Retry locating particles that start on a surface coincident with another

[GuySten]

Description

Particles split by weight windows at a surface crossing can be lost when another surface coincides with the crossed surface. This was reported in #4140, where 9 of 10 particles were lost ("Could not find the cell containing particle") and the leakage fraction was 0.1 instead of 1.0.

Cause. In the reproducer, the particle crosses x = 62125.05 (surface 3) inside a universe, and the surface weight window checkpoint splits it there. The secondaries are banked with surf_id = +3, so when they start, OpenMC knows only that they are on the positive side of surface 3. Surface 4 has exactly the same coefficients, and the root cell region is -1 | -2 | -3 | 4. After a track of about 1.5×10⁵ cm, roundoff places the banked position 2.9×10⁻¹¹ cm below the plane, more than FP_COINCIDENT (10⁻¹² cm), so 4 evaluates to false. Together with -3 being false from the surface flag, no root cell contains the particle and it is lost. The primary particle is not lost because it never re-searches the root level. Instead, the same inconsistency causes the zero-length "crossing surface 4" event visible in a trace.

Fix. When a particle that has a surface set cannot be located at the start of its history, it is now moved forward by TINY_BIT along its direction with the surface cleared, and the search is repeated. This is the same fallback Particle::cross_surface already uses when a particle cannot be located after crossing a surface. Particles without a surface set, such as source particles from point sources, are unaffected, so a genuinely undefined region is still reported as lost.

Test. test_split_on_coincident_surfaces in tests/unit_tests/test_lost_particles.py builds the model from #4140 with the Python API and checks that all of the weight leaks. It fails on develop (leakage 0.1) and passes with this change.

Note. This fixes the symptom at the point where particles are banked on a surface. The underlying issue is that the "on surface" flag covers only one surface while other coincident surfaces are evaluated from the position with an absolute tolerance, which becomes smaller than the floating-point resolution of coordinates beyond about 10⁴ cm. Treating geometrically identical surfaces as one when deciding which side a particle is on would also remove the spurious zero-length crossing. That could be a follow-up.

Fixes #4140

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

Summary by CodeRabbit

• Bug Fixes
  • Improved particle tracking at coincident surfaces, reducing the chance that particles are incorrectly marked as lost when crossing or interacting with a surface. This applies to particle cross-section calculations and secondary-particle revival, including cases where a particle is moving nearly parallel to a surface.

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📝 Walkthrough

When cell location fails for a surface-marked particle, the code resets coordinate depth, nudges the particle along its direction, and retries. Both event paths use this helper. Regression tests check neutron and photon leakage at coincident surfaces.

Changes: Surface-crossing particle recovery

Layer / File(s)	Summary
Retry cell search for surface-marked particles src/particle.cpp, tests/unit_tests/test_lost_particles.py	The helper retries cell location after a failed search for a surface-marked particle. event_calculate_xs and event_revive_from_secondary use the helper and retain their existing lost-particle handling if the search still fails. Neutron and photon regression tests check that leakage is approximately 1.0 in the coincident-surface model.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 6b4f8

The previously identified photon-loss path now retries cell location before declaring the particle lost. No actionable merge-blocking issue remains after normal checks.

Security architecture risk: 🔵 Low · up to 80fd7

The change is confined to particle recovery and regression tests, with no identified expansion of privileges or external access. However, near-grazing recovery can potentially resume transport in the wrong material instead of reporting a lost particle.

Retained concerns

• Medium · reliability · inferred: A successful retry does not necessarily preserve post-crossing material ownership. At grazing incidence, the normal displacement from TINY_BIT can leave the banked position on the pre-crossing side beyond FP_COINCIDENT. Clearing the signed surface marker then permits lookup to accept that earlier cell. Transport can calculate cross sections and score a finite path in its material before recrossing, while photon revival can assign its birth-cell energy subtraction there. This changes an explicit lost-particle failure into potentially silent continuation. The same numerical policy predates this PR in cross_surface, but these initialization and revival callers are newly exposed.

Security review details

Security Blast Radius

• inferred — The new recovery branch is reachable only after lookup fails for a surface-marked history. Its direct mutations affect that particle's position, surface marker, and coordinate depth; downstream effects include its material-dependent transport and tally contributions.

Resilience and Maintainability Implications

• observed — If the retry also fails, both callers retain lost-particle handling. That path zeros particle weight, atomically increments the lost count, and applies configured abort thresholds. The retry itself is bounded and does not loop until success.
• observed — The existing surface-crossing fallback already clears the marker, resets to root coordinates, nudges forward, and retries exhaustive lookup. This limits novelty, but does not establish that the numerical displacement always preserves the intended side in the newly covered banked-history paths.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title is concise, specific, and accurately summarizes the main change: retrying particle location when coincident surfaces are involved.
Description check		The description follows the required template and clearly explains the issue, cause, fix, testing, linked issue, and checklist status. The AI Assistance fields are blank and should be completed if AI …
Linked Issues check		The change addresses #4140. find_cell_from_surface retries exhaustive_find_cell for a particle with a surface marker after moving it by TINY_BIT along its direction. The existing lost-particle p…
Out of Scope Changes check		The source change is limited to particle cell-location retries for the reported coincident-surface loss. The added neutron and photon tests cover the same weight-window failure mode. No unrelated chan…

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⚠️ Outside diff range comments (1)

🟠 Major · Apply the surface retry before pulse-height bookkeeping. · particle.cpp:536-539

src/particle.cpp:536-539
🩺 Stability & Availability | 🟠 Major | 🏗️ Heavy lift

Apply the surface retry before pulse-height bookkeeping.

If a surface-marked photon is revived from the local bank with pulse-height tallies active, this cell search runs before the new retry in Particle::event_calculate_xs. If coincident surfaces make this search fail, mark_as_lost discards the photon before the retry can run. Share the retry logic with this lookup, then determine cell_born() for pulse-height bookkeeping. Add a regression case with a split photon and an active pulse-height tally. (raw.githubusercontent.com)

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Review comment at @src/particle.cpp around lines 536 - 539:
Update the particle cell-search flow around `exhaustive_find_cell` to apply the
same surface retry used by `Particle::event_calculate_xs` before calling
`mark_as_lost`, so revived surface-marked photons survive coincident-surface
lookup failures; then determine `cell_born()` before pulse-height bookkeeping.
Add a regression case covering a split photon with an active pulse-height tally.

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Outside diff comments:
Review comments at @src/particle.cpp:
- Around line 536-539: Update the particle cell-search flow around
`exhaustive_find_cell` to apply the same surface retry used by
`Particle::event_calculate_xs` before calling `mark_as_lost`, so revived
surface-marked photons survive coincident-surface lookup failures; then
determine `cell_born()` before pulse-height bookkeeping. Add a regression case
covering a split photon with an active pulse-height tally.

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Inline comments:
Review comments at @src/particle.cpp:
- Line 63: Update the retry logic around `p.r() += TINY_BIT * p.u()` so a
near-grazing crossing remains constrained to the crossed side, either by
ensuring the displacement exceeds the normal coordinate error or preserving the
crossed-side constraint during the retry. Add a near-grazing regression case
with a small direction component along the surface normal.

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