Chernobyl Nuclear Reactor

Chapter 1
Overview of past accidents and AP1000 plant design objectives

No doubt, nuclear reactors generate power all due to nuclear fission. A nuclear fission is a typical decay in which an atom splits into smaller parts and emits a considerably large amount of energy throughout the reaction. Typically, there are three types of nuclear reactor that are commonly used throughout the globe. They are the Pressurized Heavy Water Reactor (PHWR), Boiling Water Reactor (BWR) and Pressurized Water Rector (PWR).

Pressurized Water Reactor (PWR) accounts for 60% of nuclear reactors in the world. There are lots of countries using this type of reactor, including China, Japan, Russia, France and United States. In the following, past nuclear power plant
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Chernobyl Nuclear Power Plant Accident Figure 2. Schematic diagram of Chernobyl Power Complex (RBMK-100)

RBMK-100 was a boiling light water reactor (BWR) and designed by Soviet Union. It has built graphite moderated pressure tubes and used slightly enriched uranium dioxide fuel (2% U-235). There was no intervening heat exchanger and two loops feeding steam were directly collected to the turbines. Water acted as coolant and condensed as steam afterwards to drive the turbines.

For routine maintenance, the unit 4 reactor was designed to be shut down on 25 April 1986. By taking an advantage of this shutdown, a test was taken to access turbines spinning duration given by a loss of station power supply. The turbine spinning was to provide electricity and hence to operate the main core cooling circulating pumps.

Even though this test has been tested in the previous year, new voltage regulators was required to tested in order to cope with the problem of rapidly turbine fell off. For the test, the reactor power fell unexpectedly to about 30 MWt due to operational error (normally it should be stabilized at around 700-1000 MWt). By compensating the power lost due to operational error, lots of control rods were withdrawn and minimum operating reactivity margin (ORM) was violated. Later on, the test was carried out when the reactor was at 200
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Fukushima Daiichi Nuclear Power Plant Figure 3. Schematic diagram of Fukushima Daiichi boiling water reactor

The Fukushima Daiichi reactors are the boiling water reactors (BWR) with Mark I containment. For the reactor capacity, Unit 1 has 460 MWe while units 2-5 have 784 MWe. Unit 6 has 1100 MWe which was the largest capacity.

Due to a magnitude 9.0 earthquake, a 15-metre tsunami was created and destroyed the power supply and cooling of reactors. A nuclear accident was eventually happened on 11 March 2011. Three Fukushima Daiichi reactors core (unit 1-3) were melted in the first three days.

During that time after the 15-metre tsunami, 12 of 13 back-up generators and heat exchangers for reactor waste heat were disabled. Those three units hence lost the ability for water circulation and the reactors were not cooled properly. Furthermore, electrical switchgear was disabled. This action resulted in lack of electricity for running the Residual Heat Removal (RHR) system cooling pumps. As a result, heat could not be removed from reactors and spent fuel ponds. Core melt was achieved eventually.

1.2 AP1000 Plant Design Objectives
Several objectives for the AP1000 plant design are required to achieve, and they are:
a.

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