Since the discovery of the neutron and its applications in induced radioactivity, the latent energy of the atom has been rapidly explored and utilised. This relatively recent technology is now responsible for a large percentage of the world's electricity supply. Propelled by it's massive energy generation potential and the incentive of zero greenhouse gas emission, 71 new plants are under construction in 15 different countries as of January 2015. Although concerns about the safety and environmental effects of nuclear energy continue to propagate after the level 7 events at Fukushima and Chernobyl, the demand for more energy, low fuel costs and the carbon tax motivates the industry and further research into nuclear energy.
The Chernobyl Disaster
On the 26th of April, 1986, a combination of flawed administration, reactor design and operator errors lead to a criticality accident causing a large amount of highly radioactive core material to be released into the surrounding environment. Due to the lack of immediate and effective countermeasures taken, the consequences are quickly noticeable across Europe and the airborne radioactive cloud became detectable in America in just ten days.
The initial fire was put out by firefighters dedicated to the plant and reinforcements that were called in from nearby. Many of them soon collapsed and died later due to acute exposure to radiation. A concrete floor was then inserted beneath the reactor to prevent the molten core from contaminating the water table and risking a steam explosion. Soldiers from the Soviet army were called in to move the contaminated debris back towards the reactor which was then covered in sandbags, lead and boric acid to block and absorb the radiation. By December, the entire reactor area was entombed in a large concrete structure and further measures were taken to ensure that the core is unable to re-enter a self-sustaining reactive state. A larger, more permanent metal enclosure has been planned and will replace the current concrete version as to allow safe decommissioning and demolition of the power station.
The cause of this incident was determined to have stemmed from a safety test that was routinely carried out. Due to the unplanned postponement of the test, the preparations were made unnecessarily and the emergency core cooling system was disabled. The test called for the thermal level of the reactor to be lowered to 700 to 800 MW. But the operator mistakenly lowered the control rods too much xenon-135 started to accumulated excessively, leading to a sharp power drop. The engineers were baffled that the reactor entered a near-shutdown state and disabled the automatic systems for the control rods in order to manually raise them to restore power levels. These changes lead to variations in the water/steam drums that sounded alarms, however, these were ignored so as to preserve the 200MW thermal level that was eventually reached in an unstable equilibrium.
The test continued as planned and extra water pumps were activated. This increased flow rate meant that the thermal energy did not have enough time to be dissipated and the water is nearing nucleate boiling. It also caused the reactor energy to be lowered further as water weakly absorbed a portion of the neutrons. To counteract this, all but 18 of the control rods are removed and many safety and passive systems were bypassed to maintain the reactor temperature.
As the next step in the test, steam flow to the turbines were redirected and the water pumps that were powered by those turbines start to lose energy. This gradual decrease started a positive feedback loop in which the lowered flow rate lead to the production of steam voids, which in turn decreased the neutron absorption of water and further increased the reactor temperature. This was managed by the automatic system that controlled the few control rods that were left.
At 1:23:40, as recorded by a centralised control system, an emergency shutdown was initiated. All the control rods, include those that were manually removed, were set to return to full insertion to stop the reactor. During the reinsertion, however, an energy spike occurred and fractured the flawed graphite tips of some control rods, which did not slow down the reaction, causing them to be jammed at approximately 1/3 of the way in. This actually further increased the reactor output which began to climb uncontrollably and very rapidly[1:1]. The events following are not recorded, but it is speculated that massive steam build up in the reactor core caused a powerful steam explosion that ruptured through the roof and sent radioactive material into the atmosphere[1:2].
The effects were not immediately noticed, as people in the nearby city of Pripyat continued in their business while some reportedly fell ill. Only after an investigation team was sent to inspect the damage do they notice that the reactor core is badly damaged and the city of nearly 50,000 people was ordered to evacuate. This was done almost 24hrs after the initial explosion and two people had died and 52 were hospitalised. This evacuation was made permanent after the planned three days were deemed insufficient[1:3].
Later analysis report that nearly 400 times as much radioactive material was released than the atomic bombing of Hiroshima. A pine forest directly downwind of the power station became diseased and died. Wild animals, plants and domestic animals that were left became contaminated and stopped reproducing. The WHO attributes that cancer cases in Europe doubled due to the excess radiation levels and contamination, the effects of which will continue for at least another 100 years.
This incident ultimately stunted the growth of the nuclear energy industry and forced all the plants planned or in construction to revise safety standards and reconsider investment plans. In particular, it raised suspicions about the Soviet Union's nuclear industry and forces more transparency onto its procedures[1:4].