PE Nuclear key terms

The closed loop that carries heat from the reactor core to the steam generators and returns it to the core. It includes the reactor vessel, coolant pumps, piping, pressurizer, and steam generators, and forms a principal fission product barrier.

A tank connected to the hot leg of a pressurized water reactor loop that holds a steam bubble to control system pressure. Electric heaters raise pressure and spray lowers it during load changes and transients.

A shell-and-tube heat exchanger in a pressurized water plant that boils secondary feedwater using primary coolant heat. It separates the radioactive primary loop from the non-radioactive steam cycle.

A safety system of accumulators plus high- and low-pressure injection pumps that delivers borated water to the core after a loss-of-coolant accident to keep fuel cladding cool and intact.

A low-pressure system that removes decay heat and stored heat from the reactor coolant during shutdown, cooldown, and refueling. In some designs it also provides low-pressure injection during accidents.

The reinforced concrete and steel structure enclosing the reactor that limits release of radioactive material to the environment and is designed to withstand design-basis pressure, temperature, and missile loads.

The electromechanical device that positions and rapidly inserts control rods. In pressurized water reactors it holds rods with electromagnets so that loss of power lets the rods fall into the core.

The ratio of neutrons produced in one generation to neutrons lost by absorption and leakage in the preceding generation. A value of one is critical, greater than one is supercritical, and less than one is subcritical.

A measure of how far a reactor is from critical, commonly written as (k-1)/k and expressed in percent or in dollars. Positive reactivity raises reactor power and negative reactivity lowers it.

Neutrons released by decaying fission products seconds to minutes after fission. Though less than one percent of all neutrons, they slow the rate of power change enough for operators and control systems to respond.

The change in reactivity per unit change in moderator temperature. It is negative in power reactors, so a rise in coolant temperature reduces reactivity and tends to stabilize power during transients.

Build-up of the fission product xenon-135, a very strong neutron absorber, after power changes or shutdown. Its peak after shutdown can create a reactivity deficit that delays restart for many hours.

Long-term reactivity control by dissolving boric acid, a neutron absorber, in the reactor coolant. Boron concentration is raised by feed-and-bleed or lowered by dilution instead of moving control rods.

The controlled procedure of bringing a subcritical reactor to criticality slowly, using source-range count rates and inverse-count-rate plots to predict the critical rod position or boron concentration before reaching it.

Heat generated by radioactive decay of fission products and actinides after the fission chain reaction stops. It is roughly several percent of full power at shutdown and requires forced cooling for days.

The heat flux at which boiling changes from nucleate to film boiling, causing a sharp, potentially damaging rise in fuel cladding temperature. Designs keep local heat flux well below this value.

The ratio of the predicted critical heat flux to the actual local heat flux at the hottest point in the core. It must stay above a specified limit to prevent cladding overheating and failure.

The fraction of a flow channel's volume occupied by vapor rather than liquid. It influences both heat transfer and reactivity, since steam voids reduce moderation in a light-water core.

Simultaneous flow of liquid and vapor in a channel, occurring in regimes such as bubbly, slug, churn, and annular. The regime governs pressure drop, heat transfer, and flow stability.

The principle of keeping radiation dose As Low As Reasonably Achievable, weighing dose reduction against cost, effort, and other factors. Regulators require it for both occupational and public exposures.

The sum of the deep-dose equivalent from external exposure and the committed effective dose equivalent from inhaled or ingested radionuclides. It is the regulatory measure of an individual's overall dose.

The thickness of a shielding material that reduces radiation intensity by one half. Repeating this thickness gives rough estimates of the shield needed for gamma and x-ray sources.

Radioactive material present on surfaces, in equipment, or inside the body where it is not wanted. Unlike external radiation fields, contamination can be spread and can cause internal exposure.

The process of raising the fraction of uranium-235 above its natural level, usually by gas centrifuge using uranium hexafluoride. Commercial light-water fuel is typically enriched to three to five percent.

A deep, water-filled basin holding irradiated fuel assemblies after discharge. The water removes decay heat and shields radiation, while boron and rack geometry maintain subcriticality.

Highly radioactive, heat-generating waste consisting of spent fuel or the solidified products of reprocessing. It requires long-term isolation, typically in a deep geologic repository.

A postulated accident, such as a large-break loss-of-coolant accident, that a plant must be designed to withstand. Acceptance criteria limit fuel damage and the resulting release of radioactive material.

The minimum design requirements for nuclear power plants set out in 10 CFR Part 50 Appendix A. They cover protection systems, redundancy, barriers, and quality standards for safety-related structures and components.

A design requirement that a safety system must still perform its function despite the failure of any one component or active part. It ensures that added redundancy actually improves reliability.

A layered safety approach using multiple independent barriers and controls, such as fuel cladding, pressure boundary, and containment, so that no single failure or error leads to unacceptable consequences.

License conditions that set operating limits and surveillance requirements for a plant. They include limiting conditions for operation, required actions, and completion times when equipment is out of service.

The minimum equipment and parameter conditions specified for safe operation. If not met, prescribed actions and a shutdown clock apply until the condition is restored.

The rapid shutdown of a reactor by inserting control rods, either automatically from protection signals or manually. Rod insertion is aided by gravity, and the turbine is usually tripped at the same time.

Periodic volumetric, surface, and visual examination of pressure boundary welds and components, performed under ASME Code Section XI, to detect degradation such as cracking or wall thinning before it causes failure.

Loss of fracture toughness in the vessel beltline region caused by neutron irradiation, tracked by the reference nil-ductility transition temperature. It restricts pressure during heatup and cooldown.

Crack growth produced by the combined action of sustained tensile stress and a corrosive environment, common in stainless and nickel-base alloys. Water chemistry control and residual stress relief are main defenses.

A highly sensitive neutron detector, such as a boron trifluoride or helium-3 proportional counter, used during shutdown and startup to track the neutron population at very low levels.

The value of a measured variable at which an instrument channel or bistable initiates an action such as a trip, alarm, or valve movement. It includes allowance for instrument uncertainties.