| abs | absolute |
| ASP | accident sequence precursor (PRA term) |
| acre-ft | acre-foot |
| AE | Action Evaluation |
| AR | Action Request |
| ALJ | Administrative Law Judge |
| AL | Administrative Letter (NRC) |
| ACRS | Advisory Committee on Reactor Safeguards |
| aftercooler |
| air-cooled |
| all-inclusive |
| AVT | all volatile treatment |
| AOT | allowed outage time |
| ARC | alternate repair criteria |
| ac | alternating current |
| ACI | American Concrete Institute |
| AIEE | American Institute of Electrical Engineers |
| AISC | American Institute of Steel Construction |
| ANSI | American National Standards Institute |
| ANEC | American Nuclear Energy Council |
| ANI | American Nuclear Insurers |
| ANS | American Nuclear Society |
| API | American Petroleum Institute |
| ASTM | American Society for Testing and Materials |
| ASCE | American Society of Civil Engineers |
| ASME | American Society of Mechanical Engineers |
| ASA | American Standards Association |
| AWS | American Welding Society |
| AWG | American wire gauge |
| A/amp | ampere |
| A/cm2
| ampere per square centimeter |
| A-hr/amp-hr | ampere-hour |
| AM | amplitude modulation |
| a.m. | ante meridiem – before noon |
| ATWS | anticipated transient without scram |
| ATWT | anticipated transient without trip |
| antilog/log-1
| antilogarithm |
| antireverse |
| AVB | antivibration bar |
| ALARA | as low as is reasonably achievable |
| as-built (adj or noun) |
| atm | atmosphere |
| AEC | Atomic Energy Commission |
| AIF | Atomic Industrial Forum |
| at. no. | atomic number |
| at. % | atomic percent |
| ASLAB | Atomic Safety and Licensing Appeal Board |
| ASLB | Atomic Safety and Licensing Board |
| at. wt | atomic weight |
| AMSAC | ATWS mitigation system actuation circuitry |
| AFR | Audit Finding Report |
| AIT | augmented inspection team (NRC) |
| ANI | Authorized Nuclear Inspector |
| ACI | autoclosure interlock |
| autostart |
| autostop |
| AB | auxiliary building |
| AFW | auxiliary feedwater |
| ASW | auxiliary saltwater |
| avg | average |
| AO | axial offset |
| AOC | axial offset control |
