Escape of neutrons from a specified region before they participate in further reactions there. A leakage fraction depends on the region's geometry, surrounding materials and energy-dependent transport and absorption, rather than its dimensions alone.

A self-sustaining neutron balance in which production offsets absorption and leakage. In a steady one-group neutron diffusion equation, criticality is a compatibility condition between material parameters and the allowed spatial mode.

A cross section for removal of an incident neutron by absorption, including capture and neutron-induced fission where relevant. Its value depends on nuclide and neutron energy.

A reaction coefficient formed from target number densities and microscopic nuclear reaction cross-sections. It has inverse-length units. Multiplying it by the scalar neutron flux gives the corresponding reaction rate per volume, under the stated energy averaging.

A genomic region with elevated association with the nuclear lamina under a specified measurement. DamID reports accumulated proximity rather than continuous attachment of every copy.

uranium recovered from spent nuclear fuel. Its isotopic composition differs from natural uranium; uranium-236 can impose an additional fuel-performance penalty.

Nuclear fuel based on metal oxides. Chemical composition and the oxygen-to-metal ratio affect transport, stability and compatibility with surrounding materials.

The structural layer enclosing nuclear fuel, retaining fuel and fission products while transferring heat to a coolant. Its pressure loading, chemical compatibility and irradiation response must remain consistent with its required geometry and integrity.

A uranium–silicon compound considered in nuclear-fuel applications, including ${\mathrm{U}}_3\mathrm{S}{\mathrm{i}}_2$. High uranium loading and thermal conductivity motivate study; chemical compatibility and irradiation behavior remain essential qualifications.

A uranium–nitrogen compound investigated as nuclear fuel. High uranium loading and thermal conductivity are attractive, while chemical and irradiation compatibility constrain applications.

TRISO fuel uses coated nuclear fuel particles, each with a fuel kernel surrounded by four layers: a porous carbon buffer, an inner dense pyrolytic carbon layer, a silicon carbide layer and an outer pyrolytic carbon layer. The name abbreviates tristructural isotropic: the four layers use three coating material forms.^[ref]

The layers perform complementary functions. The porous carbon buffer accommodates released fission gas; the pyrolytic carbon supports the particle mechanically; and the silicon carbide supplies a strong barrier to fission products. Coated particles can be embedded in a graphite matrix. Their retention performance depends on coating integrity, material behaviour under irradiation, and the conditions experienced in service. The architecture makes a small particle itself part of the containment system, linking nuclear engineering closely to materials science.^[ref]

A fuel-and-cladding system designed for improved performance under specified accident conditions while maintaining acceptable normal-operation behavior. Improvements to pellet and cladding materials address different parts of the system's response.

The mass of heavy fuel elements, excluding oxygen, cladding and other structural materials. burnup and fuel-cycle prices must use a stated heavy-metal reference basis.

Energy released per initial heavy-metal mass, commonly $\mathrm{MW}\mathrm{d}{\mathrm{t}}^{-1}$. For thermal energy production $E$ and burnup $B$, the corresponding initial fuel mass is $M=E/B$.

The center-to-center spacing of neighboring rods or other repeating elements in a specified fuel lattice. Changing pitch at fixed rod dimensions changes the coolant-to-fuel volume ratio; its effect on core size additionally depends on rod count and layout.

The coupled thermal, mechanical, chemical and irradiation response determining whether fuel retains its intended function and containment during service.

A nuclear isomer is a relatively long-lived metastable nuclear state. Gamma de-excitation can be hindered by a large angular-momentum change, high electromagnetic multipolarity or small overlap with lower states. Its lifetime and decay channels depend on the actual level structure; metastability is not an absolute prohibition on decay.

A difference between the mass of a bound system and an appropriate sum of reference constituent masses. For a nuclear transformation, the initial-minus-final mass difference gives the available Q value after multiplication by $c^2$, with atomic electrons counted consistently when using neutral-atom masses.

A collective nuclear model that treats the nucleus as a nearly uniform fluid drop with short-range cohesive binding, surface energy and electrostatic self-energy. The semi-empirical mass formula incorporates these contributions, together with asymmetry and pairing terms, to describe broad binding trends. Discrete nuclear shell structure requires additional physics.

Smooth adiabatic nozzle flow without entropy production. In a steady quasi-one-dimensional ideal-gas model, $\rho uA$ and $u^2/2+c_{p}T$ are constant along the passage. A converging-diverging passage can connect subsonic and supersonic branches through a sonic throat when reservoir and back-pressure conditions select that branch.