RHR Heat Exchanger Fouling and Decay Heat Removal
During a refueling outage, the RHR heat exchanger is tested with the reactor shutdown and decay heat at 18 MW. Primary-side flow is 400 kg/s with cp = 4.30 kJ/kg·K, inlet temperature 120°C. Secondary-side flow is 500 kg/s with cp = 4.18 kJ/kg·K, inlet temperature 30°C. The heat exchanger is counterflow and its clean overall conductance is UA = 350 kW/K. Inspection suggests fouling may have reduced UA by 20%, to 280 kW/K. The RHR heat exchanger design duty is 30 MW. The outage schedule allows either cleaning now or deferring to the next outage. The system engineer asks whether the fouled heat exchanger can still remove decay heat with acceptable margin.
1. Calculate the heat removal rate for the clean and fouled conditions using the effectiveness-NTU method.
Read the worked answer
Heat capacity rates: C_primary = 400 × 4.30 = 1720 kW/K; C_secondary = 500 × 4.18 = 2090 kW/K. Thus C_min = 1720 kW/K, C_max = 2090 kW/K, and C_r = 1720/2090 = 0.823. Clean: NTU = UA/C_min = 350/1720 = 0.2035. For counterflow, ε = [1 − exp(−NTU(1−C_r))]/[1 − C_r exp(−NTU(1−C_r))]. NTU(1−C_r) = 0.2035 × 0.177 = 0.0360; exp(−0.0360) = 0.9646. ε = (1 − 0.9646)/(1 − 0.823 × 0.9646) = 0.0354/0.2061 = 0.172. Q_clean = ε C_min ΔT_in = 0.172 × 1720 × (120 − 30) = 26.6 MW. Fouled: NTU = 280/1720 = 0.1628; NTU(1−C_r) = 0.0288; exp(−0.0288) = 0.9716. ε = (1 − 0.9716)/(1 − 0.823 × 0.9716) = 0.0284/0.2004 = 0.142. Q_fouled = 0.142 × 1720 × 90 = 22.0 MW.
2. Is the fouled heat exchanger acceptable for 18 MW decay heat? State the margin and the operating parameter you would monitor most closely.
Read the worked answer
The fouled duty is 22.0 MW against 18 MW decay heat, giving a margin of 4.0 MW, or about 22% above the current decay heat. That is positive but modest, and the margin shrinks if secondary inlet temperature rises or primary flow falls. The most sensitive parameter to monitor is secondary-side inlet temperature: if it rises from 30°C to 35°C, the available ΔT drops from 90 K to 85 K and the duty falls to about 20.8 MW. I would also monitor primary outlet temperature and RHR pump flow, because a lower primary flow reduces C_min and the heat removal rate. Given the thin margin and the outage opportunity, cleaning now is the lower-risk decision; deferral should require a documented calculation showing adequate margin at the worst expected secondary temperature and flow.