LMFR, materials

An in-pile, forced-circulation loop has been built at BNL and two others at Babcock Wilcox Research Laboratory to test the corrosion stability of LMFR materials under conditions to be expected in the reactor experiment. In this loop, Bi containiiif approximately 1500 ppm of 180 ppm Zr, and 850 ppm Mg will be pumped at a rate of 5 to 7 gpm. The bulk A2 will be approximately 75°C, with a maximum temperature of 500°C. There are three sample sections in the loop one, containing samples of 1 % Cr-1/2% Mo steel, 2 % Cr-1% Mo steel. Be, and graphite, will be at the center of the reactor and will be in a flux of approximately 3 X lO " thermal neutrons one within the shield will see delayed neutrons at a temperature of 500°C and the third section will be outside the reactor at a temperature of 425°C. This test is presently being assembled, and will be operating late in 1958. 

The solution of the problem of the reliable elimination of coolant leaks is determined by the application of experimentally and analytically proved design, structural materials, manufacture and installation, as well as quality control at all stages of LMFR components manufacture. Technological procedures and approaches, as well as quality criteria, should strictly correspond to the related regulatoiy documents. 

The very low corrosion of sodium, the near atmospheric operating pressure, the use of ductile structural materials, and the reliable heat removal by a coolant having no phase change, imply that there should be nothing to provoke loss of the sodium system integrity in a LMFR. 

Because these developments are focused on intended applications in other fields and not problems of LMFR design there are significant differences in design lives, service conditions, materials, manufacturing practices, etc. The types of structures differ. The impact of these differences on such design information as constitutive models, material failure modes and models, and structural failure modes and consequences are sometimes difficult to assess. However, computer modeling, structural analyses methods, and analytic methods to understand materials behavior have advanced greatly in some of these non nuclear areas. 

The current view is that the technologies of sodium coolant and mixed oxide fuels are largely mastered, and large prototypes and demonstration LMFRs have been built and have clearly demonstrated that a LMFR is capable of sustained reliable contribution to an electricity supply system. Fast reactors will contribute not only to power production, but also to waste management and the control of nuclear materials. The world-wide investment already made in the development and demonstration of LMFR technology exceeds US 50 billions. 

If these will be eliminated on the basis of choice of available structural materials and/or development of new ones and innovative design approaches, and if operational reliability of two-circuit pool-type reactor with supercritical SG located inside the reactor vessel will be demonstrated then the most important advantage of lead and lead-bismuth-cooled reactors will be the possibility to eliminate the safety concerns of LMFR caused by sodium chemical reactivity with air and water with explosion potential. 

The reuse of this sodium for nuclear operation was not possible due to the absence of building of new LMFR in the near future. Thus the transformation of metallic sodium to a non reactive material was decided. To do so, it was decided to use the reference NOAH process that will transform metallic sodium to concentrated sodium hydroxide (10 mol/L). The treatment of primary sodium (3250 tons) and secondary sodium (1500 tons) will produce large amount of sodium hydroxide (19 000 tons) or neutralized salts such as sodium sulphate (25 000 tons). The facility envisaged will be based on the design of the SDP facility (Sodium Disposal Plant) built to treat the primary and secondary sodium of PFR [30, 32]. The estimated treatment flow rate will be 6 metric tons per day. The final destination of this by-product must be clearly defined and authorized by a decree. Two major solutions can be envisaged ... 

Reactor physic studies for the transmutation of Am in separate subassemblies of a LMFR were made using a rather high Am-concentration (to restrict the number of the special assemblies), a typical fast reactor flux (3.61589 x 10 K/cm s) and depleted UOj as a. .matrix material. The calculations were done as fundamental mode bumup calculations with the KAPROS code system. The results show that for an irradiation cycle of 6 years the total amount of Am is halved, % of the original mass was fissioned and the remaining Vi transmuted mainly to Pu 238, Pu 242 and to Cm (242 and 244) and only a small percentage of the depleted UOj was fissioned. The power densities for varying Am and U contents could be adjusted between 500 W/cm (pure UOj) and about 1200 W/cm (pure Am... 

