Nuclear energy

Review: Elements and Isotopes Elements are defined by the number of protons in the nuclei of their atoms – Eg all carbon atoms have 6 protons

Remember, isotopes are variations of atoms of an element

They vary in number of neutrons– Eg Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons

– Many elements have isotopes some are stable (never change), some are radioactive (unstable, change over time)

Radioisotopes

Radioactive isotopes experience radioactive decay (the loss of alpha or beta particles over time)

As a result of radioactive decay, atoms of one element physically change into another element.– Eg Carbon-14 decays to Nitrogen-14 by loss of negative beta particles

Radioactive half life= the amount of time it takes for 50% of the radioactive isotope in a substance to decay.

Practice:

Plutonium-239 has a half-life of 24,000 years. How much of a 4 gram sample will remain after 96,000 years?

a. 1gb. 0.5gc. 0.25gd. 0.125ge. 0.625g

Geological dating with radioactive isotopes Carbon-14 can be used to estimate the age of plant and animal remains

All living things utilize C-14 and incorporate it into their tissues– After death, C-14 changes into N-14

– Geologists can determine the age of a set of remains by comparing the ratio of C-14 to N-14

– Carbon dating is useful for remains between 1,000 and 50,000 years old

Geological dating with Uranium Uranium-238 is a very common radioisotope that decays to a stable isotope of lead

It has a half life of 4.5 billion years

This is very useful for dating rock formations that are billions of years old– Eg if there are equal parts lead and uranium in a rock, it is 4.5 billion years old

The discovery of radioactive atoms 1896 a French physicist discovered

that uranium containing minerals spontaneously & continuously gave off energy (radiation)

1898 a British physicist showed radiation to consist of high energy particles

In 1919 same British guy bombarded N nuclei with alpha particles and turned it into O

In 1938 German scientists hit uranium with neutrons, splitting U into barium and krypton, and lots of energy…FISSION!

This led to a realization of the potential power of fission and a subsequent race for a bomb and energy development (Einstein came to US during WWII to warn of impending German innovation)

Nuclear energy Nuclear reactions differ from combustion reactions

Combustion (fossil fuels): – atoms do not change, they are rearranged. – The mass of the reactants is equal to the mass of the products.

– Energy is given off as heat when chemical bonds are broken.

Nuclear reaction:– Changes occur within the nuclei of atoms.– Atoms actually transform into atoms of another element.

– Small amounts of matter are transformed into large amounts of energy.

Types of nuclear reactions Fission: larger atoms are split

into 2 smaller atoms of different elements (this is the type of reaction used to create commercial energy and atomic bombs)

Fusion: 2 smaller atoms combine to make one larger atom of a different element (this is what powers the sun and stars)

In both reactions the end product mass is less than the mass of the starting material. The remainder is converted to energy.

Fission produces 2-3 million times more energy than combustion of fossil fuels.

NUCLEAR ENERGYNuclear power plants use U-235, a radioactive isotope of uranium.– Mined uranium oxide consists of about 99.3% non-fissionable uranium-238 and 0.7% fissionable uranium-235.

– The concentration of uranium-235 is increased through an enrichment process (removing some of the U-238) to result in 97% U-238 oxide and 3% U-235 oxide fuel.

– Enrichment is very energy intensive, but the energy payoff is even greater

Electricity After enrichment, U-235 is transformed into uranium dioxide to form small fuel pellets

These pellets are placed into fuel rods, which in turn are grouped into fuel assemblies (~100 rods), of which there may be 1000s per reactor core

Nuclear power plant

NUCLEAR WASTE

NUCLEAR WASTE

Math Practice1. After 100 million years, only 1/32 of the

original amount of a particular radioactive waste will remain. The half-life of this radioactive waste is how many million years?

a. 10 b. 20 c. 30 d. 40 e. 50

2. You have 180g of a radioactive substance. It has a half-life of 265 yrs. After 1,325 yrs, what mass remains?

Nuclear waste Low level

– Radioactive solids, liquids, or gases that give off small amounts of ionizing radiation

– Sources include power plants, hospitals, research labs, and industries

– Low Level Radioactive Waste Policy Act 1980 & 1985

All states must be responsible for disposal of non-defense related waste produced w/in their borders.

High level– Radioactive solids, liquids, or gases that initially give off large amounts of ionizing radiation

– Sources include anything that was inside the reactor core (metals, water, gases, spent fuel)

Nuclear Waste Policy Act 1982 Stated that there must be a permanent site for storing high level waste by 1998– That was not met; postponed to 2010 at earliest

1987 Congress identified Yucca Mountain in Nevada as the best potential site

Feasibility studies were carried out for over a decade

In 2002 it was officially approved by Congress

Rescinded by Obama in 2009

NUCLEAR ENERGYScientists disagree about the best methods for long-term storage of high-level radioactive waste:– Bury it deep underground.– Shoot it into space.– Bury it in the Antarctic ice sheet.– Bury it in the deep-ocean floor that is geologically stable.

– Change it into harmless or less harmful isotopes.

The risks of nuclear energy Meltdown: this is when the actual metal around the reactor core melts from the heat of fission; radiation is emitted into the atmosphere in one large dose

Acute radiation syndrome= too many body cells are killed by the radiation dose to be repaired.

Daily radiation for workers (carcinogenic over time)

Radiation into groundwater from stored waste

Small scale persistent radiation to nearby communities

Radiation and health We are exposed to natural (background radiation) and artificial radiation every day– 300 millirems per year from space/the atmosphere, the soil (radon), foods we eat (radioactive potassium)

– 60 millirems from manmade radiation (radiowaves, hospitals, industries, housing materials, microwaves, cell phones, tobacco, television, smoke detectors, etc.)

Radiation is often ionizing, which is very disruptive to living cells

Chronic exposure to radiation can lead to cancer and thyroid problems

NUCLEAR ENERGY

A 1,000 megawatt nuclear plant is refueled once a year, whereas a coal plant requires 80 rail cars a day.

Figure 16-20Figure 16-20

NUCLEAR ENERGY

Building more nuclear power plants will not lessen dependence on imported oil and will not reduce CO2 emissions as much as other alternatives.– The nuclear fuel cycle contributes to CO2 emissions.

– Wind turbines, solar cells, geothermal energy, and hydrogen contributes much less to CO2 emissions.

NUCLEAR ENERGY

In 1995, the World Bank said nuclear power is too costly and risky.

In 2006, it was found that several U.S. reactors were leaking radioactive tritium into groundwater.

Figure 16-19Figure 16-19