Regaining control

Plant operators at the Fukushima Daiichi NPP seem to be slowly gaining control of the situation.

in New Delhi

THE floods unleashed by the massive tsunami following the magnitude 9.0 earthquake of March 11 rendered the critical systems of the nuclear power plants (NPPs) along the Japanese coast dysfunctional. The workers at the plants are battling to bring the situation under control: their failure could result in a serious radiological accident with widespread health consequences for the people of the region. However, as this article goes to print, the situation is serious but under control and not as threatening as media reports seem to suggest.

There are 14 reactors in the affected regions along the coast: three in the Onagawa plant operated by Tohoku Electric Power Co., six in the Fukushima Daiichi plant and four in the Fukushima Daini plant operated by the Tokyo Electric Power Co. (TEPCO), and one in Tokai run by Japan Atomic Power Co. (JAPCO). All the 14 are Boiling Water Reactors (BWRs). The most seriously affected plant, however, was the Fukushima Daiichi plant, nearest to the shore, which has a total capacity of 4,696 MWe. Unit 1 is a General Electric BWR with Mark-1 type primary containment, in operation since 1971. At the time of the earthquake event at 14-46 Japan Standard Time (05-46 GMT), Units 1, 2 and 3 were in operation, whereas Units 4, 5 and 6 were under maintenance shutdown. All the four BWRs of Fukushima Daini were, however, in operation.

In BWRs, the nuclear fuel heats water, and the steam from the boiling water drives a turbine directly to generate electricity. The reactor operates at 250oCelsius. The fuel, uranium oxide, which has a very high melting point of about 3,000oC, is fabricated in the form of pellets assembled into long tubes of zircalloy, which has a melting point of about 2,200oC, and sealed to form a fuel rod. The fuel is in the form of several bundles of these tubes packaged together, which constitute the reactor core. The core is placed in a sturdy reactor vessel (RV), which is made of high tensile steel and is designed to safely contain the fuel for several hundred degrees temperature. The entire hardware of the nuclear reactor the RV, pipes, pumps, the coolant water is then encased in a containment (primary) made of strong steel, designed to contain indefinitely a complete core meltdown.

For that purpose, a large thick concrete basin filled with graphite is cast under the RV, inside the primary containment. This is called the core-catcher'. If the core melts and the reactor vessel bursts and eventually melts, it is meant to catch the molten fuel and everything else. It is built in such a way that the nuclear fuel will spread out so that it cools down. The primary containment is then surrounded by the secondary containment, the reactor building. It should be pointed out that, unlike in Chernobyl, under no circumstances will there be an explosion though there will be significant radioactive releases in case of a meltdown. It should, however, be pointed out that the Unit 1 reactor, being a vintage Mark-1 type BWR of GE, does not have a core-catcher. So if there is a complete meltdown, the fuel melt will just leach through the metal casing and seep into the soil.Following the earthquake, all the reactors, as per design, shut down automatically. When the reactor shuts down, the chain reaction stops immediately and the power quickly (within about 90 seconds) drops to 2-3 per cent of full power, but not zero. This is because of the residual heat caused by the decay of a number of intermediate radioactive elements created during the uranium fission process, most notably iodine (I) and caesium (Cs) isotopes. This decay heat is to be removed by supplying cooling water to ensure the integrity of the fuel. For this purpose, normal cooling water systems as well as emergency (backup) cooling systems are provided in a nuclear reactor. This core cools in a matter of days.

In the Daiichi complex, the earthquake caused a very serious incident, which is still going on and can cascade into a potentially catastrophic accident. The off-site power supply from the grid was totally lost, resulting in a station blackout. This led to the automatic start-up of the Emergency Diesel Generators (EDGs), which kept the coolant pumps running. However, in less than an hour, the tsunami, which was much bigger than the plant had been designed for (6.5 metres), hit the NPP, and the wall of water destroyed all the EDGs, resulting in the complete loss of A.C. power to Units 1, 2 and 3. This led the government to order an evacuation of people living within 3 km of the site.

