Home Podcasts THMG112 – Chernobyl Case Study

THMG112 – Chernobyl Case Study

2389
0
SHARE

In this episode, Bob explores the Chernobyl disaster through an article from the Autumn 2011 edition of CBRNe World. It was written by Steve Johnson and read with the permission of the CBRNe News staff.

2:25 – Article Introduction

Firefighters risk their lives on a daily basis – heroism is in their DNA. Some acts of heroism, though, are so incredible and so selfless that it’s impossible to read or hear of the event without emotion. In recent years, we may think of the crews that lost their lives on 9/11. One that is rarely remembered, however, and made even worse by the crews knowing beyond any doubt that they would die carrying out their duty, is the response of the Chernobyl and Kiev fire crews to the Chernobyl Nuclear Disaster. I recently had the privilege of visiting the exclusion zone for research in its use for nuclear accident and dirty bomb training. In the course of the visit, I met and spoke with the fire crews at the power complex about the sacrifices their forebears made. This is the story of their response.

3:10 – Background on the Chernobyl Disaster

The Chernobyl Power Complex, lying about 130 kilometers north of Kiev, Ukraine, and about 20 kilometers south of the border with Belarus, consisted of four nuclear reactors of the RBMK-1000 design. Two more were also under construction. The nearest city, Pripyat, was three kilometers away from the reactor and had 49,000 inhabitants. The old town of Chernobyl, which had a population of 12,500, is about 15 kilometers to the southeast of the complex.

Within a 30-kilometer radius of the power plant, the total population was between 115,000 and 135,000. The RBMK-1000 is a Soviet designed and built graphite moderated boiling light water reactor with two loops feeding steam directly to the turbines and without an intervening heat exchanger. The water acts as a coolant and also provides the steam used to drive the turbines. The RBMK Chernobyl reactors had several dangerous properties that contributed to the accident.

The following are the most important:

1. Positive void coefficient – When the reactor is run at a low power level, there is increased boiling of the water in the core (due to the heat not being removed by the loss through the turbines). As the water turns to steam, there is less around the core, which leads to further power due to lack of moderation/absorption of neutrons. More power meant more steam and so on…

2. Poor control rod design – The design of the control rods was not good enough. The machinery moved them too slowly and the lower part of each rod was made of graphite, rather than boron carbide. So, for the initial period of insertion in an emergency, the rods actually accelerated the fission process until the boron entered the core.

3. Lack of containment – Strong concrete buildings surround most Western reactors, but that wasn’t the case with the Soviet reactors.

5:25 – The Day of the Accident

On the day of the accident, a test procedure approved by the reactor’s Chief Designer was run where the operating crew planned to test whether the Reactor No. 4 turbines could produce enough energy to keep the coolant pumps running until the emergency diesel generator was activated in case of an external power loss. This was an important safety test, as demonstrated by the Fukushima disaster. Even after a shutdown, there’s a lot of heat remaining in the core, which must be removed to prevent critical damage. This is usually controlled by backup generators powering the cooling system.

At Chernobyl, there were three backup diesel generators, but they took 60–75 seconds to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump. This one minute power gap was considered unacceptable, and it had been suggested that the rotational energy of the steam turbine and residual steam pressure could be used to generate electricity to run the main cooling water pumps while the emergency diesel generators were reaching the correct rotational speed and voltage. The Chernobyl power plant had been in operation for two years without the capability to ride through the first 60–75 seconds of a total loss of electric power, thus lacking an important safety feature. The station managers wished to correct this at the first opportunity, which may explain why they continued the test even when serious problems arose.

7:00 – Testing the Reactor No. 4 Turbines

The conditions to run the test were established before the day shift of 25 April 1986. The day shift workers had been instructed in advance and were familiar with the test procedures. As planned, a gradual reduction in the output of the power unit begun at 01:06 on 25 April, and the power level had reached 50% of its nominal 3200 MW thermal level by the beginning of the day shift.

At this point, another regional power station unexpectedly went offline, and the Kiev electrical grid controller requested that the further reduction of Chernobyl’s output be postponed, as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed and postponed the test. At 23:04, the Kiev grid controller allowed the reactor shut-down to resume.

This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the test. According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heat cooling systems. The night shift had very limited time to prepare for and carry out the test, so when things started to go wrong, they were ill-prepared.

