radon. introduction radon is a colorless and odorless gas produced by the decay of radium – 226 ...
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RadonRadon
IntroductionIntroduction
Radon is a colorless and odorless gas produced by the decay of radium – 226
Radon after decay produces radioisotopes known as radon daughters
Radon progenies (Po-218 and Po-214) are of health concern, as they tend to retain in the lungs causing cancer
The upper limit recommended by US EPA for radon is 4pCi/L
Radon is found in many states in the USA
SourcesSources
Sources of RadonSources of Radon
Sources of radon include
Soil
Rocks beneath or surrounding the building
Water
Building materials
Natural gas
Radon from soil moves slowly from the pores of the soil to the surface by diffusion or pressure induced flow
Radon enters the building from the cracks and joints in the foundation
Effective radon (Rn - 222) Content of SoilsEffective radon (Rn - 222) Content of Soils
Soils Range of Emanation Coefficient
Crushed rocks 0.005 – 0.40
Soil 0.03 – 0.55
Soil 0.22 – 0.32 13 % to 20 % of dry weight
Sand 0.06 – 0.18
Sandy loam 0.10 – 0.36
Silty loam 0.18 – 0.40
Heavy loam 0.17 – 0.23
Clay 0.18 – 0.40
Soil 0.09 – 0.10 Dried at 105°C for 24 h
Uranium ore 0.06 – 26 Saturated with water
Crushed Uranium ore 0.055 – 0.55 Saturated with water
Tailings from Uranium plant 0.067 – 0.072 Dried at 110° C
Source: Nazaroff et al., 1988
Factors affecting transport of Radon to the Factors affecting transport of Radon to the surfacesurface
Soil permeability
Porosity
Water content
Temperature
Pressure difference between soil and building structure
Permeability of SoilsPermeability of Soils
Soil type Permeability (m² / h)
Clay 1 x 10 E (- 16)
Sandy clay 5 x 10 E (- 15)
Silt 5 x 10 E (- 14)
Sandy silt and gravel 5 x 10 E (- 13)
Fine sand 5 x 10 E (- 12)
Medium sand 1 x 10 E (- 10)
Coarse sand 5 x 10 E (-10)
Gravel 1 x 10 E (- 8)
Source: Terzaghi, 1967: Tuma, 1973
Sources of RadonSources of Radon
Water is also one of the potential sources due to high solubility of radon
The transfer of radon from water to air decides its contribution to the indoor concentration
Building materials like granite, clay bricks, marble and sandstone are also sources of radon
Fly ash from coal-fired power plant is a major source of radon, which is used in concrete and cement
Sampling and MeasurementSampling and Measurement
Sampling MethodsSampling Methods
Radon is measured indoors by the detection of alpha, beta or gamma emissions during the decay
The sampling methods are classified as:
Grab sampling
The study is conducted for a short period indoors by using scintillating flask
This method is advantageous in sensitivity and rapidity but is less accurate
When concentration is less than 10 Bq / m³ the error is more than 30%
Sampling MethodsSampling Methods
Continuous sampling
This method gives a real time measurement at short interval over a long time
The devices available for this type are:
Flow through scintillating chamber (two-port Lucas cell)
Solid state detector (wrenn chambers)
The wrenn chamber is the most widely used device capable of measuring concentrations even below 10Bq/m³
Integrated SamplingIntegrated Sampling
The devices used in this technique are:
Alpha tract detectors
Electronic ion detectors
Charcoal canisters
The charcoal canister method is EPA recommended and widely used method
This is easy to use and can be sent through mail to lab for analysis
The disadvantage of this method is an assumption that charcoal never reaches an equilibrium with the atmospheric radon
Radon concentration calculationRadon concentration calculation
Radon concentration is calculated by:
Rn = {net CPM} / { T(s) (E) (CF) (DF)}
Where CPM – counts per minute
T(s) – exposure time
E – efficiency of detector
CF – calibration factor
DF – decay factor
This method is effective for measuring concentrations above 4pCi/L as directed by EPA
Charcoal canister is ineffective for radon below 10Bq/m³
Efficiency of Radon Detection recommended by Efficiency of Radon Detection recommended by the EPAthe EPA
Method Number of Average Error Range of Company ErrorTests (percent) (percent)
Alpha track detector 10 25 11 to 55Activated-charcoal adsorption detector 256 19 1 to 133Continuous radon monitor 99 25 0 to 658Continuous working level monitor 75 40 0 to 1353Electric ion chamber 127 31 5 to 486Grab sampling radon 66 18 3 to 75Grab sampling working level 58 29 3 to 328Radon progeny integrated sampling unit 4 27 1 to 80Source: GAO, 1989
Control StrategiesControl Strategies
Source removalSource removal
Selection of construction sites having low radium content
Knowledge of local soil characteristics such as permeability and moisture content
Removal and replacement of soil from a perimeter of 3m from the building foundation
The cost for this process is site specific and can range from $5,000 to $20,000
New construction considerationsNew construction considerations
Radon concentration can be substantially reduced by new construction techniques
Provision of soil gas outlet to the sun slab and crawl spaces
Increasing the permeability by placing minimum of 4 inches of aggregate under slab
Double barrier approach can be used for slab-on-grade and crawl space construction
Source Control by sealing Entry pathsSource Control by sealing Entry paths
Floor drains and sumps connected to drainage systems
Openings around utility lines
Hollow concrete block walls
Junction between walls and floor and slab
Cracks in building materials
Exposed soil and rocks having radon
Unpaved crawl space
Sealing agents available and their Sealing agents available and their characteristics characteristics
Caulking agents
Paints
Membranes
Cement-type materials
The sealants used should be moisture resistant
Paints for walls.
Sunslab ventilation Sunslab ventilation
The design of sunslab ventilation is house specific and depends on nature of foundation
Fan with a capability to create 50 – 100 Pa is installed on end of the pipe running from the basement
This can be made effective by placing multiple collection ports for each wall
This is good for old structures, but excessive cracks diminish its effectiveness
This is very effective if drain tiles surround the entire house
Basement pressurization and Air cleaning Basement pressurization and Air cleaning
This method is highly effective method if the basement is airtight
Over pressurization of the basement drastically reduces the radon concentration below 4 pCi / L
This method is disadvantageous where there is increased ventilation and excessive windows and doors activity
This is one of the ways of reducing the radon concentration
During this process the air exchange rates are increased using the HVAC systems
Increased ventilation and activated carbon beds can remove the radon gas and its daughter products
Electronic air cleaners and Increased ventilationElectronic air cleaners and Increased ventilation
These cleaners have the capacity of reducing the radon gas and the potential alpha energy concentration (PAEC) by a factor of 2 – 20
After various studies combination of ion generator with ceiling fan produced best results (87% reduction)
Another way of decreasing the radon from indoors is plate-out i.e. by pushing the charged progenies to walls or floors and then outdoors
Simple, but rather effective technique is to increase the ventilation rate
For homes with large crawl spaces mechanical ventilation is adopted to decreasge the radon entry into the building (four fold decrease)
Adsorption Adsorption
The radon adsorption can be another way in reducing its concentration and depends on following factors:
Air flow rates
Radon concentration
Relative humidity
Activated carbon is used as adsorbent (having high capacity for radon and minimum interference with moisture and other VOC’s)
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