iodine pit
Enable an agent to recognise an iodine-pit episode, assess its evolving xenon-related reactivity deficit, and identify what evidence is needed to judge restart constraints.
Bundle → Layer → Finding → Questions Filled
6 bundles · 11 layers · 15 findings · 27 questions
Meaning and instance boundary Define what the iodine-pit label denotes in this application.
Terminology and authority
Record the meaning used by a reference, facility or analyst.
Adopted iodine-pit definition
Record whether the term denotes the broader transient or specifically xenon-precluded startup. The latter and xenon dead time are defined in the [DOE reactor theory handbook, Xenon section](https://www.navsea.navy.mil/Portals/103/Documents/NNPTC/RD/doe_rx_theory_v2.pdf).
- Does this application require demonstrated restart preclusion before calling an episode an iodine pit? definition
- Which reference or facility terminology establishes that usage? provenance
Episode membership
Identify the reactor occurrence and exclude superficially similar conditions.
Qualifying reactivity transient
Require an identified host core, initiating power change and evidence linking the assessed deficit to xenon.
- Which core and power-change event identify this occurrence? provenance
- What distinguishes this occurrence from equilibrium xenon poisoning or a non-xenon restart restriction? boundary
Precursor inventory and trigger Capture the history and physical mechanism needed to explain the episode.
Initial inventory basis
Establish the state immediately before the initiating change.
Power history reconstruction
Record the preceding operating history and whether equilibrium inventories are justified or merely assumed.
- What power and flux history supports the initial iodine-135 and xenon-135 estimates? measurement
- What evidence supports or rejects an equilibrium assumption at the initiating time? provenance
Production and removal balance
Represent the competing processes controlling xenon inventory.
Iodine-fed xenon buildup
Iodine-135 decay supplies xenon-135; xenon removal involves decay and neutron absorption. Power reduction changes this balance. See the [DOE reactor theory handbook, Xenon section](https://www.navsea.navy.mil/Portals/103/Documents/NNPTC/RD/doe_rx_theory_v2.pdf).
- Which production and removal terms are represented in the episode calculation? definition
- Does the estimated post-change balance support rising xenon for this particular history? measurement
Reactivity deficit and preclusion Connect the xenon inventory to the core's ability to meet a defined restart criterion.
Xenon worth
Translate the assessed xenon distribution into its reactivity contribution.
Inventory-to-reactivity assessment
Record the calculation basis, reference condition and uncertainty of the xenon penalty.
- What reactor-specific analysis converts the inventory estimate into xenon reactivity worth? provenance
- What sign convention, reference state and uncertainty accompany the reported worth? measurement
Preclusion criterion
Determine whether the assessed xenon penalty prevents restart under the specified conditions.
Available reactivity comparison
Xenon-precluded startup occurs when available positive reactivity cannot counteract xenon poisoning; this is distinct from poisoning alone. See the [DOE reactor theory handbook, page 39](https://www.navsea.navy.mil/Portals/103/Documents/NNPTC/RD/doe_rx_theory_v2.pdf).
- What available-reactivity assessment and operating constraints define the comparison? provenance
- Does the comparison establish preclusion, non-preclusion or an unresolved result within uncertainty? measurement
Trajectory and recovery Track the episode over time and distinguish its different milestones.
Transient milestones
Record the initiating event and assessed progression.
Onset, peak and trend
Maintain separate timestamps for the initiating power change, poisoning peak and any preclusion boundaries.
- Which milestones are observed and which are predicted? provenance
- Does the reported peak refer to xenon concentration, xenon worth or total reactivity deficit? definition
Conditional recovery
Specify the conditions under which the episode is considered resolved.
Recovery threshold and forecast
Record the adopted recovery threshold and forecast assumptions without assigning a universal waiting period.
