Prior Risk Assessment for Work with Ionising Radiation

PRIOR RISK ASSESSMENT FOR WORK WITH IONISING RADIATION :

ROUTINE LABORATORY WORK WITH SMALL QUANTITIES OF RADIONUCLIDES

SCOPE AND DEFINITIONS

The Ionising Radiations Regulations 1999 require that a radiological risk assessment is undertaken for any work involving radioactive materials. Much of the work with open source radionuclides at Warwick University involves the use of limited quantities of radioactive materials. This assessment considers the generic radiation risks from all such work under both normal and accidental conditions.

A “limited quantity” of radioactive material may be defined in relation to the Annual Limit on Intake (ALI) for each radionuclide. For example, activity levels equivalent to up to 10 ALIs can be regarded as not requiring special precautions (controlled areas, etc). In some cases, however, routine work involves different levels of activity. Consequently, for the purposes of this risk assessment, the following activity levels are considered:

Radionuclide / Activity per procedure (MBq)
Equivalent to 10 ALIs / The limit for routine work assumed in this assessment
Tritium
Carbon-14
Sulphur-35
Phosphorus-33
Calcium-45
Phosphorus-32
Iodine-125 / 4800
340
150
140
74
63
13 / 1000
400
100
100
100
40
10

It is also assumed that the work is inherently small-scale in nature, i.e. involves the manipulation of small volumes of material, and that airborne contamination is not routinely expected.

RADIATION RISKS FROM NORMAL OPERATIONS

Radiation dose rates and external radiation risk to individuals

Tritium, carbon-14, phosphorus-33, sulphur-35 and calcium-45

These are all low energy beta-emitters readily shielded by normal laboratory containers, etc. These are not expected to give rise to a significant external radiation hazard during normal use.

Phosphorus-32

Beta dose rates during normal procedures are usually negligible due to the perspex shielding provided. The highest dose rates measured are above the open stock vial. For a typical stock vial, it is estimated that the dose rate at 10 cm above the open vial (i.e. where the fingers might be) is 1500μSv h-1. Elsewhere, dose rates are all very low. A typical procedure will require no more than a few seconds exposure of the hands above the stock vial. Assuming 10 procedures a month gives an annual dose to the skin on the fingertips of less than 0.5mSv, equivalent to 0.1% of the annual dose limit of 500mSv.

To ensure that doses remain low, the use of shielding around stock vials, pipettes, and on benches is very important, as is avoiding any direct handling of unshielded (or poorly shielded) activity. Without such precautions, it is estimated that the skin dose could be very much higher.

Iodine-125

Is a gamma/x-ray emitter and is supplied in shielded stock vials. The following dose rates are expected during the normal use of 10MBq:

Body exposure:1μSv h-1

Finger exposure: 50μSv h-1

Exposure to the above dose rates is expected to be only a few minutes per day. Assuming an annual exposure time of 100 hours gives annual doses of 0.1mSv (body) and 5mSv (fingers). These represent 0.5% and 1% of the relevant dose limits, respectively.

Contamination levels and internal radiation risk to individuals

In all cases, the work is small-scale in nature; typically involving limited quantities (a few ml) of radioactive solutions. There is the potential for contamination to arise, for example through small spills or the handling of contaminated articles. Any contamination would be expected to be low level (major spills are considered later) and highly localised (e.g. a small “spot” on a workbench). Air contamination is not expected to occur from most procedures. Ordinary laboratory procedures are considered sufficient to prevent the spread of contamination and intakes of radioactive materials.

In summary, internal radiation doses from normal operations are expected to be negligible provided that:

• Good laboratory practice is observed
• Contamination routinely arising from the work is low-level and localised
• Airborne contamination is not expected.

RADIATION RISKS FROM ACCIDENTS

Nature of accidents

The most reasonably foreseeable accident is a spill, in the worst case this would involve all the activity from a single (stock) vial, i.e. containing the activity levels described in section 1. The radiation risks associated with this are considered below.

Potential external radiation risk

Tritium, carbon-14, phosphorus-33, sulphur-35 and calcium-45

Due to the low energy of the beta emissions these are not expected to give rise to a significant external radiation hazard even where there is a total loss of shielding (e.g. due to a spill).