Unfortunately, rather than a gradual power down, there was a massive and sudden drop due to a combination of the natural production of the neutron absorber xenon 135 in the core, and an operator mistakenly inserting the control rods too far, bringing the reactor to an unintended near-shutdown state. Control-room personnel consequently made the decision to restore the power by extracting the majority of the reactor control rods to the rods’ upper limits. The power still stayed low though, which caused a further production of xenon. Slavishly following the test plan, extra water pumps were activated at 1:05, increasing the water flow. Since water also absorbs neutrons, turning on additional pumps decreased the reactor power further still. This prompted the operators to remove the manual control rods to maintain power.

All of these actions led to an extremely unstable reactor configuration prior to the start of the test. At 01:23, the test began. Four (of eight) Main Circulating Pumps (MCP) were actively pumping the water around the system. The steam to the turbines was shut off, and a run-down of the turbine generator began. The diesel generator started and sequentially picked up loads, which was complete by 01:23. During this period, the power for the four MCPs was supplied by the turbine generator as it coasted down. However, as the momentum of the turbine generator decreased, the water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.

Due to the positive void coefficient of the RBMK, the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor’s power output. This caused yet more water to flash into steam, giving a further power increase. However, during the test the automatic control system successfully counteracted this positive feedback loop, continuously inserting control rods into the reactor core to limit the power rise.

10:45 – Completing the Test and the Two Explosions

Upon completion of the test the operator pressed the emergency shutdown button, which was done simply as a routine method of shutting down the reactor. After the button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 0.4 m/s, so that the rods took 18 to 20 seconds to travel the full height of the core (about seven meters). At this stage, the graphite-tip control rod design, which initially displaced coolant before inserting neutron-absorbing material to slow the reaction, showed its flaw.

The system actually increased the reaction rate in the lower half of the core. A few seconds after the start of the shutdown, a massive power spike occurred, the core overheated, and seconds later this overheating resulted in the initial explosion. The subsequent course of events was not registered by any instruments: it is known only as a result of mathematical simulation. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building. Basically, it was a steam explosion, like the explosion of a steam boiler from excess vapor pressure.

A second, more powerful explosion occurred about two or three seconds after the first. The nuclear excursion dispersed the core and effectively terminated the nuclear chain reaction. There were initially several hypotheses about the nature of the second explosion. One view was that “the second explosion was caused by the hydrogen, which had been produced steam-zirconium” reaction. According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell onto the roof of the machine hall and started a fire. Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chernobyl Power Station firefighter brigade. The tar roof of the nearby turbine hall, which served all the reactors, was on fire. Firefighters quickly climbed to the roof and worked at extinguishing the fires.

12:55 – Responding to the Disaster and Evacuation

Their commander sounded a general alarm that would bring fire apparatus and firefighters from Pripyat and all over the Kiev region. The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 to protect No. 3. The fires were extinguished by 05:00, but many firefighters received high doses of radiation. They were unaware that they had been walking on radioactive material that was emitting 2300 Sv per hour. At least six firefighters died from this in 1986.

The resulting fire sent a plume of highly radioactive smoke fallout into the atmosphere and over an extensive geographical area, including Pripyat. Belorussian and Ukrainian civil defense received the report of the blast and fire, and several hours later the Ukrainian civil defense staff was deployed in Pripyat. Militia placed cordons, and by noon there was full control of the cordon and monitoring of radiation levels. The initial spread was to the west of the plant, but spreading of the radioactive contamination was rather insignificant because of light wind. This was to change and lead to dispersion around much of the globe.

Civil defense personnel prepared the city for evacuation, though this could only be authorized by the government of the USSR. By the evening radiation levels in Pripyat were over 100 times natural background. Although this did not cause major concern to the government physicists, an evacuation was recommended because they could not judge with confidence about the state of the reactor active area and about further development of the accident.

It was decided that the evacuation would begin on 27 April. Over 1,000 buses arrived from Kiev during the night. Weak points of the response were numerous, if perhaps understandable and similar to existing shortfalls even in the West. The local population had not been warned to shelter in place, there was no distribution of potassium iodide tablets, and there was a huge shortage of respiratory protection. Nearly 1,200 buses collected near Chernobyl, and only 36 hours after the accident, the evacuations began. Many were still unclear about the scale and nature of the emergency, but there were surprisingly few deaths considering the scale of the accident.

15:20 – Effects on Public Health

Due to the explosion, one person was killed immediately and a second died in Pripyat’s hospital soon after as a result of injuries received. Acute radiation syndrome (ARS) was originally diagnosed in 237 people on-site and involved with the clean-up, and it was later confirmed in 134 cases. Of these, 28 people died as a result of ARS within a few weeks of the accident. Nineteen more subsequently died between 1987 and 2004, but their deaths cannot necessarily be attributed to radiation exposure. Nobody off-site suffered from acute radiation effects, although a large proportion of childhood thyroid cancers diagnosed since the accident is likely to be due to the intake of radioactive iodine fallout.

