feat: add reactor control persistence and tests

src/reactor_sim/fuel.py

@@ -0,0 +1,56 @@
+"""Fuel behavior, burnup, and decay heat modeling."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+import logging
+
+from . import constants
+from .atomic import Atom, AtomicPhysics, FissionEvent, make_atom
+from .state import CoreState
+
+LOGGER = logging.getLogger(__name__)
+
+
+def fuel_reactivity_penalty(burnup: float) -> float:
+    """Simplistic model that penalizes reactivity as burnup increases."""
+    # Burnup is 0-1, penalty grows quadratically to mimic depletion.
+    return 0.4 * burnup**2
+
+
+def decay_heat_fraction(burnup: float) -> float:
+    """Return remaining decay heat fraction relative to nominal power."""
+    return min(0.07 + 0.2 * burnup, 0.15)
+
+
+@dataclass
+class FuelAssembly:
+    enrichment: float  # fraction U-235
+    mass_kg: float
+    fissile_atom: Atom = field(default_factory=lambda: make_atom(92, 143))
+    atomic_physics: AtomicPhysics = field(default_factory=AtomicPhysics)
+
+    def available_energy_j(self, state: CoreState) -> float:
+        fraction_remaining = max(0.0, 1.0 - state.burnup)
+        return self.mass_kg * constants.FUEL_ENERGY_DENSITY * fraction_remaining
+
+    def simulate_electron_hit(self) -> FissionEvent:
+        return self.atomic_physics.electron_induced_fission(self.fissile_atom)
+
+    def prompt_energy_rate(self, flux: float, control_fraction: float) -> tuple[float, FissionEvent]:
+        """Compute MW thermal from prompt fission by sampling atomic physics."""
+        event = self.simulate_electron_hit()
+        effective_flux = max(0.0, flux * max(0.0, 1.0 - control_fraction))
+        atoms = self.mass_kg / self.fissile_atom.atomic_mass_kg
+        event_rate = effective_flux * constants.ELECTRON_FISSION_CROSS_SECTION * atoms * self.enrichment
+        power_watts = event_rate * event.energy_mev * constants.MEV_TO_J
+        power_mw = power_watts / constants.MEGAWATT
+        LOGGER.debug(
+            "Prompt fission products %s-%d + %s-%d yielding %.2f MW",
+            event.products[0].symbol,
+            event.products[0].mass_number,
+            event.products[1].symbol,
+            event.products[1].mass_number,
+            power_mw,
+        )
+        return max(0.0, power_mw), event