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Uranium and Thorium Safety Management

Introduction

Uranium or thorium compounds are substances or articles in which the U-235 concentration is no more than 0.72% by mass in the case of uranium, and the thorium is in its isotopic proportions found in nature. They include uranium and thorium metals. In the case of uranium, 0.72% is the natural amount of U-235 so this applies to natural and depleted uranium (DU). In the case of thorium the natural isotopic ratios are variable, depending on the origin. The definition applies to chemically processed thorium provided the radionuclide ratio has not been modified for nuclear fuel.

Limits

Certain limits must be adhered to:

Note that the above conditions and limitations apply to the whole City Campus not just one particular building/School.

For uranium and thorium compounds, as for all other radioactive sources and waste, accurate records must be maintained by Schools/Divisions of stocks held, and waste disposed of and accumulated. These records should be sent to Safety Service once a year.

Occupational exposure pathways and risk assessment

The risks associated with exposure to unprocessed uranium and thorium are not trivial, and potentially very high if the exposure pathway is the inhalation of suspended dusts and aerosols. These radionuclides are so hazardous because they are alpha emitting radionuclides. For example, if inhaled, they would decay in the lungs via a chain of daughters that are themselves alpha emitters: alpha emitting radionuclides deposit large quantities of energy into superficial tissues such as the lining of the alveoli.

The following tables give derived data that quantify the hazards posed by intakes of uranium and thorium compounds through the two exposure pathways of concern.

 

Isotope

Specific Activity (Bq/mg)

DPUI a

(microSv/Bq)

Natural uranium

(99.3% 238U, 0.7% 235U)

26

Ingestion

0.05

Inhalation

7.3

Natural thorium (100% 232Th)

8

Ingestion

0.22

Inhalation

42

aDPUI = Dose Per Unit Intake is the dose to the most radiosensitive organ that would be irradiated via a given exposure pathway i.e. inhalation or ingestion (including skin absorption).

 

Isotope

Pathway (route taken into the body)

Derived dose from an intake of 1 mg

(microSv)

Intake required to give a dose of 1 mSv

(mg)

Dose following the ingestion of 1 ml of a 1% v/v solution b

(mSv)

Natural uranium

Ingestion

1.2

833

0.1

Inhalation

190

5.3

NA

Natural thorium

Ingestion

1.8

556

0.2

Inhalation

336

3.0

NA

b It is assuming the intake is via ingestion of a drop of the solution spilt onto the skin, which has then been taken in orally. Uranium and thorium are not readily absorbed through the skin on account of their large atomic weight and molecular mass of the compound. For risk assessment purposes assume 10% absorption.

Using the derived exposure data presented above it is possible to determine the level of risk under normal operational conditions.

Arrangements for Work with Uranium and Thorium Compounds

Scope of Arrangements

1. These arrangements apply to the use of natural uranium/thorium compounds that are in secular equilibrium (i.e. proportion of daughter products is constant) and which contain:

2. Most ‘off-the-shelf’ compounds of uranium or thorium will be in secular equilibrium i.e. uranium nitrate and uranium acetate will be formulated from ‘natural’ uranium.

3. Uranium and thorium products that have been physically produced (e.g. by milling) or chemically produced (e.g. by reaction) from naturally occurring minerals may not be covered by this guidance and may require special attention from the Radiation Protection Adviser (RPA). The keeping of geological specimens for exhibition or demonstration purposes is ‘out of scope’ (considered to be not radioactive) with regard to the Environmental Permitting Regulations, however they are, like most other radioactive materials, still subject to the prescriptive requirements of the Ionising Radiations Regulations (IRR) and must be handled with due care and attention in accordance with the IRR.

4. Any other isotopes of uranium or thorium may NOT be exempted or ‘out of scope’ with regard to EPR and their potential use must be discussed with the RPA prior to procurement of the material.

Control of Substances

5. A School may only use uranium and thorium compounds if they have first received permission to do so from the RPA. The RPA must ensure that the type and amount of radioactive material is compliant the University’s permit conditions, or is exempted from permitting under EPR or ‘out of scope’ i.e. the University is allowed to have it on site.

