Expose xenon reactivity impact on dashboard

src/reactor_sim/dashboard.py

@@ -557,6 +557,11 @@ class ReactorDashboard:
         lines.append(fmt("Xe", "Xe (xenon)", getattr(state.core, "xenon_inventory", 0.0)))
         lines.append(fmt("Sm", "Sm (samarium)", inventory.get("Sm", 0.0)))
         lines.append(fmt("I", "I (iodine)", getattr(state.core, "iodine_inventory", 0.0)))
+        try:
+            xe_penalty = -self.reactor.neutronics.xenon_penalty(state.core)
+            lines.append(("Xe Δρ", f"{xe_penalty:+.4f}"))
+        except Exception:
+            pass
         lines.append(("Neutrons (src)", f"{particles.get('n', 0.0):9.2e}"))
         lines.append(("Gammas", f"{particles.get('gamma', 0.0):9.2e}"))
         lines.append(("Alphas", f"{particles.get('alpha', 0.0):9.2e}"))

src/reactor_sim/neutronics.py

@@ -50,7 +50,7 @@ class NeutronDynamics:
             rod_term
             + temperature_feedback(state.fuel_temperature)
             - fuel_reactivity_penalty(state.burnup)
-            - self._xenon_penalty(state)
+            - self.xenon_penalty(state)
         )
         return rho
 
@@ -96,5 +96,9 @@ class NeutronDynamics:
         decay_Xe = state.xenon_inventory * self.xenon_decay_const
         state.xenon_inventory = max(0.0, state.xenon_inventory + (prod_Xe - decay_Xe - burn_Xe) * dt)
 
+    def xenon_penalty(self, state: CoreState) -> float:
+        """Return delta-rho penalty from xenon inventory (positive magnitude)."""
+        return self._xenon_penalty(state)
+
     def _xenon_penalty(self, state: CoreState) -> float:
         return min(0.03, state.xenon_inventory * self.xenon_reactivity_coeff)

tests/test_neutronics.py

@@ -1,3 +1,5 @@
+import pytest
+
 from reactor_sim.neutronics import NeutronDynamics
 from reactor_sim.state import CoreState
 
@@ -32,3 +34,12 @@ def test_poisons_accumulate_under_power():
     dynamics.update_poisons(state, dt=100.0)
     assert state.iodine_inventory > 0.0
     assert state.xenon_inventory > 0.0
+
+
+def test_xenon_penalty_caps():
+    dynamics = NeutronDynamics()
+    state = _core_state()
+    state.xenon_inventory = 50.0
+    assert dynamics.xenon_penalty(state) == 0.03
+    state.xenon_inventory = 5.0
+    assert dynamics.xenon_penalty(state) == pytest.approx(0.01)