EFFECTS OF SMALL PRESSURE DIFFERENCES BETWEEN THE STRUCTURE AND SURROUNDINGS ON RADON ENTRY

Transcription

1 EFFECTS OF SMALL PRESSURE DIFFERENCES BETWEEN THE STRUCTURE AND SURROUNDINGS ON RADON ENTRY 1,2 Akis M.C., 1 Stadtmann H., 2 Kindl P. 1 ARC Seibersdorf research GmbH, Division of Health Physics /Radiation Protection A-2444 Seibersdorf, Austria 2 Graz University of Technology, Institute of Technical Physics akis@chello.at Abstract - The Radon Test-House of ARC Seibersdorf Research GmbH was utilized to test the radon entry through the structure driven by small pressure differences ( p ~25 Pa) between the indoor space and surroundings. The Test-House was equipped with an Active Sub-Slab Depressurization (ASD) system with two suction branches and with an adjustable-power indoor electric ventilation system which has been utilized for indoor depressurization in addition to ventilation. The radon activity enrichment in the sub-floor soil was made by means of a flow-through radon-gas source of 3.7 MBq activity. The experimental conditions were established either with local natural radon flux, or with artificial radon enrichment in the sub-floor area. By making use of the indoor-ventilation and the ASD systems, the air pressure difference between the Test-House indoor and surroundings were set to the desired values. Experiments showed that depressurizations of, for example, -5 Pa, or -12 Pa inside the Test-House with respect to the sub-floor area were able to drive radon-rich air from the subfloor region to the Test-House indoor, with some considerable transfer of activity, increasing the indoor radon level. Another depressurization experiment showed that a negative air-pressure of 27 Pa inside the Test-House with respect to the atmospheric air-pressure, drove towards indoor a considerable radon-gas activity originating from the radon-gas source which was placed outside of the structure, at a position of 2 cm away from a control hole on the structure wall. The natural air-exchange rate and the indoor ventilation rate in the Test-House were documented as well regarding their constructive or destructive influence on radon entry and resulting concentration. 1. Introduction Operation of residential buildings generates small pressure differences ( p), such that the indoor air pressure is slightly lower (a few Pa< p<~25 Pa) than the air pressure in the sub-floor area [1].Therefore, small pressure differences between the building structures and surroundings are important as regards elevated indoor radon-level. The typical pressure difference which is mainly responsible for pressure driven radon flow from soil to building indoors is about p ~2 Pa. In this paper, the results of the experiments performed under such small pressure differences between the Test-House-Structure and surroundings are explained. By surroundings the sub-floor region, and the open air at atmospheric pressure are meant. 2. Methods and Materials The Radon Test-House of ARC Seibersdorf Research GmbH [7] was equipped with an Active Sub- Slab Depressurization (ASD) [2] system as shown in Figure 1 with two suction branches and with an adjustable-power indoor electric-ventilation system which has been utilized for indoor depressurization in addition to ventilation. The radon activity enrichment in the sub-floor area was made by means of a flow-through radon-gas source of 3.7 MBq activity. The experimental conditions were established either with local natural radon flux, or with artificial radon enrichment in the subfloor area. While doing the experiments, the indoor Rn-concentration, indoor air pressure, indoor temperature, and the indoor air-humidity were monitored as single measurements each with ten The present work has been a part of a project supported by ARC Seibersdorf Research GmbH.

2 2 minutes integrating time. As a radon monitor Alpha-Guard [4] (an ionization chamber used in combination with digital signal processing technology) was utilized. As radon-sources, two different flow through gas sources of 3.7 MBq and 266 kbq activities, were used. In order to control the soilgas pressure and radon activity concentration in the sub-slab area, 17 holes penetrating the concrete slab were drilled throughout the concrete slab of the test house. When a very high radon concentration is needed in the sub-slab region for different experimental purposes, we simply transferred radon into the sub-slab region through a hole on the slab, by means of a pump connected to the Rn-source. The desired pressure differences were established by making use of the indoor electric-ventilation system with adjustable power, which exists in the Test-House. Figure 1. Cross-sectional view of the Test-House showing the ASD system, the concrete slab, the control pipes penetrating the slab, and the gravel etc., (dimensions are not to scale). 3. Results As shown in Figure 2, by means of the indoor electric-ventilation system, initially the pressure difference between Test-House indoor and outdoor (open atmospheric air) was set to 7 Pa. To write this in a compact form, the notation [ p(in-out)=-7pa] was used. Here - indicates that the indoor air pressure is lower than the outdoor pressure. At exactly this point in time (138th h =>1:45) the radon level was ~9 Bq/m³. Until [ p(in-out)] was set equal to -13 Pa at 11:43 the decrease in radon level was very slow with p(in-out)=-7pa. The radon activity level started to increase with the introduction of p(in-out)=-13pa, probably because of the increased soil-gas flow rate from sub-slab region. Actually, unlike sub-slab ventilation, such an increase in radon level is expected in the very beginning stages of indoor-ventilation (=indoor-depressurization), upon introduction of a negative pressure difference to indoors with respect to surroundings. But this is not a permanent increase in radon level. As could be seen in Figure 2 after 139th h, the reduction in radon level became clear. Later on at ~14th h=>12:58, in addition to indoor-electric-ventilation, the ASD system was also made ON introducing -25 Pa pressure difference to the sub-slab region with respect to Test-House indoor [ p(sub-in)=-25 Pa]. As a consequence of ASD operation, p(in-out) became equal to -18 Pa, as well.

