Understanding radon levels in houses

Lluís Font. Grup de Física de les RadiacionsUniversitat Autònoma de Barcelona. Spain

24th International Conference on Nuclear Tracks in SolidsBologna, 1-5 Sept 2008

InterestInterest

Review of radon modelsReview of radon models

The Global Dynamic Radon Model (GDRM) conceptThe Global Dynamic Radon Model (GDRM) concept

The RAGENA modelThe RAGENA model

PerspectivesPerspectives

ContentsUnderstanding radon levels in houses

Radon is the largest single source of radiation exposure to populationRadon is the largest single source of radiation exposure to population

• Locate houses with high radon levels.• Determine de most effective mitigation methods.• Improve building design and practises to avoid high radon levels in new buildings

• Locate houses with high radon levels.• Determine de most effective mitigation methods.• Improve building design and practises to avoid high radon levels in new buildings

Radon goes through four stages until it reaches a living environment:Radon goes through four stages until it reaches a living environment:

1) Its generation in the source media2) Its migration in the source medium3) Its entry into a dweling4) Its accumulation indoors

1) Its generation in the source media2) Its migration in the source medium3) Its entry into a dweling4) Its accumulation indoors

Understanding these processes is useful to:Understanding these processes is useful to:

InterestUnderstanding radon levels in houses

Modelling, together with experimental studies, generates understandingModelling, together with experimental studies, generates understanding

• Relative importance of different parameters and processes.• Exploration of different scenarios.• Cost-effective powerful tool.

• Relative importance of different parameters and processes.• Exploration of different scenarios.• Cost-effective powerful tool.

Many parameters of different origin take part at each stage, and mostof them are time-dependent (real world).

Many parameters of different origin take part at each stage, and mostof them are time-dependent (real world).

Complexity! Partial models and/or experimental studies

InterestUnderstanding radon levels in houses

Radon entry into houses from soilRadon entry into houses from soil

Indoor radon dynamicsIndoor radon dynamics

Radon entry into houses from building materialsRadon entry into houses from building materials

Review of radon modelsUnderstanding radon levels in houses

Radon entry into houses from soilRadon entry into houses from soil

Analytical modelsAnalytical models Lumped parameter modelsLumped parameter models Numerical modelsNumerical models

• Idealised geometrical configuration

• Simplified boundary conditions

• Functional dependence of results on input parameters

• Idealised geometrical configuration

• Simplified boundary conditions

• Functional dependence of results on input parameters

Gadgil. Radiat. Prot. Dosim. 45, 373-380 (1992)Andersen. Sci Total Environ, 272, 33-42 (2001)

Review of radon modelsUnderstanding radon levels in houses

Radon entry into houses from soilRadon entry into houses from soil

Analytical modelsAnalytical models Lumped parameter modelsLumped parameter models Numerical modelsNumerical models

Gadgil. Radiat. Prot. Dosim. 45, 373-380 (1992)Andersen. Sci Total Environ, 272, 33-42 (2001)

• Analogy of pressure-driven flow of soil gas to voltage difference driving a

current in electrical circuit

• Discrete system of lumped parameters

• Analogy of pressure-driven flow of soil gas to voltage difference driving a

current in electrical circuit

• Discrete system of lumped parameters

Review of radon modelsUnderstanding radon levels in houses

Radon entry into houses from soilRadon entry into houses from soil

Analytical modelsAnalytical models Lumped parameter modelsLumped parameter models Numerical modelsNumerical models

Gadgil. Radiat. Prot. Dosim. 45, 373-380 (1992)Andersen. Sci Total Environ, 272, 33-42 (2001)

• Detailed transport of radon in soil (diffusion + advection).

• Discretisation of space (and sometimes also time).

• Finite difference, finite element and integrated finite difference models.

• Detailed knowledge of the soil-indoor interface required (cracks, gaps, holes).

• Common approach: homogeneous soil, constant soil gas-permeability and

diffusivity.

• Detailed transport of radon in soil (diffusion + advection).

• Discretisation of space (and sometimes also time).

• Finite difference, finite element and integrated finite difference models.

• Detailed knowledge of the soil-indoor interface required (cracks, gaps, holes).

• Common approach: homogeneous soil, constant soil gas-permeability and

diffusivity.