Phosphorus-32

In the event of a spill of an entire stock vial, the following beta dose rates might occur:

1 cm distance:1 000 000μSv h-1

10 cm distance:10 000μSv h-1

50 cm distance:40μSv h-1

A person cleaning up a spill (hands at 1 cm distance) could receive a skin exposure of several mSv per minute. Therefore, a serious spill could quite conceivably give rise to skin doses of several tens of mSv, (for comparison, the annual dose limit is 500mSv). Distance and shielding should both be used to minimise skin exposures when cleaning spills. For example, tongs or tweezers should be used to remove gross activity when mopping up bench spills. Thereafter, thick rubber gloves will provide protection for the skin.

Iodine-125

In the event of a spill of an entire stock vial, the following gamma dose rates might occur:

1 cm distance:30 000μSv h-1

10 cm distance:300μSv h-1

50 cm distance:14μSv h-1

A person cleaning up a major spill (hands at 1 cm distance) could receive a skin/finger exposure of no more than few mSv (for comparison, the annual dose limit is 500mSv). The use of tongs or tweezers to remove gross activity when mopping up bench spills should ensure that doses are well below 1mSv.

Potential internal radiation risk

In case of a major spill, it is possible that a person could become contaminated and ingest some activity as a result. The maximum activity in use in any one procedure is typically less than 10 ALIs and, in practice, it is unlikely that an activity equivalent to more than 1 ALI would be spilt, even in the worst case. Assuming that 1% of this is subsequently ingested (this is a very pessimistic scenario), an internal dose of 0.2mSv (i.e. 1% of the annual dose limit) would be received. In practice, simple emergency instructions in addition to the normal laboratory procedures should ensure that inadvertent ingestion is all but eliminated.

LOSS (INCLUDING THEFT) OF RADIOACTIVE MATERIALS

This is a specific type of accident involving radioactive material and is considered separately in this section.

Nature of accident

Radioactive materials do not normally need to be removed from the laboratory in which they are used, except for the disposal of radioactive waste from the premises. It is, therefore considered unlikely that sources will be lost, although this cannot be ruled out altogether. A reasonably foreseeable incident might be the accidental disposal of a radioactive material to the waste bins, for example, because it was not properly labelled. Theft is also considered possible although very unlikely.

The worst scenario would be an entire stock solution being tampered with by a member of the public. Although containers should be clearly marked, a person could still easily open such a container and become contaminated as a result.

Potential external radiation risk

Persons could potentially be exposed for much longer times than those assumed for employees. Consequently, doses greater than those estimated in 3.1 could be received. The annual dose limit for the skin/extremities of 50mSv could certainly be exceeded in the case of phosphorus-32, and this might be possible in the case of iodine-125.

Potential internal radiation risk

Doses similar to those described in 3.2 (i.e. 0.2mSv, equivalent to 20% of the annual dose limit for the public) might be expected. In the extreme case, a person (e.g. a child) could deliberately ingest the material and consequently receive an internal dose above this limit.

RECOMMENDED ACTIONS (requirements of the Ionising Radiations Regulations 1999 (IRR99))

Restriction of exposure

Basic restrictions on the work

This assessment applies to laboratory procedures that meet the following criteria:

• The maximum activity used in a single procedure does not exceed:

Tritium:1000 MBq

C-14:400 MBq

S-35, P-33 or Ca-45: 100 MBq

P-32: 40 MBq

Iodine-125 10 MBq

• Contamination of work surfaces is not routinely expected (other than small “spots” of contamination in containment such as drip trays, etc).
• Airborne radioactive contamination is not expected.

A checklist to ensure that the above criteria are met is given at the end of this assessment.

Protection during normal operations

The risk assessment indicates that radiation exposures (external and internal) from the laboratory use of small quantities of radioactive materials are likely to be very low. Notwithstanding this, the following precautions are recommended as a means of ensuring that radiation exposures are kept as low as reasonably practicable (as required by the Ionising Radiations Regulations 1999):

• All new procedures should be reviewed to determine whether the “routine work” criteria outlined above are applicable. A checklist is available for this, given at the end of this assessment. Even where the work is “routine”, it should be (briefly) reviewed to determine whether any additional practical precautions are appropriate, and these should be given on the checklist.

• A separate risk assessment should be produced for any work that does not meet the above criteria.

