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Introduction

  1. Energy Use
  2. Electricity Today and Tomorrow
  3. Nuclear Power
  4. The "Front End" of the Nuclear Fuel Cycle
  5. The "Back End" of the Nuclear Fuel Cycle
  6. Environment, Health and Safety Issues
  7. Avoiding Weapons Proliferation
  8. Australia and Canada

 

Appendices

  1. Ionising Radiation and How it is Measured
  2. Some Radioactive Decay Series
  3. Environmental and Ethical Aspects of Radioactive Waste Management
  4. Some Useful References

 

 

(Seventh edition, 2003)

Note: All material here remains Copyright Uranium Information Centre Ltd.

Appendix 1

Ionising radiation and how it is measured

The following are four kinds of nuclear radiation:

Alpha particles: These are particles (atomic nuclei) consisting of 2 protons and 2 neutrons. They are intensely ionising but can be readily stopped by a few centimetres of air, a sheet of paper, or the human skin. They are only dangerous to people if they are released inside the body. Alpha-radioactive substances are safe if kept in any containers sealed to air.

Beta particles: These are either electrons or positrons (therefore of very low mass). They can be stopped by a thin piece of wood or plastic and are generally less dangerous to people than gamma radiation. Exposure produces an effect like sunburn, but which is slower to heal. Beta-radioactive substances are also safe if kept in appropriate sealed containers.

Gamma rays: These are high energy beams almost identical with x-rays and of shorter wavelength than ultraviolet radiation. They are very penetrating, and need substantial thicknesses of heavy materials such as lead, steel or concrete to shield them. They are the main hazard to people in dealing with sealed radioactive materials. Doses can be detected by the small badges worn by workers handling any radioactive materials. Gamma activity in a substance (e.g. rock) can be measured with a scintillometer or geiger counter.

Neutrons: These are mostly released by nuclear fission, and apart from a little cosmic radiation they are seldom encountered outside the core of a nuclear reactor. Fast neutrons are very penetrating as well as (indirectly) being strongly ionising and hence very destructive to human tissue. They can be slowed down (or "moderated") by wood, plastic, or (more commonly) by graphite or water.

(X-rays are also ionising radiation, virtually identical to gamma rays, but not nuclear in origin.)

Units:

The amount of ionising radiation absorbed in tissue can be expressed in grays, 1 Gy = 1 J/kg. However, since neutrons and alpha particles cause more damage per gray than gamma or beta radiation, another unit, the sievert (Sv) is used in setting radiological protection standards. One gray of beta or gamma radiation has one sievert of biological effect, one gray of alpha particles has 20 Sv effect and one gray of neutrons is equivalent to around 10 Sv (depending on their energy).

Total dose is thus measured in sieverts (or millisieverts - mSv - one thousandth of a sievert, or microsieverts - ┬ÁSv - one millionth of a sievert). The rate of dose is measured in milli or micro sieverts per hour or year. For instance, our natural dose is around 2 mSv/yr, and maximum annual dose allowed for a uranium miner is 20 mSv/yr, though that received in Australian and Canadian mining operations is typically less than half of this.

(These levels contrast with those which are harmful in a disaster scenario: with gamma radiation a short term dose of 1 Sv causes (temporary) radiation sickness, 5 Sv would kill about half the people receiving it in a month and a burst of 10 Sv would be fatal to all in a matter of days. The 28 radiation fatalities at Chernobyl appear to have received more than 5 Sv in a few days, those suffering acute radiation sickness averaged 3.4 Sv.)

The becquerel (Bq) is the unit or a measure of actual radioactivity in material (as distinct from the radiation it emits, or the human dose from that), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). Quantities of radioactive material are commonly estimated by measuring the amount of intrinsic radioactivity in becquerel - one Bq of radioactive material is that amount which has an average of one disintegration per second, ie an activity of 1 Bq.

Older units of radiation measurement continue in use in some literature:
1 gray = 100 rads
1 sievert = 100 rem
1 becquerel = 27 picocuries or 2.7 x 10-11 curies

One curie was originally the activity of one gram of radium-226, and represents 3.7 x 1010 disintegrations per second (Bq).

Radon and radon progeny

The Working Level Month (WLM) has been used as a measure of dose for exposure to radon and in particular, radon decay products (see Appendix 2). One "Working Level" is approximately equivalent to 3700 Bq/m3 of Rn-222 in equilibrium with its decay products. Exposure to 0.4 WL was the maximum permissible for workers. Continuous exposure to 0.4 WL during working hours would result in a dose of 5 WLM over a full year, corresponding to about 50 mSv/yr whole body dose for a 40-hour week. In the underground mine at Olympic Dam, and at Ranger, individual workers' doses are kept below 1 WLM/yr (10 mSv/yr), and typically average half this.

A background radon level of 40 Bq/m3 indoors and 6 Bq/m3 outdoors, assuming an indoor occupancy of 80%, is equivalent to a dose rate of 1 mSv/yr and is the average for most of the world's inhabitants.

Some comparative radiation doses:
2 mSv/year Typical background radiation to Australian public.
3 mSv/year Typical background radiation to North American public.
2.9 mSv/year Average occupational dose to US nuclear industry employees.
5.0 mSv/year Average occupational dose to Australian uranium miners.
1.5 mSv/year Average incremental dose for aircrew.
10 mSv/year Maximum actual dose to Australian uranium miners.
20 mSv/year Current limit for nuclear industry employees (5 year average).
50 mSv/year Former limit for nuclear industry employees and U miners, current maximum limit in a single year.
350 mSv in lifetime Criterion for relocating people after Chernobyl accident.
1000 mSv as short term dose: likely to cause (temporary) radiation sickness.
10,000 mSv as short term dose: fatal within days or weeks.

Further information:
Uranium Information Centre or
contact Australian Radiation Protection & Nuclear Safety Agency, Yallambie, Vic, phone (03) 9433 2211 or e-mail arpansa@health.gov.au

 
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