AS Science In Society 1.7 Teacher notes
Page 2 ©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.7 Teacher notes
ReferencesLiving with Radiation booklet from NRPB
Textbook p.122
Introduction
This activity encourages students to consider the principles of radiation protection and how they might apply in the nuclear industry and in medicine. They discuss and answer a set of questions based on exposure data.
How Science works
Ge Several factors can influence a person’s willingness to accept a specific risk. Most people are more willing to accept a process or situation that has some risk if they get direct benefit
Gh Reducing the risk of a given hazard costs more and more, the lower we want to make the risk. Individuals or governments have to decide what level of risk is acceptable, by weighing up the probability of harm and the cost of reducing it further.
Hj Some decisions involve balancing the rights of certain individuals and groups against those of others.
Hk A utilitarian approach is to argue that the right decision or choice is the one that leads to the greatest good for the largest number. It can also be argued, however, that some actions are wrong, even if they lead to good outcomes.
The activity
Students will find it easier to discuss some of the questions in small groups before answering. The final question is either a class debating point or a homework to practice writing a longer argument.
The ideas about how science works from H, decision making, have not been identified in the activity. You may wish to ask students to note where they apply.
Suggested Answers
1. The activity of a radioactive source is measured in becquerel, Bq. Radiation protection however always uses a different unit, the sievert, Sv. Explain why this is a more suitable unit when considering radiation risk.
Activity measures the number of decays per second. However, the amount of energy absorbed by a person and the risk of harm will depend on the type of radiation (alpha, beta or gamma). The effective radiation dose, measured in Sieverts takes this into account.
2. Most radiation workers are employed in nuclear power stations or deal with the waste these power stations produce. According to the ICRP principles the radiation harm caused to these workers is only justified if the benefits of using nuclear power offset the radiation harm it causes them. Suggest how those who made the decisions might have argued in order to justify the power stations.
Need for electricity in the country, no greenhouse gases, small number of workers involved, overall risk to workers’ health is less than that to coal miners.
3. It would theoretically be possible to reduce the radiation risk for workers even lower but the costs would rise steeply. More and more precautions could be taken or they could work shorter and shorter hours. See the graph Figure 7.37 on p. 122 of the textbook. What sort of economic and social factors should regulators consider in deciding what is reasonably achievable?
Economic – the cost of the safety measures would either raise the price of the electricity too high or if subsidised by the government would take money from other needs in the country.
Social – the level of risk and the number of people involved might be relevant in deciding what is reasonable. Shorter hours would mean more radiation workers needed so more people actually exposed.
4. Why might it be reasonable to allow a radiation worker a greater dose limit than the general public?
The radiation worker chooses to do his or her job, knowing the risks. Also, they might not be able to carry out the work they need to if a lower limit was chosen, the limit would not be ‘reasonably achievable’
5. Why does using the ICRP approach justify a different limit for a worker under 18 years old?
If he/she is still growing/developing they will be more sensitive to the effects of radiation. Secondly, if they stay in the nuclear industry for their working lives their total dose will be larger. In other words the risks are greater so the balance of risk and benefit changes.
6. Explain why it would be foolish for a radiation worker to be relaxed about exposure at the end of a year in which she had only a small fraction of the maximum allowed dose.
The limit is not a measure of the safe dose but the maximum dose. It is believed that the risk is proportional to dose at all levels.
7. How many chest X-rays would a patient need to have in a year to reach the dose limit for man-made sources given in table 1?
1mSv / 0.02mSv = 50
8. Calculate the additional number of fatal cancers for every 10 000 CT scans of the abdomen/pelvis.
each one has risk of 1 in 2 000 so total number is 10 000/2000 = 5
9. Using Table 2 identify two medical procedures in which the patient is
a. irradiated, All the X-ray procedures
b. contaminated (for a short time) with radioactive material. All the nuclear medicine
c. Explain the difference between the two terms.
Irradiated means that patient is exposed to radiation, but at the end of the procedure the patient is not themselves radioactive.
Contaminated means that patient is exposed to radiation on or within their body. At the end of procedure they may be radioactive. However, the half-life of the isotopes is usually short so that the contamination is also short-lived.
10. What benefits might be expected from an abdominal CT scan to justify the risk from radiation to the exposed individual? Taking the role of the doctor use the Justification principle above to explain to the patient why you are recommending the CT scan in this case.
Symptoms suggest that you have quite a high probability of an illness that is life threatening or has very serious effects on the quality of life. The benefits of the CT scan are that we can diagnose and therefore probably treat you. The risks are 1 in 200 over your lifetime. This is less than the risk from the illness if it is undiagnosed and treated.
11. Suggest a circumstance in which the benefits from one of the medical procedures in Table 2 might not justify the increased risk.
A CT scan to reassure where there are no symptoms of cancer. The risk of cancer from the scan is greater than the chance that a healthy person has cancer.
12. A myocardial perfusion is used to investigate coronary heart disease such as the narrowing of arteries and oxygen supply to the heart.
What factors should the doctors and patient take into account before they decided to carry out a myocardial perfusion? Which of the three principles would be important here?