6. The maximum quantity allowed in one Laboratory Area is a weight authorised by the RPA/RPO to the Appointed Person. Excess quantities, spare or unused stocks can be lodged with the Safety Services for safekeeping or disposal.

7. When not being used, the solid stock and any sub-stocks should be kept in a locked radioactive source store that is suitable labelled. Stocks must not be stored with flammable or corrosive materials that could potentially lead to a loss of containment.

8. Aqueous stocks should be held in a secure location, which could be a locked cupboard or a locked lab, and they must be stored such that the risk of spillage is minimised.

9. Stained electron microscopy slides must be kept in a safe location i.e. in a suitable container that is itself kept in a locked cupboard or in a cupboard within a locked lab. The store and the container must be marked with a suitable hazard warning sign (see UT_cupboard_sign).

10. A usage log must be maintained that shows ( Pro-forma can be obtained from the RPO):

Working with Uranium and Thorium Compounds

11. Designate a suitably sized area of workbench, covered with Benchkote (absorbent side up) taped down using unmarked tape, or use a lined spill-tray. All work, equipment, lab ware, reagents etc. must be contained within this defined workstation.

12. The workstation must be cleaned (decontaminated) and dismantled immediately after use unless it is in regular use for uranium/thorium work. If the latter is the case, the workstation must be checked for contamination prior to and after use.

13. Uranium/thorium compounds may only be handled by persons who received local instruction in the safe uses of these materials. A person who has been approved to do so by the RPA would normally give this training. In particular, the worker must know how to monitor for contamination.

14. Uranium and thorium powders must only be handled in a fume cupboard. The only exception to this is where weighing out is to be done using a balance fitted with a measurement chamber. Any spillages must be wiped up using damp tissues. The contaminated tissues must be disposed of as radioactive waste (see below).

15. Work must be carried out in strict accordance with the instructions given in the risk assessment/SOP for the safe use of uranium and thorium compounds. Where not already done, a risk assessment/SOP for the safe use of uranium and thorium compounds must be drawn up and approved by the RPA prior to work commencing.

16. Great care must be exercised when handling environmental samples containing natural uranium and thorium compounds. The table below gives the maximum quantity of these materials which should be used in teaching experiments. The value quoted for uranium takes account of both chemical toxicity and radiation protection requirements.

Isotope

Activity (Bq)

Mass (mg)

Thorium

5

1

Uranium

50

5

Waste Disposal

17. As with the disposal of other radioactive wastes, the departmental   Appointed Person (Appointed by Safety Services) must maintain records of all uranium and thorium waste disposals and via which route.

     Aqueous Waste
18. All aqueous waste, including spare stock solutions, can be disposed of by decanting into running water to a laboratory sink. The disposal weight must be within the allowable quantity for that sinks annual limit. Running totals must be kept and amended after each disposal.

19. Alternatively, aqueous wastes can be transferred to Safety Services for disposal.

     Solid Waste
20. All trace amounts (i.e. less than 1g per waste container, ) of solid uranium or thorium compounds, suppliers empty stock bottles, Benchkote (contaminated or not), gloves, wipes, swabs, tissues, disposable labware, etc. must be sealed in a clear plastic zip-lock bag and place in a blue disposal bag which can be disposed of via the normal blue bag waste route. Larger quantities should be disposed of via Safety Services.

21. Scalpel blades, broken glassware, and unwanted electron microscope slides must also be decontaminated (rinsed to drains) before disposed of in a sharpsafe.

22. Before disposing, all radiation labels and/or hazard warning tape must be removed and screwed into a ball, or defaced, so that the radiation trefoils etc. can no longer be seen.

     Recycled Items
23. Laboratory glassware used for uranium and/or thorium work may be returned to general circulation only once it has been decontaminated thoroughly down to background levels e.g. by soaking in Decon or equivalent.

The following appendices show two generic risk assessments. You must use your own local arrangements to determine if these are applicable and how to modify them into your local risk assessments/SOP.

