Fort St. Vrain Generating Station

Perspective image of Fort St. Vrain from 1965 design documents

Fort Saint Vrain Generating Station is a natural gas powered electricity generating facility located near the town of Platteville in northern Colorado in the United States. It currently has a capacity of just under 1000MW and is owned and operated by Xcel Energy, the successor to the plant's founder, the Public Service Company of Colorado. It went online in this form in 1996.

The facility was built originally as a nuclear power plant. It operated as a nuclear generating power plant from 1979 until 1989.^[1]

Historical overview

Fort Saint Vrain Generating Station was built as Colorado's first and only nuclear power plant and operated as such from 1979 until 1989.^[1] It was one of two high temperature gas cooled (HTGR) power reactors in the United States. The primary coolant was helium which transferred heat to a water based secondary coolant system to drive steam generators. The reactor fuel was a combination of fissile uranium and fertile thorium microspheres dispersed within a prismatic graphite matrix. The reactor had an electrical power output of 330MW (330 MW_e), generated from a thermal power 842 MW (842 MW_th).^[1]

The Fort St. Vrain gas-cooled nuclear power plant was proposed in March 1965 and the application was filed with the Atomic Energy Commission in October 1966. Construction began in September 1968.^[1] The building was unique for U.S. commercial reactors, as it had a rectangular shape instead of the usual cylindrical domed buildings housing other reactor designs. The HTGR design was considered safer than typical boiling water designs of the time, so the typical steel-reinforced, pre-stressed concrete containment dome structure was omitted in favor of a steel-frame containment structure while the reactor core was partially contained within a prestressed concrete reactor pressure vessel (PCRV). The construction cost reached $200 million, or approximately $0.60/installed watt. Initial testing began in 1972 and the first commercial power was distributed in July 1979.^[1]

The plant was technically successful, especially towards the very end of its operating life, but was a commercial disappointment to its owner. Being one of the first commercial HTGR designs, the plant was a proof-of-concept for several advanced technologies, and correspondingly raised a number of early adopter problems that required expensive corrections.

Unique features of the design

Refueling floor at Fort St. Vrain Generating Station

The Fort St. Vrain HTGR was substantially more efficient than modern light water reactors, reaching a thermal efficiency of 39-40%, excellent for a steam-cycle power plant. Operation of the HTGR design could be readily attenuated to follow the electrical power demand load, rather than be required to generate its nameplate power all the time. The reactor was also comparatively fuel efficient, with a maximum burnup of 90,000 MW days thermal compared to Light Water Reactors with burnups of 10,000 - 40,000 MW days thermal). The basis of this improved run time is that the core design "fertilizes" the thorium pellets within the fuel with neutrons, and then burns the bred fissiles through normal neutronic processes without requiring removal from the core. Like all HTGRs, the Fort St. Vrain precluded the possibility of major core damage or radioactive releases in such a quantity that could seriously threaten public safety, and the Nuclear Regulatory Commission allowed operation with much smaller safety zones compared to LWR designs. It was also notable that plant personnel received negligible exposure to ionizing flux during the course of operations. Further, the PCRV reflected an innovative RPV that had the potential to be substantially less costly than the metallic RPVs then in service, which were made of expensive nickel-manganese superalloys (e.g. Inconel, Hastelloy, and Monel) in the case of PWRs or surgical grade stainless steel 316L in the case of BWRs. The fuel, by omitting Zircalloy sheathing (allowed due to the inert, non-aqueous core) was made far less expensive.

Fort St. Vrain worked, and once debugged, it worked well for a first of a kind facility, demonstrating a promising new concept for the future. However, the problems that occurred leading to its debugging led to its early demise.

Operational experience

Many issues occurred early in the operational experience of the Fort St. Vrain HTGR. Although these issues were never a threat to the facility or to public safety, considerable stress was placed upon the personnel, equipment, and facilities and made continued operation appear uneconomical to the plant's owner. Most of the past issues had been resolved at considerable expense and the plant was beginning to perform at a commercially viable level when an economic downturn and the history of the plant caused the owner to shut it down even though it had not reached the end of its design lifetime.

Three major categories of problems were experienced at Fort St. Vrain: first, water infiltration and corrosion issues; second, electrical system issues; and third, general facility issues.

Water infiltration and corrosion issues (Helium circulators)

Diagram of the PCRV (left) and helium circulator (right) of the Fort St. Vrain reactor

The root cause of a large part of the problems with Fort St. Vrain was one piece of equipment, in particular: the helium circulator, illustrated at right. Due to the small molecular size of helium, exceedingly close tolerances were needed to ensure that helium did not exfiltrate through the circulator while in use. Moving surfaces, in particular, were hard-pressed to provide the kind of seal required to keep the helium coolant in. Thus a water-lubricated bearing design was used to provide an adequate solution to the potential issue of helium exfiltration.

