A Criticality Accident Alarm System (CAAS) is installed as part of criticality safety management for use in reducing the radiation workers could be exposed to in the rare case of a criticality accident. For operations with fissionable material that are compliant with ANSI/ANS-8.3-2022: Criticality Accident Alarm System and the applicable ANSI/ANS-8 standards for nuclear criticality safety, the likelihood of a criticality accident is very low.
The ANSI/ANS-8.3-2022 Standard for Criticality Accident Alarms Systems
Criticality Accident Alarm Systems (CAAS) are alarm systems for prompt evacuation of personnel in the event of a criticality event. ANSI/ANS-8.3-2022 provides guidance regarding assessment, performance, design, and testing criteria that concerns CAAS. This American National Standard defines critically accident as the release of energy as a result of accidental production of a self-sustaining or divergent neutron chain reaction. It specifies that the CAAS is designed to respond to the minimum accident for system design (MASD). The MASD should be associated with actual facility operations or credible critical conditions subsequent to an upset event like a fissile-bearing solution leak, tank burst, or the addition of moderator to dry fissile material during a fire scenario. Moreover, the scope of ANSI/ANS-8.3-2022 includes operations with fissionable materials in which inadvertent criticality leading to a radiation dose to personnel immediately dangerous to life and health could occur.
This standard is not applicable to the operation of nuclear reactors or the conduct of critical experiments.
A nuclear criticality accident occurs from operations that involve fissile material and results in a sudden and potentially lethal release of radiation. According to the National Council on Radiation Protection and Measurements (NCRP), the average annual radiation dose per person in the U.S. is 6.2 millisieverts (620 millirem). Most of the average annual dose comes from natural background radiation sources:
- Space (cosmic radiation)
- Terrestrial (in the ground)
- Internal (in your body)
- Radon and thoron (radioactive gases that are created when naturally occurring elements undergo radioactive decay)
Another large source of radiation comes from medical procedures (i.e., occupational, computed tomography, nuclear medicine, interventional fluoroscopy, and conventional radiography/fluoroscopy). Hence, nuclear radiation can come from many sources, both natural and manufactured. Nuclear criticality safety is the prevention of unwanted nuclear chain reactions in fissile materials through technical and administrative control measures like those specified in ANSI/ANS-8.3-2022.
What Is the Purpose of a Criticality Accident Alarm System?
The dominant purpose of a Criticality Accident Alarm System (CAAS) is the protection of operating personnel. The CAAS aims to reduce the risk to personnel by its essential functions of criticality accident detection and alerting of personnel in areas where doses immediately dangerous to life and health might occur. A CAAS is required in nuclear facilities where an accidental criticality excursion could result from operational processes. In general, most production-oriented nuclear fuel cycle facilities require a CAAS system because the fissile material is managed in quantities that exceed critically-safe mass limits.
Interestingly, on average, nuclear power workers receive a lower annual radiation dose than flight crew, and frequent flyers in 250 hours would receive 1 millisieverts (mSv). The maximum annual dose allowed for radiation workers is 20 mSv/yr, although doses are usually kept well below this level. Ultimately, the installation of a CASS system severely helps the safety management of personal because it prompts protective actions (e.g., evacuation) after the rare occasion of a single-spike event that can save lives and/or avoid serious injuries.
ANSI/ANS-8.3-2022: Criticality Accident Alarm System is available on the ANSI Webstore.