Fort St. Vrain Generating Station
Encyclopedia
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 1977 until 1992.
and operated as such from 1977 until 1992. 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 MWe), generated from a thermal power 842 MW (842 MWth).
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 1968. 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 December 1976.
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.
that had the potential to be substantially less costly than the metallic RPVs then in service, which were made of expensive nickel
-manganese
superalloy
s (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.
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.
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, and 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 had a timed water injection system in the event of circulator trip. The designers of the circulator thus used the pressure of a fluid to counteract the pressure of other fluids. The designers of the circulator, 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, or, in the case of the helium pressure, this 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, and, in fact, 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 - the core being high-grade graphite, which did not possess the micro-porous structure of lower grade graphites providing sufficient surface area for substantial chemical reactions. It must be noted that 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, and 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 foresaw, and thus, condensed water vapor began corroding in-core and ex-core instrumentation.)
Thus, 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 ATWS, was unavailable at times due to water leaching of the boron and the subsequent unscheduled, impromptu reconfiguration of the graphite spheres into graphite sausage-shaped cylinders due to boric acid precipitation, not contemplated within the design. Steel tendons within the PCRV were found to be corroded due to precipitation of chloride and 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), adding volumes of figurative fuel to the figurative fire. Flecks of corroded steel even got into the coolant itself and lodged themselves in critical parts of critical machinery, such as control rod drives. Further, the gas cleanup train's chiller units became iced due to the deposition of water vapor on to their supercold surfaces, rendering them ineffective at times when they were most needed.
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."
, Dragon
, AVR
, HTTR
, and HTR-10
, all of which represent successful tests proving the principle of the HTGR technology, Fort St. Vrain was arguably doomed by the engineering mistake to use a first of a kind engineered
, high-complexity
steam turbine helium circulator with multiple fluid bearings instead of a simple, commercial off the shelf
, KISS principle
, low-complexity electric motor-based helium circulator, such as the electrical coolant circulators successfully used for decades in the UK's AGCR, which have stood and do stand the test of time in a similar, yet far more chemically hostile environment than that of a helium-cooled reactor core.
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. For instance, 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. Still 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 engineering mistakes made and lessons learned at Fort Saint Vrain delayed the HTGR - a very safe, affordable, highly adaptable, efficient, scalable, and perhaps immensely important nuclear technology - by decades due to the technology's buggy performance in this commercial test.
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.
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
powered electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
generating facility located near the town of Platteville
Platteville, Colorado
Platteville is a Statutory Town in Weld County, Colorado, United States. The population was 2,370 at the 2000 census. It is adjacent to Fort Vasquez on U.S. Route 85.-Geography:Platteville is located at ....
in northern Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It currently has a capacity of just under 1000MW and is owned and operated by Xcel Energy
Xcel Energy
Xcel Energy, Inc. is a public utility company based in Minneapolis, Minnesota, serving customers in Colorado, Michigan, Minnesota, New Mexico, North Dakota, South Dakota, Texas, and Wisconsin. Primary services are electricity and natural gas...
, 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
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
. It operated as a nuclear generating power plant from 1977 until 1992.
Historical overview
Fort Saint Vrain Generating Station was built as Colorado's first and only nuclear power plantNuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
and operated as such from 1977 until 1992. It was one of two high temperature gas cooled (HTGR) power reactors in the United States. The primary coolant was helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
which transferred heat to a water based secondary coolant system to drive steam generators
Steam generator (nuclear power)
Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors between the primary and secondary coolant loops....
. The reactor fuel was a combination of fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...
uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and fertile
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...
thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
microspheres dispersed within a prismatic graphite matrix. The reactor had an electrical power output of 330MW (330 MWe), generated from a thermal power 842 MW (842 MWth).
The Fort St. Vrain gas-cooled nuclear power plant was proposed in March 1965 and the application was filed with the Atomic Energy Commission
United States Atomic Energy Commission
The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...
in October 1966. Construction began in 1968. 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 December 1976.
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
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 RPVReactor vessel
In a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...
that had the potential to be substantially less costly than the metallic RPVs then in service, which were made of expensive nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
superalloy
Superalloy
A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
s (e.g. Inconel
Inconel
Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
, Hastelloy
Hastelloy
Hastelloy is the registered trademark name of Haynes International, Inc. The trademark is applied as the prefix name of a range of twenty two different highly corrosion-resistant metal alloys loosely grouped by the metallurgical industry under the material term “superalloys” or “high-performance...
, and Monel
Monel
Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...
) in the case of PWRs
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
or surgical grade stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
316L in the case of BWRs
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...
