Nuclear Power – Safety (Part 1)

1. Nuclear Power – Safety(Part 1)

2. Three factors to consider • Danger of handling radioactive materials – the fuel and the waste safely so workers and the public are safe • Danger of a ‘melt down’ – a nuclear accident • Danger of terrorist attacks – possibility of stealing nuclear material to make weapons or just blowing up the power station!

3. Let’s look at the normal operation of the plant… • Fuel rods contain uranium (both isotopes are alpha (and gamma) emitters with a long half life). • Why are radioactive isotopes dangerous to us?

4. Radioactivity • The natural decay products of the fuel are radioactive – as are the daughter nuclei…

5. Radioactivity • Note the variation in half life… • What affect does this variation have on activity? • What affect will this have on the fuel rods?

6. Radioactivity • The fission fragments are radioactive (most often neutron rich) • The natural decay products of the fission fragments are radioactive – as are their daughter nuclei.

7. Radioactivity • The fission fragments are often rare isotopes • What does that tell you about their half life? • What affect will that have on the fuel rods?

8. Radioactivity • The instruments used in the reaction vessel, moderator and control rods all become radioactive by artificial transmutation as the reactor operates – as do the walls of the vessel and the shielding around it.

9. Criteria for the Selection of a Shield Material • Theoretically any material can be used for radiation shielding as long as it is thick enough to attenuate the radiation to safe limits. • The choice of the shield material is dependent upon many varied factors. All of these need to be considered: • The final desired attenuated radiation levels, • The ease of heat dissipation • Resistance to radiation damage, • Required thickness and weight, • Multiple use considerations (e.g., shield and/or structural), • Uniformity of shielding capability, • Permanence of shielding, and • Availability and cost.

10. Shielding Materials

11. Shielding • In exam questions they expect you to name concrete as the shield as it is the principal material - but steel and lead is also employed to get the structure strong and to give a very low radiation level to workers.

12. Waste Three categories: • High level • Intermediate level • Low level

13. High Level Waste • This waste requires very heavy shielding as it is very radioactive. • The intense radioactive decay generates a large amount of heat. This needs to be carefully considered when thinking about storage and final disposal. • High Level Waste includes spent fuel and highly radioactive liquids generated during reprocessing operations.

14. High Level Waste • Spent fuel is all stored at Sellafield in high-integrity stainless steel tanks fitted with cooling coils to remove the heat generated. • Management and disposal of this waste is difficult due to: • the high levels of activity (due to the short half life nuclides), • the heat continually generated as a result of decay processes needs to be removed. 

15. Spent Fuel Rods • Spent fuel rods have to be moved by remote control as they are so radioactive. • They are placed in cooling ponds for several months to allow short half life isotopes to decay. • When they have reduced in activity enough they are reprocessed.

16. Spent Fuel Rods • The remaining waste can then be vitrified (encased within glass) and encased in air cooled steel drums for 50 years to allow the heat generated to reduce to manageable levels. They can then be buried deep underground in repositories (often deep mine shafts reinforced with concrete) that need to be highly shielded and carefully monitored at a geologically stable site.

17. Spent Fuel Rods • For transportation purposes - any casing has to be high impact resistant and transport convoys have to be secure and slow moving - in case of terrorist action or any accident occurring.

18. Intermediate Level Waste • This waste also needs to be heavily shielded, as it can be extremely radioactive, but does not generate as much heat as High Level Waste. • Some of the radioactive particles present in this waste may have very long half-lives and so require isolation for many thousands of years.

19. Intermediate Level Waste • Intermediate Level Waste includes fuel element claddings removed prior to reprocessing, various sludges and ion exchange resins from fuel storage pond water treatment; concentrates of liquid waste streams; heavily contaminated scrap equipment; plutonium contaminated materials and graphite sleeves and steel components from AGR fuel assemblies.

20. Intermediate Level Waste • Large volumes of Intermediate Level Waste are expected from decommissioning operations are expected in the coming years. • Because of the wide range of Intermediate Level Waste sources many different forms of conditioning and packaging are required.

21. Low Level Waste • This waste tends to be low in radioactivity and high in bulk. • It ranges from general rubbish (gloves, clothing, packaging, paper towels, over shoes, laboratory glassware, etc.) to some very low-level plutonium contaminated materials. • A lot of material classified as Low Level Waste, may in fact not be radioactive at all, but it is potentially radioactive through being in an active/contaminated area.

22. Low Level Waste • The low levels of radioactivity and the short-lives of the contaminants mean this waste is relatively harmless if handled properly. • However, any site used for Low Level Waste disposal will need to be subject to land use restrictions for around 300 years after the site is closed. • There is also always a risk of environmental problems if water leaching through the waste site finds its way into surface and ground waters.