobyl plant continue to be felt in the region and beyond and have been brought into focus again as the crisis at the Fukushima nuclear reactor following the earthquake and Tsunami in March 2011.

Here we look at how the Chernobyl accident led to the development of the Met Office’s emergency-response dispersion model NAME, and how this has been developed into one of the most flexible and sophisticated atmospheric dispersion models in the world.

At 01:23:44 on 26^th March 1986, four seconds into the attempted emergency shutdown, an explosive rise in steam pressure within the reactor lifted the massive shield above it, exposing the core of the reactor to the atmosphere. With the core now exposed, a second enormous explosion blew the reactor building apart.

The Soviet authorities organised a mammoth fire-fighting effort to contain the radioactive debris, using military helicopters to dump 5000 tonnes of sand onto the reactor’s remains. Releases into the atmosphere continued for ten days following the initial explosion with an estimated 4% of the radioactive core escaping. Most of this contaminated material affected Belarus and the Ukraine – it has been estimated that five million people were exposed to radiation in these areas.

Although much of the radioactivity was deposited in the region around Chernobyl, some contaminated debris would inevitably spread further afield. In the outside world, the first indication of the disaster came from Sweden when automatic radiation monitoring instruments at a nuclear power station near Stockholm sounded the alarm two days later on Monday April 28th. A plume of radioactive material was transported widely across Europe by the evolving weather patterns. The worst affected areas were those locations where rainfall intercepted the plume and washed radioactive particulates to the ground; notably, parts of Scandinavia and Eastern Europe, western parts of the UK and Ireland, and some Alpine areas.

Click on this map to see an animation of dispersion from the Chernobyl accident

The role of the Met Office at the time is described in a contemporary report… “By Tuesday morning (April 29th) the Central Forecasting Office at Bracknell were calculating forward trajectories starting from Chernobyl using forecast winds calculated at the 850mb pressure level. These trajectories suggested that whilst there was no immediate risk to the UK, there was a possibility that the plume might cross Britain by the end of the week.

“Late on Wednesday, reports of elevated radiation levels measured at laboratories in northern Italy and in Monaco were received … it became reasonably probable that the winds would carry the plume into Britain on the Friday (May 2nd), and this information was issued on Thursday morning by the Meteorological Office.”

The plume migrated across the UK from the south on the Friday and Saturday (May 2nd/3rd). It was rather unfortunate timing that an active depression in the south-west approaches pushed frontal rain northwards in combination with thunderstorm activity  advected in from France. Heavy rain washed out the radioactive debris over many parts of the country, the worst-affected areas being upland regions in the north and west of Britain.

One outcome of the Chernobyl incident has been the development of the Met Office’s pollution dispersion model NAME.

NAME has continued to be developed and applied to an ever-growing range of atmospheric transport and dispersion problems, ranging from research activities to tracing the sources of greenhouse gases under the Kyoto protocol and from air quality forecasting to numerous emergency response activities such as nuclear/radiological releases (e.g. Fukushima, 2011), volcanic eruptions (e.g. Eyjafjallajokull, 2010), industrial fires (e.g. Buncefield oil depot fire, 2005) and the spread of animal diseases (e.g. Foot and Mouth Disease and Bluetongue). NAME is now one of the most flexible and sophisticated atmospheric dispersion models in the world.

An ability to deliver sound advice for releases of all these types of contaminant requires NAME to be able to represent a wide range of physical and chemical processes and reactions. These include, but are not limited to, particulate and gaseous releases, radioactive half-life decay, radioactive decay chains, gamma radiation cloud shine, chemical reactions, biological virus decay, gravitational sedimentation and dry and wet deposition. When linked to the Met Office’s world leading numerical weather prediction model, the Unified Model, it is possible for NAME to predict the spread of atmospheric contaminants over distances ranging from a few hundred metres to the entire globe. This enables the Met Office to help inform and advise emergency responders, health protection agencies, government and the international community for events anywhere on the planet.

Internationally the Met Office is a Regional Specialized Meteorological Centre (RSMC) and a contributor to the Comprehensive Test Ban Treaty Organisation (CTBTO). As an RSMC, the Met Office, using NAME, can be called upon by any country in the European or African regions during a major atmospheric pollution incident under the World Meteorological Organisation’s (WMO) Emergency Response Activities Programme – one aspect of the Met Office’s wider international role as an RSMC. For incidents outside of this region, the Met Office continues to provide predictions but in support of the relevant lead RSMC.

For radiological incidents (e.g.Fukushima, 2011) the Met Office response, along with those of the other RSMCs, is coordinated through the International Atomic Energy Agency (IAEA). The CTBTO maintains a global network of detectors that monitor for radioactive substances that would be released during a nuclear detonation. If a substance is detected, the Met Office, using NAME, is able to show where the contaminant may have come from. By combining model results from several sensors it is then possible to identify the location where the radiological contaminant was released into the atmosphere.

More information on NAME and its full range of uses can be found on the Met Office website.