Improve reactor controls, reactions, and maintenance UI

src/reactor_sim/fuel.py

@@ -29,6 +29,7 @@ class FuelAssembly:
     mass_kg: float
     fissile_atom: Atom = field(default_factory=lambda: make_atom(92, 143))
     atomic_physics: AtomicPhysics = field(default_factory=AtomicPhysics)
+    decay_activity_base: float = 1e16  # nominal decay events per second at burnup 0
 
     def available_energy_j(self, state: CoreState) -> float:
         fraction_remaining = max(0.0, 1.0 - state.burnup)
@@ -57,3 +58,52 @@ class FuelAssembly:
             power_mw,
         )
         return max(0.0, power_mw), event_rate, event
+
+    def decay_reaction_effects(
+        self, state: CoreState
+    ) -> tuple[float, float, dict[str, float], dict[str, float]]:
+        """Return (power MW, neutron source rate, product rates, particle rates)."""
+        # Scale activity with burnup (fission products) and a small flux contribution.
+        activity = self.decay_activity_base * (0.05 + state.burnup) * (1.0 + 0.00001 * state.neutron_flux)
+        activity = max(0.0, min(activity, 1e20))
+
+        reactions = [
+            self.atomic_physics.alpha_decay(self.fissile_atom),
+            self.atomic_physics.beta_minus_decay(self.fissile_atom),
+            self.atomic_physics.beta_plus_decay(self.fissile_atom),
+            self.atomic_physics.electron_capture(self.fissile_atom),
+            self.atomic_physics.gamma_emission(self.fissile_atom),
+            self.atomic_physics.spontaneous_fission(self.fissile_atom),
+        ]
+        weights = [1.0, 1.2, 0.5, 0.4, 1.6, 0.05]
+        total_weight = sum(weights)
+
+        power_watts = 0.0
+        neutron_source = 0.0
+        product_rates: dict[str, float] = {}
+        particle_rates: dict[str, float] = {}
+
+        for event, weight in zip(reactions, weights):
+            rate = activity * (weight / total_weight)
+            power_watts += rate * event.energy_mev * constants.MEV_TO_J
+            for atom in event.products:
+                product_rates[atom.symbol] = product_rates.get(atom.symbol, 0.0) + rate
+            for particle in event.emitted_particles:
+                particle_rates[particle] = particle_rates.get(particle, 0.0) + rate
+                if particle == "n":
+                    neutron_source += rate
+
+        power_mw = power_watts / constants.MEGAWATT
+        capped_power = min(power_mw, 0.1 * max(state.power_output_mw, 1.0))
+        LOGGER.debug(
+            "Decay reactions contribute %.2f MW (raw %.2f MW) with neutron source %.2e 1/s",
+            capped_power,
+            power_mw,
+            neutron_source,
+        )
+        return max(0.0, capped_power), neutron_source, product_rates, particle_rates
+
+    def decay_reaction_power(self, state: CoreState) -> float:
+        """Compatibility shim: return only power contribution."""
+        power_mw, _, _, _ = self.decay_reaction_effects(state)
+        return power_mw