Neutrons, Electrons, Electric Current, Nuclear Power, Nuclear Chain Reaction, Residual Decay Heat and Pressurized Water Reactor

Posted by PITHOCRATES - July 18th, 2012

Technology 101

We create about Half of our Electric Power by Burning Coal to Boil Water into Steam

An atom consists of a nucleus made up of protons and neutrons.  And electrons orbiting around the nucleus.  Protons have a positive charge.  Electrons have a negative charge.  Neutrons have a neutral charge.  In chemistry and electricity the electrons are key.  When different atoms come together they form chemical bonds.  By sharing some of those electrons orbiting their nuclei.  In metals free electrons roam around the metal lattice of the crystalline solid they’re in.  If we apply a voltage across this metal these free electrons begin to flow.  Creating an electric current.  The greater the voltage the greater the current.  And the greater the work it can do.  It can power a television set.  Keep your food from spoiling in a refrigerator.  Even make your summers comfortable by running your air conditioner. 

We use electric power to do work for us.  Power is the product of voltage and current.  The higher each is the more work this power can do for us.  In a direct current (DC) system the free electrons have to make a complete path from the power source (an electric generator) through the wiring to the work load and back again to the power source.  But generating the power at the voltage of the workload required high currents.  Thick wires.  And a lot of power plants because you could only make wires so thick before they were too heavy to work with.  Alternating current (AC) solved this problem.  By using transformers at each end of the distribution path to step up and then step down the voltage.  Allowing us to transmit lower currents at higher voltages which required thinner wires.  And AC didn’t need to return to the power plant.  It was more like a steam locomotive that converted the back and forth motion of the steam engine into rotational power.  AC power plants generated a back and forth current in the wires.  And electrical loads are able to take this back and forth motion and convert it into useful electrical power.

Even though AC power allows us to transmit lower currents we still need to move a lot of these free electrons.  And we do this with massive electric generators.  Where another power source spins these generators.  This generator spins an electric field through another set of windings to induce an electrical current.  Sort of how transformers work.  This electrical current goes out to the switchyard.  And on to our homes.  Simple, really.  The difficult part is creating that rotational motion to spin the generator.  We create about half of our electric power by burning coal to boil water into steam.  This steam expands against the vanes of a steam turbine causing it to spin.  But that’s not the only heat engine we use to make steam.

To Shut Down a Nuclear Reactor takes the Full Insertion of the Control Rods and Continuously Pumping Cooling Water through the Core

We use another part of the atom to generate heat.  Which boils water into steam.  That we use to spin a steam turbine.  The neutron.  Nuclear power plants use uranium for fuel.  It is the heaviest naturally occurring element.  The density of its nucleus determines an element’s weight.  The more protons and neutrons in it the heavier it is.  Without getting into too much physics we basically get heat when we bombard these heavy nuclei with neutrons.  When a nucleus splits apart it throws off a few spare neutrons which can split other nuclei.  And so on.  Creating a nuclear chain reaction.  It’s the actual splitting of these nuclei that generates heat.  And from there it’s just boiling water into steam to spin a steam turbine coupled to a generator.

Continuous atom splitting creates a lot of heat.  So much heat that it can melt down the core.  Which would be a bad thing.  So we move an array of neutron absorbers into and out of the core to control this chain reaction.  So in the core of a nuclear reactor we have uranium fuel pellets loaded into vertical fuel rods.  There are spaces in between these fuel rods for control rods (made out of carbon or boron) to move in and out of the core.  When we fully insert the control rods they will shut down the nuclear chain reaction by absorbing those free neutrons.  However there is a lot of residual heat (i.e., decay heat) that can cause the core to melt if we don’t remove it with continuous cooling water pumped through the core. 

So to shut down a nuclear reactor it takes both the full insertion of the control rods.  And continuously pumping cooling water through the core for days after shutting down the reactor.  Even spent fuel rods have to spend a decade or two in a spent fuel pool.  To dissipate this residual decay heat.  (This residual decay heat caused the trouble at Fukushima in Japan after their earthquake/tsunami.  The reactor survived the earthquake.  But the tsunami submerged the electrical gear that powered the cooling pumps.  Preventing them from cooling the core to remove this residual decay heat.  Leading to the partial core meltdowns.)

Nuclear Power is one of the most Reliable and Cleanest Sources of Power that leaves no Carbon Footprint

There is more than one nuclear reactor design.  But more than half in the U.S. are the Pressurized Water Reactor (PWR) type.  It’s also the kind they had at Three Mile Island.  Which saw America’s worst nuclear accident.  The PWR is the classic nuclear power plant that all people fear.  The tall hyperboloid cooling towers.  And the short cylindrical containment buildings with a dome on top housing the reactor.  The reactor itself is inside a humongous steel pressure vessel.  For pressure is key in a PWR.  The cooling water of the reactor is under very high pressure.  Keeping the water from boiling even though it reaches temperatures as high as 600 degrees Fahrenheit (water boils into steam at 212 degrees Fahrenheit under normal atmospheric pressure).  This is the primary loop.