Review of radon modelsUnderstanding radon levels in houses

Radon entry into houses from building materialsRadon entry into houses from building materials

Only steady-state diffusive exhalation is consideredOnly steady-state diffusive exhalation is considered

Aging (moisture), atmospheric pressure and covering materialsAging (moisture), atmospheric pressure and covering materials

Indoor radon dynamicsIndoor radon dynamics

Constant entry rate from soil and/or building materials Constant entry rate from soil and/or building materials

Ventilation rate + inter-zone flows Ventilation rate + inter-zone flows

Mass-balance equation Mass-balance equation

Review of radon modelsUnderstanding radon levels in houses

Reasonable good understanding of the main parameters and processes afecting indoor radon levels.

Reasonable good understanding of the main parameters and processes afecting indoor radon levels.

SummarySummary

No integrated approach. Need of a “Global Dynamic Radon Model” more concerned with a global description, on which the knowledge acquired from partial models is collected.

No integrated approach. Need of a “Global Dynamic Radon Model” more concerned with a global description, on which the knowledge acquired from partial models is collected.

It is difficult to extrapolate partial model results to real inhabited houses It is difficult to extrapolate partial model results to real inhabited houses

Most of the models are steady-state or site-specificMost of the models are steady-state or site-specific

Review of radon modelsUnderstanding radon levels in houses

EmanationAdsorptionTransport

The Global Dinamic Radon Model (GDRM) conceptThe Global Dinamic Radon Model (GDRM) concept

INHABITED HOUSE

BUILDING MATERIALS

GAS SUPPLY WATER SUPPLY

SOIL OUTDOORS

EmanationAdsorptionTransport

Advective flow

Diffusive flow

Exhalation

Air-exchange

Mass-transfer

Indoor air movement

Gas-permeability

Effectivediffusion coefficient

Parameters

Ra content (soil grains and bedrock)Grain size-distributionPorosityWater content

RainfallIrrigationWater table depth

Parameters

Soil-indoor Rn concentration differenceSoil-indoor interface properties: building design, cracks, pipes, etc..

Parameters

Soil-indoor pressure differenceSoil-indoor interface properties: building design, cracks, pipes, etc..

Outdoor-indoor temperature differenceWind speed and directionBarometric pressureUse of HVAC systems

Parameters

Gas use rateGas Rn concentrationTransfer efficiency

Use of gas supply

Parameters

Water use rateWater Rn concentrationTransfer efficiency

Use of water supply

Parameters

Ra contentGrain size-distributionPorosityWater content

Effectivediffusion coefficient

Parameters

Building material - indoor Rn conc. differenceBuilding material surface coatingBarometric pressure

Parameters

Ventilation rate of each roomBuilding design

Parameters

Air-exchange rates between the different roomsBuilding design

Outdoor-indoor temperature differenceWind speed and directionBarometric pressureUse of HVAC systemsWindows and doors opening

Diagram of the sources (brown square boxes), processes (green round boxes) and parameters that a Global Dynamic Radon Model has to consider. The time-dependent parameters are in blue.

Mass-transfer

The GDRM conceptUnderstanding radon levels in houses

A Global Dynamic Radon Model (GDRM) should:A Global Dynamic Radon Model (GDRM) should:

Take into account all radon sources, processes and parameters affecting indoor radon levels

Take into account all radon sources, processes and parameters affecting indoor radon levels

1

Describe the dynamics of the indoor radon levels Describe the dynamics of the indoor radon levels2

Be adaptable to different time-scales and have the possibility of incorporating time series experimental data.

Be adaptable to different time-scales and have the possibility of incorporating time series experimental data.

3

4 Be applicable to different real sites, taking the advantage of the information available

Be applicable to different real sites, taking the advantage of the information available

5 Be able to simulate mitigation methods Be able to simulate mitigation methods

The GDRM conceptUnderstanding radon levels in houses

The RAGENA (RAdon Generation, ENtry and Accumulation)model:

The RAGENA (RAdon Generation, ENtry and Accumulation)model:

Dynamic. Time step can be fixed from seconds to years.Dynamic. Time step can be fixed from seconds to years.

Structured in sectors. Compartmental model using “efective” values.Structured in sectors. Compartmental model using “efective” values.

