Coal, Nuclear, Reactors

Nuclear decommissioning

Issue 2 and Volume 100.

Nuclear decommissioning

Shoreham decommissioning technology: Simple and effective

Decommissioning a large nuclear power plant proves to be a straightforward exercise, accomplished with existing tools and procedures

By Carl Giacomazzo, Long Island Power Authority,and Juergen K. Hadden, Raytheon Nuclear

Plans to dismantle Shoreham nuclear power embraced a simple concept: applying proven technology in new or innovative ways to get the job done. Although Shoreham`s operating history was brief, the project is significant because the plant was a large commercial unit and the technologies and methods applied to dismantle it will eventually apply to the present operating fleet of nuclear plants when their service lives come to an end. So, in several respects, Shoreham became the proving ground for planning, techniques and technologies that are likely to figure prominently in the evolution of decommissioning practices over the next two decades.

Decommissioning planning was initiated in 1989 and followed by the site`s radiological characterization in early 1990. Surveys during decommissioning planning and subsequent termination surveys identified several reactor-related structures and 24 contaminated (radioactive) systems. System and component isolation began in the first quarter of 1992 with NRC confirmation of the termination survey`s results (i.gif., decommissioning complete) in November 1994.

Two methods

Radioactive materials were removed from Shoreham by two general methods: dismantlement and removal, and in-place decontamination. Dismantlement and decontamination activities were confined primarily to the reactor containment building and the radwaste building. Minor decontamination activities also were required in the turbine building. Most of the facility (about 75 percent) was left intact. A few structures inside the containment and radwaste buildings (mostly interior walls) were demolished so contaminated systems and equipment could be easily removed.

Activated portions of the reactor biological shield were removed as well. Other systems and equipment removed included the reactor pressure vessel (RPV), except for the lower head, and major portions of radioactively contaminated plant systems. Reactor control rods, fuel channels and fuel storage racks also were removed intact and shipped directly to the low-level nuclear waste disposal facility near Barnwell, S.C. The majority (more than 4.5 million pounds) of radioactively contaminated piping and equipment was sent to an offsite vendor for volume reduction resulting in approximately 8,000 cubic feet of burial waste. A breakdown of actual costs is shown in Table 1.

The cutting edge

Many proven technologies exist to decontaminate and dismantle structures, components and systems. Early on, project engineers concluded the need to minimize the potential for the spread of contamination because Shoreham`s systems were essentially clean on the outside. A major concern included the inadvertent release of contamination which would add unnecessary expense and time to the project. Of the approximately 600 curies of radioactivity at the plant (exclusive of fuel) most resided in the reactor vessel, its internals and the control rod blades. However, sufficient activity existed in the various piping systems to require their removal and, consequently, raised valid concerns about the potential for spreading contamination while segmenting and handling those pieces.

To facilitate cost-effective waste packaging and transportation, component segmentation plans also called for subsegmenting large pieces and components. Dismantlement planning considered not only the effort to actually segment the system or component but also the labor required in handling and packaging the waste.

Pipe cutting

In an effort to minimize potential contamination, piping systems segmentation was generally accomplished via mechanical means. Hydraulically actuated, split outside diameter (OD) milling machines, bandsaws and hydraulic shears were among the pipe-cutting technologies employed. For a variety of reasons, the predominant tool used, relative to volume of piping to be segmented, was the split OD milling machine. The milling machine became the workhorse because of its high potential production rate, its simplicity, its durability and its reliability. Given that the Shoreham effort rightly favored a “surgical” dismantlement approach, the milling machine`s size and easy operating characteristics made it ideal for the job.

While union craftsmen were expected to be trained and skilled in the use of the various machines, a high production construction environment can increase the wear and tear on any equipment that relies on precision moving parts. Accordingly, close scrutiny was paid to various tool supplier`s offerings during the procurement process. With several acceptable choices available, final tool selection was made using direct experience and observation.

As a result of careful tool selection, and after approximately 4.5 million pounds of piping and component parts had been removed, there was virtually no machine downtime other than for minimal and planned maintenance and refurbishment. Of course, tool performance was enhanced by a reasonable training and indoctrination program for the workers. On a final note, all pipe ends were capped or otherwise contained promptly after cutting. This approach minimized the risk associated with transporting the material through otherwise clean areas.

Reactor internals

The reactor pressure vessel internals removal and segmentation effort began in January 1992. The effort concluded the following August after approximately 200 curies of radioactivity contained in the activated parts was removed by decontamination. During that period, the subcontractor designed, fabricated, delivered and successfully deployed a platform from which reactor internals segmentation and removal could be remotely executed from inside the reactor. (Remote refers to the fact the operation occurred underwater and the platform was manned inside the reactor vessel during the internals segmentation operation). In addition, the vendor provided a bridge and trolley assembly containing a remote cutting tool/grapple assembly which was designed to straddle and traverse the dryer/separator pool. This operation helped underwater reactor internal component volume reduction and packaging operations performed in the pool.

