18 September 1993

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The high cost of making the former Soviet Union's ageing nuclear power plants safe is forcing Western governments to set up an early warning system for radiation leaks

MONICA HORTEN

European governments are devising a cut-price solution for monitoring radiation leaks from the ageing reactors of the former communist bloc. Over the next few months, suppliers of radiation monitoring systems will be asked to tender for a 'gamma curtain' to surround these nuclear power plants.

This move to install a Europe-wide radiation monitoring system marks a major shift in policy by Western European governments. Following the accident at Chernobyl in April 1986, the European Community backed the

idea of bringing Eastern Europe's nuclear power plants up to Western safety standards. A programme that could cost up to 17 billion Ecu. Some funding for such work is available from the European Commission's

Technical Assistance to the Commonwealth of Independent States (TACIS) fund, set up to provide aid for nuclear safety, distribution and public sector projects in the former Soviet Union. However, the TACIS budget for nuclear safety amounts only to 100 million Ecu, which means that the European Community countries would be footing most of the bill.

The installation of a gamma curtain - a dense net of radiation monitors strategically placed to pick up radiation leaks from the ageing reactors - is a much cheaper option, which should cost no more than 26 million Ecu. Although the gamma curtain does not address the underlying problem of some reactors being 'unsafe' - it will warn Eastern and Western European countries of a major disaster within minutes.

Initially, the monitoring system will be tested at two nuclear power plants in Ukraine and Belarus. The aim is to get this pilot system running by August next year, and if successful, the complete curtain will be installed over the next five years. Once in place, it will be possible to detect rises in the level of background gamma radiation. Some radiation is 'normal', for example, in Minsk, the capital of Belarus, between 0.15 and 2 microsieverts per hour is regularly detected. In Western Europe typical background radiation levels vary between 0.05 and 0 microsieverts, with the level rising to 0.3 microsieverts in areas with lots of granite.

Several Western European countries, including Britain and Germany, have already started to install monitoring systems at a local level. Britain began installing the first phase of its system - called Rimnet (Radioactive Incident Monitoring Network) - in 1988. Supplied by Siemens-Plessey, this included 92 gamma dose rate monitors located all over the Britain. The monitors send hourly readings to a central database collating the monitoring data, held at the Department of the Environment in London, with a backup database centre in Poole, Dorset. The second phase of Rimnet, scheduled for completion later this year includes the addition of a software system capable of predicting the path of a radiation plume, and showing this path on a computer generated map. A technical coordination centre has also been set up at a DoE office in London. Staff at this centre will liaise with other government departments and disseminate information to the media, if a radiation plume is detected.

A larger system is in operation in Germany. The German IMIS monitoring system uses more than 2000 sensors, measuring gamma dose rate as well as alpha and beta radiation. Data from these sensors is sent to a central database at the German Environment Ministry (BMU) in Bonn, which is available to 58 state offices and four federal government environmental agencies.

The BMU is one of the main supporters of a much wider gamma curtain. In addition to the German IMIS system, the BMU is already funding pilot early warning systems in the Czech and Slovak republics, and in Russia - at a cost of £398 406 each. The Czech and Slovak systems are installed, with 20 monitors around two nuclear power plants (one in each republic) and databases in Prague and Bratislava. The Russian pilot systems will monitor the most critical reactors at Smolensk, Novovoronezh and Kursk. Data will be sent via satellite to a database in Moscow, and onwards via another satellite link, to Germany, where it will be input into the IMIS database.

The BMU is also working with the European Commission on the more comprehensive gamma curtain concept covering the whole of the Commonwealth of Independent States and Central Europe. Last year, the Commission approved TACIS funding for a feasibility study of radiation monitoring in Belarus and Ukraine. Ukraine has five nuclear plants, including the one at Chernobyl. The others are at Zaporozhye, Khmelnitskiy and Rovno. There are also the Kursk and Novovoronezh plants just across the border in Russia. Belarus has no nuclear power plants on its own territory but it is ringed by plants in neighbouring states.