The superheated water in the primary loop then flows through a heat exchanger.  Where it heats water in another loop of pumped water.  The secondary loop.  The hot water in the primary loop boils the water in the secondary loop into steam.  As it boils the water in the secondary loop it loses some of its own heat.  So it can return to the reactor core to remove more of its heat.  To prevent it from overheating.  The steam in the secondary loop drives the steam turbine.  The steam then flows from the turbine to a condenser and changes back into water.  The cooling water for the condenser is what goes to the cooling tower.  Making those scary looking cooling towers the least dangerous part of the power plant.

The PWR is one of the safest nuclear reactors.  The primary cooling loop is the only loop exposed to radiation.  The problem at Three Mile Island resulted from a stuck pressure relief valve.  That opened to vent high pressure during an event that caused the control rods to drop in and shut down the nuclear chain reaction.  So while they stopped the chain reaction the residual decay heat continued to cook the core.  But there was no feedback from the valve to the control room showing that it was still open after everyone thought it was closed.  So as cooling water entered the core it just boiled away.  Uncovering the core.  And causing part of it to melt.  Other problems with valves and gages did not identify this problem.  As some of the fuel melted it reacted with the steam producing hydrogen gas.  Fearing an explosion they vented some of this radioactive gas into the atmosphere.  But not much.  But it was enough to effectively shut down the U.S. nuclear power industry. 

A pity, really.  For if we had pursued nuclear power these past decades we may have found ways to make it safer.  Neither wind power nor solar power is a practical substitution for fossil-fuel generated electricity.  Yet we pour billions into these industries in hopes that we can advance them to a point when they can be more than a novelty.  But we have turned away from one of the most reliable and cleanest sources of power (when things work properly).  Using neutrons to move electrons.  Taking complete control of the atom to our make our lives better.  And to keep our environment clean.  And cool.  For there is no carbon footprint with nuclear power.


Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

Silicon, Semiconductor, Electrons, Holes, PN Junction, Diode, LED, Photon, 7-Segment LED and Full-Color Flat Panel LED Displays

Posted by PITHOCRATES - May 30th, 2012

Technology 101

Applying a Voltage across a PN junction to Create a Forward Bias Pushes Electrons and Holes towards the Junction

There’s gold in them thar Hills.  And silicon in the valley.  California has been a fountain of wealth.  Much of which they built from two materials located on the periodic table.  Atomic number 79.  Gold.  Or ‘Au’ as it appears on the periodic table.  And atomic number 14.  Silicon.  Or ‘Si’ as it appears on the periodic table.  Both of these metals proved to be valuable.  One by its scarcity.  One by what we could do with it.  For it was anything but scarce.  Silicon is the second most common element behind only oxygen.  But this commonly found material proved to be a greater font of wealth for California.  For it fueled the semiconductor industry.  For when we doped it with impurities we produced negatively (N-type) and positively (P-type) charged material.  Bringing the N and the P together gave us the PN junction.  Giving us the diode, transistor and integrated circuit.

The miracle of semiconductors occurs at the atomic level.  Down to the electrons orbiting the atom’s nucleus.  The nucleus contains an equal number of positively charged protons and neutrally charged neutrons.  The number of protons gives us the atomic number.  Changing the number of neutrons gives us isotopes.  Radioactive material has more protons than neutrons.  Uranium-235 is an isotope.  The stuff that made the atomic bomb dropped on Hiroshima.  Electrons orbit the nucleus.  In discrete energy levels.  The orbits closest to the nucleus have the lowest energy levels.  The orbits father away from the nucleus have higher energy levels.  Most of these orbits are ‘full’ of electrons.  The outer electron shell when ‘full’ is inert.  An outer shell that isn’t ‘full’ or has extra electrons is active.  And can chemically react.  Forming molecules.  When chemicals come into contact with each other and form molecules it is these electrons in the outer orbits (or valence electrons) that move into and out of the orbits of the different chemicals.  That is, the different elements share these valence electrons.