3 3 3 t=136 is /8:5 Vent. Syst. on : /1:45 ( " ). off : /15:58 Rn-Activity Conc. (Bq/m3) 2 1 on P(in-Out)=-13 Pa ASD Syst. on : /12:58 ( " ). off : /15:1 with ASD and Ventilation are on: P(Sub Sl.-in)=-25 Pa P(in-out)=-18 Pa ASD ON ASD OFF off P(in-Out)=-7Pa Figure 2. By the use of the indoor electric-ventilation and the ASD systems, the air pressure difference between Test-House indoor and the surroundings were set to desired values. This experiment was done under natural radon-level. The case depicted in Figure 3 is similar to the case shown in Figure 2. No artificial radon enrichment in the sub-slab area was done as in the case of Figure 2. The radon-level in the Test-House was reduced to 16 Bq/m³ by means of both ASD and indoor electric-ventilation systems, and then every possible opening in the Test-House, like the main entrance-door, the internal-door, all 3-covers of indoor electric-ventilation system were tightly closed in order to reduce the natural ventilation rate as much as possible. Under these conditions, it took about 14 hours before indoor radon-level reached 25 Bq/m³ (Figure 3). At this stage, namely at ~186th h=>11:12, all the 3-covers of indoor electricventilation system were made open to increase the natural air exchange rate. After that, the natural air exchange rate was estimated, as ~.12 1/h. Afterward, at ~189th h=>13:47, the indoor-ventilation system was made partly ON, inducing just +4 Pa to the sub-floor area. At this stage the air ventilation rate was calculated as ~.23 1/h. Figure 4 is the representation of an experimental sequence which was started with the following conditions; ASD-OFF, indoor electric-ventilation-off, all the physical openings (main entrance-door, internal-door, ventilation covers) were closed, the air pressure in the sub-slab area was 1Pa higher with respect to Test-House indoor, and the average indoor radon-level was about 25 Bq/m³. The first change in these conditions was introduced at 21th h =>11: by making all 3-covers of the indoor ventilation system and the internal door open, and making the main entrance-door just 6 cm open. As expected, this change increased the natural air-exchange rate as high as ~.65 1/h, although the atmospheric wind speed was negligibly low.

4 4 At 211th h=>12:15 the indoor electric-ventilation was made ON at a very low power such that, p(in-out)=-2 Pa, in other words indoor air-pressure was 2 Pa lower than the outdoor atmospheric pressure. Under these conditions the radon-level kept on decreasing, but slightly slower, again probably because of increased soil-gas flow rate towards indoor, induced by the indoor electricventilation system, as typically observed in the beginning stages of indoor ventilation. Rn-Activity Conc. (Bq/m³) t=144 is /16:5 ASD Sys. on /14:33 Ventil. Sys. on ( " ) P(sub - in)=-25 Pa P(in-out)=-2 Pa... ASD & Ventil. Systems are both off /17: on off Ventil. Covers open: /11:12 Internal Door open : ( " ) Ventil. Sys. on : /13:47 (Indoor ventilation is partly on. One of the three covers is open: /13:47 P(in-sub)= - 4 Pa... Ventilation System off: /16:37 All vent. covers closed:( " ) P(in-sub)= Pa (16:37) 11:12 13:47 16: Figure 3. The radon-level in the Test-House was reduced to 16 Bq/m3 and then ASD and indoor-ventilation systems were made OFF. The following stage had been an expected increase in radon-level with ~25 Bq/m³ at the maximum. At th h =>13:15 (Figure 4) while the radon-level was as low as ~34 Bq/m³ all the doors (internal and main entrance-door) were closed, by then, the indoor electric-ventilation system was made to work at a higher power, such that it induced -5 Pa into Test-House indoor [ p(in-sub)=-5 Pa]. As could be seen in Figure 4 ( from th h to 213.5th h), even such a very small pressure difference (-5 Pa) between the sub-floor region and the Test-house indoor, clearly, was able to drive radon-bearing-air flow from sub-floor region to the Test-House indoor until 213.5th h=>14:3. To be more precise, [ p(in-sub)=-5 Pa] increased the radon activity concentration level from 34 Bq/m³ until 112 Bq/m³ in 75 min (Figure 4), without artificial radon-gas enrichment in the sub-floor area. Subsequently, at 213.5th h=>14:3 the ASD system was made ON (Figure 4) inducing a -33 Pa in the sub-floor area with respect to Test-House indoor [ p(in-sub)=-33], and then the indoor electricventilation system was switched to full power such that the ventilation rate was.7 1/h. And this experimental sequence, was terminated with a resulting radon-level of 2 Bq/m³ in ~6 min.

