Page 107 - Dynamics and Control of Nuclear Reactors

8.7 Load following operation 101

The inherent feedback in power reactors creates opportunities and challenges for
the control engineer. For example, molten salt reactors have strong temperature reac-
tivity feedback effects, and to a great extent they are self-regulating systems. Control
action can alter a reactor input directly (for example, using control rods to change
reactivity) or indirectly (for example using control action to change a process vari-
able such as temperature or pressure to change reactivity feedback). Control engi-
neers select control strategies that provide the most effective responses.

8.7 Load following operation

Load following is the adjustment of a power plant production to match grid electrical
power demand. In a network of power plants, some may be assigned load following
duties and other plants do not respond to demand changes (so-called base load
plants). Decisions on assignment of load following duties depend on economic
and technical considerations.
The economic consideration is based on the operating cost contribution to power
cost versus the capital cost contribution. Nuclear plants have lower operating cost
and higher capital cost than plants fueled by coal, natural gas, or oil. Therefore, it
is desirable to run nuclear plants at maximum capacity at all times and to assign load
following to plants with smaller cost penalties for operating at reduced or zero power.
The technical consideration relates to the ability of a plant to change production
quickly and safely. For example, a hydroelectric plant can increase production
almost instantaneously by opening valves that admit water to the turbines. Even
though hydroelectric power costs are almost entirely capital costs, the excellent load
following capability often results in assigning them load following functions. The
earlier nuclear power plants (Generation II and somewhat in Generation III) were
usually expected to serve as base-load plants and there was little consideration for
designing load following capability into these plants. But in systems where nuclear
plants provide a large fraction of total production, load following capability became a
necessity. The situation in France, where nuclear power production is over 70% of
the total electrical generation, is a good example of a case where load-following
nuclear plants are needed. Load-following is also needed in hybrid power networks
where a mix of nuclear energy and renewable energy (wind and solar whose output
depends on nature) is used.
A base-load plant’s power output can be changed by operator action (changing
the power set point manually). When this occurs, other load-following plants auto-
matically change their output to maintain the desired electrical frequency.
One consideration is whether the load following strategy is based on “reactor fol-
lowing turbine” or “turbine following reactor”. In reactor following schemes, the plan
is to adjust the main steam valve first after a load imbalance to satisfy electrical power
demand. Adjustment of reactor power follows until reactor power equals demanded
power. In turbine following schemes, reactor power adjustment occurs first following
a load imbalance. Steam is admitted to the turbine after the reactor power adjustment.