THE RELIABILITY OF RADON REDUCTION TECHNIQUES. CB Howarth

Transcription

1 Radon in the Living Environment, 052 THE RELIABILITY OF RADON REDUCTION TECHNIQUES CB Howarth National Radiological Protection Board Chilton, Didcot, Oxon, OX11 0RQ, UK Tel: , Fax: , It is estimated that more than 5000 householders in the UK have taken steps to reduce high radon levels in their homes. In 1993 a number of homes with successful remedies installed were asked to participate in a study to determine the long term reliability of those systems. This involved the annual remeasurement of the radon concentration in each dwelling. Results for 26 of the dwellings which have data spanning 6 years, with a further 32 which have data over a shorter period, are discussed. The study shows that all reduction techniques can remain durable, though there are some mechanical failures in active systems. Significant variations in reliability have been seen in natural ventilation of the underfloor void due to the effects of environmental conditions, such as wind speed and direction. The overall failure rate for reduction techniques was found to be 4.5% per annum. However, most of these were noticed and corrected by the householder. There was a rate of 0.6% per annum for failures that went unnoticed. Key words: Radon, remedial measures, reliability, durability. INTRODUCTION To date, NRPB has measured radon levels in approximately homes in the UK, 40,000 of which were found to be above the UK Action Level of 200 B qm -3. An earlier study (1) of 10,174 householders with radon concentrations above the Action Level showed that between 11 and 22% had taken action to reduce high radon levels. This suggests that the total number of householders that have undertaken remedial measures in their homes is more than Various authors have discussed the effectiveness of different remedial measures (2-5). If the aim of significantly reducing total exposure to radon is to be achieved then the remedial measures must continue to be effective for several years. It is known that all radon remedial actions can fail. Therefore it is necessary to obtain some idea of the reliability of the individual radon remedial measures. Reliability is a concept with two aspects, failure rate and consistency. The first of these concerns the probability of failure (per year) of types of remedial action. The second aspect is consistency, which is concerned with the degree to which the post-remediation radon level fluctuates. If a remedial measure is prone to large annual variations in the radon level then a single measurement after remediation may not give an accurate idea of the long term average. Since 1993, NRPB has been carrying out a survey (4,6) of the durability of radon remedial actions which has involved repeated measurement of radon levels in a sample of homes in which successful remedial actions have been carried out. Fifty eight houses have so far been enrolled in the study, of which 26 have a complete set of data spanning 6 years with a further 32 which have data covering a shorter period. The complete data sets consist of an initial measurement which identified a high radon level, a measurement following remediation and six further durability measurements made after remediation at one year intervals. All measurements were carried out over a three month period using two passive etched track detectors, one in the living room and one in the bedroom. All 457

2 052 Radon in the Living Environment, measurements were corrected for duration of the measurement as well as for typical seasonal variations in radon concentrations. RESULTS The failures which occurred during this study are listed in Table 1, along with the reasons for their failure, where known. Fifteen failures occurred in the 56 houses during the period of the study, with an annual average failure rate of 4.5% ±3.2% where the uncertainty is the standard error on the mean. Of the 15 failures which occurred, 13 were discovered by the householder and fixed, and two were identified by the durability measurement. In one case (membrane across the floor) the measurement showed a result just above the Action Level but still lower than the pre-remediation level. The householder decided to await the results of later measurements before proceeding with any further action. In the second case (mechanical ventilation of the underfloor void) the radon measurement showed a return to the pre-remediation level. Once the householder knew of the increased radon level he was able to identify the problem and rectify it allowing a return to a low radon level. The rate of unnoticed failures was therefore 0.6% ± 0.6% per annum. Over the period of the study 9 out of 42 (21%) active systems have experienced a failure of some sort and 3 out of 14 (21%) passive systems have also failed at some time. This suggests that the overall failure rate for remedial measures is independent of whether the individual measure uses a fan. With the possible exception of sealing, there is no indication in the data that the effectiveness of remedial actions declines with time. This confirms the result given by Naismith (6) (see Figure 1). Table 1 shows that the rate of fan failure tends to increase with time, but this is to be expected as the fans near the end of their operational life. Even so, there are still 11 out of watt centrifugal fan systems that have operated in excess of hours without any reported problems. In the case of sealing there seems in some cases to be a slight trend towards poorer reductions over time, possibly due to widening of existing or development of other cracks in floors over a period of several years. If this trend does exist it is not yet quantifiable but requires further measurements. It is perhaps interesting to note that all of the failures that have occurred were in houses recruited at the start of the study in The houses recruited in 1995 have now yielded five year s worth of measurements without any failures so this difference is probably not simply due to lack of data. One possible explanation is better building practices as advice and workmanship improve with experience. To measure the consistency of the radon reduction achieved for a house the following procedure was adopted. First the percentage reduction for each house for each year was calculated, followed by the mean and standard deviation of these figures for each house. These data were then plotted on Figure 2. The data were also grouped by remedial type to obtain the mean, maximum and minimum percentage reductions and standard deviations for that remedial type. Figure 2 and Table 2 show the results obtained for the 56 houses for which there is four or more year s worth of data. Unlike some earlier studies (3) which reported reduction factors, the percentage reduction figure was used in this work as this is largely unaffected by small variations in very low post remediation levels. It also gives a good indication of the radon reduction that can be achieved by a particular remediation method. 458

