Guidance on completing a Radiation Risk Assessment using Unsealed Radionuclides
These guidance notes should be read before completing the radiation risk assessment form. They
should be read in conjunction with the Trust’s Risk Management Strategy and Policy (found on
Freenet under Risk management strategy and policy ). A separate risk assessment form should be
completed for each radionuclide used unless it is deemed sensible to put them altogether. If this is
the case, then an explanation should be inserted. If the radionuclides are the same but used in a
very different way, then a separate form is required. General risk assessment may be added to this
form if it is easier. The radiation risk assessment form is not meant to be prescriptive but may be
tailored to the individual situation (eg removing control measures which are not relevant and adding
others which are) so long as the format is adhered to. This form may be used as a prior risk
assessment, reviewing current ones as well as when an event occurs.
1 Persons at Risk
Consider who might be harmed and if any pregnant or young people are involved.
Enter the maximum number of radiation workers who may be involved and if they have had training.
Under the “other persons involved” note down the number of people (not radiation workers) who may
be affected (eg domestics staff, builders etc..).
2 Dose Assessment
The whole body and extremity dose rates (mSv/h) can be calculated using data from table 1 if there is
no previous data available (eg data of external exposure obtained from TLD/film badge results).
Information on radionuclides not listed may be obtained from the Radiation Safety Group (RSG).
Radionuclid
e
A
B
C
Emission
Point source
at 30cm*
Contact with
5ml syringe *
Tenth Value Layer (TVL)# of
lead (mm) [γ only]
β
γ
γ
γ
γ
γ
β
γ
γ
γ
β
β
β
β
8.62 E-2
7.29 E-4
3.90 E-4
5.16 E-4
9.94 E-4
2.61 E-4
1.08 E-1
2.79 E-4
6.04 E-5
1.97 E-4
1.18 E-1
-
1.1
11
0.6
0.6
1.2
0.4
43
0.4
0.09
0.29
24
-
<1
2
3
1
I-131
I-125
I-123
In-111
Tc-99m
Y-90
Ga-67
Cr-51
Tl-201
P-32
S-35
C-14
H-3
6
7
1
-
Table 1
* The external dose rates (mSv/h) for an activity of 1MBq
#
TVL is the thickness of shielding required to reduce the dose rates by a factor of 10
Values of WB and EX is obtained from table 1 columns A & B. Estimate the maximum time to be spent
with the radionuclide per year or per experiment for both a whole body and extremity dose and the
maximum activity to be used. There is a section at the end of the form to note down all calculations
and assumptions made.
To calculate EWB(b) and EEX(b) , the following assumptions are made:
-
Whole body dose EWB(b) : The radiation work will be at an approximate distance of 30cm from
the body. If this is not the case, then the inverse square law must
be used to correct for the distance used.
-
Extremity dose EEX(b) :
Radiation work entails handling radionuclide in a 5ml syringe
with direct contact thereby increasing the extremity dose.
Dose estimation before control measures are put in place.
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From the data in table 1 or data obtained from dose meter readings, calculate the likely annual doses
for the 2 situations before any control measures have been put in place. For those doing experiments,
it may be more suitable to calculate the dose per experiment rather than an annual dose which may be
very difficult to estimate. In the latter situation, assumptions as to how that dose has been calculated
needs to be written on the form (under section 2 – dose assessment). Examples are given below :
EWB(b) example: If I-131 is being used, then the whole body (WB) dose rate (mSv/h) (obtained from column ‘A’,
table 1) is 8.62E-2 mSv/h for 1MBq. If 10MBq is being used, then the doserate is 8.62E-2 x 10 =
0.862mSv/h. If the worker is likely to spend a maximum of 5 hrs a year working with the I-131,
then the annual dose is 0.862 x 5 = 4.3mSv. Note: Gamma radiation figures have been used to
obtain the WB doserate as it is more appropriate.
EEX(b) example: The amount of time doing radiation work that involves direct extremity exposure to the source in a
syringe may differ from the whole body exposure times. For example, the dose rate from I-131 is
1.1mSv/h (column ‘B’, table 1). But if only 10% of the 5 hours (0.1 x 5 = 0.5 hours) outlined
above involves direct manipulation, then for the 10MBq dose in a syringe, the annual extremity
dose, EX(b) = 10 x 1.1 x 0.5 = 5.5mSv
Radiation Probability Calculation
Whole Body Dose
To provide a Risk Rating parallel with the schema adopted for other risks, a pessimistic multiplicative
risk probability (P) is set up looking at the number of people (N) involved in a work activity, and their
estimated maximum annual whole body dose (E) arising from that work activity and the radiation risk
probability coefficient (C). ie P = N x E x C #
(# - Risk assessment for a radionuclide area – Nuclear Medicine communications 2003, 24: 1017-31)
For effective doses less than or equal to 200mSv, C = 0.5  10 – 4 otherwise C = 1.0  10 – 4
As the dose for most situations is < 200mSv a year, C is normally 0.5  10 – 4. Table 2 has been
simplified to include C. Therefore the Radiation Risk Probability (P) may be calculated using :
P = N E
P is for the whole body dose, not extremity or skin dose. P should be calculated before the control
measures are in place and noted.