Furthermore, large areas of Belarus, Ukraine, Russia, and beyond were contaminated in varying degrees. As a result of the Chernobyl accident, some 135,000 people were evacuated and radioactive material was widely dispersed, affecting a vast area (practically the whole of the northern hemisphere). Based on the official reports by the United Nations, up to nine million people in Belarus, Ukraine and Russia have been affected directly or indirectly by the radiation fallout. The people of the affected areas have received the highest known exposure to radiation in the history of the Nuclear Age, the full consequences of which will not be seen for at least another 50 years.

Since 1986, the rate of thyroid cancer in affected areas has increased tenfold. Specifically, there has been a significant increase in the number of thyroid cancer cases among patients age 15 or younger. About 155,000 square kilometers in Belarus, Ukraine and Russia were contaminated, which is almost half of the size of Italy. This resulted in the subsequent resettlement of 404,000 people, but millions continue to live in an environment where residual exposure has created a range of adverse effects.

As bad as the accident was, it would have been even worse were it not for the brave actions of the firefighters who rapidly got the fires under control. Without that initial control, all subsequent control measures would have been almost impossible. They were only the vanguard through of a scale of sacrifice that it is hard to find comparison with in peacetime. Nicknamed “The Liquidators,” they carried out the work of decontaminating and stabilizing the reactor structures and building the first sarcophagus.

Figures vary wildly, obfuscated by Soviet secrecy, but the UN estimated between 600,000 to one million people carried out the highly dangerous initial work. 240,000 of these worked in the first 12 months. They were awarded a special medal recognizing their bravery and sacrifice of their lives; a drop of blood with superimposed alpha, beta and gamma traces.

18:25 – Lessons Learned

The complex nowadays is over 1,000 times less radioactive and was generating power until 2000. Aid poured in in various forms, including for new dry fuel storage and a new more secure sarcophagus. The aid totaled well over 1 billion U.S. dollars, which has, like so much aid around the world, gone to contractors from the donor countries. Sadly, this seems to have been spent entirely on new uniforms and offices for the workers, as well as a wage twice the average for all the other nuclear workers in the area. There seems to have been very little spent on the fire crews who fought so valiantly. Their appliances are little different to those in action at the time of the accident, although they are meticulously maintained by the dedicated crews.

A few important differences we’ve seen since Chernobyl lie in our protective equipment. Although much of the turnout equipment seemed behind Western standards, their Dräger breathing apparatuses were in excellent condition. Also, all the men carried dosimeters and thermoluminescent detectors, correcting the gap of 1984 were the crews had no idea where the contamination was (and were walking over it in many cases).

The role of the fire service in the exclusion zone has changed in a number of ways since the accident. While on one hand the crews fulfil the basic functions one expects of a nuclear power plant fire service, there is a huge forestry fire service role for them now. The area of the exclusion zone consists of vast tracts of forest and grassland and marsh.

Water levels must be constantly monitored to prevent the drying out of water courses, which might release radioactive sediment. Equally, the seasonal risk of forest or grass fire in the exclusion zone carries the additional hazard of releasing radioactive particles and dust in to the atmosphere. This has led to an aggressive monitoring policy, obsessive weather monitoring, and a prevention strategy to envy any other major forested area in the West. Fire breaks and damping down can only do so much though, and so extreme measures are necessary at times to address the potentially vast fires.

Even so, my first sight of the armored tracked firefighting vehicles was awe-inspiring. Looking like a platoon of multiple rocket launcher systems (MLRS), but painted red, they represented the rapid solution to out of control fires. Their explosive payload eschewed for one of fire suppressant that rapidly coated wide areas of ground. More bizarrely they also seemed to have a petting zoo, with a pet boar – who I suspect may not be looking forward to barbecue season.

More detailed reports of my journey around the exclusion zone will follow, but for now I wish to pay tribute to the bravery and hospitality of the Chernobyl Fire Service – a team of brave men, with an heroic past and a praiseworthy dedication to continuing to keep the public safe from the hazards of the tragic nuclear accident of Chernobyl.

Have a question? Send an email to feedback@thehazmatguys.com or leave a message on our Haz Mat Guys comment hotline: 843-628-1484

Show Sponsors
Related Episodes

The Hazmat Guys

Author: The Hazmat Guys

LEAVE A REPLY

Please enter your comment!
Please enter your name here