Assessing Radon Exposures from NORM Materials: Regulations and Risk Assessment

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

2. 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

3. 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

4. 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

5. 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

6. Modelling radon concentrations • The airborne concentration to which the radon gas can build up inside an enclosedspace 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}

7. 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.

8. 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).

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

10. 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

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

12. Early Results

13. 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

14. 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 ?

15. 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

16. Thank you for not asking any questions