The set of coupled differential equations is solved by the 4th order Runge-Kutta numerical method.

The set of coupled differential equations is solved by the 4th order Runge-Kutta numerical method.

Inputs: experimental time-series data, constant values, probability distributionsInputs: experimental time-series data, constant values, probability distributions

Outputs: Csoil (t), CBM,i (t), Cin,j (t), ER(Bq/s) from each sourceOutputs: Csoil (t), CBM,i (t), Cin,j (t), ER(Bq/s) from each source

Font, Baixeras and Domingo. Sci Total Environ, 307, 55-69 (2003)Font. Radon Generation, Entry and Accumulation Indoors. PhD dissertation (1997)

The RAGENA modelUnderstanding radon levels in houses

Soil Build. Mat. Water Gas

Multi-compartment house

Outdoors

Meteorological parameters

Inhabitant habits

Radon Levels

Exposure

Dynamicbehaviour

Generation

Entry

Mass-balance

Ventilation

Dif. Adv. Dif.

Air-exchange

Diagram of the RAGENA modelDiagram of the RAGENA model

The RAGENA modelUnderstanding radon levels in houses

iRn

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j Rn

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Rn

iij

Rn

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Rn

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lbmd

isdsRn

aids

Rn

di

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Example: room i, build up with n different types of BM, exchangingair with outdoors and with p rooms

Example: room i, build up with n different types of BM, exchangingair with outdoors and with p rooms

The RAGENA modelUnderstanding radon levels in houses

Model behaviour Model behaviour Reference configuration

Variability analysis Sensitivity analysis Uncertainty analysis

- Capability of the model to be adapted to different sites (not site-specific)

- Range of variation of each parameter

- Relative importance of each parameter (VI)

- Site-specific.

- Step, pulses, sinwaves…

- Parameters that have to be measured with higher accuracy.

- Site-specific.

- Inputs: probability distributions

- Uncertainty with the model predictions

Font and Baixeras. Sci Total Environ, 272, 25-31 (2001)

The RAGENA modelUnderstanding radon levels in houses

Adaptation to an inhabited house

Procedure. Site description form

Spanish house Swedish house

Main source: building materials

Indoor radon levels are the balance between a steady entry from BM and a dynamic removal through ventilation, driven by indoor-outdoor pressure differences, by wind speed and by the opening and closing of windows and doors

Font et al. Radiat. Meas. 31, 277-282 (1999)

Main source: soil underneath

Main entry mechanism: advection

Font et al. Radiat. Meas. 31, 359-362. (1999)

The RAGENA modelUnderstanding radon levels in houses

WEATHER STATION (outdoor components)

Passive dosemeter (Makrofol)

Passive dosemeter (LR-115)

Active radon detector (Clipperton II)

STREET

GARAGE

Basement roomPRASSI

Living roomKitchen

Bedroom

Bathroom-1

Guest-room

GARDEN

Opening

Set F Set B Set L

Pressure transducer

Permeability device

The Spanish inhabited house experimental study

The RAGENA modelUnderstanding radon levels in houses

Some dynamic results

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Comparison model - experimental results indoors

The RAGENA modelUnderstanding radon levels in houses

Some dynamic results

Comparison model - experimental results in soil

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The RAGENA modelUnderstanding radon levels in houses

To improve the knowledge on any partial model, specially from a dynamic point of view:

- Relationship between water saturation fraction in soil and rainfall, water table depth and irrigation (in houses).

- Ventilation rate in each room of the house.- Effect of barometric pressure changes on the soil-indoor transient

pressure differences.- Effect of bioporosity on transport parameters.- Effect of barometric pressure changes on radon exhalation from

building materials.- Model outdoor radon concentration dynamics.

To improve the knowledge on any partial model, specially from a dynamic point of view:

- Relationship between water saturation fraction in soil and rainfall, water table depth and irrigation (in houses).

- Ventilation rate in each room of the house.- Effect of barometric pressure changes on the soil-indoor transient

pressure differences.- Effect of bioporosity on transport parameters.- Effect of barometric pressure changes on radon exhalation from

building materials.- Model outdoor radon concentration dynamics.

A lot of work to do!A lot of work to do!PerspectivesPerspectives

PerspectivesUnderstanding radon levels in houses