2. Comparison of Schemes of Nuclear and Thermal Power Generation

Nuclear power generation can be regarded simply as thermal power generation with a nuclear reactor in place of the boiler. The basic difference in the ways to generate steam to rotate a turbine generator is that a nuclear reactor uses nuclear energy produced by fission of uranium or plutonium whereas a thermal boiler uses thermal energy produced by chemical reaction of fossil fuel (burning). The principal differences of the schemes and designs of a nuclear reactor and a thermal boiler, which come from the fundamental difference of the steam supply systems, are described in the following.

1. Effective Reactor Fuel Loading

In thermal power generation, petroleum is pumped to the boiler and burned to generate as much steam as needed at each time to generate electricity. Hie petroleum is stored in a tank apart from the boiler. In nuclear power generation, fuels of Uranium or Plutonium are loaded in the reactor core in advance in an amount as needed for about a one-year operation and the fuels are used to generate heat by fission reaction.

The reason why such a fuel loading is possible in a nuclear reactor is that the heat generation by nuclear fission per a unit mass of uranium or plutonium is much larger by orders of magnitude than fossil fuel. During the periodic inspection shutdown time, which is regally required, spent nuclear fuels are replaced with new fuels through the opened top of the reactor pressure vessel. Nuclear fission is sometimes called “ftiel burning”, and the reactor core has sufficient “kindling coal”, meaning, more than momentarily needed. Nuclear fuel is burned slowly by nuclear fission for about one year and this is the essence of reactor control. Materials and devices such as control rods, boron solution and gadolinium are used to absorb neutrons as media in the nuclear fission chain reaction.

If the reactor temperature decreases, the nuclear fission tends to increase due to reduction of the Doppler effect of the fuel so that sufficient capacity of the control rods to shut down the reactor is required to compensate for the Doppler effect. Further, the system for the function should have

high reliability.

Although a nuclear reactor should take care at the most of its control, an enormous amount of energy can be taken from the uranium and plutonium which are loaded in a reactor core in virtue of their high energy density as explained earlier. A 1.000MW (1 million kW) NPP uses only about 35 tons of nuclear fuel every year, whereas about 1.6 million tons of petroleum are needed for the same amount of electricity production in a thermal boiler. It can be easily understood how large the difference is between the two types, taking just fuel transportation as an example.

2. Reactor Self-Regulation Characteristics

If the boiler output of a thermal power plant increases abnormally, the change is detected and the output is controlled by the boiler control system (by throttling the fuel supply). The light water reactor (LWR), which has become the principal nuclear reactor in use nowadays, is also controlled by a control system, but the reactor power is controlled rapidly by its own physical characteristics of self-controllability which is favorable for safety as well as for reactor control. That is due to three coefficients of reactivity of moderator temperature, void and Doppler effects which are described next.

In a LWR, the neutrons generated by nuclear fission are slowed down by moderators because neutrons with higher speed are less reactive with Uranium 235 f³⁵!!), etc. If nuclear fission increases and the reactor power increases rapidly for some reason, the temperature of the moderator material increases and the density decreases. The change will be even more due to steam (void) formation if the temperature of the moderator material is at saturation, which causes a reduction in the moderating function and in the fission rate itself. These are the moderator temperature effect and void effect

If the fuel temperature increases as well as the moderator temperature, the probability of neutron absorption by Uranium 238(²³⁸U) becomes higher and the rate of fission reaction by neutrons and Uranium 235(^11) decreases to reduce the fission rate. If the fission rate increases abnormally, the increase in the fuel temperature suppresses the fission rate by the Doppler reactivity effect, which

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NSRA, Japan