Fundamentals of Nuclear Power

A slow moving neutron triggers fission in Uranium 235

Expanding Chain Reaction

• A fission reaction produces more neutrons which can then triggers fission in other Uranium atoms.

Linear Chain ReactionFor controlled nuclear power, once we reach our

desired power level we want each fission reaction to trigger exactly one additional fission reaction

• When a slow moving neutron hits a U-235 atom, the U-235 atom splits into two smaller atoms and multiple fast moving neutrons.

• The neutrons must be slowed down in order to cause other uranium atoms to split.

Moderator• Neutrons are

slowed down by passing through the moderator. In most power plants, water is used as a moderator.

http://www.euronuclear.org/info/encyclopedia/images/moderator.jpg

Control Rods

• Control rods are made of a material that absorbs excess neutrons (usually Boron or Cadmium).

• Control rods can be raised or lowered to control the rate of the reaction.

Basic Ideas

-Uranium is both the fuel and the source of neutrons.

-Neutrons trigger fission

-Water acts as both the moderator and a coolant and transfer heat to the nonnuclear side of the plant.

-Control rods absorb excess neutrons which controls the rate of fission

• Each ton of Uranium ore produces 3-5 lbs of Uranium compounds

• Uranium ore is processed near the mine to produce “yellow cake”, a material rich in U3O8.

• Only 0.7% of U in yellow cake is 235U. Most of the rest is 238U which does not work for fission power.

US Uranium Deposits

World Distribution of Uranium

Enrichment• To be used in US

reactors, fuel must be 3-5% 235U.

• Yellow cake is converted into UF6 and this compound is enriched using gaseous diffusion and/or centrifuges.

• There are some reactor designs that run on pure yellow cake.

• NOTE: A nuclear bomb requires nearly 100% pure 235U or 239Pu. The 3% found in reactor grade Uranium CANNOT create a nuclear explosion!

Fuel Pellets• The enriched UF6 is

converted into UO2 which is then made into fuel pellets.

• The fuel pellets are collected into long tubes. (~12ft).

• The fuel rods are collected into bundles (~200 rods per bundle

• ~175 bundles in the core

• The material that the fuel rods are made out of is called cladding.

• It must be permeable to neutrons and be able to withstand high heats.

• Typically cladding is made of stainless steel or a zirconium alloy

Reactor is inside a large containment building

Other Options

• Other countries use different reactor designs.

• Some use heavy water (D2O) as a moderator. Some use Graphite as a moderator.

• Some are designed to use pure yellow cake without further enrichment

• Liquid metal such as sodium or gasses such as Helium are possibilities to use for coolants

Breeder Reactors

• A big problem with nuclear power is the creation of Plutonium in the reactor core.

• This is a long lived radioactive element that is difficult to store.

• Q: Why not use it as a fuel too?

Basic Idea

• Process that creates the Pu.• During fission use one of the extra neutrons to

create a Pu atom

• Somewhat difficult in that we want fast neutrons to “breed” the 239Pu out of the 238U, but we want slow neutrons to induce the fission of 235U.

• Requires a different design of reactor.

• There are no currently operating breeder reactors in the US.

-61 commercially operating nuclear power plants with 99 nuclear reactors in 30 states in the United States. -35 of these plants have two or more reactors. -4 reactors were taken out of service in 2013: the Crystal River plant in Florida with one reactor in February; the Kewaunee plant in Wisconsin with one reactor in April; and the San Onofre plant in California with two reactors in June.  - Vermont Yankee plant in Vermont, with a single reactor, was taken out of service in December 2015.

http://www.eia.gov/tools/faqs/faq.cfm?id=207&t=3

-4 new reactors were approved by the NRC in the US in 2012…the first new nuclear reactors approved in over 30 years

-6 new units may come on line by 2020

-Lower natural gas prices may jeopardize the economic viability of some existing reactors and proposed projects

-some older plants may be decomissioned

US Nuclear Power Plants 2012

http://www.eia.gov/tools/faqs/faq.cfm?id=207&t=3

World Nuclear Power

Advantages of Nuclear Energy-Does not emit greenhouse gases

-Does not emit other pollutants

-offers a stable and reliable source of electricity that is not

weather dependent

-high energy density

Disadvantages of Nuclear Energy

• Accidents can and do happen and the effects may last for centuries

• Radioactive waste must be stored for thousands of years and the US still has no permanent waste disposal site

• High cost of constructing plants compared with fossil fuel power plants

• Decommissioning costs are high

The Yankee Nuclear Power Station in Rowe, Massachusetts, took 15 years to decommission—or five times longer than was needed to build it. And decommissioning the plant—constructed early in the 1960s for $39 million—cost $608 million. The plant’s spent fuel rods are still stored in a facility on-site, because there is no permanent disposal repository to put them in. To monitor them and make sure the material does not fall into the hands of terrorists or spill into the nearby river costs $8 million per year.

Bulletin of Atomic Scientists 4/28/2014 Rising Cost of Decommissioning a Nuclear Power Plant

Major Nuclear Accidents

• Three Mile Island, Pennsylvania

• Chernobyl, Ukraine

• Fukashima, Japan