ft <?QR>JZ^ STUK-A124 SEPTEMBER 1995 Residential Radon in Finland: Sources, Variation^ Modelling and Dose Comparisons Haima Arvela STUK-A124 SEPTEMBER 1995 Residential Radon in Finland: Sources, Variation, Modelling and Dose Comparisons H. Arvela Finnish Centre for Radiation and Nuclear Safety Helsinki University of Technology, Department of Technical Physics Academic dissertation Dissertation for the degree of Doctor of Technology to be presented with due pennission for public examination and debate in Auditorium Fl at Helsinki University of Technology (Espoo, Finland) on the 6th of October, 1995 at 12 o'clock noon. FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY P.O. Box 14 FIN-00881 Helsinki Tel. + 358 0 759 881 ISBN 951-712-060-5 ISSN 0781-1705 Painatuskeskus Oy Helsinki 1995 Sold by: Finnish Centre for Radiation and Nuclear Safety P.O. Box 14 FIN-00881 Helsinki Tel. + 358 0 759 881 FINNISH CENTRE FOR RADIATION STUK-A124 AND NUCLEAR SAFETY ARVELA H. Residential Radon in Finland: Sources, Variation, Modelling and Dose Comparisons. STUK-A124. Helsinki 1995. 87 p. + appendixes 80 p. ISBN 951-712-060-5 ISSN 0781-1705 Key words: radon, radiation, radiation dose, homes SUMMARY This study deals with sources of indoor radon in Finland, seasonal variations in radon concentration, the effect of house construction and ventilation and also with the radiation dose from indoor radon and terrestrial gamma radiation. The results are based on radon measurements in approximately 4,000 dwellings and on air exchange measurements in 250 dwellings as well as on model calculations. The results confirm that convective soil air flow is by far the most important source of indoor radon in Finnish low-rise residential housing. The average flow rate of soil gas into a typical house with slab-on-grade is about 0.5 m3 h"1. This flow of radon-bearing air produces an average convective radon entry rate of 12,000 Bq h'\ increasing the indoor radon concentration close to the target concentration for new buildings, 200 Bq m'3. Consequently, in a remarkable number of houses, the concentration limit for existing houses, 400 Bq m'3, will also be exceeded. The most important reason for such elevated concentrations are permeable soil, leaking substructures or an unfavourable ventilation system. In esker areas the subterranean air flows in the esker may still increase the effect of convective flows. This situation calls for new attitudes to house construction and mitigation of existing houses. Airtightness of the bouse substructure, especially on permeable soils, is the key question in radon-safe building. The building of ground floor flats with ground contact needs similar attention to low-rise housing. Because soil permeability and the gravel layer beneath the slab have a decisive effect on soil gas flow, attention should also be paid to studies on the effect of an impermeable layer below the slab. Modern foundation structures have increased the radon-bearing soil air flow into houses. Crawl-space, nowadays seldom used, is an advantageous substructure against radon. Slab-on-grade with foundation walls made of porous light-weight concrete blocks, being the most common construction, has increased the soil air leakage into dwellings. Hillside houses with an open stairwell between the ground floor and first floor have replaced basement houses. Basements were previously more isolated and reduced the radon-rich air flow into living spaces. 3 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A124 In a notable oroportion of the dwellings the air exchange rates are lower than the recommended air exchange of 0.5 h'. Improving the air exchange rates provides great potential for decreasing indoor radon concentrations. However, improving the air infiltration has a reductive effect on indoor radon concentrations only in cases where the remedy does not increase the underpressure in the dwelling. Air exchange rates in new houses with mechanical ventilation are higher than in houses with similar structures and with natural ventilation. However, radon concentrations are higher in houses with a mechanical exhaust or a supply/exhaust ventilation system. The underpressure created by both the exhaust air flow and the elevated airtightness of houses contribute to this increase in radon concentration. Mechanical exhaust ventilation is not recommended when building on permeable sand or gravel areas, unless soil air leakages are prevented. Comparison between predicted and measured winter/summer radon concentration ratios, in houses with slab-on-grade, suggests an average diffusive radon entry rate of 6 Bq m'3 h"1 and a convective entry rate of 50 Bq m*3 h"' at an average indoor- outdoor temperature difference of 17 °C. The estimates in this study imply that both