- What threshold ends this episode under the adopted definition? boundary
- What future power history and configuration are assumed by the predicted recovery interval? measurement
- Which new observation or configuration change requires the forecast to be reassessed? action
Assessment validity and use Establish whether the available evidence supports using an iodine-pit assessment for the host reactor.
Evidence and applicability
Identify inference limits and distinguish xenon effects from other contributions.
Validated episode assessment
Record data provenance, calculation validation and whether spatial effects or competing reactivity changes undermine a whole-core interpretation.
- Which observations independently support the inferred xenon transient? provenance
- Could temperature changes, other poisons or spatial flux redistribution materially alter the interpretation? boundary
- What validation supports applying this calculation method to the identified core? provenance
Decision interface
Connect assessment results to the process that owns operational decisions.
Restart assessment handoff
Present the xenon-related conclusion, uncertainty and validity conditions to the responsible operating process; recovery is one input to restart readiness.
- Which procedure and responsible role review the xenon assessment before a restart decision? action
- What unresolved evidence prevents a reliable conclusion about xenon preclusion? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Check these first
Recalled without web access and unsourced; every item is a lead to verify.
- Which of these check these first hold for the sense of iodine pit this model covers, and on what evidence? provenance
Real-world use
Recalled without web access and unsourced; every item is a lead to verify.
- Which of these real-world use hold for the sense of iodine pit this model covers, and on what evidence? provenance
Typical measurements
Recalled without web access and unsourced; every item is a lead to verify.
- Which of these typical measurements hold for the sense of iodine pit this model covers, and on what evidence? provenance
Failure modes and hazards
Recalled without web access and unsourced; every item is a lead to verify.
- Which of these failure modes and hazards hold for the sense of iodine pit this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Recalled without web access and unsourced; every item is a lead to verify.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of iodine pit this model covers, and on what evidence? provenance
Classifiers Filled
- Family
- Thing Registry
- Category
- Information and virtual systems
- Entry kind
- thing
- Plane
- INF
- Domain
- INF.KNW
- Other names and narrower kinds
- xenon poisoning
What it is Filled
An iodine pit is a transient interval after a nuclear reactor shutdown or substantial power reduction in which xenon-135, produced by decay of accumulated iodine-135, introduces enough negative reactivity to prevent restart or the intended power increase with the available reactivity.
In scope
- Definitions distinguishing the xenon transient from the interval in which xenon prevents restart
- Power and neutron-flux history establishing the initial iodine and xenon inventories
- Evolution of iodine-135, xenon-135 and their contribution to reactivity
- Episode onset, peak poisoning, recovery and restart-preclusion interval
- Evidence, uncertainty and reactor-specific applicability of an iodine-pit assessment
Out of scope
- General reactor design, licensing and operating procedures
- Complete nuclear properties of iodine and xenon isotopes
- Radioiodine releases, iodine spiking and thyroid protection
- Other fission-product poisons and long-term fuel depletion
- Detailed spatial xenon-oscillation models
- Plant-wide restart authorisation and equipment readiness
Why it exists Filled
Enable an agent to recognise an iodine-pit episode, assess its evolving xenon-related reactivity deficit, and identify what evidence is needed to judge restart constraints.
Distinguishing features Filled
- Identify iodine-135 feeding xenon-135 as the relevant precursor-absorber relationship; elemental iodine toxicity does not establish an iodine pit.
- Establish a transient following a power reduction or shutdown rather than merely the presence of equilibrium xenon poisoning.
- For the restart-preclusion meaning, demonstrate that the xenon penalty exceeds the applicable available reactivity; a xenon concentration peak alone is insufficient.
- Separate a whole-core reactivity depression from spatial xenon oscillations, while allowing both phenomena to coexist.
- Distinguish xenon-driven restart restriction from restrictions caused by other poisons, equipment conditions or operating requirements.
What robots and AI may and may not do Filled
Must not
- Act on reactor controls, interlocks or safety systems without licensed operator authority.
- Advise operators to bypass safety limits to overcome reduced reactivity.