Using the example above, for EWB(b), if N = 2, then PWB(b) = 2 x 4.3 = 8.6
Users of H-3 and C-14 will not register an external dose but hazards from ingestion / inhalation still apply.
Security measures and control of the spread of contamination also needs to be taken into account.
3 Hazard Identification & Risk Quantification before control measure in place
List the hazards (ie something with the potential to do harm) in the boxes provided. If there are more
than 5 hazards identified, please add more rows and number them accordingly.
a Obtaining the Consequence Score
After calculating P, use Table 2 to obtain the appropriate severity level, using the column
corresponding to the number of people affected. The colour/pattern obtained corresponds to a
consequence score (1 – 5) given in table 2a. Note the score for each hazard in the boxes provided.
Risk Probability
Individual
2 – 10 people
11 – 100 people
> 2,000
> 20,000
> 40,000
200 – 2,000
2,000 – 20,000
20,000 – 40,000
20 - 200
200 – 2,000
2,000 – 20,000
2 - 20
20 – 200
200 – 2,000
0.2 – 2
2 – 20
20 - 200
0.02 – 0.2
0.2 – 2
2 - 20
< 0.02
< 0.2
<2
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Consequence score
Table 2
Table 2a
In the example given above, for 2 people and with PWB(b) =
8.6, the consequence score is 2.
5
Catastrophic
4
Major
3
Moderate
2
Minor
1
Negligible
Extremity and Skin Doses
For skin and extremity doses, the consequence score can be obtained from table 2b.
(Do not multiple by number of persons involved.)
Consequence score
Catastrophic
5
Annual Dose (mGy)
> 50,000
2,000 – 50,000
4
Major
3
Moderate
2
Minor
1
Negligible
Reasons for categorisation
Skin Necrosis
Below erythema level
150 – 2,000
500 : legal limit , < 2Gy early transient
erythema – ICRP85
50 – 150
< 150 : non classified level
(100mGy) No clinically relevant functional
impairment – ICRP 103. Also including nonradiation worker (50mGy)
< 50
Table 2b
In the example given above, the max extremity dose EEX(b) = 5.5 which gives a consequence score is 1.
b Obtaining the Likelihood Score
The likelihood score (between 1 – 5) can be obtained from the table 3 for each hazard and then
noted down in the boxes provided
Score
Frequency
1
Rare
Description
This will probably never happen or recur
2
Unlikely
Do not expect it to happen or recur but it is possible it may do so
3
Possible
Might happen or recur occasionally
4
Likely
5
Almost Certain
Will probably happen or recur, but it is not a persisting issue / circumstance
Will undoubtedly happen or recur, possibly frequently.
Table 3
c Obtaining the Risk Score
The risk is the likelihood of harm occurring together with an indication of how serious that harm
could be.
Risk rating = Likelihood score x Consequence score
Using table 4, find the risk rating (1 – 25).
CONSEQUENCE
Rare (1)
Catastrophic (5)
Major (4)
Moderate (3)
Minor (2)
Negligible (1)
5
4
3
2
1
LIKELIHOOD
Possible (3)
Unlikely (2)
10
8
6
4
2
15
12
9
6
3
Likely (4)
Almost certain (5)
20
16
12
8
4
25
20
15
10
5
Table 4
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If the likelihood score is thought to be 3 and the consequence score is 2 (from whole body dose
calculations above), then the risk rating = 3 x 2 = 6
The risk rating (between 1-25) corresponds to a risk score (between 1 – 4) as given in table 5.
Note this down in the table.
Risk Score
Risk
1
Very Low risk
2
Low risk
3
Moderate risk
4
High risk
Description
Local investigation where appropriate
Contributory factor(s) to be identified; discuss with local management the need for any
changes in practice, policies, procedures, education or training.
Identify contributory factors. Discuss at the local radiation governance meeting.
Action plans to be monitored centrally.
Report incident immediately to manager/head of department. Inform risk manager.