diffusive radon entry from soil through slab and floor coverings play an important role in diffusive radon entry in Finnish low-rise houses, more important than previously realized. In unfavourable conditions diffusive entry alone can elevate radon concentration above the target limit for new buildings, 200 Bq m'3. Therefore, attention should be paid to the quality of floor slabs in low-rise houses. The physics of diffusive radon entry into buildings, of air-flow from soil, ariven by the pressure difference, and of air infiltration, combined in the model of this study, explain to a considerable extent the observed seasonal variations in indoor radon concentration. Radon concentrations are higher in winter than in summer, because of the strong convective air flow from soil into houses in winter. Cold climatic conditions in Lapland increase indoor radon concentrations in low-rise houses, 25 % compared with the metropolitan area in Southern Finland. Correction factors for annual average radon concentration are needed to adjust measurements taken over periods other than twelve months. Indoor radon measurements in Finland are taken in the heating season, over a period of two - months. For these measurements, typical correction factors vary in the range of 0.9 - 0.6. Discrepancy between the single compartment model and the real occurrence of radon in dwellings affects the difference between measured and predicted results. 4 FINNISH CENTRE FOR RADIATION STUK-A124 AND NUCLEAR SAFETY On steep sided eskers subterranean air-flows in the esker strongly affect indoor radon concentration. Variation in both outdoor temperature and wind induce these flows which result in winter/summer radon concentration ratios of 3 - 30, in esker top areas, whereas in slope areas with amplified summer concentrations, the ratio is typically 0.1-0.5. Typical values in flat ar*as are 1.5 - 2. The estimates of the effective dose due to indoor radon and terrestrial gamma radiation are based on nationwide surveys of indoor radon, gamma radiation indoors and outdoors, including a survey of environmental dose rates after the Chernobyl accident. The annual average effective dose to Finns due to indoor radon is estimated at about 2 mSv, the range of the dose being as wide as 0.2 - 350 mSv. The annual average dose due to natural gamma radiation and external gamma radiation due to Chernobyl fallout in 1994 were 0.45 mSv and 0.03 mSv, the ranges being 0.2 -1.0 mSv and 0.001 - 0.3 mSv respectively. Special attention should be paid to prevention of the highest doses, to repairing the houses with the most elevated indoor radon concentrations and to radon-safe construction of new houses. 5 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A124 ARVELA H. Sisäilman radon suomalaisissa asunnoissa: Lähteet, vaihtelu, mallintaminen ja annosvertailu. STUK-A124. Helsinki 1995. 87 s. + liitteet 80 s. ISBN 951-712-060-5 ISSN 0781-1705 Avainsanat: radon, säteily, säteilyannos, kodit YHTEENVETO Tässä tutkimuksessa käsitellään sisäilman radonpitoisuuksia suomalaisissa asunnoissa; radonpitoisuuden lähteitä ja vuodenaikavaihtelua sekä talon perustuksien ja ilmanvaihdon vaikutusta. Radonin aiheuttamaa säteilyannosta verrataan ulkona ja sisätiloissa saatavaan gammasäteilyannokseen. Tulokset perustuvat radonmittauksiin noin 4000 asunnossa, ilmanvaihtomittauksiin 250 asunnossa sekä laskentamallin antamiin tuloksiin. Tulokset vahvistavat sen, että maaperän radonpitoisen ilman virtaus asuntoihin on kaikkein tärkein radonpitoisuuden lähde suomalaisissa pientaloasunnoissa. Tyypillisessä pientalossa, joka on rakennettu maanvaraiselle laatalle, keskimääräinen maaperän ilman virtausnopeus asuntoon on noin puoli kuutiometriä tunnissa. Tämä radonpitoisen ilman virtaus vastaa konvektiivista lähdenopeutta 12 000 Bq h"1 ja nostaa sisäilman radonpitoisuuden lähelle uusille asunnoille annettua enimmäisarvoa 200 Bq m'3. Maaperän radonpitoisen ilman vuodon seurauksena myös vanhoille asunnoille annettu radonpitoisuuden enimmäisarvo 400 Bq nr3 ylittyy merkittävässä osassa pientaloja. Tärkeimmät syyt asuntojen radonpitoisuuden kohoamiseen ovat maaperän ilmanläpäisevyys, perustusrakenteiden ilmavuodot, ilmanvaihdon riittämättömyys tai asunnon alipaineisuus. Harjualueilla harjun sisällä tapahtuvat ilmavirtaukset voivat edelleen kasvattaa radonvuotoa asuntoihin. Tilanteen korjaamiseksi tarvitaan asenteiden muutosta sekä uudisrakentamisessa että suhtautumisessa radonkorjauksiin. Perustusten ilmatiiveys, erityisesti rakennettaessa läpäisevälle maaperälle, on avainkysymys radonturvallisen asunnon rakentamiseksi. Kerrostalojen rakentamisessa tulee myös ottaa huomioon maaperästä tulevat radonvuodot, jotka vaikuttavat erityisesti alimman kerroksen maakontaktiasuntojen radonpitoisuuteen. Maaperän ilmanläpäisevyys