During the early construction of nuclear power plants, steam condensers relied upon copper base alloys – brass and copper nickel – for heat transfer capabilities. These alloys have high coefficients of thermal conductivity required in steam generation to power nuclear reactor turbines. But copper-alloyed tubes were being replaced too quickly – with an average life of eight years – because of sulphide pitting. Hardest hit were those reactors using polluted seawater to cool their reactors.
Over the past 30 years ago, nuclear utilities slowly began turning to the super austenitic stainless steels as one way to make their nuclear reactors last longer. The addition of molybdenum, initially starting with percentage of less than four percent, helped increase the thermal conductivity lacking in nickel, iron or steel. At nuclear stations which replaced the copper alloys with HPSS condenser tubes, 57 percent rated the thermal performance good and all but one rated it normal. Molybdenum had helped overcome the thermal hurdle.
A large number of the 190 nuclear reactors, which now utilize HPSS condenser tubes, reported an average life in excess of 18 years. The longest stainless steel condenser installation has remained in service more than 26 years, according to a study done several years ago. According to a report published in 2000, more than 100 million feet of super-alloy stainless steel tubes have replaced the older, copper-alloy tubing.
Condensers are large heat exchangers used in nuclear power plants. Condensers have thousands of tubes horizontally mounted to condense and recover the steam passing through turbines. Each low-pressure turbine generally has a condenser, which also maintains a vacuum to optimize the turbine’s efficiency.
Water fouling deposits were cited as a major problem at many reactors, especially with condenser tubes where seawater or high-chloride brackish water was the coolant. Pitting corrosion, tube sheet crevice corrosion and galvanic corrosion put the tubes at risk for leakage. Plugging, mud, or detritus accumulating in condenser tubes reduce a power plant’s efficiency.
Utilities use cleaning systems with small, abrasive sponge-like balls to keep the tubes clean and test for tube defectives with probing devices. Tube thinning and corrosion create the opportunity for tube leakage. This can not be tolerated because chemicals such as sodium and chlorides find their way into the reactor vessel or steam generator.
Upgrading the steam condenser tubing to stainless steel also plays a vital role in the ‘power uprate’ program utilities have used to increase generating capacity for existing reactors as we recently discussed . The more advanced uprate program could add up to 20-percent capacity to existing U.S. nuclear reactors.
There are several HPSS manufacturers for nuclear reactor condensers. The most prominent in the nuclear sector include Pennsylvania-based ATI Allegheny Ludlum and Finland’s Outokumpu. Each offers austenitic steels with chromium and nickel composition of between 20 and 25 percent for each alloy and a range of 6.2 to 7.5 percent molybdenum.
In a paper presented by Jan Olsson of Avesta Sheffield (before the company was acquired by Outokumpu), he highlighted the results of tests performed on the new super-austenitic stainless steel, 654 SMO®. Metals comprising this brand include 25-percent chromium, 22-percent nickel and 7.5-percent molybdenum. To increase pitting resistance, the manufacturers added up to 0.5-percent nitrogen and three-percent manganese (for make the nitrogen more soluble).
As with all pioneering developments – and remember that R & D breakthroughs have taken place over a two-decade-plus period, manufacturers have re-designed their metallurgical composition to find the most encouraging percentages of nickel, chromium, molybdenum and nitrogen. The earlier stainless steels relied on higher nickel content and lesser percentages of chromium and molybdenum.
At first, conventional austenitic grades, such as 316L, or high chromium-ferritic grades, were utilized. Pitting struck down widespread use of the 316L series and was replaced by higher alloy steels. For example, others, such as the 254 SMO® stainless steel, began aggressively replacing the copper alloy tubes and in some cases the 316L series. The 254 is comprised of 20-percent chromium, 18-percent nickel, 6.2-percent molybdenum and 0.20-percent nitrogen. It has also offered a high level of corrosion resistance at desalination plants without becoming cost-prohibitive.
The most significant breakthrough came after various stainless steels were tested at Scandinavian coastal reactors. In the Avesta paper, the failures of each lesser austenitic grade were checked off. Significant deficiencies included insufficient stress corrosion cracking resistance and resistance to natural seawater. Even titanium tubing was used as an interim measure because it increased total heat transfer by 17 percent, but the metal failed to stand up to high velocity steam and suffered ‘water droplet erosion.’
According to the study, “The only alloy fully resistant to all test conditions was 654 SMO®.” The results at nuclear power plants in Finland and Sweden, along the Baltic Sea, were astonishing! Four important conclusions about this super alloy were reached after the testing.
Its corrosion resistance could cope with the hostile environments existing inside condenser tubes of desalination plants and power plants. Its corrosion resistance was good enough to cop with many other hostile brine and seawater environments. Its erosion resistance was advantageous where it was exposed to high velocity streams. There was no concern about its heat transfer characteristics.
Nuclear Consumption of Molybdenu
About 48 nuclear reactors are reportedly scheduled for construction by 2013. It may be possible that up to 100 could be constructed by 2020, depending upon political and financial climates. The largest number proceeding through the proposed, planned or construction phases will be located along coastal areas to service the most populated areas. The greatest numbers of new constructions are expected from China, India, Japan, Russia, South Korea and Japan (and possibly the United States).
