Incorporating realistic conservatism into criticality alarm placement and emergency response: a regulatory perspective - 14425
- United States Nuclear Regulatory Commission, Mail Stop T-4B16, Washington, DC 20555 (United States)
Review of historical process plant criticality accidents shows that their consequences vary widely and are extremely difficult to predict accurately. Because of the wide variability and inherent uncertainty in estimating consequences, criticality detector placement analysis and consequence estimation for emergency response typically utilize very conservative assumptions with regard to the total fissions, time-dependent fission rate, and intervening shielding. For detector placement, a 'minimum accident of concern' is typically defined, whereas emergency consequence evaluations typically define a maximum fission source term. These require different mindsets because what is conservative for one type of analysis may be non-conservative for the other. While some conservatism is commonly practiced in both cases, excessive conservatism can result in a net increase in risk, such as resulting from inadvertent alarm activations, that can be costly to operations and counterproductive to safety. Conversely, reliance on very detailed calculational models to reduce conservatism could increase the number and complexity of controls, such as shielding for dose mitigation. Deterministic regulatory criteria have also been applied for determining where criticality detectors are to be deployed and what areas must be evacuated in the event of an accident. This paper explores several issues associated with criticality alarm placement and emergency response from a regulatory perspective. This paper will discuss insights gained from historical fuel facility accidents and significant events regarding the inherent risk of criticality, the most likely locations for criticality to occur, and the most probable characteristics of a critical excursion. This will include discussion of the previously disputed relevance of the United States Nuclear Regulatory Commission's (USNRC's) longstanding model for a critical excursion. While USNRC regulations deterministically require dual alarm coverage in all areas having greater than a 'critical mass' of fissionable material, a process is provided to grant exemptions when a risk-informed argument is technically justified. Similar risk-informed considerations have been used to justify compensatory measures allowing limited operation during periods of alarm system outage due to component failure or maintenance. This paper will explore the application of risk information and performance history to address requirements for criticality alarm system coverage, granting exemptions from deterministic criteria, allowing compensatory measures during system outages, determining evacuation areas and routes, and assessing the significance associated with alarm system issues. Finally, the impact of shielding on the USNRC's regulatory risk basis and approved subcritical margin will be reviewed. (authors)
- Research Organization:
- American Nuclear Society - ANS, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
- OSTI ID:
- 23100880
- Resource Relation:
- Conference: ICNC 2015: 2015 International Conference on Nuclear Criticality Safety, Charlotte, NC (United States), 13-17 Sep 2015; Other Information: Country of input: France; 18 refs.; available on CD Rom from American Nuclear Society - ANS, 555 North Kensington Avenue, La Grange Park, IL 60526 (US)
- Country of Publication:
- United States
- Language:
- English
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