This is what we do when we dope silicon with impurities.  We either remove electrons from the valence shell to create a net positive charge.  Or we add electrons to the valence shell to create a net negative charge.  Giving us P-type and N-type material.  At the PN junction the N-type material loses its excess electrons to the P-type material across the junction as the empty holes in the valence shell attract the excess electrons.  As electrons leave the valence shells in the N-type material they leave holes in the valence shell where they once were.  Or, in the world of electronics, as electrons flow one way holes flow the other.  When we apply a voltage across a PN junction to create a forward bias (negative voltage applied to N-type and positive voltage applied to P-type) we push electrons and holes towards the junction.  If the forward bias is great enough they will continue all the way through the junction and into the material on the far side.  Where electrons will combine with excess holes.  And holes will combine with excess electrons.  Creating an electric current.  If we apply a voltage to create a reverse bias we will pull electrons and holes away from the PN junction.  And there will be no electrical current. We call such a PN device a diode.  A very important and indispensible device in electronics.

Placing Seven LEDs into a Figure-Eight Pattern created the Seven-Segment LED

Now back to those discrete energy levels.  There is another useful property we get when electrons move between these energy levels.  Electrons absorb energy when they move to a higher energy level.  And emit energy when they move to a lower energy level.  We make use of this property in fluorescent lighting.  A charged plasma field in a fluorescent lamp excites a small amount of mercury in the lamp.  As electrons fall into lower orbits in the mercury atoms they release invisible short-wave ultraviolet radiation.  The phosphor coating on the inside of the lamp absorbs this radiation and fluoresces.  Creating visible light.  By using different materials, though, we could see the energy (a photon) emitted by an electron falling into a lower energy level.  We have been able to move the wavelength of this photon into the visible spectrum.  The first commercial application to convert these photons into visible light was a device that gave us a red light.  That device was that important and indispensible PN-junction.  The diode.  And the use of different materials other than silicon moved these photons into the visible spectrum.  Giving us the light-emitting diode.  Or LED.

The first LEDs were only red.  Then we developed other colors using different materials.  Shifting the wavelength of the photon through all colors of the visible spectrum.  Being low-power devices, though, the intensity of light emitted was limited.  So an LED required careful mechanical construction and optics.  To direct the light out of the material forming the PN junction.  With a reflector behind the junction.  And a lens above.  To aim and diffuse the light.  And to prevent it from reflecting back into the material where it may be dissipated as heat.  Early use of LEDs was for indicator lights.  The low power consumption meant little heat was generated as with an incandescent lamp.  Which worked well in the temperature sensitive computer world.  Placing 7 LEDs into a figure-eight pattern created the seven-segment LED display.  With a rectangular shaped piece of translucent plastic above each LED you could see a bar of light for each light emitting diode.  Creating a forward bias on certain bars in the seven-segment display created the numbers we saw on our first calculators and digital watches.

An LED could produce a similar radiation like in the fluorescent lamp.  Using that radiation to fluoresce a phosphor coating inside a lamp to produce white light.  Similar to the fluorescence lamp.  Only while using less power.  Mixing the emitted light from red, green and blue (RGB) LEDs also produced white light.  Further improvements allowed us to emit whiter and brighter lights.  Allowing brighter lamps that consumed less power than the compact fluorescent lamps which were energy saving alternatives to the incandescent lamps.  With the lower power consumption of LEDs creating less heat we expanded the lifespan of lighting sources made from LEDs.  Using them to increase the service life in lamps inconvenient to change.  Like in traffic signal lights over busy intersections.  To the taillights in tractor trailers.  Where anytime spent not hauling freight was lost revenue.

We made Full-Color Flat Panel Displays from LEDs by combining Red, Green and Blue LEDs into Full-Color Pixel Clusters

The market didn’t demand these developments in semiconductors or LEDs.  For the most part the market didn’t even know this technology existed.  But the entrepreneurs gathering in Silicon Valley did.  They had some great ideas of what they could do with this new technology.  All they needed was the capital to bring these ideas to market.  It was risky.  The technology was good.  But could they use it to make useful things at affordable prices?  And would the people be so enamored with the things they built that they would buy them?  There were just too many unknowns for conservative bankers to take a risk.  But thanks to venture capitalists those entrepreneurs got the capital they needed.  Brought their ideas to market.  Created Silicon Valley.  And the modern world we now take for granted.

They continue to advance this technology.  Creating full-color flat panel displays.  By combining red, green and blue LEDs into full-color pixel clusters.  Which, unlike an LCD flat panel display, does not need a backlight as they produce their own light.  So these panels are thinner and use less power than LCD displays.  Making them ideal for the displays in our cellular devices for they allow more battery life between charges.  They also have wide viewing angles.  People looking at these displays from near perpendicular viewing angles see nearly the same quality of picture as those viewing directly in front.  Making them ideal for use in stadiums.  The video replays you see on that mammoth flat panel display in the new Dallas Cowboy stadium is an LED flat panel display.

All of this from joining two differently-charged semiconductor materials together.  Creating that all important and indispensible PN junction.  The foundation for every electronic device.  And of Silicon Valley itself.


Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,