The University of Reading Health & Safety Office - Safety Note 7
The Use of Uranyl Acetate in Electron Microscopy:
Radiological aspects
1
INTRODUCTION
Heavy metal compounds (e.g, Lead, Osmium, Tungsten or Uranium compounds) are often used as
stains in electron microscopy because of their ability to bind to biological materials, and to provide
electron-dense contrast-enhancing areas on electron microscope grids. The use of Uranyl acetate
for this purpose is quite common, but particular problems arise because of the natural radioactive
properties of the Uranium atoms – even though the radioactive properties are incidental to its use in
electron microscopy.
2
REGULATORY BACKGROUND
The requirements of the Radioactive Substances Act (RSA) apply to the use of a (naturally)
radioactive substance if the activity is greater than 11.1 Becquerels per gram (solid) or 0.74 Bq g-1
(liquid)1. In addition, the Ionising Radiations Regulations apply to any “practice” (which includes
production, use, handling, storage, transport and disposal of radioactive materials) if the limits
given in Schedule 8 to the Regulations are exceeded. For Uranium, the limit is given as 10 Bq g-1,
or a total of 104 Becquerels.
Typically, Uranyl acetate as supplied is likely to have a specific activity of between 11,000 - 25,000
Bq g-1, depending on whether or not the salt is prepared from Uranium depleted in the 235U isotope2,
and the time that has elapsed since the compound was prepared (see below) – thus, both sets of
requirements will apply if the total amount of Uranium held by the employer is more than 1g.
The Radioactive Substances Act (see Safety Guide 16 for more details) requires organisations to
be registered and to hold appropriate licences before they may acquire, use or dispose of radioactive
materials. The application of the RSA is however modified by the Radioactive Substances
(Prepared Uranium and Thorium Compounds) Exemption Order 1962, which permits an
organisation to hold up to 2 Kg Uranium without having to be registered under RSA. This
Exemption Order also applies to the disposal of waste Uranium and Thorium compounds – see
section 4, below.
One important requirement of the Ionising Radiations Regulations is a general duty to ensure that
exposures to ionising radiation are as low as reasonably practicable [ALARP], and to this end,
users should:
(i) be able to produce a justification for using any material (or process) which emits ionising
radiation; and
(ii) use appropriate techniques and quantities of radioactive material that will minimise
exposure to ionising radiation.
A risk assessment should therefore be prepared to cover the use of Uranyl acetate in electron
microscopy (etc.) [IRR, Regulation 7] and should be recorded. The purpose of the risk assessment
1
If the specific activity is less than these limits, the material is deemed not to be radioactive under the RSA.
Uranyl acetate prepared from natural Uranium will have a greater specific activity due to the presence of
daughter decay elements, in addition to those derived from 238U.
2
235
U and its
under the IRR is to assess ,and identify the measures necessary to control, the risks arising from the
use of radioactive materials (and hence, exposure to ionising radiation). However, the assessment
should also take into account the highly toxic nature of Uranyl acetate, as required under the
Control of Substances Hazardous to Health Regulations (COSHH) (see below). Provided this is
done, there is no need to undertake two separate risk assessments.
Some of the activity in Uranyl acetate is due to the presence of Thorium-234 and Protoactinium234m/ Protoactinium-234 isotopes (produced by decay of 238U atoms) which reappear after
chemical purification – thus, any freshly purified Uranyl acetate will increase in specific activity as
the elements of the decay series accumulate, until equilibrium is reached between the production of
Thorium and Protoactinium and their own decay to Uranium-234 (half-life 24.5 days and 1.14
minutes respectively). Published data indicate that it takes about a year for this equilibrium to be reestablished, hence it is likely that any Uranyl acetate purchased from a supplier would have
regained equilibrium by the time of receipt.
In addition, a EURATOM Directive requires that if an organisation such as The University has
more than 2Kg of Uranium (as metal, or in the form of one or more compounds), then the
organisation must keep a detailed inventory (accurate to the nearest milligram) of the quantities and
locations of all such material. To avoid this onerous security requirement, the total amount of
Uranium permitted at the University is restricted to less than 2kg (see section 5, below.)
Note that Uranyl acetate is classified as “highly toxic” (especially by inhalation) and that the
COSHH Regulations apply to its use. The occupational exposure standard for soluble
Uranium compounds is 0.2 mg m-3 in air (as U).
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SAFE USE IN ELECTRON MICROSCOPY
Since inhalation is the most serious route of intake leading to internal exposure, precautions should
be adopted to prevent any possibility of breathing in dust, especially when weighing out the
compound – this should be done in a fume cupboard. Any stock of solid Uranyl acetate should be
marked with “Radioactive” warning tape (if not already so marked by the supplier), and stored in a
safe place, such as a locked store, or in a locked room. There is effectively no external risk from
the alpha radiation emitted by stored material (as the particles are readily stopped, e.g., by a sheet of
paper); the risk arises mainly from internal exposure to radiation3 should any of the material be
ingested or inhaled. Use of solutions of Uranyl acetate in electron microscopy therefore entails little
risk unless any of the material is ingested or inhaled – it is essential to adopt good occupational
hygiene practices when using the material. Gloves should always be worn, and dosimetry is
not required.
Any spills of solution should be carefully cleaned up and monitored for residual radioactivity to
ensure cleanliness.
4
DISPOSAL CONSIDERATIONS
One of the major problems associated with the use of Uranyl acetate is concerned with the disposal
of waste solutions: the University does not have any authorisation to dispose of alpha-emitters.
However, the Radioactive Substances (Prepared Uranium and Thorium Compounds) Exemption
Order 1962 permits an organisation to hold up to 2 Kg Uranium without having to be registered
under RSA, but more importantly for the University, permits a limited amount of disposal of
Uranium without a specific waste authorisation under RSA.
According to the Exemption Order, up to 100g Uranium waste may be disposed of per day, but this
exemption has been partly overtaken by the Special Waste Regulations 1996, whereby Uranium and
Uranium salts (which are classified as “highly toxic”) are defined as “Hazardous waste”. Such
materials must be disposed of via a registered Waste Disposal Contractor unless the concentration
3
There is a small external risk from the gamma radiation that accompanies the alpha emissions, and from the beta
radiation from the daughter elements, but this can be regarded as negligible in comparison with the inhalation risk.
of Uranium is less than 0.1%. (Even this concentration would give an activity of up to 25 Bq g-1,
and still complies with the definition of “radioactive material”: however, if the concentration is in
the “parts per million” range or less, it is likely to be deemed “non-radioactive.”)
Provided that the concentration of U in the waste (after completing all the staining and washing
steps in the staining procedure) is less than 0.1% w/v, sink disposal may be possible. However, note
that the liquid waste arising from electron microscopy may also contain other toxic materials which
may preclude disposal down the sink.
Contaminated solid objects (such as paper towels, etc) may be disposed along with “ordinary”
waste provided that the disposal limits as described above are followed.
5
ORDERING AND STOCK CONTROL
Anyone wishing to order Uranyl Acetate for electron microscopy should consult the University
Radiation Safety Officer (URSO) before purchase to make sure that the overall limit of 2 Kg is not
exceeded and the quantity to be purchased may be added to the U&Th stock inventory held by the
Health & Safety Office. No single Department or School should have more than 100g in stock at
any one time (including waste awaiting disposal) to assist in complying with this limit.
Malcolm Iosson
University Radiation Safety Officer
28 May 2004
Ref: NOT07NEWUranylAcetateMay2004