NUCLEAR ENERGY

Historical:

Late 1800’s Pierre and Marie Curie determined that uranium minerals emitted invisible radiation capable of passing through solid objects.

Subsequently, it was determined that the radiation was a result of atomic disintegration as atoms of radioactive minerals break down spontaneously over time (DECAY)

RADIOACTIVE DECAY

Parent radioactive material decays to daughter material.238U 4He + 234Th

Radioactive decay is described in terms of half life – the time it takes for ½ of the radioactive atoms to decay = Fission (break apart and lose mass).

Fusion is when atoms combine to form new atoms with less mass.1H + 1H 2He (Thermonuclear rxn in the sun)

How to solve half-life problems

½^x = fraction remainingX = number of half-lives elapsed

1/32 of a radioactive sample is left after 100 years. What is the half life of this sample?

½^x = 1/32x = 5 100 years/5 = 20 years

How Can We Fuse?

Need 1,000,000 degrees!

BENEFIT: waste product is helium…the stuff you put in children’s balloons! No air pollution.

Impossible to maintain for more than 1 minute – not practical as energy source yet

IONIZING RADIATION

Definition: Emissions from radioactive nuclei collide with other atoms or molecules and hence ionizes that atom or molecule. Three types of ionizing radiation:

Alpha radiation – physically identical to the nuclei of He atoms. They travel 10,000 mi/s and interact with other atoms. Energy dissipates when alpha particles make contact with skin cells; only at risk when these ions are inhaled, ingested, or absorbed in a wound.

Beta Radiation – just electrons that are emitted from a radioactive atom. They lose energy at a much slower rate than alpha particles and can penetrate human tissue. Internal organs are usually protected, but not external ones like eyes

Gamma Radiation – photons from the nucleus of radioactive atoms. Bad for any tissue

HOW DOES A NUCLEAR FISSION REACTOR WORK?

THERMAL REACTOR

PARTS OF THE REACTOR

Core – 35,000 – 40,000 long, thin fuel rods each packed with pellets of uranium oxide fuel. Each pellet is the size of a cigarette and is packed with 97% of 238U (nonfissionable) and 3% 235U (fissionable).

Control Rods – are moved in/out of the reactor core to absorb neutrons and regulate the rate of fission and the amount of power the reactor produces.

Moderator (liquid water, graphite, or deuterium) – slows down the neutrons emitted by the fission process so that the chain rxn can continue.

PARTS OF THE REACTOR

Coolant- Usually water, circulates through the core to remove heat to keep the fuel rods and other materials from melting and to produce the steam to turn the turbines for generating electricity.

After 3-4 years, the concentration of fissionable uranium (235U) in the fuel rods becomes too low to continue the chain rxn OR the rods become damaged from ionizing radiation. They are removed and placed in large, concrete lined pools of water to act as a shield/coolant.

WHAT DO WE DO WITH NUCELAR WASTE?

High Level Radioactive Waste (HLRW)– reactor fuel; gives off large amounts of ionizing radiation for a short time and small amounts for a long time. It must be stored for a very long time, 240,000 years if plutonium (239 Pt) is not removed by processing.

Reprocessing – removal of Pt from fuel rods. The remaining radioactive waste must be stored for at least 10,000 years. USA does NOT do this anymore due to high operating costs and ample supplies of uranium.

Long-Term Waste Storage Facilities

Carlsbad New Mexico – Waste isolation Pilot Plant (WIPP); a government run underground storage facility for waste from nuclear weapons – does NOT accept fuel from nuclear plants. Cost = 2 billion dollars.

Problems: active fault zone, water seepage, non-uniform strata.

1996 USA accepted and manage highly spent fuel rods from 40 countries to prevent them from extracting highly enriched uranium for nuclear weapons. Terrorists only need 1kg of plutonium oxide to contaminate an area 3 mi2 with dangerous ionizing radiation for 100,000 year

LOW-LEVEL RADIOACTIVE WASTE

Low –Level Radioactive Waste (LLRW) – contaminated equipment, etc; gives off small amounts of ionizing radiation and must be stored safely for 100-500 years before decaying to levels that don’t pose a health risk to the public.

1940’s – 1970’s USA and most other countries put LLRW into steel drums and dumped it into the ocean. (Pakistan and UK still do)

Since 1970 – LLRW has been buried in government-run landfills (2 remaining) and in above ground storage containers.

OTHER PROPOSALS

Bury it deep in the ground

Shoot it into space or the sun - abandoned

Bury it under Antarctic ice sheet or Greenland - abandoned

Dump into descending subductions zones in the deep ocean – active study

Change it into less harmful isotopes – not achieved yet

Three Mile Island

March 29, 1979 - #2 reactor in Harrisburg, PA lost its coolant water b/c of a series of mechanical failures and human operator errors for safety measure.

The reactor core became partially exposed and 50% of it melted and fell to the bottom of the reactor. Unknown amounts of radiation was released into atm.

50,000 people evacuated, 1.2$ billion in law suits, increase in cancer rates over the years (stress and radiation).

CHERNOBYL

April 26, 1986 – Total meltdown of a graphite moderated nuclear fission power plant. Released enormous amounts of radiation due to loss of coolant around fuel rods and they melted through the core.

Health Effects: thyroid, skin, liver, ovaries, muscles, lungs, spleen, kidney, bone. Caused mutations and cancer.

Fukushima Daiichi

One of 15 largest reactors in world Half the plant was shut down ahead of tsunami for

maintainance; other half was automatically turned off Tsunami waves breached defenses and swamped backup

generators cooling the plant, resulting in explosions/partial meltdowns

Quick reaction so limited health risks; 156,000 people still displaced due to meltdown

LIFESPAN OF A REACTOR

After 15-40 years, a nuclear reactor becomes dangerously contaminated with radioactive material. They can be decommissioned or retired by:

1. Dismantling and storing large volumes HLRW in appropriate storage facilities (don’t exist).

2. Construct a physical barrier for security for 30-100 years before the plant is dismantled.

3. Enclose plant in a tomb that must last for thousands of years.

ADVANTAGES OF NUCLEAR POWER

No air pollutants emitted.

Land disturbance is low when no accidents are involved.

Construction and backup safety systems decrease the likelihood of a catastrophic event.

Chernobyl “only” caused the known premature deaths of 32,000 people; coal burning causes premature deaths of 65,000 – 200,000 people in the USA each year!

DISADVANTAGES OF NUCLEAR ENERGY

1. Always the danger of a meltdown

2. Waste disposal?

3. How do we effectively decommission the facilities after only 17 years of use?

4. Only 17% efficient

5. Extremely high costs associated with using “safe technology”.