Pressurized Thermal Shock (PTS) — How Repressurization after Overcooling Threatens an Embrittled Reactor Vessel

Principle and mechanism

Three factors combined (two event elements plus one material condition)

1. Inner-surface quench: cold safety-injection water or secondary-side overcooling drops the inner-wall temperature quickly. The inner layer tries to contract but is held by the warmer outer layers, producing tensile thermal stress at the inner surface and compression at the outer surface.
2. Sustained pressure or repressurization: when pressure rises again through the high-pressure safety injection pumps, charging pumps, pressurizer heaters or the reclosure of a stuck-open valve, hoop stress from pressure is added.
3. Irradiation embrittlement (material condition): fast-neutron irradiation (E > 1 MeV) raises the reference nil-ductility transition temperature RT_NDT of the beltline material, so at the same temperature it sits on the lower-shelf side with low fracture toughness.

RT_NDT is the material reference temperature set from drop-weight (NDT) and Charpy tests. After irradiation, the shift ΔRT_NDT (a function of Cu and Ni content and fluence) and a margin M are added to the initial value.

Fracture-mechanics test: K_I racing K_IC

The driving force and the resistance are compared at the tip of an inner-surface crack.

• Driving force K_I = K_I(thermal stress) + K_I(pressure): it grows with thermal stress early in the overcooling and jumps again on repressurization.
• Resistance K_IC(T − RT_NDT): ASME Sec. XI lower-bound curve K_IC = 36.5 + 22.783·exp[0.036(T − RT_NDT)] MPa√m (T in °C). It falls as the crack-tip temperature falls and as RT_NDT rises.
• If K_I ≥ K_IC, the crack initiates; it arrests if, in the deeper and warmer layers, K_I does not exceed the arrest toughness K_Ia. If it cannot arrest, it can become a through-wall crack.

On the same overcooling path, the material state decides the outcome. At a crack-tip temperature of 100 °C, an unirradiated material with RT_NDT = 0 °C has so much margin that the lower-bound value exceeds the cap assumed in the figure (220 MPa√m), whereas a material embrittled to RT_NDT = 132 °C (an assumed value at the level of the screening criterion) drops to K_IC of about 44 MPa√m. A crossing means that a crack may initiate from the assumed flaw; it does not mean immediate through-wall failure.

An axial flaw faces the full hoop stress (about twice the axial stress), so it is more severe than a circumferential flaw. That is why the screening criteria differ: 270 °F for axial welds and 300 °F for circumferential welds. A crack that was first loaded at high temperature tends not to initiate while K_I is decreasing (the warm prestress, WPS, effect), so the moment K_I rises again on repressurization is especially dangerous.

What it means in a plant

Event types that cause PTS

1. Main steam line break (MSLB) and secondary-side steam release, including stuck-open turbine bypass or atmospheric dump valves and main steam safety valves: the primary side overcools, shrinks and depressurizes → safety injection → repressurization.
2. Small-break LOCA (SBLOCA): cold injection water and stagnant loop flow create cold plumes in the cold leg and downcomer.
3. Stuck-open pressurizer safety valve or PORV that later recloses: depressurization and overcooling, then repressurization when the valve closes.
4. Excess feedwater or excess auxiliary feedwater: overcooling of the primary side through the steam generators.
5. Steam generator tube rupture (SGTR): cooldown and depressurization actions with possible repressurization.
6. Medium LOCA: large cold injection (in the 50.61a re-evaluation the dominant event classes were primary pipe breaks, stuck-open primary valves that later reclose, and main steam line breaks).

Equipment that works against you

Operator mitigation actions

The basic framework is the reactor coolant system integrity function-restoration procedure of the emergency operating procedures, for example the Westinghouse ERG family's "response to imminent PTS".