Factors such as: possible severity of disease, family history of disease, family history of other cancers, how will knowing more about the disease affect treatment/survival rate, cost of test etc. Must balance out the additional risk of fatal cancer from the procedure with the benefits gained from the procedure. The fist principle of justification.
13. In order to calculate the figures for additional risk due to radiation exposure scientists made use of model. Using the data from table 2 suggest the mathematical relationship that may have been used in the model.
When dose rate is 0.01mSv (teeth) then additional risk is 1 in 2 million
When dose rate is 0.02mSv (chest) then additional risk is 1 in a million
When dose rate is 0.1mSv (lung ventilation) then additional risk is 1 in 200 000
When dose rate is 1mSv (kidney scan) then additional risk is 1 in 20 000
When dose rate is 10mSv (CT scan abdomen) then additional risk is 1 in 2 000
Linear (directly proportional) relationship – if you double dose, then you double risk; if you increase dose rate by a factor of 10, then you increase risk by a factor of 10.
14. Newer CT machines usually deliver lower doses for the same procedure. Economic factors apply here as in other ALARA decisions. A hospital has only enough money for either a new CT machine or for incubators for premature babies. How would it decide what it is reasonable to spend it’s money on?
There is not unlimited money available to reduce risks. Which is reasonable? This is an economic decision, there is only enough for one of the two
Factors might include: how much the new machine reduces radiation risk and so how many cancers might be prevented. The number of babies that might be saved. How many people have CT scans and what benefits do they gain.
15. Another group of radiation workers, not mentioned so far, consists of those who work making and maintaining nuclear weapons. What is your opinion on whether their exposure is justified?
A class or small group discussion or a homework activity to practice presenting an argument.
Page 2 ©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.7 Student sheets
Some groups of people receive radiation doses from artificial sources that are higher than the average. This is usually for medical reasons or because they work in the nuclear industry. It is important that these doses are carefully managed and monitored; decisions have to be made about the level of exposure permitted in each case. The decisions are made using a system developed by the International Commission on Radiological Protection, ICRP, see textbook p.122. The three principles of the system can be summarised as follows:
· Justification
No practice involving radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation harm it causes.
· ALARA
All radiation exposures shall be kept As Low As Reasonably Achievable, economic and social factors being taken into account.
· Dose limits
The radiation dose to individuals shall not exceed the limits recommended for the circumstances.
In this activity you will think about how the principles are applied.
Table 1: Dose limits from man-made sources in any calendar year.
Dose Limit per year / Additional annual risk of deathMembers of the public / 1mSv / 1 in 20 000
Radiation workers over 18yrs / 20mSv / 1 in 1 000
Trainee Radiation workers under 18yrs / 6mSv / 1 in 3 000
(data taken from: www.gla.ac.uk/services/radiationprotection/rp5.doc)
In addition to the man-made sources the radiation dose due to natural causes (background radiation) is about 2mSv per year.
1. The activity of a radioactive source is measured in becquerel, Bq. Radiation protection however always uses a different unit, the sievert, Sv. Explain why this is a more suitable unit when considering radiation risk.
2. Most radiation workers are employed in nuclear power stations or deal with the waste these power stations produce. According to the ICRP principles the radiation harm caused to these workers is only justified if the benefits of using nuclear power offset the radiation harm it causes them. Suggest how those who made the decisions might have argued in order to justify the power stations.
3. It would theoretically be possible to reduce the radiation risk for workers even lower but the costs would rise steeply. More and more precautions could be taken or they could work shorter and shorter hours. See the graph Figure 7.37 on p. 122 of the textbook. What sort of economic and social factors should regulators consider in deciding what is reasonably achievable?
4. Why might it be reasonable to allow a radiation worker a greater dose limit than the general public?
5. Why does using the ICRP approach justify a different limit for a worker under 18 years old?
6. Explain why it would be foolish for a radiation worker to be relaxed about exposure at the end of a year in which she had only a small fraction of the maximum allowed dose.
Table 2: Typical radiation doses due to medical procedures
Diagnostic procedure / Typical effective doses (mSv) / Equivalent period of natural background radiation / Lifetime additional risk of fatal cancer per examinationX-ray examinations:
Teeth (single side) / <0.01 / <1.5 days / 1 in a few million
Teeth (panoramic) / 0.01 / 1.5 days / 1 in 2 million
Chest / 0.02 / 3 days / 1 in a million
Hip / 0.3 / 7 weeks / 1 in 67 000
Barium meal / 3 / 16 months / 1 in 6 700
Barium enema / 7 / 3.2 years / 1 in 3 000
CT scan head / 2 / 1 year / 1 in 10 000
CT scan abdomen/pelvis / 10 / 4.5 years / 1 in 2 000
Nuclear medicine studies:
Lung ventilation (Kr-81m) / 0.1 / 2.4 weeks / 1 in 200 000
Kidney scan (Tc-99m) / 1 / 6 months / 1 in 20 000
Myocardial perfusion (Tl-201) / 18 / 8 years / 1 in 1 100
(Data taken from: http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1195733826941?p=1158934607708 )