A Functionally Graded Composite for Service in High-Temperature Lead- and Lead-Bismuth–Cooled Nuclear Reactors—I_ Design

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A FUNCTIONALLY GRADED COMPOSITE FOR SERVICE IN HIGH-TEMPERATURE LEAD- AND LEAD-BISMUTH–COOLED NUCLEAR REACTORS—I: DESIGN NUCLEAR PLANT OPERATIONS AND CONTROL KEYWORDS: corrosion, composite, lead-bismuth MICHAEL PHILIP SHORT* and RONALD GEORGE BALLINGER Massachusetts Institute of Technology, H. H. Uhlig Corrosion Laboratory 185 Albany Street, Cambridge, Massachusetts 02139 Downloaded by [Australian Catholic University] at 03:26 17 August 2017 Received January 24, 2011 Accepted for Publication May 13, 2011 A material system that resists lead-bismuth attack and retains its strength at very high temperatures has been developed that enables increased outlet temperature and the promise of allowing increased coolant velocity and efficiency of lead- and lead-bismuth–cooled reactors if the behavior reported here is confirmed by long-term tests. The development of this system represents an enabling technology for lead-bismuth–cooled reactors. The system is a functionally graded composite (FGC), with separate layers engineered to perform corrosion resistance and structural functions. Alloy F91 was chosen as the structural layer of the composite because of its strength and radiation resistance. An Fe-12Cr-2Si alloy was developed based on previous work in the Fe- Cr-Si system, and was used as the corrosion-resistant I. INTRODUCTION A deviation from the evolutionary path of light water reactor design will soon be required to meet the rising energy demands of the world, in order to more efficiently extract energy from the same fuel while simultaneously improving safety, reducing the possibility for proliferation, and simplifying reactor design. These advanced systems, designated as Generation IV ~Gen IV! systems, promise higher efficiencies, improved uranium utilization, and increased safety margins. 1 However, each Gen IV system also poses unique challenges to successful implementation. In the case of lead-bismuth–cooled systems, the primary issue is the corrosion of structural materials. *E-mail: hereiam@mit.edu cladding layer because of its chemical similarity to F91 and its superior corrosion resistance in lead and leadbismuth in both oxidizing and reducing environments. The availability of the FGC will have significant impacts on lead-bismuth reactor design. The allowable increases in outlet temperature and coolant velocity lead to a large increase in power density—either to a smaller core for the same power rating or to more power output for the same-size core. In this paper, we report on the overall design of the FGC. We also discuss the general implications for lead-bismuth–cooled reactor design. In a future paper, we will discuss the fabrication and the initial evaluation of the actual product produced using commercial processing methods. I.A. Comparison of Proposed Gen IV Reactor Coolants Six principal coolants are being investigated by countries pursuing Gen IV nuclear technologies as viable options for reactor coolants. 1–7 These coolants have been identified as possessing superior properties to others considered, and they allow for reactor designs that meet the goals of the Gen IV program. 1 Each coolant has specific advantages and disadvantages, all relating to their physical properties, which are summarized in Table I. The liquid metals and molten salts have superior heat conductivities and heat capacities relative to the other coolants. The high boiling points of the liquid metals ensure that the system will not need to be pressurized to remain in the liquid phase. Instead, an unpressurized, inert cover gas, such as argon, is all that is necessary to exclude air from the system. This means that structural 366 NUCLEAR TECHNOLOGY VOL. 177 MAR. 2012
Short and Ballinger A COMPOSITE FOR HIGH-TEMPERATURE LEAD- AND LEAD-BISMUTH–COOLED REACTORS—I TABLE I Relevant Physical Properties of Selected Gen IV Reactor Coolants Coolant Melting Point ~8C! Outlet Temperature ~8C! Boiling Point ~8C! rc p ~J0cm 3 {K! a k ~W0m{K! a Reactor Pressure ~MPa! Viscosity ~Pa{s! a Helium 8 272 950 269 0.0143 ~Ref. 9! 0.0357 7.0 4.111 10 5 Lead-bismuth ~Ref. 10! 123.3 700 1670 1.37 17.5 0.1 1.0 10 3 Lead 10 327.4 700 1737 1.51 19.9 0.1 1.3 10 3 Sodium 11 97.8 550 892 1.03 ~Ref. 12! 64.9 ~Ref. 13! 0.1 2.2 10 4 ~Ref. 13! Molten salt 11 b 396 700 2500 1.72 0.39 0.1 1.18 10 3 Supercritical H 2 O ~Refs. 6 and 14! 