Contact dose rates on reactor internals exceeded 200 roentgen equivalent man (rem) per hour. Suspended within the reactor vessel, the platform served as a work area for the removal of internal components from the reactor structure. The platform`s design was such that unattached components, securely rigged to the platform, were removed from the vessel by the platform`s polar crane and then transported to the dryer/separator pool (the “wet cutting station”) where they were immersed and disconnected from the platform. Final reactor internals segmentation was accomplished in the wet cutting station using another plasma arc cutter mounted on the bridge crane`s mast.

All underwater operations were monitored via remote cameras. While the bridge had indexing capability, the in-vessel platform`s location was fixed using local markings. Platform location was also judged by the known elevation of the water level managed by operations personnel. Managing the water`s level proved effective in reducing the exposure accumulated by technicians and other crafts people working in this area.

RPV segmentation

Because of the RPV`s close proximity to the fuel in the spent fuel pool, segmenting operation exposure rates were a critical parameter, one with the greatest potential to impact the project. Load paths, in compliance with federal regulations, were detailed, approved and tightly controlled, especially considering the 12-ton segments which had to pass within a few feet of the pool. The fuel`s presence impacted other aspects of the plan including a given segment`s ultimate destination on the refuel floor.

The project`s original plans called for a temporary enclosure on the refuel floor within which reactor vessel subsegmentation could occur. This enclosure was supposed to facilitate and contain various emissions generated by cutting large amounts of carbon steel. Subsequent analysis of the enclosure`s design, however, revealed a number of cost prohibitive issues which precluded its use. The more cost-effective option, it was decided, involved converting the wet cutting station into a dry cutting station following the completion of the reactor internals segmentation process. So, in August 1992, the dryer/separator pool`s conversion to a dry cutting station began in parallel with the final testing and delivery of the reactor shell dismantlement assembly. The conversion consisted of establishing a habitable, ventilated/monitored location using the entire dryer/separator pool within which large vessel segments could be further segmented using an oxypropane torch, motorized and mounted on a track assembly.

The design of the dry cutting station included a retractable fire retardant cover mounted on the rails previously installed for the internals segmentation platform. Ventilation and exhaust filtering were accomplished using portable high-efficiency particulate air filter (HEPA) units set upon the refuel floor and exhausting to the refuel floor atmosphere.

Segmentation tool

The reactor segmentation “tool” was a unique 18-foot inside diameter (ID) mill provided by the same vendor that supplied the split OD milling machines. This cutter was an improved version of the cutting device used in a recent steam generator replacement project that separated the steam domes from the evaporator sections. Assembled on the refuel floor and lowered into the cavity via the polar crane, the cutter frame rested on the lip of the reactor vessel while the cutting heads were suspended approximately 8 feet into the reactor vessel. Hydraulically powered, the heads rotated around the inside circumference of the vessel to remove material at predetermined cut locations. The kerf of the cut was maintained by split brackets welded across the cut locations while the cutter`s location was maintained by “standoffs” forced against the inside of the vessel wall. After an initial manual setting, the depth of the cut was automatically set based on the revolutions of the cutting assembly around the inside of the vessel. Given the nature of the material being removed (stainless steel cladding on carbon steel) the cutter proved reliable and effective.

After a ring section cut was completed, the cutter and its integral platform were lifted from the vessel via the polar crane, followed by the ring section (previously modified to facilitate rigging) itself. Finally, the ring section was moved to the dry cutting station for further segmentation.

Plasma arc cutting

The dry cutting station was also used for segmenting the steam dryer and moisture separator. To do this, hand-held plasma arc torches were employed. This operation proved to be one of the most significant industrial safety challenges encountered during the decommissioning process. The plasma arc cutting process causes the ionization of materials being cut resulting in a large amount of industrial pollutants; in the case of stainless steels (the material of which the dryer and separator are made), the pollutants include various heavy metals. Although these contaminants were produced during underwater cutting operations, they were contained by floating exhaust hoods.

During dry (i.gif., open-air) plasma arc cutting operations, significant attention must be placed on individual worker safety to ensure breathing air quality. That means periodic air samples must be taken to judge the character and concentration of contaminants and planning adequate access and egress points in the case of emergency. High-efficiency particulate air filtration must be closely assessed given the particulate sizes encountered and the need to minimzie work stoppages to change out filter elements.

Concrete cutting

Planning called for the removal of two fairly small wing walls to facilitate material handling to and from the reactor building truck bay. Using conventional means such as impact hammers and cutting torches, the reinforced concrete walls were removed with vigor and a determined sense of practicality. However, this “low budget” approach, though seemingly prudent at the time, proved to be a housekeeping nightmare.

