Lead coolants

The thermohydraulic features of lead-bismuth and lead coolants are high boiling temperatures and the relative inertness compared with sodium. The melting and boiling points of sodium are respectively 98°C and 883 C. For lead-bismuth eutectic the respective figures are 123.5°C and 1670°C and for lead 327 C and 1740°C at atmospheric pressure. The boiling points are well above cladding failure temperatures. The specific heats per unit volume of lead-bismuth and lead are similar to those of sodium but the conductivities are about a factor of 4 smaller. 

A careful control of the purity of the coolant is required to avoid the formation of such deposits. It was necessary to develop corrosion resistant steels and to pre-treat the surfaces of components and also to use special inhibitors in the lead-bismuth coolant. More extensive studies are required for lead coolant to demonstrate the corrosion-resistance of structural material. 

The most important contributor to specific long-lived residual radioactivity of a lead coolant is Pb-205 (half-life = 1.51 x 10 a), which generates in the reaction Pb-204 (n, y)Pb-205. The specific p-activity of a pure lead coolant is significantly less than one of lead-bismuth. Activation of lead-bismuth and lead coolants will increase in every cycle of reuse, if reuse is possible in principle [2.27]. 

V I. OUSANOV, D. V. PANKRATOV et al, Long-lived Radionuclids of Sodium, Lead-bismuth and Lead Coolants at Fast Reactors . Atomnaia Energia,V.87, 9,pp. 204-210, 1999(Rus). 

Presence of impurities in lead coolant is injurious for at least two reasons, namely ... 

Direct application of data gained on lead-bismuth alloy to lead coolant is doubtful, and it should be noted, that further long-term studies are required. 

ROUSSANOV, A., et. al.. Corrosion resistance of structure materials in lead coolant with reference to reactor installation paper presented at the International Seminar on Cost, Competitive, Proliferation Resistant Inherently and Ecologically Safe Fast Reactor and Fuel Cycle for Large Scale Power , MINATOM, Moscow, 29 May-1 June, (2000),pp. 65-67. 

Qualitative comparison of some thermohydraulic characteristics of sodium and lead coolants is presented in [6.1] with BREST reactor design taken as a reference [6.2]. Results of optional assessments performed by the author [6.2] are in a good agreement with the data presented in [6.1]. In the analysis, the following parameters are assumed identical reactor power, average temperature in the circuit, cross section for coolant flow, coolant temperature rise in the reactor, primary circuit pressure drop and coolant volume. 

Even in case of lead coolant the problem of polonium contamination exists because of Bi-209 formation by neutron capture in Pb-208 (abundance 52.3%) [7.5] ... 

Long term residual activity of sodium, lead-bismuth and lead coolants of fast reactors have been studied and compared by V. Ousanov, D. Pankratov, et al. [7.4]. It was found, that specific a-activity of typical lead-bismuth coolant is determined by Bi-210m (half-life = 3.6 x 10 years) generated in reaction Bi-209 (n,y) Bi-210m. The long lived p-activity of Bi-208 (half-life = 3.65 XlO years) is produced in the reaction Bi-209 (n, 2n) Bi-208. 

Thus, the residual activity of lead-bismuth and lead coolants is expected to be as high as millions of years. As it is pointed out in [7.4], purification of lead-bismuth and lead coolants from the long lived radionuclides of bismuth and lead (if it is possible) would be too expensive. 

The Problems of Increasing the Overall Bismuth Production and Using the Lead Coolant... 

RDIPE proposes to consider lead coolant as an alternative to lead-bismuth alloy because the scales of lead production do not limit the rate of large-scale NP development. 

Taking into accoxmt all mentioned above, use of lead coolant would be justified only if the rates of power capacities increase for the NPPs with the Rls considered were high enough, and expenditures for increasing the annual bismuth production and its cost were put up as much that the increase of specific capital costs of NPP construction would not be economically reasonable. 

It should be highlighted that there is no sharp boundary between lead-bismuth eutectic alloy and pure lead. As bismuth has been in deficiency, one can consider non-eutectic alloy with bismuth content decreased up to 10% (versus 56% in eutectic alloy). Being compared with lead coolant, its melting temperature is decreased by 77°C (to 250°C) and that facilitates RI operation and reduces maximal temperatures of fuel elements claddings up to the values tested for eutectic coolant under the conditions of long-term operation tests. 

Mastering the technology of alloy with 10% of bismuth content further ensmes, if necessary, gradually to introduce lead coolant use. 

