Add delayed kinetics and steam-drum balance

tests/test_neutronics.py

@@ -43,3 +43,15 @@ def test_xenon_penalty_caps():
     assert dynamics.xenon_penalty(state) == 0.05
     state.xenon_inventory = 0.2
     assert dynamics.xenon_penalty(state) == pytest.approx(0.01)
+
+
+def test_delayed_precursors_follow_rod_banks():
+    dynamics = NeutronDynamics()
+    state = _core_state(power=600.0, flux=5e6)
+    dynamics.step(state, control_fraction=0.2, dt=1.0, rod_banks=[0.2, 0.2, 0.2])
+    initial_sum = sum(state.delayed_precursors)
+    assert len(state.delayed_precursors) == 3
+    assert initial_sum > 0.0
+
+    dynamics.step(state, control_fraction=0.95, dt=2.0, rod_banks=[0.95, 0.95, 0.95])
+    assert sum(state.delayed_precursors) < initial_sum

tests/test_thermal.py

@@ -0,0 +1,37 @@
+import pytest
+
+from reactor_sim import constants
+from reactor_sim.state import CoolantLoopState
+from reactor_sim.thermal import ThermalSolver, saturation_temperature
+
+
+def _secondary_loop(temp_in: float = 350.0, pressure: float = 0.5, flow: float = 200.0) -> CoolantLoopState:
+    nominal_mass = constants.SECONDARY_LOOP_VOLUME_M3 * constants.COOLANT_DENSITY
+    return CoolantLoopState(
+        temperature_in=temp_in,
+        temperature_out=temp_in,
+        pressure=pressure,
+        mass_flow_rate=flow,
+        steam_quality=0.0,
+        inventory_kg=nominal_mass,
+        level=constants.SECONDARY_INVENTORY_TARGET,
+    )
+
+
+def test_secondary_heats_to_saturation_before_boiling():
+    solver = ThermalSolver()
+    loop = _secondary_loop(temp_in=330.0, flow=180.0, pressure=0.5)
+    sat_temp = saturation_temperature(loop.pressure)
+    solver.step_secondary(loop, transferred_mw=50.0, dt=1.0)
+    assert loop.steam_quality == pytest.approx(0.0)
+    assert loop.temperature_out < sat_temp
+
+
+def test_secondary_generates_steam_when_energy_exceeds_sensible_heat():
+    solver = ThermalSolver()
+    loop = _secondary_loop(temp_in=330.0, flow=180.0, pressure=0.5)
+    sat_temp = saturation_temperature(loop.pressure)
+    solver.step_secondary(loop, transferred_mw=120.0, dt=1.0)
+    assert loop.temperature_out == pytest.approx(sat_temp, rel=0.02)
+    assert loop.steam_quality > 0.0
+    assert loop.steam_quality < 1.0