Concrete cutting is not difficult. However, even in the best of situations, the slurry (the mixture of concrete cutting residue and cooling water) developed during cutting flows/drips some distance to the point at which it is collected for decanting. Despite the fact that the solids are contained and the liquid is generally recycled back into the cutting/cooling process, the slurry`s characteristics are problematic at best.

If too little water is used, the cooling water evaporates and the residue dries to a dust and produces a significant airborne contamination problem. Too much water and a liquid waste disposal opportunity exists. Concrete cutting is a sometimes necessary practice in a nuclear environment and it pays to have a healthy respect for the process.

Nevertheless, there remained two forms of concrete removal that had to be addressed: small, localized easily manageable necessities which could be reasonably handled with skills on hand (and a relatively minor outlay for the basic tooling) and specialty tasks requiring specialized skills.

In the former category, the technology of choice was a hand-held, water-cooled hydraulically powered chain saw with diamond tipped teeth. While not quite like running a hot knife through butter, the tool allowed for the efficient dispatch of small jobs while keeping slurry problems at bay.

Large cutting job

The largest concrete cutting job at Shoreham involved segmenting and removing most of the biological shield wall. The technology of choice here involved using the diamond wire cutting method. Diamond wire is best described as a series of diamond coated balls interspersed between springs with the whole group of parts strung on a fairly fragile cable; the cable employed at Shoreham was about one-eighth of an inch diameter. Assembled as a continuous loop, the cable is driven through a series of idler pulleys that direct the location of the cut and maintain the cable`s tension.

The bioshield was cut along predetermined lines considering the location of the imbedded structural steel and the size/weight of the pieces. Wall segments were handled to specially fabricated carts by a crane modified for the purpose: the crane was located inside the bioshield installed on what remained of the reactor lower head.

Acid tests

Initial tests of “soft” chemical decontamination processes at Shoreham were conducted on the reactor water cleanup system. The acid decontamination did lower contamination levels but not enough to achieve Nuclear Regulatory Commission release limits. Consequently, chemical decontamination was not considered a viable tool.

When in doubt, cut it out

Cost and schedule considerations being equal, contaminated systems or structures should be removed from the facility instead of attempting to decontaminate them in place. There was, however, a noteworthy exception to this rule of thumb. Approximately 15,000 feet of drain piping, of which 6,000 feet were embedded in structural concrete, required either removal or decontamination. The scope of this endeavor was truly significant and removing this piping could have added months to the schedule and millions of dollars to budget. However, a pipe crawler developed by an employee gave project personnel the ability to measure radioactivity throughout the piping system and consequently provided enough information to decontaminate the piping in place. Hydrolazing at 20,000 pounds proved effective to remove the contamination to acceptable levels.

Volume reduction

Except for the reactor vessel internals, volume reduction was primarily conducted off site. The approximately 6 million pounds of waste shipped (exclusive of the fuel) resulted in slightly more than 8,000 cubic feet of waste which actually went to burial. Using a smelting method, the Shoreham metals were converted to slightly radioactive metal blocks suitable for shielding. Table 2 includes a summary of Radioactive Material and Radwaste Shipment Totals.

Conclusion

In the final analysis, the selection of the proper vendor to provide the technology, technique or service must be based on a clear assessment of the vendor`s history and ability to perform in the project`s environment and given application. Moreover, with a project of this size and scope, planning and managing engineers must thoroughly assess costs and alternatives, but decisions shouldn`t be led by their costs alone. Finally, in a dismantlement/decontamination situation, when in doubt, cut it out. z

Click here to enlarge image

A guiding philosophy for the project was “When in doubt, cut it out.” Here, a crane lifts a section of the reactor`s primary containment clear of the wetwell.

Click here to enlarge image

Click here to enlarge image

Temporary shielding, such as these tarps, and other measures were used to limit worker`s and dismantled section`s exposure to radioactivity.

Click here to enlarge image

Shoreham`s short-lived history

The Shoreham nuclear facility, originally owned by the Long Island Lighting Co. (LILCO), was constructed on the north shore of Long Island. The construction permit was issued for the plant in April 1973 and by mid-1984, construction of the plant was substantially completed. Shoreham achieved initial criticality on Feb. 15, 1985, and low power testing was performed intermittently until June 1987 when the plant was shut down for a variety of economic and political reasons. During this period the plant was operated for less than two effective full power days at or below a 5-percent power level. Nevertheless, the National Regulatory Commission (NRC) issued the 850-MW General Electric boiling water nuclear plant a full-power operating license on April 21, 1989. However, the die had been cast and the unit`s nuclear fuel was unloaded from the reactor and placed in the spent fuel storage pool on Aug. 9, 1989.

LILCO never did operate the reactor commercially, and on June 14, 1991, the license was amended from an operating license to a possession-only license. The plant was subsequently placed in a lay-up mode per NRC commitments and routine radiological decontamination was conducted to prepare the plant for decommissioning.