Backgrounder on Reactor Pressure Vessel Issues
Reactor pressure vessels are thick steel containers that hold nuclear fuel when the reactors operate. The vessels provide one of several barriers that keep radioactive fuel contained and out of the environment. U.S. nuclear power plants follow NRC regulations to inspect, maintain and repair reactor pressure vessels. Several technical issues must be addressed as the vessels operate over long periods of time.
Reactor operation generates subatomic particles called neutrons. Some of these neutrons hit atoms in the steel as they leave the core. These neutron impacts can make the steel brittle and less able to handle the stresses of operation. Embrittlement usually occurs in a vessel's middle section, closest to the reactor fuel.
Vessels in pressurized-water reactors can face more neutron impacts and are more susceptible to embrittlement than vessels in boiling-water reactors. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel's embrittlement. The percentage of copper and nickel in the steel also affects how slowly a vessel becomes embrittled. The NRC's regulations regarding embrittlement include 10 CFR Part 50, Appendix G "Fracture Toughness Requirements" and Appendix H, "Reactor Vessel Material Surveillance Program Requirements."
Pressurized-water reactors also take embrittlement into account because of a phenomenon called pressurized thermal shock, or PTS. This is an accident scenario where cold water enters a reactor while the vessel is pressurized. This rapidly cools the vessel and places large thermal stresses on the steel. Under these conditions an embrittled vessel could crack and even fail. This would seriously challenge the plant's ability to keep the public safe. The NRC created 10 CFR part 50.61 and 50.61a – the "PTS rule" and "alternate PTS rule" – to limit how brittle pressurized-water reactor vessels can get. These rules require additional evaluations or other actions if embrittlement reaches certain limits. While PTS doesn't affect boiling-water reactors, there are very limited conditions where those vessels could overpressurize at low temperatures.
The NRC summarized these issues in an October 2000 supplement to NUREG-1511, "Reactor Pressure Vessel Status Report." The NRC expects all reactors will maintain adequate toughness throughout their operating lives.
Cracking of Upper Reactor Vessel Head Nozzles
Reactor vessels have a removable cap, or "head," for loading fuel or doing maintenance inside the vessel. The heads at pressurized-water reactors include access nozzles for devices such as rods that control the nuclear reaction. These nozzles are welded to the head. The reactors' combination of high temperature, pressure and chemicals in the water can lead to cracks in the welds. This could lead to safety issues, since a nozzle with enough cracking could break off during operation. This would breach the reactor vessel boundary, one of three primary barriers that keep radioactive material out of the environment. A broken nozzle could also eject a control rod and damage other safety systems.
The NRC has worked to ensure U.S. reactors properly deal with this issue. In 2001, the NRC issued a Bulletin to U.S. pressured water reactors, asking about nozzle structural integrity. The plants responded with plans for inspecting their nozzles and the outside of the vessel heads to spot any cracked or leaking nozzles. Plant inspections later that year found nozzle cracking at four plants, all of which repaired their nozzles.
In 2002 the NRC issued another Bulletin that suggested plants might need to supplement their visual inspections of vessel heads with machine-based examinations. The staff used this Bulletin to ask pressurized-water reactors about their inspection programs, as well as any plans to supplement visual inspections. U.S. pressurized-water reactors described their immediate inspection plans, with many plants saying their long-term inspections would follow guidance from an industry research effort, the Materials Reliability Program.
Many pressurized-water reactors repaired vessel head nozzle and weld cracking after inspections in 2002. Most plants took the additional step of purchasing new vessel heads made of an alloy much less susceptible to cracking. Based on the amount of cracking found, the NRC issued an Order in February 2003 for specific inspections of U.S. pressurized-water reactor vessel heads and associated nozzles. The NRC updated the Order in February 2004. In 2008, the NRC issued a rule requiring all pressurized-water reactors to follow the American Society of Mechanical Engineer's Boiler and Pressure Vessel Code Case N-729-1, with NRC conditions. By replacing the Order, the rule established long-term inspection requirements for vessel heads and associated nozzles and welds. Today 7 plants have vessel heads with repaired nozzles and approximately 40 U.S. pressurized-water reactors have completed replacing their vessel heads.
Reactor Vessel Head Damage at Davis-Besse
The Davis-Besse nuclear reactor in Ohio, responding to Bulletin 2001-01, identified in March 2002 a football-sized cavity in the reactor vessel head. The cavity was next to a cracked, leaking nozzle, in an area of the vessel head covered with deposits from years of leaks. A few days after Davis-Besse discovered the cavity, the NRC issued a Confirmatory Action Letter to the plant owner, First Energy Nuclear Corporation. The letter confirmed the plant would remain shut down until the company evaluated and resolved damage to the vessel head. Later analysis concluded the leaking nozzle created boric acid, which corroded the vessel head's steel and created the cavity.
In 2003, the NRC issued a Bulletin to address the generic implications of the Davis-Besse information on the safe operation of U.S. pressurized-water reactors. The NRC reviewed the industry's responses to the Bulletin in order to ensure proper actions were taken. More information on the agency and industry response to the Davis-Besse corrosion is available on the NRC website.
Cracking of Lower Reactor Vessel Head Nozzles
In 2003, inspections at the South Texas Project Unit 1 pressurized-water reactor discovered small deposits of boron (found in reactor coolant) at the bottom of the reactor vessel. The deposits centered on two bottom-mounted instrumentation nozzles. Further examination of all bottom-mounted nozzles at the South Texas nuclear power plant confirmed only those two nozzles were cracked. The plant repaired the cracked nozzles and returned normal operations later in 2003.
The South Texas plant reviewed the inspection results and analysis of a material sample from one of the leaking nozzles. The plant concluded the nozzles had small defects from their original construction. These defects most likely created conditions that allowed the reactor's high temperature, pressure and water chemistry to crack the nozzles.
On August 13, 2003, the NRC issued NRC Information Notice 2003-11, "Leakage Found on Bottom-Mounted Instrumentation Nozzles," to inform members of the U.S. nuclear power industry and members of the public of this event. Information Notice 2003-11 is available on NRC's web site.
On August 21, 2003, the NRC issued NRC Bulletin 2003-02, "Leakage from Reactor Pressure Vessel Lower Head Penetrations and Reactor Coolant Pressure Boundary Integrity," in order to address the generic safety implications of the South Texas cracking experience on operations of pressurized water reactors in the U.S. and specifically as the operations relate to maintaining the health and safety of the public. The staff is currently in the progress of reviewing the industry's responses to NRC Bulletin 2003-02, which is available on NRC's web site.