Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the pressure. Thermal stresses exist whenever temperature gradients are present in a material, and different temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures cooled by a cold fluid. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials.
By contrast, the reactor pressure vessel and piping are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves, such as spray nozzles and surge lines, are used in some cases to minimize thermal stresses. Heat up and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heat up and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include, for example, heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps to reduce the temperature difference between injected water and the material of RPV.
One safety issue that is a long-term problem brought on by the aging of nuclear facilities is pressurized thermal shock (PTS). PTS is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change.
Special Reference: Reactor Pressure Vessel Status Report, U.S. NRC. NUREG-1511. Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission, Washington, 1994.
Pressurized Thermal Shock – PTS
In general thermal shock is a mechanical load caused by a rapid change of temperature at a certain point. The change in temperature causes stresses on the surface that is in tension, which can encourage crack formation and propagation. Usually, ceramics materials are susceptible to thermal shock, but under some circumstances, pressurized vessels alsos suffer from thermal shocks. With rapid heating (or cooling) of a thick-walled vessel such as the reactor pressure vessel, one part of the wall may try to expand (or contract) while the adjacent section, which has not yet been exposed to the temperature change, tries to restrain it.
Pressurized Thermal Shock, PTS, means an event or transient in pressurized water reactors (PWRs) causing severe overcooling (thermal shock) concurrent with or followed by significant pressure in the reactor vessel. In this accident scenario, cold water enters a reactor while the vessel is pressurized, and this rapidly cools the vessel and places large thermal stresses on the steel. Severe reactor system overcooling events that could be accompanied by pressurization or repressurization of the reactor vessel can result from various causes. Pressure in the reactor system raises the severity of the thermal shock due to the addition of stress from the pressure. Transients, which combine high system pressure and a severe thermal shock, are potentially more dangerous due to the added effect of the tensile stresses on the inside of the reactor vessel wall. PTS-related transients include:
- stuck-open valves in the primary system,
- stuck-open valves in the secondary system,
- small-break loss-of-coolant accidents with subsequent injection of emergency core cooling system (ECCS) water,
- main steam line breaks,
- feedwater line breaks.
The NRC created 10 CFR Part 50.61 and 50.61a – the “PTS rule” and “alternate PTS rule” – to ensure the vessel’s steel remains strong enough to protect the vessel’s integrity. These rules require additional evaluations or other actions if embrittlement reaches certain limits.
RTNDT = RTNDT(U) + M + ΔRTNDT
- RTNDT means the reference temperature for a reactor vessel material under any conditions. For the reactor vessel beltline materials, RTNDT must account for the effects of neutron radiation.
- RTNDT(U) is the reference temperature for a reactor vessel material in pre-service or unirradiated conditions.
- ΔRTNDT is the increase in RTNDT caused by irradiation
- M is a margin added to cover uncertainties in the initial properties, copper and nickel contents, fluence, and calculation procedures. The greater the amounts of copper, nickel, and neutron fluence, the greater the increase.
As long as the fracture toughness of the reactor vessel material is relatively high, such events will not threaten RPV integrity. However, the fracture toughness of reactor vessel materials decreases with exposure to fast neutrons during the life of a nuclear power plant. Suppose the fracture toughness of the vessel material has been reduced sufficiently. In that case, severe PTS events could cause the propagation of small flaws that might exist near the inner surface of the vessel. The assumed initial flaw might propagate into a crack through the vessel wall to a sufficient extent to threaten vessel integrity and, therefore, core cooling capability.
While PTS doesn’t affect boiling-water reactors, there are very limited conditions where those vessels could overpressurize at low temperatures.
Special Reference: NUREG-1511, Reactor Pressure Vessel Status Report. U.S. Nuclear Regulatory Commission, Washington, DC, 1994.
Special Reference: DOE FUNDAMENTALS HANDBOOK MATERIAL SCIENCE Volume 2 of 2, DOE-HDBK-1017/2-93, Washington, DC, 1993.