New Provisional INES Rating + A Chernobyl Primer

Today the Japanese Nuclear and Industrial Safety Agency revised its INES rating of the Fukushima Daiichi event. The previous assessment treated the events at each of the ailing reactors as separate: the core damage to units 1-3 resulted in an assignment of a 5 (accident with wider consequences) for each reactor; the problems at unit 4’s spent fuel pool were assigned a 3 (serious incident). NISA is now treating the situation as a single event, assigned a rating of 7 (major accident). This rating is still being assessed as information about the disposition of radioactive materials originating at the reactor site comes in.

Because the rating is now the same as that assigned to the Chernobyl accident, the blog has received a number of questions about how the events at Fukushima differ from it. We present a sequence of events at Chernobyl, along with links to some denser technical matter for interested readers, and an IAEA report on the human costs of the disaster. For comparison, it’s been estimated that the radiation released by the Fukushima reactors is 1/10th that released to the environment at Chernobyl.


On April 26th 1986, the most serious nuclear accident in history took place at Unit 4 of the Chernobyl power plant located 130 km north of Kiev, Ukraine.  The site had four RBMK-1000 reactors.  These reactors are graphite moderated boiling water reactors and did not have a containment structure. Reactor containment is the large and thick concrete and metal structure surrounding the nuclear reactor. Its purpose is to protect the reactor from external damage, and to contain radioactivity in case of a significant reactor failure. By regulation, all western BWR and PWR reactors have to have a containment.  Additionally, the RBMK design also had a very large and positive coolant void reactivity coefficient, meaning that as the coolant (i.e. water) temperature increases, the reactor power increases. This positive coefficient is not present in BWRs or PWRs.

A brief summary of the events is presented here, a detailed description can be found at    The following document ( also includes information about the health and environmental effects of Chernobyl accident.

On April 25th, unit 4 was to shut down for routine maintenance, prior to which a safety test was to be performed.  The test was to evaluate how long the turbine would spin and supply power to the main water pumps in the event of a loss of electrical power.  This test had been tried in the past and new adjustments were made to allow the pumps to be powered longer.

After a few delays due to demand for electricity from the grid, the test was performed by an inexperienced crew.  The test was to be performed at ~30% of full power.  A series of operator actions, including the disabling of automatic shutdown mechanisms, positioned the reactor in a very unstable condition, in which the reactor was at very low power. As operators withdrew control rods in an attempt to increase the power to the level necessary for the test, the reactor heated up. The reactor’s positive void reactivity coefficient resulted in a rapid increase in power.  Control rods were inserted in order to staunch this increase in power. The unusual design of these control rods, which had graphite “followers” (recall that graphite is a moderator) worsened the situation by increasing power at an even more rapid rate. The result was a power excursion of between 100 and 500 times full power as the rods were inserted into the reactor.

This large power surge caused the fuel to disintegrate.   As the fragmented fuel interacted with the steam/water mixture, a steam explosion occurred. This blew off the reactor’s massive vessel top (1000 tons) which penetrated the reactor building concrete, and dispersed burning graphite and fuel.   This initial explosion and the subsequent fire sent a plume of radioactive gas and particulates into the environment. Further explosions were caused by production of hydrogen in clad/steam chemical reaction.

The radiological consequences of the Chernobyl incident were severe. The radioactive plume that emanated from the reactor contained not only volatile radioactive nuclides (such as Iodine-131, Cesium-137) which have been observed around Fukushima, but also many non-volatile ones, which were in the disintegrated fuel pieces. This plume got carried far away by wind and deposited radioactive particulates over many places in the northern hemisphere. 31 of the plant operators and firefighters got lethal radiation doses. The risk of cancer to surviving staff members and to residents of the 30 km evacuation zone is predicted to have approximately doubled as a result of exposure. An important thing to note about the Chernobyl accident is that the evacuation was not started until a nuclear reactor in Sweden (1000 km away) detected elevated radiation levels.

About 97% of the radioactive nuclides found in spent or partially spent fuel remain inside the fuel rods, as long as they do not melt. Of those, a fraction are noble gases (such as Xe-135), and many are solid materials. When the fuel melts, the noble gases escape the fuel and leak to the environment; however, due to being noble gases they do not react chemically, and disperse in the atmosphere. Iodine-131 (deposits in thyroid), Cesium-137 (30-year half-life) and Strontium-90 (replaces calcium in bones) are the three most significant non-gaseous fission products. Due to the explosion of the reactor vessel in the Chernobyl accident, these products were released as well, thus significantly contributing to the dose to the public.

Chernobyl unit 4 is now enclosed in a large concrete shelter which was erected quickly (by October 1986) to allow continuing operation of the other reactors at the plant.  The last reactor, unit 3, was shut down in 2000.  A New Safe Confinement structure is due to be completed in 2014. It is being built adjacent to the facility and then will be moved into place on rails.


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