David Orr david.orr@hpa-rp.org.uk Assessing Radon Exposures from Materials Containing Naturally Occurring Radioactive Material (NORM) David Orr david.orr@hpa-rp.org.uk

Application of regulations to NORM UK Regulations are Ionising Radiations Regulations 1999 Apply to NORM (not part of practice) if doses > 1mSv/annum - ALL PATHWAYS Doses assessed by risk assessment

Risk assessment for work with NORM External gamma dose rates can be measured Internal doses from dust inhalation can be determined from dust sampling campaigns Internal doses from radon emanating from NORM usually estimated and often pessimistic assumptions made

Estimation of radon doses Cannot distinguish between radon from the ground and radon emanating from the NORM Most measurement techniques involve long measurement periods and give average radon gas levels Assumptions have to be made on equilibrium factor to establish radon daughter concentration and hence calculate dose

Measurement of radon from NORM Exhalation rate of radon from NORM can be measured experimentally Measured exhalation rate can be used to model rate of build up of radon and potential final levels in warehouse/ships hold

Modelling radon concentrations The airborne concentration to which the radon gas can build up inside an enclosed space at measured exhalation rate is: XRn = ( 3.6 E A )/V k Where: XRn = radon-222 activity concentration (Bq m-3) A = the surface area of the source (m2) V = the volume of the storage space (m3) k = the ventilation rate (h-1) {Dixon, 1984}

Experimental Method Measurements of the radon concentration were carried out using a steel exhalation chamber with a Perspex lid. The chamber was connected to a pump and Lucas Alpha-Scintillation Flask by a closed loop of rubber hoses. At intervals, the air in the Flask was refreshed with that from the chamber, and the Flask removed for counting.

Derivation of exhalation rate By plotting radon concentration against time, the initial slope of the curve can be obtained, in units of Bq/m3/h The gradient is multiplied by V (net volume of the chamber) before dividing by the sample open surface area, A, to obtain the exhalation rate (Bq/m2/h).

Parameters to be investigated Radionuclide composition of material Depth of sample Moisture content Reproducibility of measurements

Material Th-232 chain U-238 chain Ilmanite 0.7 0.2 Materials analysed Material Th-232 chain U-238 chain Bq/m3 Bq/m3 Ilmanite 0.7 0.2 Synthetic Rutile 0.5 0.2 Bastnasite 5.0 0.1

Radon Concentration Graphs Weight: 2.42 kg Depth: 1.5 cm Weight: 4.27 kg Depth: 3 cm

Early Results

Summary of results Significant radon concentrations obtained in chamber (> 4000 Bq/m3) Concentration reaches plateau concentration after a number of days Significant concentrations can build up in first 24 hours of sample chamber being sealed Final radon concentration and exhalation rate function of depth of sample

Radon = 2 Bq m-3 ? But, in practice…..? Storage room 10m x 3m x 3m 1 m deep NORM 2 air changes/hour 0.2 Bq/g U-238 Exhalation rate = 0.3 Bq m-2 h-1 per cm depth? Radon = 2 Bq m-3 ?

Further work Complete lab tests Model potential rate of radon build up in ship’s hold/mineral warehouse Carry out in situ radon monitoring to test model Measure effects of ventilation on radon levels Produce guidance for users

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