ISO 18077:2022

Overview - ISO 18077:2022 (Reload startup physics tests for PWRs)

ISO 18077:2022 defines the minimum acceptable startup reactor physics test program for commercial pressurized water reactors (PWRs) after refuelling or other core alterations that require nuclear design calculations. The standard is limited to reload/alteration startup testing (not the initial core) and focuses on demonstrating that the as-built core behaves consistently with design predictions before returning to normal operation. It does not cover in‑operation surveillance, mechanical system tests, fuel loading visual verification, or instrument/control system calibration.

Key topics and technical requirements

• Scope and intent: Establishes the baseline set of physics tests and selection criteria needed to confirm core behavior following core modification.
• Minimum test program: Specifies required measurements and the basis for each test to show agreement between measurements and design predictions within predetermined review criteria (the document uses review criteria only; acceptance criteria are outside its scope).
• Primary physics characteristics to confirm:
  • Reactivity balance (e.g., boron endpoint checks to confirm fuel inventory and poisons)
  • Reactivity control (notably the isothermal temperature coefficient (ITC))
  • Power distribution (core power shape verification)
  • Capability to shutdown and requirements to shutdown (control rod worth and shutdown margin confirmation)
• Test methods and evaluation: Clause 6 describes test method requirements (objectives, initial conditions, methods, and evaluation). Annex A provides user guidance with acceptable methods, typical test criteria, and practical recommendations; alternative methods may be used if they meet Clause 6 requirements.
• Operational assumptions and limitations: Assumes the same analytical methods are used for design and startup predictions and that operation remains within the plant’s historical database. Major design changes may require expanded testing.
• Updated content in the 2022 edition: Clarifies review vs. acceptance criteria, adds guidance that progression to the next power plateau requires acceptable results at the current plateau, and includes editorial/clarifying changes.

Practical applications and users

Who uses ISO 18077:2022:

• Reactor physics engineers and core design teams
• Nuclear power plant operators and startup test engineers
• Regulatory authorities and safety assessors
• Vendors and consultants conducting reload testing or validation

Practical uses:

• Designing and executing reload startup physics test programs
• Verifying that a refuelled or altered PWR core matches safety analysis and licensing assumptions
• Supporting plant return-to-service decisions after refuelling

Related standards

ISO 18077:2022 is informed by and related to several ANSI/ANS standards referenced in the document, including:

• ANSI/ANS-19.6.1 (basis for minimum startup physics program)
• ANSI/ANS-19.4 (reactor physics measurement quality)
• ANSI/ANS-19.11 (moderator temperature coefficient methods)
• ANSI/ANS-3.2 and ANSI/ANS-3.1 (administrative, QA, and personnel qualification aspects)

Keywords: ISO 18077:2022, reload startup physics tests, PWR, pressurized water reactor, startup reactor physics test program, reactivity balance, isothermal temperature coefficient, power distribution, control rod worth.