Add mild thermal/measurement lag and expose margins on dashboard

src/reactor_sim/constants.py

@@ -79,6 +79,10 @@ HX_FOULING_HEAL_RATE = 5e-6  # cleaning/settling when cool/low steam
 HX_FOULING_MAX_PENALTY = 0.25  # fractional UA loss cap
 BORON_WORTH_PER_PPM = 8e-6  # delta rho per ppm relative to baseline boron
 BORON_TRIM_RATE_PPM_PER_S = 0.02  # slow boron trim toward setpoint when near target
+# Mild thermal/measurement lags
+FUEL_TO_CLAD_TIME_CONSTANT = 0.3  # seconds (mild lag)
+CLAD_TO_COOLANT_TIME_CONSTANT = 0.2  # seconds (mild lag)
+POWER_MEASUREMENT_TIME_CONSTANT = 6.0  # seconds
 # Threshold inventories (event counts) for flagging common poisons in diagnostics.
 KEY_POISON_THRESHOLDS = {
     "Xe": 1e20,  # xenon

src/reactor_sim/control.py

@@ -41,7 +41,8 @@ class ControlSystem:
         if self._filtered_power_mw <= 0.0:
             self._filtered_power_mw = raw_power
         if raw_power > 0.7 * self.setpoint_mw:
-            alpha = clamp(dt / 6.0, 0.0, 1.0)  # ~6s time constant
+            tau = constants.POWER_MEASUREMENT_TIME_CONSTANT
+            alpha = clamp(dt / max(1e-6, tau), 0.0, 1.0)
             self._filtered_power_mw += alpha * (raw_power - self._filtered_power_mw)
             measured_power = self._filtered_power_mw
         else:

src/reactor_sim/dashboard.py

@@ -784,6 +784,15 @@ class ReactorDashboard:
             reliefs.append("Secondary")
         relief_attr = curses.color_pair(2) | curses.A_BOLD if reliefs else 0
         lines.append(("Relief valves", ", ".join(reliefs) if reliefs else "Closed", relief_attr))
+        lines.append(("DNB margin", f"{state.core.dnb_margin:5.2f}" if state.core.dnb_margin is not None else "n/a"))
+        lines.append(("Subcooling", f"{state.core.subcooling_margin:5.1f} K" if state.core.subcooling_margin is not None else "n/a"))
+        lines.append(
+            (
+                "SG level",
+                f"{state.secondary_loop.level*100:5.1f}%",
+            )
+        )
+        lines.append(("SG pressure", f"{state.secondary_loop.pressure:5.2f}/{constants.MAX_PRESSURE:4.1f} MPa"))
         lines.append(
             (
                 "SCRAM trips",

src/reactor_sim/thermal.py

@@ -114,8 +114,16 @@ class ThermalSolver:
         dt: float,
         residual_power_mw: float | None = None,
     ) -> None:
+        def _lag(prev: float, new: float, tau: float) -> float:
+            if tau <= 0.0:
+                return new
+            alpha = min(1.0, max(0.0, dt / max(1e-6, tau)))
+            return prev + alpha * (new - prev)
+
         if residual_power_mw is None:
             residual_power_mw = power_mw
+        prev_fuel = core.fuel_temperature
+        prev_clad = core.clad_temperature or primary.temperature_out
         temp_rise = temperature_rise(power_mw, primary.mass_flow_rate)
         primary.temperature_out = primary.temperature_in + temp_rise
         # Fuel heats from total fission power (even when most is convected) plus any residual left in the coolant.
@@ -136,6 +144,9 @@ class ThermalSolver:
         # Keep temperatures bounded and never below coolant outlet.
         core.fuel_temperature = min(core.fuel_temperature, constants.MAX_CORE_TEMPERATURE)
         core.clad_temperature = min(clad, constants.MAX_CORE_TEMPERATURE)
+        # Apply mild lags so heat moves from fuel to clad to coolant over a short time constant.
+        core.fuel_temperature = _lag(prev_fuel, core.fuel_temperature, constants.FUEL_TO_CLAD_TIME_CONSTANT)
+        core.clad_temperature = _lag(prev_clad, core.clad_temperature or prev_clad, constants.CLAD_TO_COOLANT_TIME_CONSTANT)
         core.subcooling_margin = max(0.0, saturation_temperature(primary.pressure) - primary.temperature_out)
         chf = self._critical_heat_flux(primary)
         heat_flux = (power_mw * constants.MEGAWATT) / max(1.0, self._core_surface_area())