| Ibid. | Ibidem (in the same place) |
| i.e. | id est (that is) |
| id. | idem (the same) |
| ITS | important to safety |
| ITS | Improved Technical Specifications |
| in process |
| in progress |
| in situ |
| in. | inch(es) |
| in. /s | inches per second |
| incore |
| IDVP | Independent Design Verification Program |
| ISC | Independent Safety Committee |
| ISFSI | independent spent fuel storage installation |
| ITR | independent technical reviewer |
| in-depth (adj), in depth (adv) |
| IPE | individual plant examination |
| IPEEE | individual plant examination for external events |
| IN | information notice |
| in-house (adj) |
| in-leakage |
| in-line (adj) |
| in-phase (adj) |
| in-place (adj) |
| in-plant (adj) |
| inservice |
| ISI | inservice inspection |
| ILT | inservice leak test |
| IST | inservice testing |
| ID | inside diameter |
| IR | Inspection Report (NRC) |
| IEEE | Institute of Electrical and Electronics Engineers |
| INPO | Institute of Nuclear Power Operations |
| I&C | instrumentation and controls |
| ISA | Instrument Society of America |
| intake structure |
| ICS | integrated communications system |
| IIT | Integrated Inspection Team |
| ILRT | integrated leak rate test |
| ILS | integrated living schedule |
| IPAP | integrated performance assessment process (NRC) |
| ISAP | Integrated Safety Assessment Program |
| inter alia (among other things) |
| IDAP | Inter-Departmental Administrative Procedure |
| IGA | intergranular attack |
| IGSCC | intergranular stress corrosion cracking |
| ICRP | International Commission on Radiation Protection |
| SI | International System of Units (Standard International) |
| IOM | interoffice memorandum |
| intertidal |
| isophase |
| INVDIO | issue needing validation to determine its impact on operability |
