chernobyl

Foolhardy Test with a Faulty Reactor — Chernobyl 30 Years Ago

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September 2016

While driving north from Kiev, could not help of thinking the bus convoys from Pripyat coming opposite direction 30 years ago. A test in Chernobyl reactor #4 had gone horribly wrong, and had triggered series of events that still resonate today. 49 thousand people living in Pripyat were elite workers of Soviet famed nuclear industry, and their families, a prestigious position that guaranteed better living compared to average Soviet citizens.

IMG_20160917_093235Soviet era photo of reactor inspection.

After being exposed to high levels of radiation for a day without knowing about the danger, they were told few hours in advance to gather most important belongings and board on busses. Not much else were known by anyone inside and outside Soviet Union, except the political elite in Moscow and specialists that were hurriedly dispatched to the nuclear plant.

001_Ukraine_Chernobyl_1701Road to Chernobyl.
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Quotes bellow are from World Nuclear Association website.

Quote: On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested. A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. A peculiarity of the design of the control rods caused a dramatic power surge as they were inserted into the reactor.

042_Nexus5_2287Schematic model of RBMK1000 (High Power Channel-type Reactor), Chernobyl Museum in Kiev.

Quote: For this test, the reactor should have been stabilised at about 700-1000 MWt prior to shutdown, but possibly due to operational error the power fell to about 30 MWt at 00:28 on 26 April. Efforts to increase the power to the level originally planned for the test were frustrated by a combination of xenon poisoning, reduced coolant void and graphite cooldown. Many of the control rods were withdrawn to compensate for these effects, resulting in a violation of the minimum operating reactivity margin (positive void coefficient) by 01:00 – although the operators may not have known this. At 01:03, the reactor was stabilised at about 200 MWt and it was decided that the test would be carried out at this power level.

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Quote: At 01:23:43, the power excursion rate emergency protection system signals came on and power exceeded 530 MWt and continued to rise (Wikipedia: the last reading on the control panel was 33,000 MW, ten times the normal operational output). Fuel elements ruptured, leading to increased steam generation, which in turn further increased power owing to the large positive void coefficient. Damage to even three or four fuel assemblies would have been enough to lead to the destruction of the reactor. The rupture of several fuel channels increased the pressure in the reactor to the extent that the 1000 t reactor support plate became detached, consequently jamming the control rods, which were only halfway down by that time. As the channel pipes began to rupture, mass steam generation occurred as a result of depressurisation of the reactor cooling circuit. A note in the operating log of the Chief Reactor Control Engineer reads: “01:24: Severe shocks; the RCPS rods stopped moving before they reached the lower limit stop switches; power switch of clutch mechanisms is off.”

003_Ukraine_Chernobyl_1714Moon rover from Soviet space program that was used during cleanup efforts. It was only equipment that could sustain the levels of radiation, all other remotely controlled equipment stopped working after few hours. Most of cleanup effort were done by army reservists, that have since been called as bio-robots.

Quote: Two explosions were reported, the first being the initial steam explosion, followed two or three seconds later by a second explosion, possibly from the build-up of hydrogen due to zirconium-steam reactions. Fuel, moderator, and structural materials were ejected, starting a number of fires, and the destroyed core was exposed to the atmosphere.

007_Ukraine_Chernobyl_1746Radiation level today, next to Reactor #4 is above normal background radion. About the same as in passenger jet in cruising altitudes.

Quote: Fires started in what remained of the unit 4 building, giving rise to clouds of steam and dust, and fires also broke out on the adjacent turbine hall roof (bitumen, a flammable material, had been used in its construction). A first group of 14 firemen arrived on the scene of the accident at 01:28. Over 100 fire-fighters from the site and called in from Pripyat were needed, and it was this group that received the highest radiation exposures. Reinforcements were brought in until about 04:00, when 250 firemen were available and 69 firemen participated in fire control activities. The INSAG-1 report states: “The fires on the roofs of units 3 and 4 were localized at 02:10 and 02:20 respectively, and the fire was quenched at 05:00.” Unit 3, which had continued to operate, was shut down at this time, and units 1 and 2 were shut down in the morning of 27 April.

002_Ukraine_Chernobyl_1710Monument for the firemen that were first to arrive the plant after accident. Not knowing what they were dealing with, they were unprotected against the massive levels of radiation released from reactor.

