PRIOR RISK ASSESSMENT FOR WORK WITH IONISING RADIATION :
IODINATIONS
SCOPE AND DEFINITIONS
A generic risk assessment for routine work with small quantities of radioisotopes in laboratories has been undertaken. This risk assessment only considers the additional radiation risks arising from iodinations.
There are two main differences between the generic assessment and this work, as follows:
· Higher activity levels of iodine-125 may be used. This assessment considers the use of up to 400MBq iodine-125 per experiment; and
· It is assumed that iodinations carry the risk of producing airborne contamination.
RADIATION RISKS FROM NORMAL OPERATIONS
Dose rates and external radiation risk to individuals
Iodinations involve up to 400MBq of iodine-125 and gamma dose rates are expected to be higher than those in the generic assessment. For example, dose rates around an unshielded vial containing this activity are as follows:
Exposure conditions
/ Dose rate (μSv h-1)Whole body exposure
1 metre from vial
0.5 m from vial
Finger exposure, contact with vial / 6
24
16 000
Access to high dose rates is possible, especially to the fingers. If iodinations were regularly undertaken persons could receive significant exposures, especially to the fingers. Exposure to the above dose rates is expected to be, at most, a few minutes per iodination. Assuming one iodination a month gives annual doses of 0.02mSv (body) and 10mSv (fingers). These represent 0.1% and 2% of the relevant dose limits, respectively.
Contamination levels and internal radiation risks to individuals
Although higher levels of activity are involved, the work is of a similar scale (i.e. in terms of solution volumes, etc) to that assumed in the generic assessment for small quantities of other open radionuclide sources. As such, normal laboratory precautions should provide sufficient protection against the possible ingestion of radioactive material.
There are, however, two main differences:
· Iodinations typically involve activities equivalent to about 300 ALIs. During the procedure, there is the potential for a significant fraction of the activity to be released in a vapour form.
· Internal exposures can also arise from the direct absorption of iodine-125 through the skin.
It is considered that significant internal doses (by inhalation and skin adsorption) could be received unless additional precautions are taken.
RADIATION RISKS FROM ACCIDENTS
Spillage of radioactive material
This accident scenario is considered in the generic risk assessment for small quantities of other open radionuclide sources. For iodinations, it is considered possible that higher radiation risks could be presented, as follows:
External radiation – In the worst case, dose rates in close to spills of the stock solution could be several tens of mSvh-1. Even so, it is still considered that the use of tongs or tweezers to remove gross activity when mopping up bench spills should ensure that (finger) doses are below 1mSv. (for comparison, the annual dose limit is 500mSv).
Internal radiation – Following the same exposure scenario as used in the generic assessment, internal doses as high as 40% of the annual dose limit are predicted. Although this is a pessimistic assessment, it does not include possible intakes from inhalation or skin absorption. It is considered, therefore, that significant internal doses could be received in the event of a major spill.
Loss (including theft) of radioactive materials
This scenario is considered in the generic assessment, and the conclusions are similar. Certainly it is considered possible for the public dose limits to be exceeded from both external (skin/hand) exposures and internal exposures.
RECOMMENDED ACTIONS (requirements of the Ionising Radiations Regulations 1999 (IRR99))
Note: All the relevant precautions arising out of the generic risk assessment for routine work with radioisotopes should be considered to also apply to work with iodinations. This section lists the additional precautions considered necessary.
Restriction of exposure
Protection during normal operations
· It is recommended that only persons trained to RPS standard should undertake iodinations. No other work should be undertaken in the laboratory while iodinations are being carried out.
· Local shielding (e.g. lead shielded stock vials) and distance (e.g. the use of tongs or tweezers) must be used when handling stock solutions of iodine-125. In addition, local shielding should also be considered around any laboratory equipment that is expected to contain significant levels of activity. As a guide, shielding should be considered wherever the dose rate exceeds 7.5μSv h-1.
· Iodination reactions must be carried out inside a fume cupboard or equivalent ventilated enclosure.
Accidents – prevention and mitigation
In addition to the generic precautions to avoid and restrict spills, the following precautions are also considered appropriate:
· Persons are advised not to attempt to clean up major spills without first seeking the advice of the relevant RPS and University RPO. The priority should be to decontaminate any affected personnel (with assistance) and remove from the area without the further spread of contamination. The subsequent clean-up should be discussed with the RPO and be subject to a separate risk assessment.
· As a precaution, any person that has been involved with a major spill should be subject to a thyroid measurement to estimate possible internal exposures. This may be done using a Mini Instruments monitor with a 5.44 probe on the day after the suspected intake. Advice on monitoring techniques and dose estimation will be provided by the RPO.
Designated Areas and restrictions on access
It is concluded that special working procedures are required to restrict exposures. Consequently, it is recommended that any laboratory in which iodinations are carried out and the fume cupboard be designated as controlled areas.
Controlled area warning signs should be posted at the laboratory entrance for the duration of the work. These signs should clearly indicate the names of the persons authorised to work in the controlled area.
Note: This risk assessment, and the following section in particular, provide the basis for the “suitable written arrangements” for non-classified persons working in controlled areas, as required by IRR99 Regulation 18(2).
Classified persons and individual monitoring
Under normal operating conditions, radiation exposures are expected to be well below three-tenths of the dose limits for employees specified in IRR99, provided that:
· The precautions listed in this risk assessment are observed; and
· No individual carries out more than 12 iodinations per year.
· However, to ensure that radiation doses are being adequately restricted, the following methods of individual monitoring are recommended:
· Individual dosemeters (body and finger) must be worn whenever iodinations are undertaken; and
· Thyroid measurements (i.e. to estimate internal exposures) should be undertaken following any incident in which an intake is suspected to have occurred. Note: this form of monitoring should be undertaken routinely if the number of iodinations undertaken increases to greater than 12 per year.
Working procedures and supervision
Specific local rules for iodinations (i.e. in addition to the rules for routine work) are recommended.
Personal protective equipment
In addition to normal laboratory clothing, Persons undertaking iodination should wear two pairs of disposable gloves to minimise the possibility of skin absorption.
Respiratory Protection is not considered necessary during routine work. However, face masks with activated charcoal filters should be made available for use in cleaning up major spills.
Workplace monitoring
The following, additional monitoring is recommended:
· A formal contamination survey should be undertaken after each iodination and the results recorded in the normal manner; and
· A dose rate survey should be undertaken at the start of each new iodination procedure. This should be used to determine where local shielding is appropriate. The results should be recorded.