Tidal Current Energy

The Pebble Bed Modular Reactor
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application, the helium flow rate down through the core is about 184 kg � s and
the pressure about 9 MPa.
An account of the evolution of the PBMR from the HTR-Modul concept to
today’s design is given in Ref. [6] .
2.1 .
Controlling the reactor
The routine method for raising and lowering the reactor power output is unexpected.
It depends on the so-called nuclear Doppler effect. This is an aspect of
the physics of uranium, more specifically of the isotope uranium-238, which is
of great importance for reactor control and particularly for reactor safety. During
normal operation, about 30 � 10 9 nuclei of uranium-235 (which initially constitutes
9.6% of the uranium in the equilibrium core) undergo fission every second
for every watt of heat being produced. In each fission event, two or three highenergy
neutrons are emitted. If at least one of these fission neutrons goes on to
cause a further fission, the nuclear chain reaction ‘ diverges’ and the power level
rises. If not, the reaction dies away.
Counter-intuitively, the neutrons are effective in causing further fission only
if they are first slowed down to very low ‘ thermal ’ energies. This happens in the
course of multiple collisions within a moderator, for example with carbon nuclei
in the graphite comprising the fuel spheres and reflector blocks of the PBMR, or
with hydrogen nuclei in the coolant water in light water reactors.
The obliging property of uranium-238 is that it absorbs some of these neutrons
before they have slowed down sufficiently to cause further fissions in uranium-
235. More particularly, the hotter the fuel becomes, the more intermediateenergy
neutrons are absorbed by nuclei of uranium-238 and thus removed from
the chain reaction process. There are then fewer low-energy neutrons flying
around in the core to cause fission, the fission rate in the reactor falls away and
the reactor power level drops. Such is the totally reliable Doppler effect.
Now consider the effect of reducing the mass of helium in the reactor pressure
vessel. The helium flow down through the fuel bed is reduced and the
temperature of the fuel spheres therefore rises. Thanks to the Doppler effect, the
reactor power level inevitably drops. Conversely, increasing the mass of helium
in the system reduces the fuel temperature and therefore causes the power level
to rise.
The routine way of controlling the reactor power level is thus to adjust the
mass flow of helium through the core either by releasing helium from the coolant
circuit or by injecting more helium into it. If it is required to change the
power level more rapidly, valves can be opened to allow a fraction of the helium
to bypass the reactor core. This has the same effect as reducing the helium pressure,
but acts faster.
A further degree of control is afforded by raising or lowering the 24 control
rods of the reactivity control system (RCS) surrounding the core. These are used
principally to maintain the core temperature while power-level changes are
being made by adjusting the helium pressure and also when it is necessary to