Development and Benchmarking of Mechanistic Channel Deformation Models in RELAP/SCDAPSIM/MOD3.6 for CANDU Severe Accident Analysis
Bibliographic record
Abstract
In different stages of postulated severe accidents in CANDU reactors, the fuel channels may experience a series of thermomechanical deformations, some of which may have significant impacts on accident progression; however, they have not been mechanistically modeled by integrated severe accident codes such as MAAP-CANDU and SCDAP/RELAP5. This paper focuses on the development and benchmarking of mechanistic models for pressure tube (PT) ballooning and sagging phenomena during the fuel channel heatup phase as well as for the sagging of fuel channel assemblies during the core disassembly phase. These models, which are based on existing phenomena in literature, are coupled with RELAP5 and/or integrated into RELAP/SCDAPSIM/MOD3.6 as new SCDAP subroutines to provide more robust treatment of the deformation phases of severe accidents.The ballooning of a PT will lead to contact with its calandria tube (CT) and occurs during conditions where the coolant pressure is moderately high. At initial contact the high contact thermal conductance and the large temperature difference between the two tubes result in a large transient heat flux that challenges the channel integrity through potential film boiling on the outer calandria surface if moderator subcooling is low. A one-dimensional ballooning and contact model (BALLON) has been developed. BALLON calculates the ballooning-driven transverse strain of PT and CT and modifies the effective conductivity of the annulus before and after contact.Pressure tube sagging is the dominant deformation mechanism at low pressures and occurs at relatively high PT temperatures. A model based on simple beam theory (SAGPT) has been developed. SAGPT calculates the longitudinal strain and the deflection of PT, and it also determines PT-to-CT sagging contact. The sagging and disassembly of the entire fuel channel assembly occur when the fuel channels are uncovered and the moderator heat sink is lost; thus, the entire PT-CT assembly sags together, possibly contacting channels at lower elevations. A model named SAGCH is created to track fuel channel assembly sagging after moderator boil off and also determines the extent of channel-to-channel contact, channel disassembly, suspended debris bed characteristics, and eventual core collapse.This paper presents detailed descriptions of the models, the coupling schemes, and their benchmark against experiments, together with an extensive review of relevant studies in the literature.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.003 |
| Meta-epidemiology (narrow) | 0.002 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.002 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.005 | 0.001 |
| Research integrity | 0.001 | 0.002 |
| Insufficient payload (model declined to judge) | 0.005 | 0.002 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".