Improve persistence and reactor dynamics

tests/test_neutronics.py

@@ -0,0 +1,26 @@
+from reactor_sim.neutronics import NeutronDynamics
+from reactor_sim.state import CoreState
+
+
+def _core_state(
+    temperature: float = 950.0,
+    flux: float = 2e7,
+    burnup: float = 0.01,
+    power: float = 500.0,
+) -> CoreState:
+    return CoreState(
+        fuel_temperature=temperature,
+        neutron_flux=flux,
+        reactivity_margin=0.0,
+        power_output_mw=power,
+        burnup=burnup,
+    )
+
+
+def test_reactivity_increases_with_rod_withdrawal():
+    dynamics = NeutronDynamics()
+    state = _core_state()
+    rho_full_out = dynamics.reactivity(state, control_fraction=0.0)
+    rho_half = dynamics.reactivity(state, control_fraction=0.5)
+    assert rho_full_out > 0.0
+    assert rho_full_out > rho_half