Unfortunately, in satisfactorily preventing helium exfiltration, the designers caused another issue: water infiltration. Circulator bearings featured a timed water injection system in the event of circulator trip. The designers of the circulator thus used the pressure of one fluid, water, to counteract the pressure of other fluids. The designers, however, had not fully appreciated the transient variations that could occur in the pressure of either fluid, especially the pressure of the bearing water. As such, when bearing water was injected into the circulators, problems could occur if steam or helium pressure which opposed the pressure of the bearing water was not within expected parameters. For instance, the steam pressure could vary considerably due to changes in circulator speed, water flow through the steam generator, stop valve closure or throttle valve actuation. In the case of the helium pressure, these variables could vary based on the level of reactor power generation and core pressurization or depressurization. Thus, during certain plant evolutions, the bearing water infiltrated into the PCRV due to variable pressures of plant fluids.

FSV did have a gas cleanup train that could rapidly remove certain contaminants from the helium, but was limited in volume and was not greatly effective in removing water vapor from the gas within the PCRV. The gas cleanup could be prevented from working by water vapor icing the chillers within the gas cleanup train, and so, when the reactor descended from power and cooled, the water condensed upon equipment within the PCRV. Neither the PCRV nor the equipment thereof was designed to resist the effects of water-induced corrosion.

FSV's gas cleanup train was driven around regulatory concerns pertaining to theoretical core graphite-water interactions at high temperatures and pressures, which did not occur due to the core's construction from high-grade graphite: the core did not possess the micro-porous structure of lower grade graphites, and therefore did not provide sufficient surface area for substantial chemical reactions. Even though the core proper was not reactive, there was some erosion of low-grade ex-core graphite support blocks due to water-gas shift processes, but the core's graphite was not subject to these. The slight erosion detected did not substantially impact operations, absorb all the infiltrated water or evolved steam, or induce major gas cleanup considerations. Instead, the vast majority of entrained steam and water vapor in the coolant failed to react as the regulators intended, and thus, condensed water vapor began corroding in-core and ex-core instrumentation.)^[2]

By these mechanisms, water entered the sealed volume of the PCRV and caused havoc with numerous operations-critical systems. Though safety was assured to a substantial level by the design, numerous severe operability problems emerged quickly. Control rod drives rusted, and consequently rapid shutdowns failed when called upon to function. The reserve shutdown system, consisting of borated graphite spheres to be released into the core in the event of an Anticipated Transient Without SCRAM (ATWS), was unavailable at times due to water leaching of the boron. The subsequent unscheduled, impromptu reconfiguration of the graphite spheres into graphite sausage-shaped cylinders due to boric acid precipitation was not contemplated within the design. Steel tendons within the PCRV were found to be corroded due to precipitation of chloride, and were not to specification upon routine surveillance. Steam generator leaks due to corrosion of the steam generators also occurred, probably due to the original water infiltration problems. Flecks of corroded steel migrated into the coolant itself and lodged into critical parts of critical machinery, including control rod drives. Further, the gas cleanup train's chiller units became iced due to the deposition of water vapor onto their supercold surfaces, rendering them ineffective at times when they were most needed.^[2]

Some of the blame for the corrosion debacle has to be laid on the regulators, who maintained a consistent improper regulatory focus on chemical reactions involving steam with the high-grade core graphite, as this was the area that drove design of the gas cleanup train; it was foreseeable that the memorandums from Rockville, Maryland regarding this obviously consumed countless man-hours and drove the designers to distraction on peripheral issues whose occurrence was physically infeasible. Some of the blame for the corrosion debacle has to be laid on the owner of FSV, whose staff failed to respond to moisture alarms that had been going off for months in critical parts of the plant, instead assuming that the moisture alarms were defective. (Licensee staff sent to remove the "defective" moisture alarms for "repair" discovered that the moisture alarms were not defective, for when they removed the "defective" alarms, they got sprayed with a large volume of water.) Still, a large part of the blame must be laid on the designers of the plant themselves, who should have been able to foresee that large scale water infiltration was possible with the complex, buggy circulator design; who should have been able to foresee that the cleanup train should have reserve capacity for steam and water extraction; who should have been able to foresee that since this was not present, that major corrosion of in-core instrumentation and systems could occur and severely degrade the performance and systems of the total plant. Further, though the literature does not suggest what sorts of motivations or concerns drove the designers of the circulators to choose such a high-complexity, low-tolerance, leak-prone design, this was the major cause of the major plant problems; the designers themselves admitted this, stating: "The FSV circulators have 'met all design specifications', however, the bearings, seals, and support systems for the water-lubricated bearing have caused many problems. Further, the circulators employed a steam turbine drive that adds complexity to system operations. These unique design features (emphasis added) resulted in water ingress to the core, the primary reason for poor plant availability."^[3]