The operators then switched to emergency battery power. But this battery backup could hold only for eight hours. So mobile diesel generators were brought in, but these external generators failed to work for reasons not clear. As a result, in the absence of cooling, the reactor vessel began to heat up and pressure in the RV of Unit 1 started to increase. According to TEPCO, the pressure had increased to 600 kPa against the reference value of 400 kPa. The Japan Atomic Energy Agency had initially rated the emergency arising from the loss of coolant event at 4 on the 0-7 International Nuclear Event Scale (INES) of the International Atomic Energy Agency (IAEA). The Three Mile Island accident was rated at 5 and Chernobyl at 7.

Unless cooling could be restored and the fuel temperatures kept below 2,200oC, the core would eventually melt, leading to a core meltdown. But the next line of defence is to relieve the pressure by passing this high-pressure steam through a large pool of water called suppression pool maintained in a torus-shaped vessel connected to the containment below the RV. But this has only a limited capacity, and passing the steam through the suppression pool repeatedly eventually led to a pressure build-up again in the vessel up to 840 kPa. The reactor has 11 pressure release valves and so, to keep the pressure under control, the operators then resorted to venting steam, which is at about 550oC, periodically.

Besides caesium and iodine, radioactive materials with a very short half-life are also created, like radioactive nitrogen (N-16) and xenon. Neutrons that leak out of the core and get captured by the coolant water or by air in the water produce these radioactive elements outside the fuel assembly. So the pressure valve is opened to relieve pressure inside the vessel by venting steam not directly into the environment but between the primary containment and the reactor building. Thus, while venting does result in radioactive release, it is not dangerous as both radioactive nitrogen and noble gases decay within hours and hence do not pose any health hazard.

At 14-40 JST on March 12, the operators released steam from the containment. In about an hour after release, the radioactivity at the site boundary, as measured by a moving car, showed a spike at 1,015 microsievert (microSv)/hr (= 1,01.5 millirem/hr) at one location. This localised spike is equal to the dose permissible annually. At other locations, however, the field around the same time continued to be low at 7-40 microSv/hr.

A little before this increased radioactivity, at 13-30 JST, the presence of low levels of Cs-137 and I-131 was also detected near Unit 1. This was ominous, as it was indicative of possible overheating of the fuel elements and zircalloy cladding. It also implied that the water level around the core had dropped, exposing the fuel elements to air, and that there might have been a partial meltdown. Around 18-00 JST, an explosion, raising a cloud of white smoke, was seen around Unit 1; it damaged the external building structure, which does not act as a containment structure. The explosion is believed to have been caused by a build-up of hydrogen in the reactor building. When exposed to high temperatures, zirconium in the cladding reacts with water to form zirconium oxide (corrosion reaction) and hydrogen. This hydrogen perhaps found its way into the reactor building, accumulated close to the ceiling, and ignited with air. Although the explosion was significant, it did not compromise the integrity of the primary containment or the RV. To somehow restore cooling and limit the damage, the desperate measure of cooling the core with seawater mixed with boron (a neutron absorber) was initiated at 20-00 JST. On March 14, a similar situation arose in Daiichi Unit 3, and seawater cooling was resorted to. Unit 3 also had a hydrogen explosion at 11-00 JST on March 14, and as in Unit 1, only the reactor building was damaged; the containment vessel had not been breached and there was no significant radioactivity. Unlike the other units, Unit 3 used mixed plutonium-uranium oxide (MOX) fuel, and overheating or melting of this would be more dangerous because of the presence of the long-lived plutonium. In accordance with the emergency procedures at NPPs, the evacuation of the population within a 20-km radius of the Daiichi site was initiated. Since injecting seawater was not adequate, plant operator TEPCO decided to douse Unit 3 with water sprayed from helicopters.

As was noted in a separate story, the BWR Mark-1 has a lower containment to power ratio, and so the impact of accumulating hydrogen will be more dangerous in this than in other GE designs with a larger containment volume. But, more pertinently, the U.S. Nuclear Regulatory Commission had also recommended rendering the containment inert with nitrogen, which was perhaps not implemented at the Daiichi units. On March 15, at 06-20 JST, an explosion occurred in the lower part of the primary containment of Fukushima Daiichi Unit 2 near the suppression pool, perhaps because of heating. The radiation field showed a sudden increase of up to 8,217 microSv/hr. This was followed by a fire in the reactor building of Daiichi Unit 4, which was in a maintenance shutdown state at the time of the earthquake/tsunami and had no fuel in its core. Though the fire, the reason for which was not clear, was extinguished, it seemed to have led to the on-site radiation levels shooting up to 400 milliSv/hr between Units 3 and 4. This led to the evacuation of people up to 20 km from the site and the sheltering of inhabitants within another 10 km.

