NRPB-W1: Non-technical summary

Non-technical summary

When radioactivity is released into the atmosphere, a proportion of the activity will deposit onto surfaces. Some of the deposited material will become airborne again, when disturbed by wind or human activities such as walking or vehicle movement. This process is known as resuspension (see Jargon Watch). In the event of an accidental release, people may continue to inhale resuspended activity after the initial cloud of radioactivity has dispersed, and so receive additional doses (resuspension doses). A review of experimental studies of resuspension has been carried out in order to recommend an appropriate method for estimating likely resuspension doses after an accident, in UK conditions. This information can be used to assist decisions on how best to protect people in the immediate aftermath of an accident, i.e. during the emergency phase (see Jargon Watch).

The report summarised here recommends a mathematical formula for estimating resuspension doses from measurements of radioactive contamination on the ground. In addition, guidance is given on how to apply that formula in different situations (such as very windy conditions, in towns with a lot of traffic) and how to modify the formula for resuspension that might occur inside buildings.

Introduction

If radioactivity is in the air, it is possible that tiny particles will be inhaled into the lungs. The reason why resuspension is important for emergency response is because it makes inhalation a possible exposure route, even after the initial airborne radioactivity has gone.

The resuspension factor is the ratio of the concentration of radioactive material in the air due to resuspension, to that on the surface (see Jargon Watch), and is used to calculate the amount of radioactive material a person may inhale. There are several methods used to calculate the quantity of radioactivity that will be resuspended.These methods, however, can give very different results, especially in the first few days following a release of activity. These methods were reviewed to judge how accurate they would be for the UK, and if they could be applied in emergency response situations.

What methods were reviewed?

Linsley reviewed a number of studies and recommended a method to be used for resuspension calculations. However, a lot of the data reviewed were from dry climates, and because resuspension is higher in dry climates than moderate climates the resuspension factor predicted may be too high for the UK.

A method suggested by Garland included results from many measurements in the UK. These measurements looked at resuspension in typical UK weather conditions. The method was tested for resuspension immediately after the particles landed on the ground and also for long time periods. After the Chernobyl accident, when many measurements were made around Europe, this method was re-tested, and was found to be accurate.

Another method, known as 'dust loading', was reviewed. This method is used in many studies to calculate the extent of resuspension. When measurements of the activity deposited on the ground are taken immediately after an accident, values tend to be reported in units of Bq m ^-2 (see Jargon Watch). To use this method, the activity on the ground is required in units of Bq kg ^-1 (see Jargon Watch). This is difficult to calculate without additional measurements, and therefore it was decided that this method was not suitable for emergency response situations.

It was decided on the basis of these reviews, that the Garland method would be recommended. However, a small adjustment was made to the formula suggested by Garland, to include a term to account for aged deposits. Garland's method is time-dependant, and the resuspension factor is calculated to decrease with time, until the resuspension factor reaches a value of zero. Garland's experiments did not last longer than a year. It may be necessary when making decisions about protective countermeasures (see Jargon Watch) after an accident to use his methods to calculate resuspension for 10 years and longer. There is enough measurement data to suggest that the resuspension factor does not decline below a particular value for many years and so Garland method was adapted to reflect this.

What else influences resuspension?

As mentioned before, many things can influence resuspension. Studies were reviewed to estimate whether different factors should be considered when applying Garland's method.

Wind speed

Although several authors have reported effects of the wind, measurements have been too scattered to make an accurate recommendation.

Nature of surface

There appears to be a difference between resuspension from grass to that from bare soil, but the difference is small, when compared to other uncertainties in resuspension. There does appear to be higher resuspension in towns and cities, but this is mainly due to vehicle movement on roads.

Size of contaminant

Particle size influences whether it will be resuspended. Very small particles (0.1µm (see Jargon Watch) or less) and very large particles (50 µm) are unlikely to be resuspended.

Indoor resuspension

The mechanisms for particle resuspension indoors are essentially the same for those outdoors, except that effects due to soil movement are absent. The movement of people and the operation of equipment, as well as ventilation within the room may cause resuspension. Studies of indoor resuspension were reviewed, and the recommendation in this report is to adopt a constant resuspension factor.

Uncertainty

(see Jargon Watch)

Despite many measurements, there is still a lot of uncertainty about the extent of resuspension. This uncertainty is due to several factors: wind speed, surface structure, time since deposition, size of particles and human activities. It is important to be aware of this when using the resuspension factor for a particular situation. This report reviews the size of this uncertainty.

Conclusions

This report concludes by recommending that Garland's method be adopted for emergency response, using the additional term for aged deposits. Scaling factors are suggested which can be used if there are influences present that could alter the result, such as heavy traffic or strong winds. Tables of results for different radionuclides are also included.

Jargon Watch

Last reviewed: 1 September 2009