0 500 100 0.225 0.09 22.1 3 10 5 a Selected property is at the given outlet temperature and pressure. b The salt chosen for this comparison is ~3Na–2K–5Mg!Cl x . Downloaded by [Australian Catholic University] at 03:26 17 August 2017 materials in the reactor do not have to be able to tolerate a higher operating pressure; they need only to withstand their own weight and that of the coolant. The liquid metals and molten salts also have higher atomic masses, and therefore less neutron moderation, than the other three proposed coolants, which make them particularly suitable for fast reactors. Helium shares the benefit of staying in one phase with the molten metals and salts; however, the temperatures required to raise the efficiency levels of helium-cooled reactors ~up to 10008C! put enormous demands on the materials of the reactor. Supercritical water possesses heat transfer properties between those of helium and the molten metals and salts, but it must be highly pressurized, again putting enormous strain on reactor materials. While some physical and thermal properties of sodium exceed those of lead and lead-bismuth, it should be noted that these latter two possess two main benefits in the context of fast reactors. First, the atomic number of lead ~and the average of lead-bismuth! is higher than that of sodium, leading to a lower amount of neutron moderation. 15 This results in fewer neutrons losing energy and being captured by resonances in lower-nergy regions. Second, lead and lead-bismuth do not possess the chemical incompatibility with water or moist air that sodium does. This leads to less of a safety concern in the event of a coolant leak for lead and lead-bismuth. I.B. Material Compatibility Issues with Lead and Lead-Bismuth The most significant drawback to using lead-bismuth as a high-temperature coolant is corrosion, which has been extensively discussed in the framework of mediumpower lead-cooled reactors. 16 As with many working fluids, oxygen is soluble to some degree in lead-bismuth. The presence of dissolved oxygen in the working fluid requires materials either to be protectively coated or to form their own coating via passivation—the formation of a protective oxide film. Many materials including chromia, alumina, and silica formers possess the ability to do this, and many working fluids can carry sufficient oxygen to induce passivation in these materials. The main difference that makes lead-bismuth so incompatible with some of these materials, and nickel in particular, is their behavior in environments that are below the chemical potential ~a combination of oxygen concentration and temperature! for forming protective oxide films. Many metals have high solubilities in lead and leadbismuth. Without an oxide layer protecting the surface of the metal, materials are free to dissolve into the working fluid. Nickel has a particularly high solubility of 4.8 wt% in lead-bismuth at 7008C ~Ref. 17!. Alloys with even a few percent nickel are subject to severe liquid-metal attack and dissolution in lead and leadbismuth systems. 18 This problem is further aggravated by the fact that the coolant is flowing in a thermal loop. This creates a transport cycle where because of the temperature dependence of solubilities, material in the hot leg is removed and deposited in the cold leg, simultaneously weakening the hot-leg structural materials and clogging the pipes in the cold leg. This was a problem with past lead-bismuth–cooled systems, experienced most dramatically in Soviet lead-bismuth–cooled submarines. 19 Pure iron oxide is a poor barrier to corrosion over the proposed operating temperature range ~6008C to7008C! due to a phase transformation of the oxide from magnetite to wüstite at 5708C ~Ref. 20!. Wüstite is a porous, brittle oxide phase that does not protect the underlying metal. Both of the situations described above would lead to accelerated corrosion in regions of oxygen depletion, such as in pits or crevices, which function as powerful corrosion accelerators for materials in lead-bismuth. As oxygen inside a crevice is consumed, it is not replenished by oxygen-bearing fluid because of restricted transport of oxygen into the crevice. As a result, the oxygen potential can drop dramatically inside the crevice, thinning or even eliminating the protective passive oxide layer and leaving bare metal exposed to a very reducing environment. This could cause crevices or pits to grow, resulting in stress concentrators and localized wall thinning. NUCLEAR TECHNOLOGY VOL. 177 MAR. 2012 367
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A Functionally Graded Composite for Service in High-Temperature Lead- and Lead-Bismuth–Cooled Nuclear Reactors—I_ Design

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