Tuesday, April 1, 2014

The Truth About Nuclear Power - Part Five

Subtitle: You Cannot Simply Turn Off a Nuclear Power Plant

Fission-based commercial nuclear power plants are expensive for many reasons, but the main reason is due to the fact that nuclear heat is not like fossil heat; it produces deadly radiation, and one can’t just turn it off very easily.   The additional expense is a prudent, carefully-weighed and negotiated response to those basic facts. 
Palo Verde Nuclear Plant
source: NRC

Not much has changed in nuclear engineering since I first studied it in undergraduate school more than forty years ago.  Put simply, the uranium atom is so large, so unstable, that not only does it emit high-energy particles itself, it will easily split when it receives a neutron from an outside source.  The splitting of the atom is known as fission.  When fission occurs, several more neutrons are released.  The additional neutrons can be absorbed by other uranium atoms, causing them to undergo fission in what is termed a chain reaction.  This is, of course, greatly simplified.  Detailed discussions are available in many textbooks and on the internet.
The great benefit of fission is the tremendous amount of heat released, according to the famous Einstein equation E = M C^2.  (energy equals the mass times the speed of light, C, squared).    One of the great curses of nuclear fission is that heat must be removed as fast as it is produced, or many very bad things happen. 
In sharp contrast to gas-fired, oil-fired, or coal-fired power plants, which can be shut off easily and quickly, nuclear power plants cannot be easily shut off.  A nuclear reactor, once it has begun the fission process, continues to emit heat for a long time.  This residual heat must be removed to prevent the dreaded nuclear melt-down.  Nuclear industry proponents insist to this day that their plants are safe, they are designed with multiple safety systems, they are operated safely, they are routinely inspected and tested, and a melt-down will not happen.  Yet, we have seen clear evidence of reactor melt-downs; they have indeed happened.  The most infamous is probably the multiple-reactor melt-downs in Japan at the Fukushima complex.  But, one reactor in the US also suffered a partial melt-down, that one being the Three Mile Island plant.  (see link)   Back to the heat emitted and melt-downs, the safety systems are there to ensure the heat is removed.  
Perhaps it will be useful to go into some detail on how a nuclear reactor is designed and constructed, with a view toward why the multiple safety systems are required, and how this increases the construction cost.   A modern nuclear plant designed to meet the US Nuclear Regulatory Agency (NRC) requirements must have three levels of containment.  Containment is the word used to describe a physical barrier between the nuclear fuel pellets and the atmosphere.   The first containment is the fuel rod, a long metal cylinder that contains the nuclear pellets.  The pellets are short cylinders, roughly the size of the tip of a man’s index finger.  The metal walls of the fuel rod serve as the first containment.   The metal rods are made of an expensive alloy known as zircalloy.    From the NRC website, a fuel pellet is “[a] thimble-sized ceramic cylinder, 3/8 inches diameter by 5/8 inches long, consisting of uranium (typically uranium oxide, UO2), which has been enriched to increase the concentration of uranium-235 to fuel a nuclear reactor.  Modern reactor cores may contain 10 million pellets, stacked in the fuel rods that form fuel assemblies.”  see link  
The fuel rods are grouped in what is known as a fuel assembly, and multiple fuel assemblies are placed in the second containment, the reactor vessel.  The reactor vessel is a vertical metal cylinder with a bottom and top, made of thick alloy steel.   The reactor vessel is made of thick metal to withstand high temperatures and high pressures when the plant is operating.    The reactor vessel is very expensive, due to the wall thickness and the metal alloy.  There are also various pipes that must be connected to the reactor vessel, therefore the reactor wall has holes cut into it and short pieces of pipe and flanges welded onto it.  The fabrication of such a vessel is quite expensive.
Finally, the third containment is a reinforced-concrete room with a domed ceiling in which the reactor vessel and other plant equipment are placed.  The containment structure, as it is called, is “a gas-tight shell or other enclosure around a nuclear reactor to contain fission products that otherwise might be released to the atmosphere in the event of an accident.  Such enclosures are usually dome-shaped and made of steel-reinforced concrete.”  (NRC glossary)   The containment structure is also made of thick walls, floor, and ceiling.  Note the requirement that the containment structure be gas-proof.   The requirement for an enclosure, thick walls made of steel-reinforced concrete, and the gas-proof feature also makes the nuclear power plant very expensive.
Having now looked at the three levels of containment, fuel rods, reactor vessel, and containment structure, all made of expensive materials and by expensive methods, one can contrast this with a gas-fired power plant’s heat source.   In a steam plant, natural gas is piped to a boiler, where the gas is burned inside the boiler.  The boiler is typically a vertical, square or rectangular box with openings to allow air to flow into it, and an opening at the top to allow the gases formed from combustion to flow into the atmosphere.  That’s it.  There are no special alloy rods, no thick-walled alloy reactor, but there is a walled combustion chamber.   The combustion chamber, or boiler, has a compound wall that typically has an inner layer made of refractory to reflect and radiate heat back into the combustion chamber, a layer of insulation outside that, and a weather-resistant outer layer. 
