The Reasons Of Chernobyl Accident
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1. INTRODUCTION
Throughout the world there are several different designs of nuclear reactor to power turbines to generate electricity for a national grid. The reactors at Chernobyl, of which there were four by April 1986, with two more planned for the future, are known as RBMK-1000 and are only to be found in the ex-Soviyet Union. Building work began in 1971, and first reactor went critical in August 1977. By December 1983 all four were on steam.
On the night of 25-26 April 1986, there were on the power plant site 176 duty operational staff and workers from various departments and maintenance services. In addition to this number, there were also 268 builders and assemblers working on the night shift of the constructions of units Nos.5 and 6.
The fatal accident sequence was initiated by a decision of the plants management and specialists to make an overnight experiment to test the ability of the turbine generator to power certain of the cooling pumps while the generator was freewheeling to a standstill after its steam supply had been cut off. The purpose of the experiment was to see if the power requirement of unit No.4 could be sustained for a short time during a power failure.
The nuclear power plant staff conducting the experiment knowingly departed from the experimental programme which was already of poor quality. This created the conditions for the emergency situation which no one believed could ever happen.
2. DESCRIPTION OF THE CHERNOBYL ACCIDENT
2.1 Introduction
At 01.23 hours on 26 April 1986, Unit No.4 of the Chernobyl Nuclear Power Station was operating at low power prior to a scheduled shutdown when a sudden, very rapid and uncontrolled power increase occurred
. This resulted in the destrution of the reactor core and severe damage to the reactor building. The accident occurred
as a result of a test which was being carried out by the operators. The manner in which the test was conducted, including the diabling of all the safety systems provided to protect the plant, was the immediate cause of the accident. However, aspects of the reactor design resulted in the opposite response of the reactor which caused its destruction.
2.2 The Planned Test
Prior to the scheduled shutdown of the No.4 Unit, a test on the turbo-generator was planned. The test was intended to confirm the operation of a modification made to the generator control circuits. This would enable it to continue to provide power during its rundown period if the connection to the main electrical distribution grid was lost. If succesful, this would enable power for emergency pumps to be provided until the emergency diesel generators on the site started. The results would have been applicable to all of the RBMK reactors. The test was seen as concerning the generator and its electrical systems and so little consideration was given to the safity of the reactor. The test plan did not go into detail on necessary safety measures and had not been checked and approved by the safety authorities. However, similar tests had been carried out safely in the past and, had the test plan been followed, the accident would probably not have occurred
2.3 Events Prior To The Accident
Power reduction for the scheduled shutdown began on 25 April 1986. By 23.05, 50% power was reached and one of the two turbo-generators was switched off. Electrical supplies for the reactor systems were switched off to the other. Shortly after, the units emergency core cooling system was switched off. This was in accordance with the (unapproved) plan, but in violation of units safity principles. However, this is not thought to have significantly affected subsequent events. At this point the test was delayed because of an unexpected request to continue supplying power to the grid at the 50% level.
The programmed power reduction was resumed at 23.10 with a target level of 700-1000 MW (thermal). However, due to an operator error in the manipulation of the control system, the power fell to 30 MW (thermal). Below and difficult to control. Therefore, operation below 700 MW (thermal) is not permitted. The operator stabilised the power at 200 MW (thermal) by manuel control at 01.00 on 26 April. In regaining this power level, the operator was forced to run further out from the core then was permitted. This made the reactor even more unstable and at the same time significantly reduced the effectiveness of the shutdown system, since the control rods would have to be driven back into the core before they started to shut down the chain reaction. The operators, according to their own rules, should not have continued to operate to operate the reactor under these conditions; however, there was no automatic system to prevent them.
At 01.03 the operator started up additional main reactor cooling pumps as part of the schedule. Four pumps were connected to the main power grid and four to the remaining turbo-generator. This was intended to ensure adequate reactor cooling after the test when the four connected to the grid would be still operating. However, the water flow in the core was now higher then the permitted value. This caused several problems. Firstly, the water content of the reactor increased because the increased coolant flow resulted in a lower proportion being turned to steam. Normally the steam displaced some of the water in the upper part of the core. The additional water