Page 43 - Energize December 2022

TECHNICAL

Many MSRs can automatically shut down if their temperature becomes too high, as Pebble bed modular reactor
they are designed to rapidly reduce power in case they begin generating too much heat. Pebble bed reactors are graphite-
This feature makes MSRs ideal for meeting dynamic electricity demand by allowing for moderated reactors in which the
quick output adjustments. Some MSRs can burn spent nuclear fuel (SNF) from current fuel is in the form of tennis ball-sized
power reactors, reducing the amount of radioactive waste that must be stored. Figure 2 spherical “pebbles” encased in a
shows a proposed molten salt unit. graphite moderator. New fuel pebbles
are continuously added at the top of
Dual fluid MSR a cylindrical reactor vessel and travel
The dual fluid concept is a fast reactor that aims to combine the advantages of the molten slowly down the column by gravity, until
salt reactor with those of a molten lead or molten salt cooled reactor in a completely they reach the bottom and are removed.
new design. The key innovation of Dual Fluid, as the name suggests, lies in using two The capacity of the PBMR is generally
liquids in the reactor core. There, liquid fuel can develop its full power, at around 1000°C less than 100 MW. Two reactor types
(compared with 320°C for a typical LWR), while molten metal or salt handles the heat can be designed with pebbles (carbon
transfer. Dual fluid reactor is under development in Canada, and claims are made that spheres) as the reactor core: the pebble-
separating the heat generation and heat transfer functions allows a higher energy density bed high-temperature gas-cooled
than a conventional MSR (Figure 3). reactor (PB-HTGR), and the pebble-bed
fluoride-salt-cooled high-temperature
reactor (PB-FHR). There are two basic
versions of the gas-cooled reactor.
In one, the hot helium gas is used to
drive a gas turbine directly, and the
second version uses a heat exchanger
to develop steam that drives a steam
turbine. Figure 4 shows the principle of
the steam turbine version.
The most widely used spherical
fuel is made up of thousands of coated
particles known as tristructural-isotropic
(TRISO) particles. As shown in Figure
5, the centre of the particle is typically
uranium dioxide, and is 0,5 mm in
diameter. The fuel kernel is coated with
a layer of porous carbon which serves
to capture any fission product particles
emitted. Three additional layers of
carbon are then applied to each particle:
Figure 2: Molten salt SMR (OECD Nuclear Energy Agency)
an inner layer of pyrolitic carbon; a mid-

Figure 3: Integral MSSMR (Terrestrial energy) Figure 4: Pebble bed reactor principles

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