Electrical system issues

Diagram of how high-temperature gas reactor in Fort St. Vrain worked

The plant electrical system was challenged on numerous occasions, and the resolutions were frequently expensive. Transformers experienced faults. Backup generators sometimes failed to engage when activated, and on other occasions, side channel issues occurred during operation, preventing them from generating power. Failure of backup power also led to some of the moisture infiltration problems, by variously disrupting the logic of the bearing water injection systems and the helium circulator trip logic. Interestingly, failures of transformers and consequent failure of backup power occurred on at least one occasion due to moisture infiltration into electric cables and subsequent ground faulting when the plant was at low power to remove water from previous moisture infiltration issues. It is believed that this electrical fault led to further moisture infiltration.^[2]

General facilities issues

Facility contractors introduced safety concerns on several occasions. In one of the more serious incidents, contractor personnel damaged hydraulic units, allowing hydraulic fluid to spray over reactor control cables. The same crew then performed welding operations to equipment located above the control cables. Hot slag fell onto the material used to contain the hydraulic fluid and ignited it, along with the fluid on the control cables. The fire involved the cables for five minutes, and 16 essential control cables were damaged. The contractor personnel then failed to inform plant personnel of the situation and the reactor was in operation for several hours in this condition. On another occasion, contractor personnel using improperly grounded welding apparatuses tripped neutron protection circuits, leading to a nuisance trip of the entire plant.^[2]

Operational improvement and closure

Due to the water-induced corrosion problems and electrical problems, plant shutdowns were common. As a result, Public Service Company of Colorado began to question the economics of continued commercial operation. An increase in performance was observed from 1987–1989, suggesting some of the problems had been worked out of the system, but Public Service was not persuaded. In 1989 Public Service indicated that the plant was under consideration for closure. Later that same year a critical part of the reactor was found to have long-term corrosion and required replacement. The replacement cost was deemed excessive and the plant was shut down. The decommissioning and removal of the fuel was completed by 1992. Fort St. Vrain thus became the first commercial-scale nuclear reactor in the US to be decommissioned.^[4]

Analysis

Lessons learned at Fort St. Vrain have led more recent reactor designs of the HTGR type to adopt different strategies to confront issues that occurred there. More recent HTGR designs have tended to avoid large per-unit cores (in favor of more compact modular units), tended to avoid concrete reactor pressure vessels (in favor of proven carbon or alloy steel reactor pressure vessels), and tended to avoid steam cycles without an intermediate non-water based circuit between the core and the steam generators. Others, such as the Adams Atomic Engine (using nitrogen), the Romawa Nereus (using helium), and General Atomics GT-MHR (using helium) have favored simplification of the high-temperature gas-cooled reactor concept as much as possible, down to practically a reactor and a gas turbine linked together with the reactor using a right-sized, inherently safe core with no water used in the plant design. The GT-MHR, however, is large enough that it has a system for residual heat removal using convected air.

The reactor concept of Fort St.Vrain experienced a resurrection in form of AREVA's Antares reactor. This is a high temperature helium cooled modular reactor and thus is conceptually similar to the reactor at Fort St.Vrain. The INL approved AREVA's Allegro reactor as the chosen Next Generation Nuclear Power Plant(NGNP) to be deployed as prototype by 2021.^[5]

Reuse as combustion power facility

Following the reactor decommissioning, Fort St. Vrain was converted to a combustion facility. The first natural gas combustion turbine was installed in 1996. Two more turbines were installed by 2001. Heat recovery steam generators (HRSGs) allow the plant to operate in combined-cycle mode, in which waste heat recovered from combustion-turbine exhaust gases is used to make a second stage of steam capable of driving the facility's original steam turbine and generator. As of 2011, the nameplate generating capacity of the plant is 965MW.^[4]

Notes

External links

• Fort Saint Vrain History
• Fort Saint Vrain - INSC Database
• Fort St. Vrain - Xcel Energy Website
• Fort St. Vrain HTGR (drawing)

Coordinates: 40°14′40″N 104°52′27″W / 40.24444°N 104.87417°W / 40.24444; -104.87417