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

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 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}

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. 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
Synthetic Rutile
Bastnasite

11. Radon Concentration Graphs
Weight: kg
Depth: 1.5 cm
Weight: 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. 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 ?

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