Appendix 1 - Risk Assessment: Use of Uranium and Thorium compounds in invitro studies

 

Procedure

  1. Uranium/thorium compounds are made into an aqueous solution. The solid (powder) is weighed out using a balance fitted with a measurement chamber or a balance in a fume cupboard. The solid is transferred into glass in a fume cupboard where it is made up to volume.
  2. The labelled stock solution is kept on a drip tray in a cupboard, or in some other location, where it is unlikely to get knocked over. Normal precautions are taken to minimise the effect of potential spillages.
  3. Aliquots are dispensed either in a fume cupboard or on the open bench.

 

Waste

Before disposing of any uranium and/or thorium wastes, contact SafetyServices with the details of the waste and how you are planning to dispose of it.

All aqueous wastes can usually be disposed of to the designated sink. Note that the Exemption limit for aqueous disposal is approximately 10g per week by this route and this limit applies to the whole University.

Solid wastes must be bagged and labelled. Safety Services will collect these for subsequent disposal.

Note: Records of all wastes accumulated and disposed of must be maintained by the departmental RPS.

 

Major Risks

A. Inhalation of uranium or thorium dust during the weighing out procedures

Route of intake: Inhalation.

Prevention: Weighing out using a balance fitted with a measurement chamber or a balance in a fume cupboard.

B. Contamination of the hands during staining, or following a spillage

Route of intake: Ingestion of contamination transferred from hand to food, drink, cigarettes etc. Note that uranium/thorium will not pass through unbroken skin.

Prevention: Use of nitrile gloves to stop contamination of the hands and washing the hands thoroughly at appropriate times during work and particularly after work.

C. Intake by other persons exposed to contaminated work surfaces, equipment or through the spread of contamination (cross contamination).

Route of intake: Ingestion of contamination inadvertently picked up as a result of cross-contamination then transferred from hand to food, drink, cigarettes etc.

Prevention: Work carried out under (appropriate) containment i.e. in designated areas on surfaces marked out and laid with BenchkoteÒ (absorbent side up) or using lined drip trays. Work surfaces and hands are monitored for contamination during and after work. Worker changes gloves frequently or if contamination is suspected.

Appendix 2 - Risk Assessment: Use of Uranium compounds as electron microscopy staining agents

Procedure

  1. Uranium compounds are made into an aqueous solution. The solid (powder) is weighed out using a balance fitted with a measurement chamber or a balance in a fume cupboard. The solid is transferred into glass in a fume cupboard where it is made up to volume (usually to solution strength of between 1 and 3% v/v or occasionally as a saturated solution).
  2. The labelled stock solution is kept on a drip tray in a cupboard, or in some other location, where it is unlikely to get knocked over. Normal precautions are taken to minimise the effect of potential spillages.
  3. Slides bearing microscopy specimens are stained by immersing in the uranium solution and air-dried.

 

Waste

Before disposing of any uranium and/or thorium wastes, contact Safety Services with the details of the waste and how you are planning to dispose of it.

All aqueous wastes can usually be disposed of to the designated sink. Note that the Exemption limit for aqueous disposal is approximately 10g per week by this route and this limit applies to the whole University.

Solid wastes must be bagged and labelled. Safety Services will collect these for subsequent disposal.

Note: Records of all wastes accumulated and disposed of must be maintained by the RPS.

 

Major Risks

A. Inhalation of uranium or thorium dust during the weighing out procedures

Route of intake: Inhalation.

Prevention: Weighing out using a balance fitted with a measurement chamber or a balance in a fume cupboard.

B. Contamination of the hands during staining, or following a spillage

Route of intake: Ingestion of contamination transferred from hand to food, drink, cigarettes etc. Note that uranium/thorium will not pass through unbroken skin.

Prevention: Use of nitrile gloves to stop contamination of the hands and washing the hands thoroughly at appropriate times during work and particularly after work. Thereafter slides must only be handled when wearing gloves.

C. Intake by other persons exposed to contaminated work surfaces, equipment or through the spread of contamination (cross contamination).

Route of intake: Ingestion of contamination inadvertently picked up as a result of cross-contamination then transferred from hand to food, drink, cigarettes etc.

Prevention: Work carried out under (appropriate) containment i.e. in designated areas on surfaces marked out and laid with BenchkoteÒ (absorbent side up) or using lined drip trays. Work surfaces and hands are monitored for contamination during and after work. Worker changes gloves frequently or if contamination is suspected.

 

 

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