5 5 8 t=24 is /4:5 Rn-Activity Conc. (Bq/m³) vent. covers, int. door, & outdoor (6 cm) open: /11: 1 vent. on (other covers closed): /12:15 P(in-out)=-2 Pa, P(in-sub)= P(in-sub)=-5 Pa ( /13:15) Int. door & Outdoor are closed: /13:15 ASD & ventilation systems are on : /14:3 P(in-sub)=-33 Pa (while ASD is on at 14:3) ASD & ventil. systems all doors and covers are closed: /17:1 18.7/11: 18.7/12: /14:3 18.7/13:15 18:7/17: Figure 4. Variation of radon-activity-concentration under the effect of small pressure differences (few Pascals) due to natural air-exchange. From 18.7/14.3 until 18.7/17:1 ASD and the indoor electric-ventilation systems were ON. In Figure 5, a rather interesting result was represented.we simply placed our low activity radon source (266 KBq) outside (in atmospheric air) at a position of ~2 cm away from the control-hole-2 on the wall of the Test-House (in Figure 1 right hand side wall.the hole diameter is ~1 cm), end then the following sequential pressure adjustments were done. Firstly, the radon-source (266 KBq) were made ON at 14:45 (Figure 5), while the indoor air-pressure was 9 Pa lower with respect to outdoor atmospheric-air [ p(in-out)=-9 Pa] due to indoor electric-ventilation system working at a low power. At 1st h=>14:5 the indoor-ventilation system was made to work at a higher power such that the indoor air pressure was 27 Pa lower than the outdoor pressure, 7 Pa of which were due to the contribution of the ASD system which was ON since 12:3 to make the sub-floor area radon free. This was important to make sure that the radon-level increase (if it happens!) in the Test-House could only be from the radon-source placed outside, but not from the sub-floor area. As shown in Figure 5, at 1st h=>14:5 the radon level started to increase from ~ 45 Bq/m³ and reached ~ 6 Bq/m³ in 4 min. The position of the control-holes on the Test-House side walls, can be seen in Figure 1. As the last example of radon-rich-air flow driven by small pressure differences towards indoors, the experimental results shown in Figure 6 are explained. The radon-gas enrichment was made in the sub-floor area by means of a radon-gas source of 3.7 MBq of activity.

6 6 14 t= is /13:5 Rn-Activity Conc. (Bq/m³) Ventilation! P(in-out)=-9 Pa; /14:45 P(in-out)=-27 Pa; /14:5 Ventilation off! P(in-out)=-1 Pa; /15:37 (outdoor) Rn-S on 14:45 15:37 ASD on 14:5 Ventilation on! P(in-out)=-25 Pa; 16:52 16:52 Rn-S off! 17: Figure 5. The graphic above shows the radon-concentration peak which formed after air pressure reduction in the Test-House, with the radon-source placed outdoor ~2cm away from the control hole-2 on the wall (see Figure 1-left hand side). In this experiment, like the one before, the radon-gas source was placed outside the Test-House as well. The radon-gas outlet of the source, was extended by means of a silicon hosepipe until the gravel in the sub-floor area, through the pressure, flow, and Rn-control orifice on the ASD-pipe while the ASD system was OFF. After that, the radon-gas-source was flushed outside the Test-House to eliminate the possibility to transfer excessive amount of activity into the sub-floor area. When it was 48th h=>13:55 (Figure 6), both, the radon-gas outlet valve of the source (without pumping), and the indoor electric-ventilation system were made ON. By then, the air-pressure in the sub-floor area was measured as 7 Pa lower than the atmospheric air pressure or [ p(sub-atm)=-7pa], and the Test-House indoor air pressure, with all the doors were closed, was measured as 12 Pa lower than the sub-floor area air pressure, [ p(in-sub)=-12]. From 13:55 until 14:45 (Figure 6) the radon-gas-outlet valve of the source was ON without pumping, and the amount of the radon-activity driven towards indoor by [ p(in-sub)=-12 Pa] from the subfloor area, was ~13 KBq. From 14:45 until 15:5 (Figure 6) the electric pump was made ON to increase the radon transfer process into the sub-floor area, therefore the amount of activity driven by [ p(in-sub)=-12 Pa] was ~18 KBq in this 2 min of time span; and the indoor radon-level was as high as 3 Bq/m³ (Figure 6). At 15:12 the ASD system was made ON, in addition to the indoor electric-ventilation system, which was already working to maintain the condition p(in-sub)=-12 Pa. The ASD operation manifested itself as a sharp reduction in radon-activity-concentration level from ~31 Bq/m³ to ~31 Bq/m³ in 24 min, besides indoor electric-ventilation.

