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THE PEBBLE BED MODULAR REACTOR (PBMR)

 

THE PBMR: "OLD WINE IN A NEW BOTTLE"

The Pebble Bed Modular Reactor (PBMR) is being re-introduced in an industry effort to revive an all-but-moribund nuclear power technology. The PBMR’s basic design concept, the high-temperature gas-cooled reactor (HTGR), has been commercially abandoned time and again without tangible benefit over the past thirty years in England, France, Germany and with the 1967 and 1989 closures of the Peach Bottom Unit 1 and Fort St. Vrain reactors in the United States. Small HTGR non-power research reactors currently operate in Japan and China. For as many years, the concept has been offered as an "inherently safe" design.

The current PBMR project is a hybrid of these past efforts and is piloted by an international conglomerate of U.S.-based Exelon Corporation (Commonwealth Edison, PECO Energy, and British Energy), British Nuclear Fuels Limited and South African-based ESKOM as "merchant" nuclear power plants. The consortium plans to begin the construction by 2002 of a full-size prototype of a 110 MW modular unit in Koeberg, South Africa. If successful, commercial operation would begin in 2006.

Exelon hopes to use this prototype to obtain a license through the Nuclear Regulatory Commission to begin construction of seven new reactors on an unspecified site in the U.S. by the summer of 2004. The PBMR is proposed as a standardized design that can be built in as little as two years, with multiple modular units combined onto a single site.

NO REACTOR CONTAINMENT BUILDING AND REDUCED SAFETY SYSTEMS CUT PBMR COSTS

Unlike light water reactors that use water and steam, the PBMR design would use pressurized helium heated in the reactor core to drive a series of turbine compressors that attach to an electrical generator. The helium is cycled to a recuperator to be cooled down and returned to cool the reactor while the waste heat is discharged to the environment. Designers claim there are no accident scenarios that would result in significant fuel damage and catastrophic release of radioactivity.

These industry safety claims rely on the heat resistant quality and integrity of the tennis ball-sized graphite fuel assemblies or "pebbles," 400,000 of which are continuously fed from a fuel silo through the reactor "little by little" to keep the reactor core only marginally critical. Each spherical fuel element has an inner graphite core embedded with thousands of smaller fuel particles of enriched uranium (up to 10 %) encapsulated in multi-layers of non-porous hardened carbon. The slow circulation of fuel through the reactor provides for a small core size that minimizes excess core reactivity and lowers power density, all of which is credited to safety.

However, so much credit is given to the integrity and quality control of the coated fuel pebbles to retain the radioactivity that no containment building is planned for the PBMR design. While the elimination of the containment building provides a significant cost savings for the utility—perhaps making the design economically feasible—the trade-off is public health and safety.

The protective containment building also is nixed because it would hinder the design’s passive cooling feature of the reactor core through natural convection (air cooling). Exelon also proposes a dramatic reduction in additional reactor safety systems and procedures (i.e. no emergency core cooling system and a reduced one-half mile emergency planning zone as compared to a 10-mile emergency planning zone for light water reactors) to provide for further reducing PBMR construction and operation costs.

To date, however, Exelon has not submitted to the Nuclear Regulatory Commission descriptions of challenges that could lead to a radiological accident such as a fire that ignites the combustible graphite loaded into the core. Fire and smoke then become the transport vehicle for radioactivity released to the environment from damaged fuel.

In addition, the lack of containment would require 100%-perfect quality control in the manufacture of the fuel pellets—an impossible goal. Imperfections in fuel pellet manufacture could lead to higher radiation releases during normal operation than is the case with conventional reactors.

"INHERENTLY SAFE" GERMAN PBMR COVERS UP RADIATION ACCIDENT AND SHUTS DOWN

As Dr. Edward Teller, the father of the H-bomb said, "Sooner or later a fool will prove greater than the proof even in a foolproof system." Accidents can and do happen in the inherently dangerous business of splitting the atom. Human error occurs at every level of development, construction and operation of the process. Material and component failures along with aging can break down or defeat operational and safety systems.

In 1985, the experimental THTR-300 PBMR on the Ruhr in Hamm-Uentrop, Germany was also offered as accident proof--with the same promise of an indestructible carbon fuel cladding capable of retaining all generated radioactivity. Following the April 26, 1986 Chernobyl nuclear reactor accident and graphite fire in Ukraine, the West German government revealed that on May 4, the 300-megawatt PBMR at Hamm released radiation after one of its spherical fuel pebbles became lodged in the pipe feeding the fuel to the reactor. Operator actions during the event caused damage to the fuel cladding.

Radioactivity was released with the escaping helium and radioactive fallout was deposited as far as two kilometers from the reactor. The fallout in the region was high enough to initially be blamed on Chernobyl. Government officials were then alerted by scientists in Freiburg who reported that as much as 70 % of the region’s contamination was not of the type of radiation leaking hundreds of miles away in Ukraine. Dismayed by an attempt to conceal the reactor malfunction and confronted with mounting public pressure in light of the Chernobyl accident only days prior, the state ordered the reactor to close pending a design review.

Continuing technical problems including a lack of quality control resulting in damage to unused fuel pebbles and radiation-induced bolt head failures in the reactor’s gas channels resulted in the unit’s closure in late 1988. Citing doubts about reliability, the government refused to further subsidize utility funding and instead approved plans for decommissioning the reactor.

NUCLEAR WASTE REMAINS INTRINSICALLY DANGEROUS

A single 110-megawatt PBMR will produce 2.5 million irradiated fuel elements during a 40-year operational cycle. Nuclear waste remains dangerous over geological spans of time and a threat to life from radioactive contamination would persist long after a PBMR has closed. The health and environmental uncertainties associated with a historically mismanaged radioactive legacy from continued operation of nuclear technology is yet another reason the public will not accept the PBMR.—Paul Gunter, March 2001

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