(M2) 16 - Nuclear power

Question 1

Natural background radiation and its sources

Answer 1

The UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) identifies 4 major sources of public exposure to natural radiation:
1. Cosmic radiation (e.g., Sun)
2. Terrestrial radiation (e.g., minerals)
3. Through inhalation (e.g., gases, minerals)
4. Through ingestion (e.g., veggies, drinking water)

Question 2

Radiation dose examples and health issues related to radiation exposure

Answer 2

1000
- dose that may cause symptoms of radiation sickness

500
- annual dose limit for persons carrying out emergency work

> 100
- average annual exposure to astronauts working on the International Space Station

100
- five-yr dose limit for nuclear energy workers

50
- annual dose limit for nuclear energy workers

<10
- Typical chest CT scan

1
- annual public dose limit
- typical annual dose received by a worker in a uranium mine or nuclear power plant in Canada
- average annual dose from natural background radiation in Canada

<1
- typical chest x-ray
- typical cross-Canada flight
- typical dose from living one year within a few km of an operating nuclear power plant in Canada

Radiation can damage DNA, leading to:
1. Nothing, and cells are properly repaired
2. The cell dies because the DNA is too damaged
3. The cell repairs itself improperly because of the damaged DNA (= risk of cancer and/or abnormalities)

Question 3

Nuclear energy:
- How it produces electricity
- Main parts of a nuclear reactor
- Types of nuclear reactors (PWR, CANDU) and their differences

Answer 3

HOW
- Nuclear fission: the splitting apart of atomic nuclei (opposed to nuclear fusion, the forcing together of atomic nuclei)

NUCLEAR REACTOR
1. Moderator: substance used to slow down neutrons bombarding the uranium, usually water or graphite. Also used to transfer heat energy.
2. Control rods: used to absorb excess neutrons to slow down reactions, usually made of a metal (e.g., cadmium, silver).
3. Containment structure: building housing the reactor core, reactor vessel, and steam generator. Usually, super thick concrete and steel.

PWR - “Standard” Pressurized Water Reactors
- Pump water into reactor core where it is heated as nuclear fission takes place; water is then pumped out into tubes in a heat exchanger that heat another source of water that produces steam.

CANDU - Breeder Reactors, all Canadian plants
- Fxn is similar to PWR except:
- Fewer neutrons absorbed by HEAVY WATER vs. LIGHT WATER and can use UNENRICHED URANIUM as fuel
- The excess 238U can absorb a neutron, form 239Pu, which in turn can be used as fuel
- Can be refueled while operating at full power

Question 4

Uranium Mining
- Methods
- Problems

Answer 4

Methods
- Open Pit
- Underground
- In-situ Leaching
- Heap Leaching

Problems
- Uranium Tailings
- Often stored in water to prevent oxidation, and to contain radiation
- Excess water is treated as nuclear waste
- Must be properly maintained

Question 5

Components of radioactive waste and classification (low, intermediate, high)

Answer 5

Components of Radioactive Waste
1. Spent fuel rods
2. Used PPE
3. Old equipment
4. Contaminated soil
5. Contaminated water

Classification by Radioactivity
1. Low-level
- Items exposed to radiation that require <300 yrs to lose radioactivity

2. Intermediate-level
   - Items exposed to radiation that require > 300 yrs to lose radioactivity
3. High-level
   - Spent nuclear fuel

Question 6

Steps to store and manage high-level radioactive waste (wet and dry storage)

Answer 6

Low and Intermediate-Level Storage
- In Canada, the producers of low and intermediate-level radioactive waste are responsible for disposal:
1. Store on site
- medical isotopes usually need max a few days to decay
- others stored in shielded areas
2. Pay the Canadian Nuclear Laboratories (CNL) to take care of it

Storage of High-Level Radioactive Waste: 2 Step Process
1. Wet Storage: initial storage after use, held in water tanks at power plants for 7-10 yrs
2. Dry Storage: long-term storage in either concrete canisters, Modular Air-cooled Storage units, or concrete/steel dry storage containers

Question 7

How are deep geological repositories a possible solution?
- Requirements for construction

Answer 7

How do you store waste for millennia?
- We are still figuring that out
- Currently dry storage containers in Canada have a lifetime of 50 yrs
- Problem: High-level waste takes a lot longer than that to decay

Proposed Solution: BURY IT
- Deep Geological Repository
- Bury in crystalline or sedimentary rocks; radiation shielded from surface, 500m deep
- Geologically stable; no earthquakes/tsunamis
- Easy to access; have to easily transport the waste there

Question 8

Examples of nuclear accidents (Fukushima and Chernobyl)

Answer 8

The two nuclear power accidents with widespread, measurable environmental damages.

1. Fukushima Daiichi, March 2011
   - Plant damaged by tsunami following an earthquake
   - Disabled generators cooling, causing them to overheat
   - Lead to 3 reactor meltdowns, one wet storage pool overheating
   - High levels of radioactive isotopes measured in waters off the coast near Fukushima; these isotopes have short half-lives, but some are continuously being leaked due to structural damages to the 3 reactors
   - Strong ocean currents moved contaminated water throughout the Pacific Ocean
   - Elevated radiation levels measured in Bluefin Tuna; migratory, caught in CALI
2. Chernobyl, April 1986
   - Worth nuclear accident in history
   - 49 direct fatalities, plus thousands of cases of related cancers
   - Why? Human error; turned off safety systems during a test, then power surge occurred
   - Ash from the fire was radioactive, spread throughout Europ
   - Area in and around Chernobyl is still irradiated and uninhabitable

Enviro Impacts of Chernobyl: Local Wildlife & Forests
- Evidence of defect in wildlife
- Some wildlife populations are now thriving due to minimal human interference
- Trees grow slower and dead trees are not decaying… (lack of decomposers)
- More dead trees = more fires