After the accident, several investigations have been conducted to find causes of the accident. The first one still during Soviet Union laid all the blame on operators of the plant. Second one after couple years in turn blamed the reactor design. Consensus today seems to be combination of both:

  1. Errors done by the reckless and inexperienced control crew. Even with quirks of reactor that delivered the final blow, operators demonstrated that they didn’t fully control the reactor, and created conditions for the accident. For example the inability to stabilise the reactor at 700-1000 MWt before starting the test, power level dropped to measly 30MWt in their hands, and led to extremely unstable reactor configuration. Another example is manually overriding several automated safety systems, to be able raise the reactor output that had plummeted.
  2. Reactor design, described earlier.

Several contributing factors have also been identified, for instance:

  • Bad luck. Inexperienced night shift was doing the test, instead of day/evening shifts as originally planned. Kiev power grid controller needed power until 23pm, which postponed the test later than expected.
  • General lack of respect to safety procedures and wider operating margins, as well as withholding important information can be attributed as products of paranoid, totalitarian and dysfunctional society of Soviet Union after decades of Brezhnevian stagnation. For instance same type reactor had already demonstrated its tendency for power spikes in Ignalina nuclear plant (Lithuania), four years prior Chernobyl 1986. This important information was not spread anywhere, and Chernobyl operators were unaware of it.

011_Ukraine_Chernobyl_1757Pripyat town center today.

Two days after the accident, radiation detector alarms went on in Swedish Forsmark nuclear plant, over 1200 kilometers away from Chernobyl. After checking their own plant several times, without finding the cause of high levels of radiation, Swedish started to look causes abroad and contacted Soviet authorities (link for more info). This finally forced Moscow to admit to the rest of the world what had happened.

022_Ukraine_Chernobyl_1830Pripyat town center today.

All resources of super power were employed to contain the radiation, disconnect reactor from atmosphere and prevent even larger damage. Superheated core was slowly eating its way to lower sections of destroyed building, which risked yet another explosion if it reached large water pools under. Helicopter pilots from Afghan front were rushed to Chernobyl and drop sand, lead, clay, and neutron-absorbing boron onto the burning reactor. Miners from Russia were brought to dig tunnel under the reactor and replace water in pools under the reactor with cement. Thousands of army reservists were used as bio-robots because equipment available broke down. They had to clean the highly radioactive debris and destroyed buildings. Large sarcophagus to seal reactor from air was hurriedly designed and built in highly radioactive environment.

027_Ukraine_Chernobyl_1854Ghost town of Pripyat is touching place to visit and see.
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019_Ukraine_Chernobyl_1818Pripyat.
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039_Nexus5_2373Abandoned Duga radar near Chernobyl, nicknamed “Russian Woodpecker”. It was a Soviet over-the-horizon radar system, used as part of the early-warning network of oncoming American ballistic missiles. Two operational Duga radars were deployed, one near Chernobyl and Chernihiv in Ukraine, the other in eastern Siberia.

038_Ukraine_Chernobyl_1723Starting in 1976 a new and powerful radio signal was detected worldwide, and quickly dubbed the Woodpecker by amateur radio operators, due to its sounding like a sharp, repetitive tapping noise at 10 Hz. In late 1980’s Duga system was made irrelevant by satellites. The satellite system provides immediate, direct and highly secure warnings, whereas any radar-based system is subject to jamming.

037_Ukraine_Chernobyl_1865Duga radar can be easily seen from higher buildings of Pripyat, so it was hardly a secret for civilians living there. What was secret though was its purpose. Locals were told it being television antenna!

Just two years after Chernobyl, Gorbachev government found itself again at odds with harsh realities of Soviet system. December 1988 an earthquake 6.8 Ms magnitude, rocked northern regions of Soviet Republic of Armenia. Casualties have been estimated 25000—50000 dead. Much of casualties were attributed to substandard construction of buildings during Breznevian decades, such as excess amount of sand used because cement had been stolen or sold to black market. Also the Soviet war in Afghanistan was going wrong big time, and eventually they would have to recognise the defeat and withdraw. These hardships were used by Gorbachev as evidence that Soviet system needed fundamental changes, enter the glasnost and perestroika reforms. However as history then witnessed, it was too little too late. In November 1989 Berlin Wall fell, and dominoes started to fall for Soviet block.

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Statue of Lenin still stood at firmly in Kiev, September 2012. In public view it had transitioned from being Communist monument, to a symbol of Ukraine’s linkage to Moscow. After EuroMaidan protests in 2013-2014, old gentleman finally had to step down from his podium.

008_Ukraine_Chernobyl_1748Chernobyl today: the New Safe Confinement is getting ready. It is a structure intended to contain the destroyed nuclear reactor #4. Besides better prevention of radiation leaks, secondary goal is to allow partial demolition of the old sarcophagus built 1986. The total cost of the project is estimated to be around 2.15 billion Euros. November 2016, NSC moving has started: Link.


Interesting clip about starting and stopping a scientific nuclear reactor.