Existing reactors along coastal areas in Asian countries presently breaks down as follows: Japan (57), South Korea (26), China and Taiwan (19) and India (11). Because these are the most prone to seawater or brackish corrosion, they are also the likely candidates for upgrading existing condenser tubing to high alloy stainless steel. And their new reactors are likely going to be constructed along their coasts, requiring the super austenitic grades. As an aside, of the previously mentioned 190 nuclear power plants which had replaced their condensers with HPSS, 45 percent used fresh water as coolant. Those plants chose the high alloy steel as a ‘fail-safe’ measure to prevent interrupted service or a potential reactor incident.
The United Nations estimates that two-thirds of the planet’s population will be living with water stress by 2025. Global freshwater scarcity may demand the use of brackish or seawater as nuclear reactor coolant. To prevent the accompanying corrosion, the higher-percentage molybdenum alloy, specifically the 654 SMO®, could emerge as the condenser tubing material of choice. Either the 254 SMO® or the 654 would be utilized in desalination plants required to overcome water shortages in the hardest hit areas: North Africa, the Middle East and West Asia.
Typically, nuclear power plant condenser tubing requires approximately 520,000 feet of stainless steel. According to the International Molybdenum Association [IMOA], larger reactors could utilize up to one million feet of stainless steel. With the higher molybdenum grades found in the super alloys, new nuclear reactors could require tens of thousands of metric tons of molybdenum.
By comparison, nuclear waste containers proposed for the Yucca Mountain nuclear waste repository were forecast to consume about 15,000 metric tons of moly. While this project may or may not proceed as planned to the construction phase, the Nuclear Energy Institute [NEI] has proposed regionalized storage of spent fuel.
Should comparably designed storage canisters be utilized to ‘temporarily’ contain the nuclear waste, it is likely molybdenum will play a key role. According to the U.S. Government’s Energy Citation Database, as published by the Department of Energy’s Office of Scientific and Technical Information, “Alloys with combined chromium plus molybdenum contents greater than 30 percent were the most resistant to general and local attack.” This was the conclusion reached after corrosion scouring tests were performed on stainless steel and nickel-based alloys to immobilize high-level, radioactive waste.
Another aspect where high-percentage molybdenum stainless steel would double up is with the expansion of nuclear desalination plants. In the past we have discussed the rise of nuclear desalination across those coastal areas, requiring far more freshwater than can possibly be transported through other means. The World Nuclear Association [WNA] has reported of numerous such desalination projects in progress.
From nearly every energy project – oil, gas, coal and nuclear, and for water, molybdenum demand will continue increasing. Super austenitic grades demand a higher moly content to combat corrosion and provide reliability of service. Of course, there will be substitution in the face of future supply shortfalls. In some instances, there are reports the Russians have substituted vanadium for molybdenum in some of their oil and gas pipelines to conserve on moly consumption. ATI Allegheny Ludlum has argued for the substitution of two-percent manganese for every percent of nickel, but in the lower grade austenitic groups which do not demand the corrosion resistance of energy projects.
While reviewing the anticipated new projects from the molybdenum mining sector, we foresee the high probability of supply inadequacy. Aside from China Moly’s Sandaozhuang molybdenum mine, which the company hopes could produce 28,000 tonnes of molybdenum concentrate this year and perhaps grow by another 17 percent the following year, there is a paucity of new molybdenum projects coming fully online before 2009.
Based upon China’s voracious appetite for molybdenum – one research firm estimated compounded annual growth rate over the previous five years at 17 percent, whatever excess moly production comes from China Moly’s mining efforts could very well be domestically consumed.
Future North American molybdenum producers may need to ramp up their projects to meet the growing demand. During 2006, demand grew above the historical norm of four percent; most of the consumption came from China. This is unlikely to stagnate or decrease, and could interfere with North American and European consumption of molybdenum.
Only one company is scheduled to commence molybdenum mining in 2007, Roca Mines. Because the company is limited to a small-mining permit, anticipated production could not exceed three million pounds. By late 2008, or early 2009, Adanac Molybdenum hopes to commence its start-up efforts to reach eight-figure moly production. Later, Blue Pearl Mining hopes to commence high-grade molybdenum mining at the Davidson deposit in British Columbia. Around this time, the Climax molybdenum mine could re-open and begin production in Colorado. Moly Mines hopes to begin production at the company’s Spinifex project. Possibly, before the decade ends, Idaho General might commence operations in Nevada. Perhaps before those 48 nuclear reactors come online, US Energy’s moly deposit may be mined in Colorado.
Many of these projects are subject to environmental permitting and/or financing, putting any material amount of forecasted supply in jeopardy. And this comes at a time when some experts believe byproduct molybdenum production at copper mines could be constrained. There are many conditional requirements which do not necessarily guarantee a reliable supply from the new breed of primary moly producers. We have witnessed comparable obstacles in the uranium sector, which has since been accompanied by a hyperbolic price rally in this metal.
There could come a time in the molybdenum sector when the silvery white metal could mimic such a breakout scenario. Nearly three years ago, we featured a forecast of US$100/pound uranium. No one believed that prediction at the time. On Friday, TradeTech announced a spot price of US$113/pound.