3 Radon in the Living Environment, 052 The graph shows a clear inverse relationship between the variability (expressed as percentage standard deviation) and the percentage reduction in radon levels achieved in a particular house. Hence smaller reductions in radon levels are associated with larger year to year variations in the radon level after remediation. The correlation coefficient obtained for the entire set of data is Remedial measures have been split into two types on the graph, active systems (those that use a fan) and passive systems. It is clear that in general active systems produce better reductions in radon levels and that these post-remediation levels are less variable. Table 2 shows that the reductions achieved can be remarkably consistent for active systems over the period of the study. The most variable results came from passive systems. In the case of natural underfloor ventilation this may be due to variations in factors such as wind speed and direction, which affect this method more than others. CONCLUSION It is clear that active measures that use a fan are far more effective than passive measures and maintain a more consistently low radon level after remediation. There is no evidence of declining effectiveness with time of remedial measures that have not suffered a failure. There was little or no difference in failure rate between passive and active systems, although the small numbers involved make this a tentative conclusion. The failure rate for all remedial measures was 4.5% per annum. In all but two cases the failure was noted by the householder and corrected prior to any measurement, resulting in an unnoticed failure rate of 0.6% per annum ACKNOWLEDGEMENTS The work for this study was part funded by the European Commission, contract number F13P- CT d. The author would also like to thank S Naismith for the contribution he has made over the years to this study. REFERENCES [1] Bradley, E. J. and Thomas, J. M. An Analysis of Responses to Radon Remediation Advice. National Radiological Protection Board, Chilton, M-707 (1996) [2] Cliff, K. D., Green, B. M. R. and Lomas, P. R. Domestic Radon Remedies. Radiat. Prot. Dosim., 45, (1992). [3] Naismith, S. P. Efficacy of radon remedial measures. Radiol. Prot. Bull., 152, (1994). [4] Cliff, K. D., Naismith, S. P., Scivyer, C., Stephen, R. The efficacy and durability of radon remedial measures. Radiat. Prot. Dosim., 56, (1994). [5] Woolliscroft, M., Scivyer, C., Welsh, P. Radon remediation in dwellings in the UK: Technical solutions and field experience. Presented at Healthy Buildings 94, Budapest, August [6] 6 Naismith, S P, Durability of radon remedial actions. Radiat. Prot. Dosim., 71, , (1997) 459

4 052 Radon in the Living Environment, Table 1: Failures of remedial measures Remedial action Reason for failure Solution Year into study Sump Fan failed Replaced fan * 4 Sump Fan failed Repaired fan ** 4 Sump Fan failing Replaced fan * 1 Sump Fan failed Replaced fan* 5 Sump Fan failing Replaced fan* 5 Positive ventilation Positive ventilation Blocked inlet filter Blocked inlet filter Replaced filter *** 4 Replaced filter *** 3,4 Mechanical underfloor ventilation + Mechanical underfloor ventilation Natural underfloor ventilation Blocked inlet filter Removed filter 3 Fan failing Replaced fan* 6 Blocked airbricks Removed leaves 2,4 Membrane across floor + Not known 6 Sealing Opening of cracks Resealed cracks ** 4,6 * An additional cost of approximately 100 ** An additional cost of approximately 50 *** An additional cost of approximately 20 + Failure was not detected by the householder 460

5 Radon in the Living Environment, 052 Table 2: Percentage reduction factors achieved and variability Remedial measure Percentage reduction achieved Percentage standard deviation Mean Minimum Maximum Mean Minimum Maximum Sump Positive ventilation Mechanical underfloor ventilation Natural underfloor ventilation Sealing

6 052 Radon in the Living Environment, 1200 Figure 1: Geometric mean of absolute reduction achieved by remedial measures 1000 Sumps Radon reduction (Bq m-³) Positive Ventilation Mechanical ventilation of underfloor void Natural ventilation of underfloor void Sealing Membrane across concrete floor Year of measurement 462

7 Radon in the Living Environment, F igure 2: V ariability of radon reduction with reduction achieved 25 % SD in radon reduction Active systems Passive systems A verage % radon reduction 463

8 052 Radon in the Living Environment, 464