State of the IndustryThe reality around the globe is that nuclear energy is here to stay not necessarily because we’re convinced it is the best option, that it does not carry with it short- and (very) long-term consequences or it is easy, but because it is the most sustainable energy source capable of meeting near-term energy demands (Fig. 1). In most countries concerns over safety, pollution in the form of waste and proliferation continue to create controversy as to how and when to proceed.
In March 2007, for the first time in more than three decades, the U.S. Nuclear Regulatory Commission granted an Early Site Permit (ESP) for a new nuclear power plant. The ESP is the first stage toward a new plant and allows Exelon Generation Company to start construction work in Clinton, Ill., to determine if the site is suitable for a nuclear power plant. This is the first permit granted under a new licensing process that was established in 1989 but had not previously been used. The initial application was filed in September 2003. Decisions on two other plant applications are expected in early 2010 with another pending application still in the early stages.
In all, over 100 power reactors with a total net capacity of almost 120,000 MW are planned and over 250 more are proposed. Rising energy prices and environmental (greenhouse) constraints have combined to put nuclear power back on the agenda for projected new capacity in both Europe and North America with Asian capacity growing rapidly to keep pace with their fast growing energy demands.
Brief Overview of Nuclear PowerTo provide the power for an electric generator, nuclear power plants rely on the process of nuclear fission. In this process, the nucleus of a heavy element, such as uranium, splits when bombarded by a free neutron in a nuclear reactor. The fission process for uranium atoms yields two smaller atoms, one to three free neutrons and an amount of energy (Fig. 5). Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self-sustaining, creating the familiar chain reaction under controlled conditions and yielding a tremendous amount of energy.
In the vast majority of the world's nuclear power plants, heat energy generated by burning uranium fuel is collected in ordinary water and is carried away from the reactor's core either as steam in boiling-water reactors (BWR) or as superheated water in pressurized-water reactors (PWR).
Boiling-water and pressurized-water reactors are so-called light-water reactors because they utilize ordinary water to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs, pressurized heavy water, gas or other cooling media transfers the heat energy.
In a typical commercial boiling-water reactor (Fig. 6) the sequence of events are: (1) the reactor core creates heat; (2) a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat; (3) the steam-water mixture leaves the top of the core and enters two stages of moisture separation, where water droplets are removed before the steam is allowed to enter the steam line; and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to a condenser where it is returns to liquid form. The resulting water is pumped out of the condenser with a series of pumps, reheated, and pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are cooled by water, which is force circulated by electrically powered pumps. Emergency cooling water is supplied by other pumps that can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power.
In a typical commercial pressurized-water reactor (Fig. 7) the sequence of events are: (1) the reactor core generates heat; (2) pressurized-water in the primary coolant loop carries the heat to the steam generator; (3) inside the steam generator, heat from the primary coolant loop vaporizes the water in a secondary loop, producing steam; and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to the condenser where it returns to liquid form. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the steam generator. The reactor’s core contains fuel assemblies that are cooled by water, which is force-circulated by electrically powered pumps. Emergency and other safety systems are similar to those in a boiling-water reactor.
This design is also called the CANDU reactor (Fig. 8) because it originated in Canada. It is very similar to the PWR with respect to the pressure system. However, the main difference is that these reactors use natural uranium as the fuel. There is, therefore, not a lot of fissionable U235 in the fuel, so a moderator is needed to control the speed of the neutron fission products. As many neutrons as possible are required to bombard the limited U235 in order to continue the fission chain reaction. Light water, though it slows down neutrons efficiently, also has a tendency to absorb the neutrons. These reactors therefore use heavy water (deuterium oxide, D2O or 2H2O) as the moderator. Deuterium oxide is less likely to absorb neutrons because deuterium already has twice the amount of neutrons as the hydrogen in light water. However, a large quantity of heavy water is needed to have an influence on the neutron speed. Another disadvantage of heavy water is that if it absorbs neutrons it produces the radioactive isotope of hydrogen called tritium. Though heavy water is expensive, costs are cut down by use of natural uranium. Furthermore, the PHWR can be refueled while in operation.
Gas-cooled reactors are of European origin (Fig. 9). The coolant is carbon dioxide (CO2), and the moderator is graphite. Carbon dioxide is an effective coolant because it can be heated to high temperatures and can thus maximize heat transfer to the steam-generation process. Also, because it is gaseous, CO2 will not absorb the neutrons that are involved in the fission chain reaction. Gases are poor moderators, however, so graphite is needed because it is inexpensive, readily available and not damaged by high temperatures. The thermal efficiency of these reactors is very high but the fuel efficiency is low.
The Light Water Graphite Reactor is a Soviet invention (Fig. 10). It was uniquely designed to generate power and produce plutonium. The coolant is light water and the moderator is graphite. This coolant/moderator combination is unique to the LWGR. The light water vaporizes as it passes through the reactor core. This steam is used to drive the turbines. A major advantage of the LWGR is that it can be refueled while in operation.
Valves (Pressure, Steam, Safety)The main steam system used in any power plant provides steam from the source (reactor) to the turbine. The system normally has several other functions:
- To provide the ability to prevent over-pressurization of the steam source (if the source puts out more heat than the turbine can accept)
- To provide the ability to prevent overcooling of the reactor coolant system (if the steam system draws off more heat than the source can provide)
- Reactor steam line (BWR), steam generator (PWR) or Turbine Steam Separator (GCR).
- Main steam isolation valve – usually an air-operated or motor-operated valve used to isolate the steam source from the turbine.
- Safety valves – large relief valves that will open if steam pressure gets too high (same purpose as the pop valve on your hot water heater).
- Power operated relief valves – large air or motor operated valves that usually lift at a setpoint lower than the safety valves – in order to keep the steam pressure from getting too high.
- Non-return valve – a large valve that prevents backward steam flow in the steam line
- Pressurizer safety relief (primary)
- Main steam safety relief (powered)
- Main steam safety relief (manual)
- Main steam isolation
- Main feedwater isolation
- Heater drain flow control
- Damped feedwater reverse flow check
- Feedwater regulation
- Aux-feedwater startup flow control
- Steam driven aux-feed pump flow control
- Chemical-injection flow control
- Main steam isolation valve
- Main steam safety relief
- Damped feedwater reverse flow check
- Residual heat removal low cooling control
- Reactor core isolation cooling control
- Reactor level flow control
- Turbine bypass flow control
- Feedwater pump recirculation control
- Main feedwater regulation flow control
- Feedwater water startup flow control
- Low-pressure core spray flow control
- High-pressure core injection flow control
- Reactor-water cleanup flow control
- Heater drain flow control