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Chapter 3 Systems of PWR Nuclear Power Plants

relatively small, the fuel center temperature at the maximum core output rises to about 2,00013 (Figure 3.3.9).

The melting point of the pre-irradiated uranium dioxide fuel is about 2,80013. As the fuel burnup proceeds, the melting point of the fuel decreases; approximately, 32 °C per 10,000 MWd/t burnup. The drop in fuel melting point is somewhat worsened by adding gadolinium poison into the fuel. In the step 2 high burnup fuels, these effects as well as the design and fabrication errors are taken into consideration and the design limits on the fuel center temperature are determined at 2,58010 for the pre-irradiated (fresh) fuel and at 2,35013 for the irradiated fuel at 71,OOOMWd/t burnup. For gadolinium-loaded fuel, the design limits are 2,48013 and 2,44013 for the pre-irradiated (fresh) fuel and irradiated fuel (at about 10,000 MWd/t burnup), respectively. These design limits must not be exceeded during the normal operation and under anticipated transient conditions of the reactor core.

Design parameters of the fuel rod and fuel assembly are given in Section 3.3.1-(2).

  1. Dynamic characteristics of the nuclear reactor

The control system of a PWR power plant is based on the turbine follow-up control philosophy

Figure 3.3.9 Temperature inside fuel rod (example)

in which, based on the electric power system demands, the power output of the turbine-generator is regulated (either automatically or manually) and then, the reactor output is controlled by matching it The power regulation of the nuclear reactor is accomplished through the increase or decrease in core reactivity by adjusting either the position of control rods or the concentration of boron in the reactor coolant In a PWR plant, at power operating conditions, both the power reactivity coefficient and the moderator temperature reactivity coefficient are designed to be negative. This self-regulation characteristic of the reactor and the function of the reactor control system maintain continuous and stable heat generation, heat transfer, and steam generation by attenuating the reactor power transients caused by the design load changes and by restoring the equilibrium condition of the reactor without trip. The reactor control system for the PWR plant is designed to enable the nuclear reactor to accept the following standard load changes within the load range of 15% to 100% of the rated power:

Q) ±10% step load changes

  1. ±5% per minute ramp load changes

  2. the function of the turbine bypass control system permits the plant to accept a 50% (or 95%) step load reduction.

  3. Core stability

To provide sufficient attenuation in response of the reactor system to any load perturbation and power fluctuation at the normal operation condition, the following stability requirements are considered in the core design:

(D core stability against reactivity feedback effects;

  1. stability of the whole reactor system;

  2. reactor stability against control-induced power oscillations; and

© core stability against xenon-induced spatial oscillations.

a. Stability of the whole reactor system and the reactor core

Dynamic characteristics of a PWR including the reactivity feedback are very stable due to the immediate effect of the negative fuel temperature reactivity coefficient (Doppler coefficient), and the moderator temperature coefficient which is

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NSRA, Japan