Guidance on completing a Radiation Risk Assessment using Unsealed Radionuclides

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).

		A	B	C
Radionuclide	Emission	Point source at  30cm*	Contact with 5ml syringe *	Tenth Value Layer (TVL)# of lead (mm) [γ only]
I-131	β	8.62 E-2	1.1	11
	γ	7.29 E-4
I-125	γ	3.90 E-4	0.6	<1
I-123	γ	5.16 E-4	0.6	2
In-111	γ	9.94 E-4	1.2	3
Tc-99m	γ	2.61 E-4	0.4	1
Y-90	β	1.08 E-1	43	Table 1
Ga-67	γ	2.79 E-4	0.4	6
Cr-51	γ	6.04 E-5	0.09	7
Tl-201	γ	1.97 E-4	0.29	1
P-32	β	1.18 E-1	24	-
S-35	β	-	-	-
C-14	β	-	-	-
H-3	β	-	-	-

* 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 E_WB(b) and E_{EX(b) ,} the following assumptions are made:

-       Whole body dose E_WB(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 E_EX(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.

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 :

E_WB(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.

E_EX(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 E_WB(b),  if N = 2, then P_WB(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

Consequence score
5	Catastrophic
4	Major
3	Moderate
2	Minor
1	Negligible

Table 2                                                                                                  Table 2a

In the example given above, for 2 people and with  P_WB(b) = 8.6, the consequence score is 2.

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		Annual Dose (mGy)	Reasons for categorisation
5	Catastrophic	> 50,000	Skin Necrosis
4	Major	2,000 – 50,000	Below erythema level
3	Moderate	150 – 2,000	500 : legal limit , < 2Gy early transient erythema – ICRP85
2	Minor	50 – 150	< 150  : non classified level
1	Negligible	< 50	(100mGy) No clinically relevant functional impairment – ICRP 103. Also including non-radiation worker (50mGy)

Table 2b

In the example given above, the max extremity dose E_EX(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	Description
1	Rare	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	Will probably happen or recur, but it is not a persisting issue / circumstance
5	Almost Certain	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	LIKELIHOOD
	Rare  (1)	Unlikely  (2)	Possible  (3)	Likely  (4)	Almost certain (5)
Catastrophic (5)	5	10	15	20	25
Major (4)	4	8	12	16	20
Moderate (3)	3	6	9	12	15
Minor (2)	2	4	6	8	10
Negligible  (1)	1	2	3	4	5

Table 4

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	Description
1	Very Low risk	Local investigation where appropriate
2	Low risk	Contributory factor(s) to be identified; discuss with local management the need for any changes in practice, policies, procedures, education or training.
3	Moderate risk	Identify contributory factors.  Discuss at the local radiation governance meeting.  Action plans to be monitored centrally.
4	High  risk	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.

E_WB(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 E_WB(a)  = 4.31/ 10 = 0.431 mSv.

6mm Perspex is also added before the lead shield to absorb the beta particles.

E_EX(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 E_EX(a) = 5.5/2 = 2.75 mSv.

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.

	Non-classified Radiation workers    (mSv)	Members of the public (mSv)
Whole body	6	1
Extremities	150	50
Skin (Contamination)	150	50

Table 6

Calculate the risk probability (described above) using the dose estimations post control measures E_WB(a) and E_EX(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

Supervised Area:           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.

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.