Page 42 - Energize December 2022

TECHNICAL

fuel form, especially in molten salt reactors (MSRs).
Thermal breeding with thorium requires that the neutron economy in the reactor
has to be very good (i.e., there must be low neutron loss through escape or parasitic
absorption). The possibility to breed fissile material in slow neutron systems is a unique
feature for thorium-based fuels and is not possible with uranium fuels. Thorium is the
only element that allows for a breeder reactor in the thermal spectrum. This means that
the power output per kg of fissile inventory is greater than any other reactor. 5
Another distinct option for using thorium is as a ‘fertile matrix’ of mixed thorium-
plutonium oxide, that serves as the fissile driver while being consumed. Production of
all actinides is lower than with conventional fuel, and negative reactivity coefficient is
enhanced compared with U-Pu MOX fuel. Mining of other materials produces enough
thorium to power all global energy production. 5

Small modular reactors
While most of the planned NPPs are large Generation III, LW reactors, there is a new
development that is gaining momentum and could change the future of nuclear power
generation. This is the field of small modular reactors, units with limited size and power,
which are anticipated to reduce cost, build time and increase safety, and allow the
deployment of nuclear power in a distributed generation arrangement.
SMRs are newer generation reactors designed to generate electric power up to 300
MW, whose components and systems can be shop fabricated and then transported as
modules to the sites for installation as demand arises. Most of the SMR designs adopt
advanced or even inherent safety features and are deployable either as a single or
multimodule plant. SMRs are under development for all principal reactor technologies: Figure 1: Light water SMR ( Nuscale )
water cooled reactors (LW, BW) and high temperature gas cooled reactors, liquid-metal,
sodium and gas-cooled reactors with fast neutron spectrum, and molten salt reactors. Molten salt reactors
The key driving forces of SMR development include the need for flexible power Molten salt reactors (MSR) were
generation for a wider range of users and applications, replacing ageing fossil-fired units, developed in the 1950s and 1960s,
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enhancing safety performance, and offering better economic affordability. There are over and were billed as an alternative
50 different SMRs at various stages of development worldwide. to light water reactors due to their
Small reactors have been around for a long time, mainly in the mariner propulsion expected smaller size and improved
sector (submarines, navy vessels and icebreakers), but have generally been custom safety. The designs at the time
designed for a particular application in terms of size and technology used. Most are a ultimately proved unusable for their
scaled-down version of the larger reactors. original purposes, and development
The SMR differs in that the design is modular, i.e., of a fixed size and a fixed design, was abandoned in favour of the LWR.
allowing multiple units to be combined to provide the required capacity and also Renewed interest has resulted in
catering for modular growth at a particular site. The SMR is specifically designed for modern designs that have overcome
utility generation but can also be used as a heat source or as a combined heat and most of the problems associated
power unit. The technologies used as a well as the small size offer improved safety, and with MSR, as well as introducing new
the standardisation of design can reduce cost when used in a “fleet” or batch type of concepts.
production. Construction time is also reduced, as much of the equipment is assembled in In a modern MSR design, the fuel,
the factory and transported as such to site. SMRs also have a longer refuel cycle running which could be uranium or thorium,
from 5 to 10 years. is contained in molten salt, which is
The SMR concept has been adopted by several countries, with government supporting circulated through the core of the
development, and different versions are close to pilot or production stage. Although LWR, reactor. In the conventional design, the
PWR, and BWR designs have reached licensing stage, most of the development is focused molten salt circulates both through
on MSRs and pebble bed reactors. It is foreseen that The SMR could make the entry the core and a heat exchanger. In
in nuclear power usage easier for many countries and speed up the change from fossil more advanced designs, the fuel salt
fuelled generation plant to carbon free electricity. In this article we will concentrate on is circulated through the core only,
the main types of SMR. and a separate integral molten salt (or
molten lead) loop removes the heat.
Light water SMRs MSRs under development by a number
SMR versions of existing light water, pressurised water and boiling water reactors are of companies and research institutes
under development. Some have reached licensing stage. Sizes are in the range 100 to worldwide have capacities in the range
300 MW (Figure 1). up to 200 MW.

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