THE ELECTRICITY SUPPLY SYSTEM ASPECT

Technical issue

Nuclear reactor regulation mode

Reactor output power regulation is complex process based on reactivity factor changes. Reactivity is a factor, describing reactor deviation from critical state, what means from conditions under which number of neutrons generated in each fission cycle is the same. Power increase or decrease does not occur with fuel amount changes, but along with fission reaction intensity. It can be regulated by putting control rods in or out the reactor's core. Moreover output power does not depend on reactivity factor. It depends on time. In theory, it means, that with any positive reactivity factor value, output power can increase over reactor capabilities, leading to reactor destroy (RBMK, Chernobyl accident was caused by uncontrolled reactivity increase ). Important is only the sign of reactivity. When reactivity is negative, the reactor is in subcritical state. Its power decreases. If reactivity is positive (overcritical state), reactor power increases. Although output power can be regulated basing on reactivity sign and its margin, control and protection systems in the reactor operates with power growth rate.

It is worth to admit, that the fission reactor, which actually is not operating due to maintenance work or schedule of power system, has to be controlled all the time. With its minimal power, fission can occur accidentally with spontaneous fission or cosmic radiation. Output power regulation need advanced control systems to compensate reactivity changes caused by external radiation and temperature. In old gen I and II reactor's regulation process was difficult and those reactor were inflexible. Today's reactors, being offered at the marker, are equipped with systems, which allow to relative simple power output change. They can operate in load following mode as well. Nevertheless such operation has specific consequences and can not be executed without limits.

Gen III reactors are the most controllable just after refueling. At 65% fuel burn up, reactor's flexibility is limited and at 99% fuel burn up, power changes is not allowed. These restrictions are caused by physics of the reactor. As it operates, uranium is split and forms particles with large cross section for neutron absorption. Such particles are named reactor's poisons and they significantly affect power plant's operation. The reactor, to operate with constant output power must have positive reactivity factor to ensure additional neutron emission for poison's compensation. Fortunately poisons are burnable and they disappear after two days from generation. It is why, reactor start - up takes two days. After this time reactivity stops changing and steady state is achieved. Then, the most important are transient states such as power regulation. When power is being reduced , poisons are being still generated and theirs burning is less effective. Reactivity losses is increasing and if reactivity margin is not high enough to compensate the losses, output power can not be increased. Next reactor start - up is possible after time, when poisons undergo natural decay. Ensuring enough high level of radioactivity margin is not economical and can be dangerous because of fuel amount and its enrichment increase.

So nuclear power plants even after serious development in regulation area are not fully flexible and they make troubles in load following operation mode. Additionally its start - ups cost and duration cause further limits in fulfilling the electricity supply system schedule.

During output power changes, temperature gradient arising in the fuel pellet is reaching 1200°C in PWR reactor. It decreases significantly mechanical strength of the fuel cladding, as the result of different thermal expansion factor. This process is called as pellet cladding interaction (PCI). Broke of fuel elements leads to radioactive particles penetration of the coolant. After such incident, the reactor has to be shut down for decontamination.

In spite of very large capital cost of nuclear power plant, to achieve possible maximum ROI (return on investment) and to minimize O&M costs, nuclear reactors operate with high load factor (near 90 - 93%). This requirement causes, nuclear reactors in most cases operate to cover base electricity demand.

The gen III and III+ designs offer possibilities to flexible output power regulation, but in marketing prospects, serious limits are being omitted . Conventional technologies are more suitable for variable covering the electricity demand day after day. [19],[68]

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