. 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 past 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)
The root cause of a large part of the problems with Fort St. Vrain was one piece of equipment, in particular: the heliumHelium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
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, and 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 had a timed water injection system in the event of circulator trip. The designers of the circulator thus used the pressure of a fluid to counteract the pressure of other fluids. The designers of the circulator, 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, or, in the case of the helium pressure, this 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, and, in fact, 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 - the core being high-grade graphite, which did not possess the micro-porous structure of lower grade graphites providing sufficient surface area for substantial chemical reactions. It must be noted that 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, and 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 foresaw, and thus, condensed water vapor began corroding in-core and ex-core instrumentation.)
Thus, 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 ATWS, was unavailable at times due to water leaching of the boron and the subsequent unscheduled, impromptu reconfiguration of the graphite spheres into graphite sausage-shaped cylinders due to boric acid precipitation, not contemplated within the design. Steel tendons within the PCRV were found to be corroded due to precipitation of chloride and 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), adding volumes of figurative fuel to the figurative fire. Flecks of corroded steel even got into the coolant itself and lodged themselves in critical parts of critical machinery, such as control rod drives. Further, the gas cleanup train's chiller units became iced due to the deposition of water vapor on to their supercold surfaces, rendering them ineffective at times when they were most needed.
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
Red tape
Red tape is excessive regulation or rigid conformity to formal rules that is considered redundant or bureaucratic and hinders or prevents action or decision-making...
from Rockville, Maryland
Rockville, Maryland
Rockville is the county seat of Montgomery County, Maryland, United States. It is a major incorporated city in the central part of Montgomery County and forms part of the Baltimore-Washington Metropolitan Area. The 2010 U.S...
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
General Atomics
General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...
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."
Electrical system issues
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.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.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.Analysis
Unlike Peach BottomPeach Bottom Nuclear Generating Station
Peach Bottom Atomic Power Station, a nuclear power plant, is located southeast of Harrisburg in Peach Bottom Township, York County, Pennsylvania, on the Susquehanna River on the Maryland border....
, Dragon
Dragon reactor
Dragon was a high temperature gas cooled reactor at Winfrith in England operated by UKAEA. Its purpose was to test fuel and materials for the European high temperature reactor programme, and was built and managed as an OECD/NEA international project...
, AVR
AVR reactor
The AVR reactor was a prototype pebble bed reactor at Jülich Research Centre in West Germany. Construction began in 1960, first grid connection was in 1967 and operation ceased in 1988....
, HTTR
HTTR
The high temperature test reactor is a graphite-moderated gas-cooled research reactor in Oarai, Ibaraki, Japan operated by the Japan Atomic Energy Agency. It uses long hexagonal fuel assemblies, unlike the competing pebble bed reactor designs....
, and HTR-10
HTR-10
HTR-10 is a 10 MWt prototype pebble bed reactor at Tsinghua University in China. Construction began in 2000 and it achieved first criticality in January 2003.In 2005, China announced its intention to scale up HTR-10 for commercial power generation...
, all of which represent successful tests proving the principle of the HTGR technology, Fort St. Vrain was arguably doomed by the engineering mistake to use a first of a kind engineered
FOAK
FOAK is an abbreviation for First of a Kind. It is used in engineering economics where the first item or generation of items using a new technology or design can cost significantly more than later items or generations....
, high-complexity
Complexity
In general usage, complexity tends to be used to characterize something with many parts in intricate arrangement. The study of these complex linkages is the main goal of complex systems theory. In science there are at this time a number of approaches to characterizing complexity, many of which are...
steam turbine helium circulator with multiple fluid bearings instead of a simple, commercial off the shelf
Commercial off-the-shelf
In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...
, KISS principle
KISS principle
KISS is an acronym for the design principle Keep it simple, Stupid!. Other variations include "keep it simple and stupid", "keep it short and simple", "keep it simple sir", "keep it simple or be stupid" or "keep it simple and straightforward"...
, low-complexity electric motor-based helium circulator, such as the electrical coolant circulators successfully used for decades in the UK's AGCR, which have stood and do stand the test of time in a similar, yet far more chemically hostile environment than that of a helium-cooled reactor core.
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. For instance, 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. Still others, such as the Adams Atomic Engine (using nitrogen), the Romawa Nereus (using helium), and General Atomics
General Atomics
General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...
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 engineering mistakes made and lessons learned at Fort Saint Vrain delayed the HTGR - a very safe, affordable, highly adaptable, efficient, scalable, and perhaps immensely important nuclear technology - by decades due to the technology's buggy performance in this commercial test.
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-cycleCombined cycle
In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...
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
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
and generator. As of 2011, the nameplate generating capacity of the plant is 965MW.