With a wider-spaced fuel lattice in the LFR it is possible to increase the coolant flow area and to reduce its velocity. As a result, the hydraulic resistance and the pumping power requirements in the LFR are a factor of 3—5 smaller than in the SFR. The lead coolant temperature gain reduced down to 100-120°C allows raising its temperature at the core inlet to 400—420°C and providing a sufficient margin to its crystallization point (T<-fyst = 327°C). Even so, the fuel cladding temperature in the hot spot will not exceed 650° C, whereas in the sodium-cooled reactors it is higher than 700°C. 

For the properties of natural safety to be fully and consistently implemented, it is not only nitride fuel and lead coolant but also certain other options that should be translated into the reactor design, such as ... 

Lead coolant circuit of high heat capacity, capable of accumulating heat during accidents and transients without a perceptible increase in the temperature of the core and safety-related circuit components. 

Use of the chemically inert and high-boiling lead coolant allows giving up the three-circuit arrangement of heat removal in favor of a two-circuit system. 

The reactor facility of pool design, which incorporates the core with reflectors and control rods, the lead coolant circulation circuit with steam generators, pumps, equipment of the fuel reloading system, as well as safety and auxiliary systems, is arranged in a steel-lined thermally insulated concrete vault O Fig. 58.16). The concrete temperature is maintained within the permissible limits by natural circulation of air. 

The natural properties of lead coolant and mononitride fuel and the neutronic characteristics of the fast reactor combined with the design of the core and cooling circuits raise the BREST reactor to a radically different level of safety and provide for its stable behavior without involving active safety features in the severe accidents unmanageable in any one of the existing reactors, such as ... 

This p r postulates that new technologies, or novel combinations of existing technologies are necessary to the design of safe and economic small reactors. The paper then suggest a set of requirements that must be satisfied by a small reactor design, and defines a pool type reactor that utilizes lead coolant and TRISO fuel which has the potential for meeting these requirements. 

The above efforts identified the potential for a lead (or lead alloy) cooled, graphite moderated reactor utilizing the TRISO coated feel kernels. Essentially, a MHTGR without pressure vessels, utilizing lead coolant. 

However, unlike the MHTGR which utilizes pressure vessels to house both the steam generating equipment and the reactor, and helium coolant at high pressure, LEADIR-PS incorporates the reactor and heat removal equipment in a pool of lead coolant at near atmospheric pressure. 

LEADER-PS thereby avoids the cost and safety concerns related to pressure vessels, and any prospect of the burning of graphite core materials by maintaining these materials submerged in the lead coolant. 

General It is necessary to identify materials for coolant, moderator, and fuel, and for the structure and components of the reactor and reactor systems, which are compatible under all operating conditions. Basic compatibility was established for the combination of lead coolant, graphite moderator, and TRKO fuel, and for lead and the reactor and reactor systems, structures and components. Specifics are discussed below. 

The primary lead coolant pool is surrounded by a secondary lead pool consisting of ordinary lead, contained within the reactor vessel structure. The secondary lead pool and steel Welding provides a short term heat sink for the most severe design basis events, allowing the volume of the primary pool to be minimized it also reduces heat loss during normal operation. 

The inner and outer wails of the reactor vessel are steel, connected by reinforcing webs each wall is fully capable of containing the lead coolant in the event of a mptme in the other wall. These walls, together with the steel balls in the innerspace, provide neutron shielding for the concrete. 

Incidence during which the primary lead coolant pool solidifies are very infrequent. However, in the event that the lead coolant solidifies during a cold shutdown, coolant flow is re-established by providing steam at 350 C to the steam generators, or by the use of electric heaters that can be positioned in the reflector. 

The lead coolant serves as an excellent lubricant during the refuelling operation, and allows for sufficient clearance between the prismatic blocks to assure refuelling capability. Even under severe seismic conditions, the lead between the prismatic blocks precludes damage due to column impact. 

LEAD COOLANT — Lead coolant has several advantages including a boiling point well above the assured shutdown temperature of the core therefore minimal coolant level measurements are required and pump cavitation caimot occur. Further, there are no significant reactivity effects associated with the lead (208) (it is essentially transparent to neutrons) and no chemical reaction between coolant and fuel or moderator is possible. 

Steam generator tube ruptures are also accommodated. Immediately following the ruptures significant steam generation occurs however, the lead coolant quickly solidifies in the region of the rupture and steam releases approach those due to the flashing of feedwater only. There is no lead-water reaction, and no public safety concern results. 

To minimize development cost and time, maximum use of existing technologies and expertise is required. Hence, cooperation between countries and institutions with relevant experience, for example, with graphite moderator, lead coolant, TRISO fuel, and plant and equipment design is essential. There are two principal areas requiring development, both related to the coolant These are ... 

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