Of the countries 103 nuclear power reactors as many as 2/3 of them use pressurized water reactors, such as the one used at the Waterford 3 plant, and contain components made of the crack-prone metal Alloy 600. Such cracks if left unchecked have the potential to be devastating. The coolant system of the plant, which prevents the reactor from overheating and melting down the radioactive core, is dependent upon the integrity of these nozzles.
The discovery of similar cracks and an associated leak at FirstEnergy Corp.’s Davis-Besse plant near Toledo, Ohio in March of 2002 illustrate just how dangerous the situation can become. Water from the coolant system at this plant, which normally circulates through the reactor system at temperatures reaching up to 600 degrees, leaked through a crack and created a football size cavity on the outside of the reaction vessel. The thin stainless steel inner lining of the reactor vessel was all that remained to prevent the reaction vessel from bursting and the core from melting down.
The leak at the Davis-Besse plant apparently went undetected for approximately six years. Its discovery shocked the industry and validated the concern of industry watchdogs who complain that not enough is done by plant operators and regulators to monitor and repair faulty Alloy 600 components in our nations nuclear power plants.
David Lockbaum, chief nuclear safety engineer with the Union of Concerned Scientists, said, “These plants are getting older, and the older they get the more susceptible they are to this type of cracking. A small crack today can grow into a big crack in a year or two”.
Don Hintz, Entergy Corp. President lauded the effectiveness of the company’s inspection program by referencing the discovery of the cracks. He said, “It wasn’t the cracking that created the problem (at Davis-Besse). It was the fact that it was left undetected for such a long period of time. From a safety standpoint, we think all of the inspections that we are doing are adequate”.
Lockham however believes that “ a better (plant) aging management programs would have found the cracks sooner. Lockham says that while the cracks at the Waterford plant were discovered before they developed into a serious threat inspection efforts should have been more aggressive and the fissures should have been found before they fully penetrated the pipe wall.
Additionally, last November worker inspecting an Entergy plant in Russellville, Arkansas found cracks in a reactor vessel lid. These cracks were discovered only after a new inspection regime was ordered by the NRC following the discovery of cracks in the Davis-Besse plant.
Prior to the crack discovery at the Davis-Besse plant nozzles in reactor lids were only inspected visually and were not thought to be as vulnerable to cracking as other nozzles because they are not exposed to the same high stress levels that nozzles around reactor pressurizes experience. Bill Eaton, vice president of engineering for Entergy Nuclear’s reactors in the South, said that before the Davis-Besse incident “ the industry was using visual examination techniques (on reactor lids) that in some cases didn’t require the removal of insulation”.
NRC’s new inspection regime requires that all Alloy 600 areas be inspected by plant operators during each maintenance outage, even if the work requires the time-consuming removal of insulation material.
In other major nuclear power countries the growing frequency of Alloy 600 cracks in the late 80’s and early 90’s prompted more aggressive safely responses than that seen in the United States. For example, in France, Japan, and Sweden regulators and plant operators have used ultrasonic and electrical-current scanners to detect cracks that haven’t yet broken the surface of nozzles, replaced cracked reactor lids, and are using advanced monitoring systems in reactors to better detect water leakage.
The Union for Concerned Scientists in 2001 issues a paper criticizing the NRC and nuclear plant operators in the U.S. for not following the lead of these other nuclear power countries with regard to safety. The paper concluded by saying that it was unlikely that the U.S. would follow the lead of these countries until “after an accident at a U.S. reactor demonstrates that the Band-Aid fix experiments are inadequate”.