A combustion turbine gas power plant is even simpler: the gas is piped into a small combustion chamber where it is mixed with compressed air and burned.  The hot combustion gases then flow through the blades of a power turbine, and from the exit of the power turbine, either into the atmosphere or into a heat-recovery steam generator. 
When one wants to shut down a gas-fired power plant, one simply shuts the valve that the natural gas flows through.  That’s it.  The fire goes out, the system begins to cool down.  Typically, water is pumped through the boiler tubes for some time to prevent the tubes from overheating, but this does not take long. 
To summarize to this point, the nuclear plant has millions of uranium fuel pellets stacked in individual rods, mainly to keep the pellets away from each other to prevent melt-downs.  The rods are enclosed in the reactor vessel that is very thick to withstand the temperature and pressure of the water that flows past the rods.  The flowing water removes the heat from the rods.  The containment structure is there “just in case” either of the first two containments fail and radioactive gases, steam, or liquids escape. 
It is the close proximity of millions of uranium fuel pellets that have split uranium atoms and produce various forms of radiation and heat that causes the next expensive safeguards.  If a nuclear reactor suffers a loss of reactor coolant, then bad things happen.  This situation is so important, the NRC has a special acronym for a Loss Of Coolant Accident, LOCA.  Much research has been conducted (see link) and thousands of pages have been written on LOCA.  The primary reactor coolant, as mentioned above, is flowing water that is pumped into the reactor and flows past the rods.  The rods are submerged in the water.  The hot water flows out of the reactor and is piped to one or more steam generators.  The steam generator is somewhat analogous to a gas-fired boiler’s steam separator drum.  However, the nuclear steam generator is far more expensive.  The steam generator is a vertical, U-tube heat exchanger with the hot water from the reactor on the tube side (flows through the U-tubes), and water and steam for the turbine on the shell side (flows past the U-tubes). 
More expense is required because a nuclear reactor of the pressurized water design as described, the modern design, has two separate water loops and separate pumping systems for each loop.  A gas-fired boiler has only one water loop and one set of pumps.  However, to be fair, an older design of nuclear reactors also has only one water loop, that is the boiling water reactor design.  But, modern designs use the pressurized water reactor with its two separate water loops.  The first water loop circulates water from the reactor to the steam generator, through a pump and back to the reactor.  The second water loop circulates water through the shell side of the steam generator where steam is produced, the steam flows through the turbine, through the condenser where it returns to water, then through a pump and back to the steam generator. 
The LOCA issue is so serious, the NRC also requires a reactor auxiliary cooling system, in case the primary cooling loop (the first water loop described above) fails.   The auxiliary cooling system requires more pumps, piping, valves, and a means to remove the reactor heat. 
Still more expense is required for the water pressurizer, the spent and fresh fuel storage areas, the spent fuel cooling system, and others.   None of these systems are required for a gas-fired power plant.
One reason nuclear power plants are so expensive is the nature of the fuel, uranium undergoing fission, can and does create deadly materials that must be contained within the reactor system.  Secondly, the plant cannot be shut down quickly.  The heat generated by the nuclear pellets is so intense that multiple provisions must be made to keep the fuel rods cool, keep the pellets from melting the rods and causing a melt-down.  The concern over a failure of the primary coolant system, LOCA, requires an expensive, redundant auxiliary cooling system.  The entire system could fail, which requires the expensive containment structure.   The inability to simply stop the heat, as by shutting a valve on the gas line in a gas-fired plant, requires that cooling systems continue to operate after the reactor is nominally shut down.  Residual heat continues to be generated by the fuel pellets.  A cooling system is required for the spent fuel storage area also. 
Additional reasons for the high cost of nuclear power plants will be discussed in future articles.  Such reasons include over-sized equipment, economies of scale, large commercial aircraft collision, and the high water consumption due to the nature of a nuclear reactor system. 
Previous articles in the Truth About Nuclear Power series are found at the following links.  Additional articles will be linked as they are published. 
Part Four – Nuclear Power Plants Use Far More Fresh Water

Roger E. Sowell, Esq.
Marina del Rey, California 

No comments: