A decision on DOE's new production reactor (NPR), ' the largest U.S. nuclear power plant order in the foreseeable future, inched closer last week as an expert panel said that a heavy water reactor (HWR) at the Savannah River" Plant (SRP) has the best chance of quickly producing ***** tritium for nuclear weapons. However, the panel noted there are technical uncertainties in every design reviewed.:

The recommendation by a special Energy Research, Advisory Board (ERAB) panel appears to give two teams; of HWR proponents-led by Ebasco Services, Inc. and 1 Westinghouse Electric Corp.—an early leg up in the com-, petition for the contract, estimated to be worth $5- to $10billion over perhaps 10 years. The panel stopped short, .......i however, of a full-fledged endorsement of the HWR tech-. nology, noting that other “passively safe” designs would do more to advance reactor technology than an HWR.

After Energy Secretary John Herrington announces by August 1 a preferred technology and site for the NPR, DOE will begin preparing an environmental impact statement (EIS), which could take two years, DOE spokesman, Will Callicott said. DOE will issue a request for proposals. (RFP) after the EIS is finished.

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The NPR eventually would replace DOE's 34-yearold SRP reactors, which are shut down for seismic fixes, routine maintenance, or to comply with state environmental laws (NW, 30 June, 11). The power levels of all three reactors were halved more than a year ago after the Na-, tional Academy of Sciences (NAS), the first outside review team to examine them, raised questions about the... capacities of their emergency cooling systems in the event of a severe accident (NW, 26 March '87, 3).

The Reagan administration postponed the decision to... build an NPR because of budget cuts. The administration elected in November could change, whatever decision Herrington makes.

Many in DOE have wanted to build a new HWR, on rather than gambling on other technologies. DOE statistics show the reactors performed well unul safety concerns 'n prompted power cutbacks: as measured by reactor "in-"" nage," or operational days per given year (with 365 days"! equaling 100%), the SRP reactors have dipped below 60% (to 58%) during only three fiscal years. During 18 fiscal " years, the reactors averaged between 80% and 84%?~} ??»

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But pressure from Congress and cabinet officials prompted DOE to look at LWRs-as a large plant, a converted commercial reactor, and an "advanced" LWR, at modular high-temperature gas-cooled reactors (MHTGRS), and at liquid metal reactors (LMRs). According to ERAB, the major flaw of the other reactors is that they are unproven tritium producers. The new HWRs would use the SRP fuel and targets, but the other technologies require demonstrations.

For example, Westinghouse, which is proposing all the LWR technologies, would need $91- to $137-million to qualify its targets, depending on how many must be developed and tested and how many tests need to be done, according to Robert Vijuk, Westinghouse manager of space, defense, and nuclear programs. General Atomics' MHTGR target has been developed but would require another $25-million for full-scale target tests and building and operating pilot target fabrication and recovery lines, said Linden Blue, GA's vice chairman of the board.

The major drawback of the HWR is that it is totally new to the safety review process. The SRP reactors have been reviewed by only one outside group; that review

resulted in the current power restrictions (NW, 5 Nov., '87,4). Recent revelations of missing engineering drawings and hand-calculated safety calculations (NW, 9 June, 2) have fueled the debate over the reactors' safety.

Many of the other concepts have been at least partially reviewed by NRC, the Advisory Committee on Reactor Safeguards (ACRS), or both. NRC has fully licensed one HTGR and reviewed two large HTGRS through the construction permit stage, ERAB said. The proposed MHTGR is a scaled-down version of the Fort St. Vrain plant that more closely resembles Peach Bottom-1, both of which were licensed by NRC. The ACRS has reviewed DOE's Fast Flux Test Facility, an LMR, although DOE facilities are not subject to NRC review. The large LWR concept is based on Union Electric Co.'s Callaway, which was licensed in 1984.

In addition, the HTGR, LMR, and, to a lesser extent, the small LWR are "passively safe" concepts that rely mainly upon gravity, natural convection and circulation, and a few engineered safety features. Choosing one of those concepts would advance nuclear technology far more than would building another HWR, and should be considered despite somewhat higher initial capital costs, ERAB said.

Safety of Current HWRs Uncertain

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The six-loop Savannah River HWRs operate at 2,350. megawatts thermal at full power and are cooled and moderated by heavy water. Each loop contains a pump and a heat exchanger. The coolant enters the top of the reactor through a plenum, flows down through the fuel assemblies, and exits the bottom of the reactor. A secondary loop of light water cools the primary coolant.

The low-temperature (230 degrees F) and low-pressure (5 pounds per square inch gauge) reactors currently use two types of fuel assemblies, several types of target assemblies (for tritium, plutonium-239, and Pu-238), and one type of blanket assembly (for tritium).

The Mark 16B (for plutonium) and Mark 22 (for tritium) fuel assemblies contain either two or three concentric fuel tubes of highly enriched uranium-235 in an aluminum sleeve housing with an inner lithium target. Depending on the type, the assembly also contains either a permanent or removable lithium-aluminum outer target. These fuel assemblies are used in conjunction with depleted uranium targets to produce plutonium-239, with neptunium targets to produce plutonium-238, or with lithium targets to produce tritium. In the plutonium production mode, some tritium is produced in the lithium blanket assemblies, in the inner targets of the fuel assemblies, and in the control rods.

The Mark 16 fuel is in the reactor for five to six months at full power, and the targets are in the reactor for about one month at full power, according to Roger Rollins, chief of the reactors branch at DOE-Savannah River. The Mark 22 fuel and target assemblies are each good for nine months at full power. Loading of fresh targets takes about five days, and reloading of both fuel and target assemblies takes 10 days. The reactors are in annual maintenance outages that last from 60 to 75 days, Rollins said.

Partial and full-length control rods, made of either cadmium poison or lithium or cobalt for isotope production, control the flux across the reactor core. In an emer

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gency, the reactors can be shut down through the use of safety rods, a gadolinium nitrate poison-injection system, or a computer-generated scram. Three of the cooling loops also could function as an emergency cooling system (ECS) to inject light water in to the top of the core during an emergency.

After 25 years of full-power operation, SRP operator E.I. du Pont de Nemours & Co. discovered that the loss of cooling capability in the fuel assemblies could occur during a loss-of-coolant accident (LOCA), even with the ECS operating, at lower power than previously thought. During a LOCA, the reduced downward coolant flow could be cut off by steam produced from bulk boiling in the core. The power was reduced to prevent bulk moderator boiling, and DOE plans to add a fourth ECS line to each reactor.

DOE also has had problems with oxygen-induced intergranular stress corrosion cracking (IGSCC) because of the large amount of carbon in the stainless steel vessels and piping. Cracks in C reactor's tank were exacerbated by the unique, curved construction of the vessel, which was designed differently than the other reactors in order to allow ultrasonic testing for cracks. The curving and welding stressed the metal before it was irradiated, Rollins said. The cracks in C reactor have defied repair attempts.

DOE's reactors also lack containments. Rather than "" contain radionuclides, the confinement systems rely on a high-efficiency, particle-air filter to remove particles, a charcoal-bed filter to absorb iodine and halogens, and demisters to remove water droplets, Rollins said. "The filters are designed to remove all radioactivity except for noble gases," such as xenon, krypton, and tritium, Rollins said.

The potential problem with the filters is that they may not withstand the loading that results from a severe accident. Because the source term calculations were done when little was known about radioactive releases, DOE underestimated the amount and form of iodine and the amount of cesium that would be released. The biggest threat to filter integrity would be large amounts of cesium: recent tests on Savannah River fuel have shown that nearly all iodine would be released from damaged fuel and that cesium and iodine will combine into the particulate cesium iodide, which could collect on the filtration unit instead of the charcoal bed.

New HWR Designs In Progress

The industrial proponents of the new HWR designs say they have solved the problems with DOE's SRP reactors. Two industrial teams--from Ebasco, Babcock & Wilcox, and NUS Corp., and from Westinghouse/ Bechtel-made presentations to ERAB. DOE also has been designing an improved HWR design with the help of DuPont and Savannah River Laboratory scientists. Both Ebasco and Westinghouse said they could build smaller versions of their proposed designs rather than one large plant.

All the designs are updated versions of the current SRP reactors and would use the same design, fuel, and target technologies and facilities. All would rely heavily on safety codes being developed and validated by DOE for the current SRP reactors, representatives from each team acknowledged-a task complicated by the lack of documentation for the old reactors' designs.

DOE has been designing a new HWR for a number of years, said M.C. Kirkland, director of the DOE-Savannah River project engineering division. It is "basically the same reactor" but would contain "all the safety features we've learned about over the years," he said.

Three changes are being considered. DOE "may go to underwater refueling instead of dry refueling," could add a containment, and might change the direction of

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coolant flow, Kirkland said. "The downflow does work," he said, "but people thought it (might be) better to have the upflow because natural convection allows the water to rise and allows cooling in a shutdown condition without pumps.... We haven't decided whether (the containment) will be rectilinear like a conventional containment building or circular with a dome," he said. "Either one will work and the cost of either is about the same. But it will be a low-pressure containment because of the low temperature" of the system.

According to DOE and industry sources, however, the Ebasco/Babcock & Wilcox/NUS Corp. "enhanced heavy water reactor" most impressed the ERAB panel. The Ebasco HWR would be a four-loop, 2,500-MWI low-temperature (250 degrees F), low-pressure (45 psig) reactor with separate heavy water moderator and coolant loops, according to Robert lotti, Ebasco's vice president, advanced technology. The reactor vessel and the primary system would be made of Type 316 austenitic stainless steel which, lotti says, is less susceptible to IGSCC than carbon or low-alloy steels. Tighter control over oxygen concentrations in the primary and moderator circuits, as well as other precautions that can be taken during manufacture and installation, will minimize IGSCC potential, Ebasco said.

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Tritium would be removed in two streams from the primary coolant and moderator through cryogenic distillation, which permits a "very small but reasonable leak rate into the containment vessel," said lotti's assistant Vincent Walker. The tritium would be removed from the containment atmosphere by a heating, ventilating, and air-conditioning system that uses air dryers. This tritium control and removal system would result in a plant personnel dose of only 10% of current SRP levels, Ebasco said.

To eliminate potential filter and confinement problems, Ebasco's vessel would be housed in a large, dry, post-tensioned reinforced concrete containment vessel with a steel liner and a design pressure of 60 psig. To control potential hydrogen generated during a severe accident, the design would use a combination of containment sprays, hydrogen recombiners, and hydrogen igniters, lotti said.

Ebasco plans to conduct level one, two, and three Probabilistic Risk Assessments (PRA) on the design should it land the contract, lotti said.

Design enhancements have climinated problems experienced by the SRP reactors, lotti said. The condition limiting operating power would be resolved in the Ebasco design because the primary coolant would enter the reactor vessel through a downcomer surrounding the moderator and core region, and flow up through the assemblies, as in commercial PWRS, lotti said. The upper and lower end fittings would be changed to accommodate the change in flow direction, Walker said.

Other issues raised by NAS, including questions? related to human performance, liquid effluent relcases, and emergency planning would be resolved during the design and licensability review processes, Ebasco said.

Ebasco would plan an annual outage of about 30 days, lotti said. Total core replacement could be ac-. complished in 14 days using robotized charging/discharg-. ing machines, he said. The remaining 16 days would be used for preventive maintenance and in-service inspection activities, he said. The Ebasco design incorporates two redundant refueling machines and two transfer tubes from the containment transfer pool to the spent fuel pool to minimize possible schedule delays due to machine failure, he added.

The Westinghouse HWR design would be a six-loop. 3,300-MWt plant using the same target design as the SRP reactors but 762 fuel assemblies of 8% enriched U-238. The Westinghouse design would use 210 blanket assemblies containing lithium-aluminum target materials for

tritium production. The target materials and fuel would be changed out at 10-month intervals.

Westinghouse also would go to automated refueling equipment to speed refueling and reduce personnel exposures. The reactor tank would be made of type 304L stainless steel, which also has a lower carbon content than the SRP reactor tanks, and Westinghouse also would control water chemistry to inhibit IGSCC.

The major difference between the Westinghouse design and the current SRP reactors is the incorporation of a robust containment with a design-basis pressure of 45 psig. The metal containment would transfer any decay heat through the shell, and the outer surface of the containment would be cooled by a passive air system operating by convection, Vijuk said.

DOE's scheduled August 1 decision on a new weapons materials production reactor (NPR) has sent industrial teams scrambling for the contract, worth $5- to $10-billion and the largest U.S. nuclear plant order in the near fu

ture.

DOE says it will pick a new reactor technology by giving equal weight to the goals of production efficiency and safety. But an Energy Research Advisory Board (ERAB) panel, which reviewed several designs for DOE, recommended DOE opt for a heavy-water reactor (HWR), built next to the existing but shut-down HWRs at the Savannah River Plant (SRP), to attain quick tritium production (NW, 9 June, 1). The report contradicted earlier studies showing that all the technologies, including, the passively safe designs, had roughly the same construc-" tion times.

But all of the technologies entail schedule risk. For the HWR, the risk is in the safety review process. DOE is developing policies on safety objectives, severe accidents, and probabilistic risk assessments (PRAs) in response to the first-ever outside review of the SRP reactors. The safety effort, begun in fiscal year 1988, will take four years and $12-million to develop, according to DOE documents. Each new HWR design would use the safety: 10 codes to be developed in that program. The SRP codes, which use revised commercial industry codes as a base, must be recalculated for their higher fuel enrichments, lower operating pressures, and heavy water moderation. Other complications include missing documentation for the SRP reactors and proposed HWR design changes reversing the direction of coolant flow (NW, 14 July, 9). For the other designs-LWRs, modular high-temperature gas-cooled reactors (MHTGRs), and liquid metal reactors (LMRs)—the schedule risk lies in development of tritium production capabilities. All the designs have been or will be reviewed by NRC for safety, but they are unproven tritium producers, though the MHTGR has what is considered the most mature technology. Proving that capability could delay those reactors, ERAB said.

DOE should consider an advanced reactor despite potentially higher costs, ERAB said. All the non-HWR concepts could sell steam to offset costs. The MHTGR and the LMR would advance nuclear technology more than a HWR or even a large LWR, the panel said. The MHTGR and LMR and, to a lesser extent, a small LWR, are "passively safe" designs that rely on gravity and natural convection and circulation, rather than operator in

tervention, for shutdown. The smaller, modular designs should cut costs and reduce hasty design changes.

But linking LWRs to weapons is a connection many in the industry want to avoid. The two LMR proponents, who are also trying to avoid that connection, have sacrificed efficient breeding ratios for bigger safety margins. And Public Service Co. of Colorado's (PSC) Fort St. Vrain, the only U.S. commercial HTGR, has been plagued with operational problems, although the plant was taken off NRC's "problem plant" list last week.

DOE officials, who openly prefer the HWR, have argued that they cannot gamble the nation's nuclear deterrent on unproven technologies. Proponents of non-HWR technologies argue that DOE must upgrade its public image by choosing a "safer" reactor. What is clear, say DOE observers, is that the ERAB panel told DOE exactly what it wanted to hear. It is also clear that the reactor safety debate is far from over.

MHTGRs: Will the technology work?

The MHTGR, which by most accounts presented the only serious challenge to the HWRs, impressed the ERAB panel because of its passive safety features, the maturity of its target technology, and the ability of its modular construction to provide redundancy and flexibility in materials production, sources said.

General Atomics' (GA) MHTGR would consist of eight, 135-MW, helium-cooled, graphite-moderated modules. Each module would contain a carbon steel reactor vessel, an interconnecting crossduct, and a vessel housing a helical coil steam generator and a motor-driven main helium circulator. Each module would have its own turbine, which would allow production of different isotopes as well as staggered refueling and target changeout times. The carbon steel would not exhibit the intergranular stress corrosion cracking shown in the SRP reactors because the MHTGR system does not contain water, GA said.

According to Linden Blue, GA's vice chairman of the board, every performance problem experienced by Fort St. Vrain has been linked to the unique design of the helium circulators. The Fort St. Vrain circulators have bearings that are cooled by water, rather than oil, which has resulted in water leakage into the helium coolant. The bearings for the proposed circulators would be conventionally cooled, Blue said.

The fuel would consist of highly enriched uranium

dicarbide particles with a TRISO coating consisting of three layers of pyrolytic carbon, silicon carbide, and pyrolytic carbon. The coating is designed to retain fission I products. The particles are mixed with graphite, formed into fuel rods, and inserted into graphite blocks to make fuel elements. A total of 660 fuel elements make up one module's core. The fuel is identical to that used in Fort St. Vrain and has performed well, according to PSC. The fuel has a design life of three years; there is no degradation of the fuel up to 3,600 degrees F.

GA's target technology, the most advanced of all the non-HWR concepts, would use TRISO-coated lithium aluminate kernels. The coating retains tritium, which diffuses rapidly through most materials. The target particles are mixed with graphite, assembled into graphite sleeves, and inserted into the fuel assemblies. The tritium targets are designed to last six months and retain their integrity up to the same temperature as the fuel.

GA anticipates a two-week maintenance and inspection outage each time the targets are removed, Blue said. The reactor is refueled through control rod drive penetrations in the top of the vessel. A refueling machine is inserted into the penetrations and entire graphite blocks-one-third of the core-are removed and replaced. The removal of the entire block every three years precludes the neutron-induced graphite distortion that plagued DOE's Hanford, Wash., N reactor, Blue said (NW, 18 Dec. '86, 7).

The tritium recovery technology consists of crushers to fracture the target particles and a vacuum furnace with a purification system. Reprocessing of driver fuel could make major use of existing facilities at SRP and the Idaho National Engineering Laboratory (INEL), GA said.

The two key passive safety features of the MHTGR, Blue said, are passive decay heat removal and a negative temperature coefficient. Decay heat would be dissipated from the core by conduction and radiation to the silo cooling system or to the surrounding earth. The large negative temperature coefficient would shut the reactor down in the event of a loss of forced circulation at full power. At Fort St. Vrain, forced circulation could be lost for 18 hours without fuel damage (NW, 15 Oct. '87, 8).

The reactors would be housed in underground silos, with the "containment" being unlined reinforced concrete horizontal blowdown tunnels designed for a maximum 15 pounds per square inch gauge (psig). The low contained energy within the system would preclude a breach of the tunnel containments, Blue said.

LWRs: Building on a technology base

The LWR technologies have a theoretical advantage because of their widespread use in the commercial power industry, ERAB said. The problem with the LWRs is that they lack a proven technology for producing tritium, sources said.

A Westinghouse/Bechtel team, which also proposed an HWR concept, proposed two technologies: a "special water reactor" (SWR) based on the design of Union Electric Co.'s Callaway, and conversion of the Washington Public Power Supply System's (WPPSS) mothballed WNP-1. Late in the selection process, ERAB also considered a small, "inherently safe" Westinghouse spin-off of its AP-600 concept, called a Small Advanced Special Water Reactor (SASWR). The SASWR would be redesigned for weapons production, according to Robert Vijuk, Westinghouse's manager of space, defense, and nuclear programs.

The SWR would be very similar to Callaway, a fourloop, 1,170-MW (net) PWR. The SASWR would be a two-loop, 700-MW (net) modular PWR. Like the AP-600, the SASWR would have fewer valves, fewer large pumps, and less piping than current LWRs. The SASWR would rely on passive safety injection systems, passive decay

heat removal systems, and passive containment cooling using gravity and natural convection.

The SWR would use a 19x19 zircaloy-clad fuel assembly, but the uranium would be 8.5% enriched. Callaway's fuck is in a 17x17 zircaloy-clad assembly and is 2% to 5% enriched. The SASWR also would use 19x19 fuel assemblies, but the fuel enrichment would be 8.8%. WNP-1 is designed to use 17x17 fuel assemblies, but the enrichment would be upped to 10%.

The fuel for each reactor would have a three-year life, with one-third of the core changed out at each refueling. Westinghouse began its SWR design using highly enriched uranium, but the enrichment was gradually lowered to 8.5% to preclude recriticality problems, Vijuk said.

The ceramic lithium-aluminum target rods would be contained in a stainless steel tube with a tritium diffusion barrier and a getter in the tube to retain the tritium during irradiation. Westinghouse has tested small-scale targets in INEL's Advanced Test Reactor (ATR) and would test fulllength simulated targets in an operating commercial reactor, Vijuk said. The targets would have a one-year lifetime. Unlike GA's targets, the Westinghouse targets have not been demonstrated, although the target concept is feasible, Vijuk said.

Westinghouse anticipates 42 days per year outage time for target change-out, refueling and maintenance for its SWR concepts. The company expects an annual 80% availability rate for all its LWR concepts, Vijuk said.

A disadvantage for the LWRs is that they would require new fuel fabrication facilities, assuming that DOE wanted that capability on-site, Vijuk said. Otherwise, he said, the fuel could be fabricated in existing commercial or government fuel facilities. "The targets could be fabricated anywhere," he said, but tritium extraction facilities would have to be built.

"The SWR design, through the Callaway plant, is 100% complete, "so we have complete plant drawings and specifications, and only the changes need to be engineered in the same detail," Vijuk said. The SASWR is still a concept. The SWR reactor vessel would be slightly larger than Callaway's, and there are other, minor differences, he said.

WNP-1, a 1,250-MW Babcock & Wilcox PWR, would appear to be a perfect solution to DOE's current dilemma: at the 63% completion level, the plant's major systems have been installed, and the reactor's design is 98% complete. WPPSS has been careful to keep a skeleton maintenance crew on site to keep up the plant, so the reactor could theoretically be brought on line in five years, or half the estimated time required to build a new reactor from scratch. WNP-1 is located on federal land at DOE's Hanford reservation and could use the N reactor's work force. A Bechtel study last year concluded the conversion was feasible, and the company had proposed finishing the plant on a $1.6-billion fixed-price contract.

But WPPSS sold $2.1-billion in bonds to finance the plant, and any attempt by DOE to purchase or otherwise acquire the unit could result in bondholder litigation that could tie the issue up in court for several years (NW, 22 Oct. '87, 2) and dramatically increase DOE's costs. And, although the Arms Control & Disarmament Agency and the State Department have said that the conversion would not violate U.S. law or treaty obligations, significant political opposition to the conversion of a commercial reactor exists.

LMR Advocates: We're Not Interested

DOE requested early this year that General Electric and Rockwell International make presentations to the ERAB panel on their proposed commercial LMR designs, which are sponsored by DOE. But the companies would just as soon not have their reactors linked to weapons

production, as were earlier liquid metal fast breeder reactors. GE's reactor would not breed, and Rockwell's literature makes only a passing reference to a "breeding ratio slightly greater than unity to make up for fuel cycle losses." Both reactors rely on the inherent safety features of liquid sodium as a coolant and use natural convection for decay heat removal.

GE's Power Reactor Inherently Safe Module (PRISM) would be a 1,395-MW (net) pool-type reactor plant.comsisting of three, 465-MW power blocks. Each block would consist of three 155-MW modules, which include a nuclear steam supply system and a turbine, so targets could be changed in one module while the rest operated, providing the production flexibility so attractive in the MHTGR.

The reference commercial fuel is Argonne National Laboratory's (ANL) metallic fuel, although oxide fuel could also be used. Like the MHTGR, the reactor would

be housed in an underground silko, and the reactor module would be factory-fabricated and shipped to the site in one piece.

Rockwell's Sodium Advanced Fast Reactor (SAFR) would be a 1,800-MW pool-type plant consisting of four, 450-MW modules. SAFR also would use metallic fuel, but could use oxide fuel as well. The reactor would be housed above grade, fabricated in a factory, and shipped to the site in one piece.

"DOE is our customer, and our customer requested that we make the presentation," said Robert Bergland, GE's manager of advanced nuclear technology. "But we're not interested."

"Boch SAFR and PRISM are being proposed and advocated as super-sale advanced systems for electric power generation,” said Richard Oldenkamp, director of power plant projects for Rockwell's Atomics International Rockeldyne Division. DOE might want to use a LMR design