CHERNOBYL

Students currently in this course along with me mostly are in their final year and thus time constraint is an inevitable factor due to our final year project. For those who are interested in facts on Chernobyl incident and 3 mile island can simply seek it in the web as frankly I haven’t done any studies or literature reviews regarding this matter but if you prefer a story tale, this might be a good start.

Recently I watched a documentary program on national geography channel on Chernobyl accident. Let me try to explain it as easy as possible as far as my understandings according to the program I watched. Chernobyl nuclear power plant uses an active system where all of the power plant including components such as pumps and control rods are powered by electricity. Electricity used is the one produced by the nuclear power plant itself.

One day, they scheduled to run a simple test which is to make sure that the turbine will keep on running when the reactor power level is at its minimum with aid of auxiliary system on reactor number 4. During daylight, at least 3 senior engineers are on duty and therefore the test can be monitored closely by experts but unfortunately this is when the grid demand reaches its peak. Instead they postponed the test decided to run the test past midnight when the grid demand is at minimum, leaving the task to 2 inexperienced young engineers on duty at that time.

Preparation for the test took more than 5 hours to complete and these 2 engineers faces several difficulties during this phase in terms of balancing the reactor. Unfortunately I can’t remember either names and therefore let me designate them as Mr Ku and Mr Bu. As Mr Ku in the control room lowers the core power by inserting 114 control rods, Mr Bu in the other building increase the water (moderator) flow into the reactor causing the reactor power and temperature to decrease rapidly. As more water pumped in, insufficient heat means that not enough steam produced to run the turbine. At this time, moderator to fuel ratio is decreasing meaning that the reactor is under moderated.

Mr Ku in the control room quickly compensate this condition by leaving only 6 control rods in the reactor despite the fact that the manual stated that the reactor should operate with a minimum number of 26 control rods. Due to communication breakdown, Mr Bu who just realized that the steam production is insufficient intends to help by reducing the water (moderator) flow rate and thus causing the power and temperature of the reactor to rise rapidly. At this time, moderator to fuel ratio is increasing meaning that the reactor is over moderated.

Temperature changes in a reactor affect reactivity in a matter of seconds where at this point, catastrophe is inevitable. At a time, the core power stabilized for a while before it suddenly burst up. Mr Ku reacts quickly by inserting all available control rods to reduce the power but unfortunately it is too late. 

Analysis said that the moderator used is made of graphite, while the coolant used is water which means that even without coolant, neutrons are still thermalized causing fission to occur. Furthermore graphite is  a material that can catch fire at extremely high temperature. If it catches fire, it will actually increase temperature and results in positive feedback reactivity coefficients. The reactor exploded and instantly more than 60 firemen came to put off the fire without noticing that it is highly radioactive and results in lost of 31 brave firemen. Residential around 25km radius is evacuated until today. Fortunately no leak detected at the bottom part of the reactor core or it might contaminate soil,ground water and everything available. Why is the damage due to the blow is so vigorous and how can radiation leak so easily? Because they didn't build a reactor containment building yet at that time.

My lecturer will explain the physics behind this incident in the next class.