Overview
ISO 16117:2013 - "Nuclear criticality safety - Estimation of the number of fissions of a postulated criticality accident" provides a structured methodology for estimating a reasonably maximal value of the number of fissions (fission yield) for a postulated criticality accident. The standard is intended to support criticality accident analysis that underpins emergency planning, dose assessment, and radioactive material release estimates. It applies to facilities, plants, laboratories, storage, and transportation of fissile material - but not to nuclear power reactor cores.
Key topics and requirements
- Scope and purpose: Defines how to estimate a bounding fission number for postulated accident scenarios and links that estimate to downstream consequence analyses (dosimetry, releases, emergency planning).
- Postulated accidents: Each estimate is tied to a single postulated accident (scenario + evolution); assumptions must be physically plausible and documented.
- Assumptions to consider: geometry, confinement/environment, fissile material form and quantity, total and rate of reactivity addition, initial neutron source, expected accident duration and human interventions.
- Estimation routes:
- Simplified models - preferred first route for order-of-magnitude estimates; should use collective experiment/accident experience (Annexes B, C) and simplified formulae (Annex D).
- Calculation tools - may be used when needed, but require documented tool implementation, verification, applicability justification, and comparison to experiments where possible.
- Uncertainty and sensitivity: Perform sensitivity studies on input data and parameters; associate each fission-number estimate with an approximated duration and uncertainty bounds.
- Documentation and justification: All assumptions, areas of applicability, and any discrepancies between simplified models and calculation tools must be explained and recorded.
- Warnings: The standard highlights challenges in fully validating calculation tools because of limited experimental data and emphasizes iterating between fission estimates and consequence analyses.
Practical applications and users
- Nuclear criticality safety engineers performing criticality accident analyses and fission-yield estimation
- Emergency planners and radiological protection specialists using fission estimates for dose assessments and release modeling
- Facility designers and safety assessors for laboratories, fuel cycle facilities, storage, and transportation of fissile materials
- Regulators and auditors evaluating the adequacy of criticality safety analyses and emergency preparedness
Related standards
- ISO 27467 - provides the broader criticality accident analysis framework and flow diagram referenced by ISO 16117
- ISO 7753 - covers criticality accident detection (separate from fission-number estimation)
Keywords: ISO 16117:2013, nuclear criticality safety, number of fissions, fission yield, criticality accident analysis, emergency planning, dosimetry, simplified models, calculation tools.
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