For the most promising low activated materials, such as Ti and TiH2, irradiated in the blanket of LMFR, the content of the natural uranium and niobium impurities should not exceed several thousandths of ppm, while for thorium and dysprosium this limit is several hundredths of ppm. 

The present status report intends to provide comprehensive and detailed information on LMFR technology. The focus is on practical issues that are useful to engineers, scientists, managers, university students and professors, on the following topics experience in constmction and operation, reactor physics and safety, core structural material and fuel technology, fast reactor engineering and activities in progress on LMFR plants. 

Acoustic transducers for LMFR plications are usually based on piezoelectric materials. The material used depends on the temperature at which it is to operate, because the piezoelectric properties are lost above the Curie tonperature. At low tonperatures, up to about 2S0°C, lead zirconate titanate ("PZT"), whidi has a Curie point of 3S0X, is acceptable. At higher temperatures lithium niobate, with a Curie point of 1200X, is used. 

The two-fluid externally cooled LMFR (Fig. 18-2) is somewhat more complex because it has a physically separate core and blanket, but higher conversion ratios are possible. The blanket can be made in a variety of ways, making use of either solid or liquid blanket materials. In exploiting the LMFR concept to the full, a fluid blanket consisting of a slurry of ThBi2 or Th02 in bismuth is used. 

The liciuid-metal system that has received the greatest emphasis to date is of the heterogeneous, circulating fuel type. This reactor, known as the Liquid iMetal Fuel Reactor (LMFR), has as its fuel a dilute solution of enriched uranium in liquid bismuth, and graphite is u.scd as both moderator and reflector. With as the fuel and Th as the fertile material, the reactor can be designed as a thermal breeder. Consideration is restricted here to this reactor type but, wherever possible, information of a general nature is included. 

Metals. Alloy steel. For maximum power production, it is desirable to operate an LMFR at the highe.st possible temperature consistent ivith the mechanical properties and corro.sion resistance of the materials of construction. A maximum temperature of 500°C or higher is deemed desirable for economically attractive operation of the reactor. No materials have yet been found that are mechanically strong at these temperatures, readily fabricable, and also completely resistant to corrosion by the U-Bi fuel. 

Graphite. In the LMFR, graphite is considered as the principal choice for the moderating material because of its availability, cost, and knowledge of its characteristics under radiation. However, there arc additional special requirements for the graphite in the LMFR system. It not only is the moderator, but is also the container material for the U-Bi solution in the reactor. Hence it. should be impervious to the liquid metal and mechanically strong. 

Forc( d circulation loops are used to. study materials under environments more closely approximating LMFR conditions. Three such loops are now in operation at BNL and two more are under eon.struction. A very large loo]) ( 4 in. ID) which will circulate U-Bi at 360 U.S. gpm and tran.sfer about 2.V X 10 watts of heat, is now under construction and is expected to go into oijeration late this year. 

Dynamic tests of the reaction between Bi and steel in the presence of a radiation field must be completed before a final selection can be made of materials for the LMFR. The effect of velocity on corrosion is not certain from the out-of-pile studies, so that no exact analogy can be made between out-of-pile forced circulation loops and in-pile capsules. There has been limited work done at Harwell [6] with thermal convection loops in and out of a radiation field. These loops had no U but did contain Ca and Zr inhibitors. The data suggest that pile radiation may have induced some acceleration of mass transfer. 

W. E. Miller and J. R. Weeks, Reactions between LMFR Fuel and Its Container. Materials, USAEC Report BNL-2913, Brookhaven National Laboratory, 1956. 

In the process of Fig. 22-11, a typical two-fluid 500-Mw LMFR would have a blanket of about 400 tons of material containing approximately 10% Th and 90% Bi. The material balance shows that 5.52 tons/day would be withdrawn and processed. In the first step, an additional 5.52 tons/day of bismuth containing fresh thorium is added to the stream. 

General description. The two-fluid externally cooled LMFR concept consists of a relatively small core surrounded, for the most part, by a blanket containing fertile material. The core is composed of high-density, impervious graphite through which vertical channels arc drilled to allow circulation of the fuel coolant. The fuel in the core is dissolved or dissolved and suspended in liquid bismuth. The fluid fuel also acts as coolant for the core system. The required coolant to moderator ratio is obtained by proper size and spacing of the fuel coolant channels. 

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