1. Isolate the source of overcooling: isolate the faulted steam generator, stop feedwater to it, and isolate stuck-open valves.
2. When the safety injection throttling/termination criteria are met (subcooling margin, pressurizer level, secondary heat removal, RCS pressure stable or rising), reduce injection flow → prevent repressurization. The SI termination criteria of Westinghouse ERG FR-P.1 are relaxed compared with other procedures, so injection can be cut earlier.
3. Stop pressurizer heaters and lower pressure with auxiliary spray or the PORV to stay within the P-T limits.
4. Stop further cooldown and hold (soak) the cold-leg temperature. In the Westinghouse ERG this lasts about one hour, counted from the time the temperature has stabilized; the cooldown rate afterwards follows the unit procedure. 56 °C/h (100 °F/h) is an example of a normal-operation technical specification cooldown limit, not the post-soak cooldown rate.
5. If SI cannot be terminated, restart an RCP when conditions allow, mixing the injected water to break up the cold plume.

Design and operating countermeasures

• Flux reduction: low-leakage loading (burned fuel at the core periphery) and shielding assemblies facing critical welds slow the growth of fluence.
• Annealing: heating the beltline to several hundred °C recovers irradiation damage (10 CFR 50.66). There is experience on VVER-440 vessels.
• Materials and design: low Cu and Ni specifications, ring forgings that eliminate axial welds (APR1400 and other advanced reactors), and correction with measured surveillance-specimen data.
• Refined evaluation: plant-specific probabilistic fracture-mechanics assessment, and switching to 50.61a.

Regulatory and design criteria

The criteria below are US NRC requirements, given as an example of how PTS is regulated:

• 10 CFR 50.61 (issued 1985, revised 1991): RT_PTS = RT_NDT(U) + M + ΔRT_PTS (EOL fluence at the inner surface), M = 2√(σ_U² + σ_Δ²), σ_Δ = 28 °F (welds) / 17 °F (base metal).
• Screening criteria: plates, forgings and axial welds 270 °F (132 °C); circumferential welds 300 °F (149 °C).
• If exceedance is projected: reasonably practicable flux reduction → if exceedance is still projected, submit a safety analysis at least three years before the projected date → case-by-case approval. Annealing under §50.66 is an available alternative.
• 10 CFR 50.61a (2010, voluntary): alternative screening criteria (RT_MAX-X) and nondestructive examination requirements, based on a probabilistic re-evaluation (NUREG-1806/1874) targeting a through-wall crack frequency of about 1×10⁻⁶ per reactor-year. The RT_MAX limits depend on wall thickness (for example, at 9.5 in or less: axial welds 269 °F, plates 356 °F, circumferential welds 312 °F).
• RG 1.99 Rev. 2 (1988): prediction of irradiation embrittlement; RG 1.154 (1987): plant-specific PTS analysis; 10 CFR 50 Appendices G and H: P-T limits and upper-shelf energy, surveillance testing.

Easily confused pairs

Key points at a glance

1. Definition: an event of rapid overcooling of the vessel inner surface with concurrent or subsequent high pressure (repressurization) (10 CFR 50.61(a)(2)).
2. Risk factors: crack risk grows when overcooling plus pressure (the event) meets beltline irradiation embrittlement (higher RT_NDT, a material condition).
3. Stress: restrained inner-surface contraction → tensile thermal stress at the inner surface + hoop stress from pressure.
4. Criterion: at the crack tip, K_I ≥ K_IC(T − RT_NDT) → initiation; arrest is judged with K_Ia.
5. Initiating events: MSLB, SBLOCA, stuck-open PORV or safety valve that recloses, excess feedwater, SGTR.
6. Screening criteria: RT_PTS ≤ 270 °F (plates, forgings, axial welds), 300 °F (circumferential welds).
7. If exceedance is projected: flux reduction → if still exceeded, a safety analysis three years ahead; annealing (50.66) is optional; 50.61a (TWCF 10⁻⁶) is an alternative route.
8. Operator actions: isolate the overcooling source, throttle or terminate SI, reduce pressure, stop cooldown and hold temperature, restart an RCP.
9. Design: low-leakage loading, low Cu and Ni, ring forgings without axial welds, surveillance specimens.