Codes and Standards Corner

Safety in the handling of heavy loads at nuclear power plants and other nuclear facilities is of critical importance. Dropping or otherwise losing control of a load in an area where the result could be damage to safety-critical equipment or spent fuel is unacceptable. With this in mind, the ASME Committee on Cranes for Nuclear Facilities (CNF) has written standards for overhead cranes-the equipment that handles the great majority of heavy loads.

The Committee on Cranes was established in 1976, shortly after the US Nuclear Regulatory Commission (NRC) first issued written guidelines for safety-critical, or single-failure-proof (SFP) cranes. In 1980, the committee's scope was broadened from nuclear power plants to nuclear (and other critical load handling) facilities. Its first standard, ASME NOG-1, Rules for Construction of Overhead and Gantry Cranes, was issued in 1983. Its second standard, NUM-1, Rules for Construction of Cranes, Monorails, and Hoists, followed in 1996. Currently the CNF Standards Committee consists of some 20 members representing a broad spectrum of power plant and nuclear facility owners and operators, engineers, regulatory personnel, constructors, equipment suppliers and crane manufacturers.

No other industry or government standards, including those of the Crane Manufacturers Association of America (CMAA), have addressed issues important for nuclear facility cranes such as quality assurance, dynamic seismic analysis, and SFP crane features. NOG-1 and NUM-1 cover all these and more.

It would be difficult to overstate the importance of using the safest crane design for handling heavy loads in nuclear facilities.

In a recent Nuclear News magazine article [Safety: The foundation upon which economic value is built", Nuclear News, August 2001], NRC Commissioner Jeffrey Merrifield said "I believe that the future of the nuclear industry does not hinge on corporate decisions about new plants-it hinges on the safety of the existing fleet of reactors" and also, "Anyone who believes that safety and economic value are mutually exclusive goals is simply blind to the realities that history has unmistakably, and sometimes painfully, taught this industry." In today's political and social climate, when the potential for construction of new nuclear plants in the U.S. is being voiced in high places and numerous operating plants are applying for operating life extensions, a heavy load drop would be potentially disastrous for the industry, even without an offsite release.

The NRC defines a "heavy load" as anything weighing more than a nuclear fuel assembly (approximately one ton). A spent fuel cask may weigh 100 to 150 tons or more, and if dropped into the spent fuel pool could cause major damage to the pool and/or the fuel, resulting in a radioactive release. Dropped elsewhere, it could damage safe shutdown equipment or rupture the cask itself. The empty hook block on such a crane can weigh 4 or 5 tons and can itself cause severe damage if it falls into the fuel pool or onto safety-related equipment.

Two of the primary causes of load drops identified by the NRC are severe types of overloads known as "two blocking" and "load hangup." Two blocking occurs when control malfunction or maintenance error permits unintentional overhoisting, resulting in solid contact between the hook block and the head block or its supporting structure.

The typical consequence is immediate failure of the wire rope and dropping the load or empty hook block. Load hangup is abrupt snagging of the moving load, resulting in similarly severe effects on the equipment. Other causes of dropped loads include "mis-reeving" of the hoist wire rope and failure of the hoisting machinery itself.

It is the intent of the NRC guidelines for SFP cranes as well as ASME NOG-1 "Type I" and NUM "Type I" SFP standards to have a crane designed either to withstand all such incidents without damage or loss of load, or to make the likelihood of their occurrence extremely small. Type I NOG-1 and NUM-1 crane standards in particular incorporate design requirements specific to these concerns, going well beyond those of other crane standards.

The work of the CNF Committee, and the application of its standards, has never been more relevant. The last few years have seen a tremendous increase in handling of spent fuel casks to move fuel from plant fuel pools to independent spent fuel storage installations (ISFSI). Numerous operating plants have reached or are approaching the time when they must remove spent fuel from their pools in order to continue operation. Many plants delayed addressing heavy load handling (crane) concerns until faced with this imminent need. Decommissioning plants face many of the same issues as operating plants when handling spent fuel.

The CNF Committee has a wealth of technical information on cranes, and can be contacted through its Secretary, Shannon Burke by e-mail, at burkes@asme.org or by phone at 212-591-8514. Information on the Committee is also available on the ASME website at www.asme.org.