- Present simulated or estimated reactivity figures as measured values.
- Hide or downplay abnormal reactor behaviour in reports to operators or regulators.
Only with a human decision
- Any decision to restart or raise power after a shutdown or power reduction.
- Any change to reactor operating procedures.
May
- Explain the iodine pit and xenon poisoning in general terms from reactor physics sources.
- Report reactor status to licensed operators on systems the agent is authorized to read.
Moral aspects Filled
- Pushing a reactor against xenon effects contributed to a major nuclear accident; safety culture must come first.
- Nuclear incidents harm people and land far beyond the plant.
Who is affected
- Plant workers
- Nearby communities
- The wider public and environment
Owners Filled
Steward
The licensed operator of the reactor, under the oversight of the national nuclear regulator.
Links to other meta-models Filled
parent
- Q19977811 - registry parent class
What else AI and robots need to interact with it Filled
Identity and identifiers required Filled
- Vercy registry: vr.tr.iodine-pit
- Wikidata: Q15649597 (https://www.wikidata.org/wiki/Q15649597)
Direct properties not applicable Not applicable
- Definition adopted: post-reduction xenon transient | xenon-precluded restart interval | unresolved
- Host core and configuration: reference to reactor, core loading, cycle state and dated configuration
- Preceding power and flux history: time series in MW thermal or percent rated power, and neutrons/cm²/s where available
- Elapsed time from initiating change: hours from a specified timestamp
- Iodine-135 and xenon-135 inventories: atoms or atoms/cm³, with spatial basis and estimation uncertainty
- Xenon reactivity contribution: pcm or Δk/k, with sign convention and reference state
- Restart reactivity balance: pcm or Δk/k relative to the stated restart criterion
- Episode phase: building | near peak | recovering | resolved under adopted criterion | indeterminate
- Predicted preclusion interval: start and end timestamps or hours after initiating change, with uncertainty bounds
Plane INF: no invented physical properties.
Recognition not applicable Not applicable
Not applicable
An iodine pit has no directly visible identifying features and must be established from reactor operating history, measurements and reactor physics analysis.
Capabilities and actions required Filled
- Classify a reported event against an explicitly selected iodine-pit definition.
- Assemble the power history and inventory estimates needed for a qualified transient assessment.
- Compare observed trends with predicted xenon buildup and recovery.
- Estimate a conditional restart-preclusion interval using a validated reactor-specific analysis.
- Flag missing evidence or competing causes of a reactivity deficit.
- Refer the assessment to the applicable operating procedure and responsible reactor personnel.
Hazards and failure modes required Filled
- Attempts to force a restart can leave a reactor in an unstable state.
- Uneven xenon distribution can cause power oscillations in large cores.
Standards and interfaces required Filled
- IAEA Safety Standards for the operation of nuclear power plants, International Atomic Energy Agency.
Context of use required Filled
- An iodine pit is encountered in nuclear reactor operation after shutdown or a substantial reduction in power when xenon-135 temporarily limits available reactivity.
- The concept is used in reactor physics assessments and operating reviews to explain why a planned restart or power increase may be temporarily unavailable.
Sources Filled
- Wikidata item Q15649597: iodine pit - identity and sense of the item
- Wikipedia: Iodine pit - general description of the item
Open questions
- Does the registry intend iodine pit to cover every post-reduction xenon transient or only the interval of xenon-precluded restart?
- Which authoritative references establish the relationship among iodine pit, xenon pit, xenon poisoning and xenon dead time across reactor communities?
- What reactor-specific evidence and uncertainty criteria are sufficient to classify an episode as confirmed rather than predicted?
- When is a whole-core assessment adequate, and when must this model link to a spatial xenon model?
- Does an existing Vercy world model already own this phenomenon, requiring this registry entry to link to that publication?
Machine files
Provenance
thing registry research (pass 2) · unreviewed
Built from: models/things/publications/thing-q15649597/spec.json