Local procedures for radiation monitoring do exist, but they rely on hand-held monitors, which are often up to twenty years old. At 9 am, readings of temperature, rainfall, wind speed and direction and gamma dose rate count are relayed to a central computer centre in Minsk or Kiev, usually from a phone in the local post office, but sometimes by telegraph.

Automatic reaction

With the introduction of the gamma curtain, this manual approach will be replaced with strategically placed automatic sensors. In Britain, it is usual to have between 8 and 10 gamma dose rate sensors on the boundary fence around a nuclear power plant. However, Illya Lukhtariov, a Russian radiation expert, suggests that for the CIS project there should be between 50 and 60 gamma dose rate sensors placed at varying heights and distances around the perimeter of each plant.

Lukhtariov points out that radiation plumes which break out from a nuclear plant in the event of a leak can travel vertically or horizontally. Vertical plumes can rise quickly - well out of the reach of ground-based sensors - and travel long distances before they are detected. Some may even rise to the top of the inversion layer in the atmosphere, and wait there until rain clouds form before the radiation falls to the ground. In the Chernobyl accident, two plumes did just that. One travelled almost 200 kilometres to the town of Gomel in Belarus, before it fell as radioactive rain. Another travelled all the way to Black Sea, around 800 kilometres away. Horizontal plumes can be extremely narrow - as little as a few metres across - and may slip between the sensors.

PA Consulting Group, the British management consultancy that carried out the preliminary study for the gamma curtain, put forward a scheme based on Luktariov's advice. However, the idea of having 60 sensors around each plant was deemed 'grandiose' and impractical so the pilot project will use fewer sensors, placed in three bands around the reactor. Between five and eight sensors will be put on the roof of the reactor, while the second band of some 10 sensors will be 1 kilometre from the plant, and the third band of between 10 and 20 sensors will be anywhere between 10 and 30 kilometres away.

Other types of sensors will also be placed in strategic positions near the reactors. Gamma dose rate sensors only indicate radiation increases over and above the existing background radiation, they don't give information on the 'type' of radiation, which can be very useful for identifying the seriousness of a leak. The presence of iodine-131, for example, would indicate a serious accident in the reactor core, and civilian authorities would need to take swift action, such as evacuating the area.

These specialised sensors typically use a dust filter to collect a sample and measure levels of alpha and beta radiation over a time period of an hour. If the level exceeds a pre-set limit - 50 millibequerrels per cubic metre per hour for alpha particles and 100 millibequerrels per cubic metre per hour for beta - they will raise the alarm. In the gamma curtain such sensors will be placed on the side of a reactor nearest to a population centre.

The team from PA is also looking at using nucleid specific sensors, which can give even more precise information about the radionucleids present in a radiation sample. Nucleid specific sensors, are an advanced form of gamma spectrometer, which operate at -196 °C. Until recently these sensors were limited to laboratory use where liquid nitrogen could be used for cooling. But EG&G, an electronics firm based in Boston, Massachusetts has developed an advanced electrical cooling system which makes the sensors more portable.

Information from these sensors can be transferred to the company's automatic analysis system, which runs on standard personal computers. This can analyse 100 different nucleids and detect radiation levels of less than 50 millibequerrels per cubic metre. EG&G's system is expensive, however, with a price tag of more than £60 000 per unit. The high cost - compared with an average of £3000 for a gamma dose rate sensor or even £20 000 for an alpha/beta or iodine sensor - means it can only have limited use in the gamma curtain, for example, downwind from a reactor near a town.

Companies will be invited to tender for the pilot system in Ukraine and Belarus next month. If the system goes well, TACIS will be asked to approve the 22 million Ecu funding - a relatively small price to pay for public reassurance and safety.

From issue 1891 of New Scientist magazine, 18 September 1993, page 23

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