Full investigation to be undertaken including interview with staff and identification of
root causes. Action plans to be monitored centrally. To be reported to the Radiation
Board.
Table 5
From the example above, with the risk rating being 6, the risk score (obtained from table 5) = 2
4 Control Measures
State the type of shielding suitable for the radionuclide used (e.g. lead for I-131, Perspex for P-32). If
shielding is adopted, it should have an appropriate thickness:
Gamma Radiation: The thickness of lead should be greater than or equal to the TVL of the
radionuclide (see table 1, column C). However, the attenuated dose should still
be checked to ensure that it does not exceed any dose limit given in table 6.
Beta Radiation:
10mm of Perspex should provide adequate shielding for all Beta emitters listed in
table 1 excluding Y-90 and I-131.
A list of control measures have been given as a prompt. These may or may not be applicable to the
situation being assessed. If it is not applicable or not being carried out, explain why not under the “If
no, explain why not” column. Otherwise give details under the other column. For other control
measures which are relevant but not listed, please add extra rows.
The chance of contamination spread can be significantly reduced by ensuring that gloves are used and
that all work is carried out on spill trays which are lined with absorbent materials. Movement of
radioactive materials also needs to be considered to ensure that accidental spillages are not a regular
occurrence and the chance of significant contamination from the spillage is minimised by adopting an
appropriate movement procedure
5 Radiation Risk Quantification after control measures are in place
Assess what the doses will be (as above) after control measures are put in place and note these down.
Again these values should be annual doses.
EWB(a) example: There is a significant reduction in dose rate after introducing shielding (control measure) to stand
behind during manipulations and shielded storage when the activity is not being used. If a shield
containing 11mm of lead is introduced, then the whole body annual dose can be calculated as
EWB(a) = 4.31/ 10 = 0.431 mSv.
6mm Perspex is also added before the lead shield to absorb the beta particles.
EEX(a) example: Dose-sharing techniques are now introduced during syringe manipulation resulting in half the
time spent being exposed from direct extremity contact. The annual extremity dose is then
calculated as EEX(a) = 5.5/2 = 2.75 mSv.
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The limit for non-classified workers and members of the public are given in table 6. Pregnant staff may
come under the category of “members of the public” in this exercise. If any of the annual dose values
after control measures have been adopted exceeds the respective limits given in table 6 then further
action needs to be taken reduce this to within the limits. If this is not possible, then the member of staff
would need to be classified. This should be noted in section 6.
Whole body
Extremities
Skin (Contamination)
Non-classified Radiation
workers (mSv)
6
150
150
Members of the public
(mSv)
1
50
50
Table 6
Calculate the risk probability (described above) using the dose estimations post control measures
EWB(a) and EEX(a). Obtain the consequence score, the likelihood rating and the risk score as before.
If any further action is needed to reduce the risk score for each hazard, note that in the appropriate
column. The number of hazards identified in section 3 should be the same number as those in section
5 after control measures are in place.
6 Hazard Identification & Control Measures for Incident Situations
Note down situations where incidents could occur. Identify the best action to take when the incident
occurs so that people are as safe as possible first. ( This could mean shutting down an area !)
7 Action Plan
Any action as well as improvements required to reduce the risks should be included in the “Action
required” section. The person responsible for carrying out the action and the date the action should be
done needs to be inserted. When the action has been carried out, the “date completed” needs to be
added as well as a signature from the person who carried it out.
8 Area Designation
Risk assessments are a way of indication what an area should be designated as. Sometimes after
carrying out the risk assessment, the area may change its designation. All this needs to be addressed
in the space provided. An area can be designated as controlled or supervised for various different
reasons. Areas need not be permanent. Some reasons for designating an area are as follows :
Controlled Area:
-
If increased security and restriction of access to that area is required.
To reduce the spread of contamination which is likely.
If a person is likely to receive a dose higher than the dose limits given in table 6
-
to keep conditions under review to determine if the area needs to be controlled
In labs where only small quantities of very low energy radioactive materials are used.
To reduce the spread of contamination which may occur.
Supervised Area:
9 Approval
Once the form is completed, the person(s) carrying out the risk assessment should sign and date it and
get the Head of department / section or lab manager to approve it. A member of the Radiation Safety
Group should then sign it to say that it has been noted.
10 Summary
All the hazards and control measures (group them if possible) far should noted in the summary table. A
small list has been provided but it may not all be relevant or sufficient and extra columns may need to
be added. A tick should be placed in the appropriate box to demonstrate how the each hazard has
been addressed by at least one control measure.
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