7 7 Rn-Activity Conc. (Bq/m³) t=44 is / 9:5 Rn-transfer in sub-slab area OFF: 15:5 14:45 13:55 ASD on 15:12 Rn-S (3.7 MBq) is connected to the sub-slab area by means of an ASD exhaust pipe, and made ON without pump; 13:55 (all doors are closed) Indoor Electric Ventilators on; 13:55 P(sub-atm)=-7 Pa P(in-sub)=-12 Pa Rn-Pumping to sub-slab region through ASD sub-sl. pipes on; 14:45 ASD & ventilators off ; 15:36 Vent. Covers are closed; 15: Figure 6. Between 13:55 and 14:45 about 13 KBq of activity transported with radon source connected to sub-floor area but without pumping. With the pump-on connected to the source (from until 15:5), an additional ~18 KBq more of radon-activity from sub-floor area were driven into the Test-House indoor, due to 12 Pa of negative indoor air pressure with respect to the sub-floor area. 4. Conclusion Radon is the heaviest known monatomic gas with a roughly estimated average mass fraction of 16 ~ 1 % [5] of the elements in the earth s lithosphere (upper 16 km) together with hydrosphere and atmosphere. Estimation of the magnitude order regarding radon activity transport towards indoor areas, particularly driven by small (a few Pa p ~25 Pa) negative pressure differences, deserves a special attention. The experimental results given in this paper are in good agreement with the previous works [1] [3] [6], concerning how even small air pressure differences between the structure and surroundings, can be critical as regards the transported activity of such a high density and extremely low natural mass fraction gas. For instance, a slight pressure difference [ p(in-sub)=-5 Pa] increased the radon activity concentration level from 34 Bq/m³ until 112 Bq/m³ in 75 min (Figure 4). This case was one of the best evidences concerning indoor radon-level increase due to soil-gas flow driven by pressure, even with very low natural-radon content in the soil-gas, moreover, under slightly negative indoor air pressure with respect to sub-floor area of the Radon Test-House.

8 8 References 1. Nero A. V., Nazaroff W.Characterizing the Source of Radon Indoors. Radiation Protection Dosimetry Vol. 7 No. 1-4 pp , (1984). 2. U.S. Environmental Protection Agency, Office of Research and Development. Radon-Resistant Construction Techniques for new Residential Construction. EPA / 625 / 2-91 / 32 Feb. (1991). 3. Eaton R.S., Scott A.G., Understanding Radon Transport into Houses. Radiation Protection Dosimetry Vol. 7, No. pp 1-4: , (1984). 4. Genrich V., Alphaguard PQ2/MC5:Multiparameter Radon Monitor. Genitron Instruments GmbH, Heerstraße 149 D-6488 Frankfurt am Main Germany. 5. Periodic Table of Elements, ISBN Wiley VCH, P.O. Box 11161, D Weinheim, Germany. 6. Nero, A.V., A.J. Gadgil, W.W. Nazaroff and K.L. Revzen,. Indoor Radon and Decay Products: Concentrations Causes, and Control Strategies. Department of Energy, Office of Health and Environmental Research, Washington, D.C., DOE/ER-48P, (199). 7. Akis M. C., Stadtmann H., Kindl P., Steger F., Performance Test of an Active Sub-Slab Depressurization System Developed for a Radon Test-House. European Commission Radiation Research Unit, ERRICCA Workshop: Radon in the Living Environment Athens, Greece, (19-23 April 1999).