• Good laboratory practice should be observed when handling radioactive materials. This includes the use of drip trays and absorbent bench cover (e.g.’Bench-kote’), where appropriate, and the wearing of laboratory coats and disposable gloves.
• Local shielding (Perspex bench shields, boxes, etc) should be used for all work involving phosphorus-32.
• Local shielding (e.g. lead shielded stock vials) and distance (e.g. the use of tongs or tweezers) should be used when handling stock solutions of iodine-125.
• Training should be provided to all persons that handle radioactive materials. This training should explain the conclusions of this risk assessment and the precautions expected.

Accidents – prevention and mitigation

In addition to the measures to avoid and restrict spills outlined above, the following precautions are also considered appropriate:

• Stores for radioactive materials should be designed to prevent unauthorised access and minimise the effects of fire. Stock solutions should be kept in closed non-combustible containers, and secondary containment should be provided to collect any leaks. The store and containers should be suitably labelled.
• Procedures for cleaning up spills should be provided. These should include the use of shielding and distance, as appropriate, when dealing with major spills of P-32 or gamma emitters. Training in these emergency procedures should be provided for all relevant employees.

Designated Areas

IRR99 require the designation of a controlled area where either:

a)radiation doses are likely to exceed three-tenths of a dose limit; or

b)special working procedures are required to restrict radiation exposures.

For the routine work described in this risk assessment neither of these conditions is present, and the designation of controlled areas is not considered necessary.

The Regulations also require that a supervised area should be designated where it is considered necessary to keep the radiological conditions under periodic review. This is considered appropriate and hence it is recommended that all areas where this work is undertaken be designated as Supervised Areas.

Classified persons and individual monitoring

Internal radiation exposures are expected to be negligible and the designation of classified persons and individual monitoring for intakes of radioactive material are not considered appropriate. Provided that simple shielding precautions are taken when dispensing P-32 and I-125 materials from the stock vial, external radiation exposures are also expected to be very low. Consequently, classification and individual monitoring for external radiation are also considered unnecessary.

Working procedures and supervision

Local rules are considered appropriate as a means of ensuring that good laboratory practice is observed. In particular, local rules should stress the importance of the precautions listed in sections 5.1 and 5.2. The appointment of a Radiation Protection Supervisor (RPS) in each department is considered appropriate, as a means of confirming that the local rules are observed.

Personal protective equipment

Standard laboratory coats and disposable gloves are considered appropriate.

Access restrictions and training

Only laboratory workers are allowed to gain access to the area (visitors are always accompanied). Further restrictions are not regarded as necessary. All persons who work with unsealed radioactive materials should receive basic training in radiation protection and good laboratory practice.

Workplace monitoring

It is considered appropriate for surface contamination monitoring to be carried out:

• in individual work areas during and after the work
• on individuals to check for personal contamination before leaving the area

A formal monthly survey of each work area (with the results recorded) is also recommended. Routine surface contamination levels are expected to be negligible. Any reading above normal background will, therefore, require action i.e. decontamination.

Checklist.

Work with radioactive materials

A generic risk assessment for routine work with radionuclides in laboratories has been carried out which concludes that radiation exposures of employees are expected to be minimal provided that certain criteria are met. The purpose of this checklist is to ensure that the proposed work complies with these criteria.

Protocol description:
Nuclide and activity per procedure:
H-3 (max = 1000MBq) MBq
C-14 (max = 400 MBq) MBq
S-35 (max = 100 MBq) MBq
P-33 (max = 100 MBq) MBq
Ca-45 (max = 100 MBq) MBq
P-32 (max = 40 MBq) MBq
I-125 (max = 10 MBq) MBq / Proposed by:
Department:
Location(s) of work:
RPS for work:
Surface contamination: Indicate whether surface contamination is expected to occur on work surfaces as a result of normal operations, and also from any likely accidents. An indication of the activity and spread of contamination should be given where applicable:
Air contamination: indicate whether airborne radioactivity (as a gas, dust or vapour) might be expected at any stage during this work:
Waste: Provide estimates of the activity of waste expected to be disposed of by the following routes:
Gaseous waste: MBq per day = MBq per month =
Aqueous waste: MBq per month =
Organic liquid waste: MBq per week = Litres per week =
Solid waste (to bins): Max kBq per item = Max kBq per 0.1 m3 =
Solid waste (for incineration) MBq per week =
Is the activity used and waste within departmental limits?
Is the procedure consistent with the generic risk assessment?
Signature: / Date:
Additional precautions required
List below any additional precautions that should be taken during the work (e.g. any additional containment, equipment for transferring radioactive materials, spill containment measures, etc)
Additional Comments