Reduce secondary pump head and ease secondary heating

src/reactor_sim/state.py

@@ -18,12 +18,17 @@ class CoreState:
     reactivity_margin: float  # delta rho
     power_output_mw: float  # MW thermal
     burnup: float  # fraction of fuel consumed
+    clad_temperature: float | None = None  # Kelvin
     xenon_inventory: float = 0.0
     iodine_inventory: float = 0.0
     delayed_precursors: list[float] = field(default_factory=list)
     fission_product_inventory: dict[str, float] = field(default_factory=dict)
     emitted_particles: dict[str, float] = field(default_factory=dict)
 
+    def __post_init__(self) -> None:
+        if self.clad_temperature is None:
+            self.clad_temperature = self.fuel_temperature
+
     def update_burnup(self, dt: float) -> None:
         produced_energy_mwh = self.power_output_mw * (dt / 3600.0)
         self.burnup = clamp(self.burnup + produced_energy_mwh * 1e-5, 0.0, 0.99)
@@ -111,6 +116,10 @@ class PlantState:
         core_blob = dict(data["core"])
         inventory = core_blob.pop("fission_product_inventory", {})
         particles = core_blob.pop("emitted_particles", {})
+        # Backwards/forwards compatibility for optional core fields.
+        core_blob.pop("dnb_margin", None)
+        core_blob.pop("subcooling_margin", None)
+        core_blob.setdefault("clad_temperature", core_blob.get("fuel_temperature", 295.0))
         turbines_blob = data.get("turbines")
         if turbines_blob is None:
             # Compatibility with previous single-turbine snapshots.