Page 111 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

84 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Indeed, the choice of lead as primary coolant has a number of positive aspects with
regard to safety and in simplifying the design:
– The nonexistence of exothermic reactions between lead and water or air provides favorable
conditions for the elimination of an intermediate circuit that reduces the footprint of
the plant.
– High boiling point of lead (1749°C at 1bar) reduces the risk of core voiding due to coolant
boiling.
– High density of coolant favors fuel dispersion phenomena when compared with fuel com-
paction phenomena in case of core destruction; this reduces the likelihood that fuel collects
within the primary system, especially at the reactor vessel bottom in such a way that rec-
riticalities may occur; a core catcher is probably not needed.
– Low vapor pressure of lead makes it possible to operate the reactor at low primary system
pressure (subatmospheric), which allows reduced reactor vessel thickness.
– High thermal capacity allows a significant grace time in case of cooling-system-related
accidents, for example, the loss of heat sink.
– Lead appears to form compounds with iodine and cesium at temperatures up to 600°C; this
reduces the source term to the confinement/containment during accidents in which volatile
fission products are released from the fuel matrix.
– Lead shields gamma rays effectively.
– The low moderation effect of lead permits a greater spacing between fuel pins, resulting in a
low core pressure drop of about 1–2bar.
– The combination of a simple flow path and the low core pressure drop enhances the estab-
lishment of natural convection of the coolant in the primary system for heat removal from
the core, thereby reducing the risk of overheating accidents during, for example, a loss-of-
flow accident.
Concerning the development of lead-cooled nuclear system technologies, it has how-
ever to be pointed out that the state of the art of the technology is still not fully proved.
The most challenging issues to be thoroughly analyzed are the compatibility of the
structural materials, coolant chemistry, and fuel assembly and pool thermal fluid
dynamics. While for the first two it is quite obvious that new material combinations
pose unknown problems and research issues, for the thermal hydraulics, it needs to be
noted that typical design tools and models for water or sodium cannot simply be trans-
ferred to lead.
R & D efforts are thus necessary for completing the design, supporting the
prelicensing, and starting with the construction of such systems. Such activities
require the identification of the technological gaps, which are design-dependent,
and the key topics for lead fast reactor developments.
The lead fast reactor technological issues have been divided into the following
main topics (Agostini and Del Nevo, 2011; Hering et al., 2011; Vermeeren et al.,
2011; Juricek et al., 2011; Va ´la et al., 2011; Cinotti, 2011):
– Material studies and coolant physical chemistry
– Studies of core integrity, moving mechanisms, instrumentation, maintenance, in-service
inspection, and repair
– Steam generator functionality and safety
– Thermal fluid dynamics