Nuclear Engineer

(7) Subchannel: For a PWR, BWR or LMFBR, the smallest flow area which is bounded by fuel
rods or a combination of fuel rods and a fuel assembly can or housing is called a subchannel. Several subchannels make up the flow area for a single fuel assembly.

(8) Structure: The geometry and integrity of the reactor core is maintained by structural elements such as support plates, spacer grids, or the metallic tubes used to clad the fuel in some reactor designs. The structural materials may also serve a dual role by moderating neutrons such as the graphite in an HTGR.

(9) Control elements: Absorbing material inserted into the reactor to control core multiplication. Although most commonly regarded as movable rods of absorber (control rods or blades), control elements may also consist of fixed absorbers or absorbing materials dissolved in the coolant. Common absorbing materials include boron, cadmium, gadolinium, and hafnium.

(10) Reactor core: The total array of fuel, moderator, and control elements.

(11) Reactor blanket: In a breeder or high conversion reactor the core is usually surrounded by a blanket of fertile material that more effectively utilizes the neutrons leaking out of the core.
For PWRs and BWRs, fuel blanket material may exist at the top and the bottom of the fuel rods within the fuel assembly.

(12) Reflector: A material characterized by a low absorption cross section used to surround the core in order to reflect or scatter leaking neutrons back into the core.

(13) Shielding: The reactor is an intense source of radiation. Not only must operating personnel and-the public be shielded from this radiation, but reactor components must as well be
protected. Hence absorbing material is introduced to attenuate neutron and gamma radiation. Radiation shielding is used to attenuate the emergent core radiation to levels that do not result in significant damage in reactor components. Cooling is usually required for such shields. Biological shielding reduces the radiation still further to acceptable levels for operating personnel.

(14) Support structure: The support plates that serve to maintain the core geometry.

(15) Reactor pressure vessel: The high pressure containment for reactor and associated primary coolant system.

(1) Fuel: Any fissionable material. This can be either fissile material such as 233U, 235U, 239Pu, or 24lPu or fissionable material such as 232Th, 238U, or 240Pu. Most modern power reactors utilize this fuel in a ceramic form either as an oxide such as UO2, carbide such as UC, or a nitride, UN.

(2) Fuel element: The smallest sealed unit of fuel. In an LWR or LMFBR the fuel element is a metal tube containing ceramic pellets of fuel (such as UO2). In an HTGR the fuel element can be regarded as either a tiny (300 μm diameter) particle of uranium carbide coated with pyrolytic graphite layers, or as a cylindrical fuel pin composed of these fuel particles bound together with a graphite binder.

(3) Fuel assembly or bundle: The smallest unit combining fuel elements into an assembly. For example, in a LWR the fuel assembly is composed of several hundred fuel elements fastened together at top and bottom with coolant nozzle plates and with several spring clip assemblies along the length of the fuel. In an HTGR the fuel assembly is a hexagonal block of graphite with holes into which the cylindrical fuel pins are inserted. Fuel is usually loaded into a reactor core or replaced one fuel assembly at a time. A typical power reactor core will contain hundreds of such fuel assemblies.

(4) Moderator: Material of low mass number which is inserted into the reactor to slow down or moderate neutrons via scattering collisions. Typical moderators include light water, heavy water, graphite, and beryllium.

(5) Coolant: A fluid, which circulates through the reactor removing fission heat. The coolant can be either liquid, such as water or sodium, or gaseous, such as helium or carbon dioxide. It may also serve a dual role as both coolant and moderator, such as in the LWR.

(6) Coolant channel: One of the many channels through which coolant flows in the fuel lattice. This may be an actual cylindrical channel in the fuel assembly, as in the HTGR, or an equivalent channel associated with a single fuel rod, as in a LWR.

To benefit mankind without imposing unacceptable burdens, new sources of energy need to provide a number of attributes. The best would provide all of the following. They should be clean, which means non-emitting with compact wastes that can be easily and inexpensively controlled. They should be affordable, especially for the impoverished, but they should not be cheap. As Californians demonstrated during the recent electricity crisis and blackouts, economics is the best incentive for conservation. New sources of electricity must also be reliable. No one wants the power to fail while their children or grandchildren are on the operating table, and electricity disruptions have crippled California’s, and probably the nation’s, economy in the past two years. Energy has in the past become more and more environmental (not the same as clean), and it must continue to do so.
In addition to these attributes, massive new sources of energy must also be safe, secure, and sustainable. The initials of these attributes—clean, affordable, reliable, environmental, safe and secure, and sustainable—make up the acronym CARESS. It is obvious from a review of recent global activities and proclamations that the world is now CARESSing nuclear energy as a top choice to meet global demand for new sources of electricity.

The design and operation of a large-nuclear power plant is an enormously complex task and involves the coordination of a remarkably diverse range of disciplines. Each major component of the plant has a separate and distinct design basis and is usually the responsibility of a specific engineering operational team. For example, the design of the reactor pressure vessel or steam generators is usually performed by the reactor supplier, while the turbogenerator and switchgear design is the responsibility of the electrical equipment manufacturer. Understanding these different designs is extremely important, since the designs frequently interact to a very high degree, and impact the safe operation of the plant. The primary responsibility for the nuclear design of the reactor core rests with the nuclear engineer either at the fuel supplier or at the utility. This design must be accomplished meeting numerous conflicting constraints imposed on the reactor operation.
The nuclear analysis and design of a reactor core is highly dependent on other areas of core design, including thermal hydraulic design, economic performance, and so on. The criteria for a design effort encompass considerations of safety, performance, reliability and economics. These criteria are frequently contradictory in nature, and hence require optimization, without compromising safety.
The complete nuclear design of a given core configuration is performed in an iterative manner, initially to survey design parameters, identify constraints, then to refine the design while interacting with other facets of the plant design, and finally, to establish a reference reload design that provides a calculational base against which further optimization calculations can be compared. The core design process requires complex digital computer programs that model the reactor core and plant. One important task of the nuclear reactor engineer is to develop models of the reactor core such that can then be analyzed on the computer. Such models result in large computer programs or "codes" which can then be used by other nuclear engineers in reactor design. Most of our emphasis in this course is on learning how to develop component models for reactor behavior and to use them in a suitable form for reactor design calculations. These calculations will determine the state and behavior of the reactor core or fuel for different situations.
The relationship between computers and reactor design cannot be overstressed. It is almost impossible for the nuclear engineer to function without a background in computer techniques (both in programming and numerical analysis). The increasingly heavy reliance of the nuclear reactor industry on computational models for reactor performance makes it even more imperative that the nuclear engineer possess a thorough background and knowledge in the fundamental physical and mathematical concepts underlying reactor core and system models.

Nuclear Engineering is the application of the principles of nuclear science for the benefit of humankind. It is a broad field, based on the natural laws governing radioactive decay, nuclear interactions including fission and fusion, and the interaction of radiation with matter.

Nuclear Engineers are problem solvers who may develop and use computer models, sophisticated monitoring systems, or software that uses artificial intelligence principles to improve safety. The nuclear engineer, therefore, requires knowledge of energy removal and conversion, reactor theory, materials science, instrumentation and control, nuclear fuel cycle, radioactive waste management, nuclear safety and risk assessment, neutron physics, chemistry, and the energy-environment interface covers.