On the same day, the water level in Unit 5 began to drop, and it was found to have decreased to 201 cm above the top of the fuel, a 40 cm drop from the normal level. Again the following morning a fire was noticed in Unit 4 on the plant's fourth floor, where the spent fuel pool (SFP) is located. However, the fire could not be located half an hour later, but the radioactivity level at the plant boundary, which was 8.99 microSv at 00-00 JST on March 16, shot up to a high 3,391 microSv/hr at 11-00 JST. It was later seen to drop to 1,937 microSv at 14-30 JST. Further, since radiation levels above the plant site were greater than 50 mSv, perhaps because of the increased radioactivity release from Unit 4, the planned dousing of Unit 3 from helicopters was postponed. Cooling failure in SFPs is more dangerous because this will expose the highly radioactive fission products directly to the atmosphere. Normally, SFPs are maintained at 25oC.

At 10-00 JST on March 15, the temperature in the storage pool of Unit 4 was 84oC, while in those of Units 5 and 6, it was 62.7oC and 60oC respectively. To deal with the situation of low water level in the SFPs of Units 3 and 4, the Self-Defence Forces (SDF) of Japan started the operation to fill the pools with water by directly dropping huge quantities of seawater from a helicopter. The value at the site boundary at 16-20 JST on March 16, before the start of Operation Helicopter, was 1,472 microSv.

On March 17 morning, helicopters started pouring water on reactor buildings 3 and 4 from a height of 300 m. In addition, more than 10 water cannons were to be used to spray water over RB 3 and 4. But now the water level in Unit 5 reactor was falling and pressure was increasing. So now the SDF started cooling operations at the Daiichi plant from the ground as well with fire engines and high-pressure water cannons. There is no significant evidence that this was having any effect, but a slight decrease in radioactivity was observed.

Attempts were also being made to install a cable line from the Tohoku Electric Power Company to Units 1 and 2, which should be connected by March 19. This should greatly help restore cooling to the stricken reactor vessels. On March 18, the coolant loss event was rated on the INES scale for each Fukushima Daiichi reactor unit separately. Core damage due to loss of cooling in Units 2 and 3 was rated as 5 on the INES scale, whereas that of loss of cooling function in the SFP in Unit 4 was rated as 3. Similarly, the loss of coolant event in Units 1, 2 and 4 was rated as 3. The EDG for Unit 6 has been made operable now, which has begun to supply electricity to Units 5 and 6. Water injection into the RV and the SFP is also progressing, and the water level of the SFPs seems to have been restored.

As regards the Daini units, cold shutdown conditions have been achieved for all of them. Thus, the situation as of March 18 was that while there were unpredictable situations that brought the reactors at Fukushima Daiichi NPP perilously close to the threshold of a catastrophe, the plant operators seemed to be slowly gaining control of the situation. And with measures to get off-site power through cable lines from another power company, the operators should soon be able to bring the situation to safety within a month or so.

The possible radioactive fallout from the reactors has, however, caused widespread public concern. Besides the periodic radioactive releases through steam venting and the single instance of detection of long-lived Cs-137 and I-131, which did show up as spikes in the radioactive monitoring in cars, the situation does not seem to be posing any undue radiological danger to the public, though some on-site workers have got exposed beyond their annual limits and have been isolated. The on-site levels, expectedly, continue to be above normal radioactivity levels. There was a single instance of increased levels of radioactivity beyond normal levels observed in Tokyo, which is towards the south-west of Fukushima. It seemed to be the result of unusual wind conditions for a short period on that day. This, too, has since returned to normal levels.

The Japanese authorities have been meticulously monitoring on a daily basis the radioactivity situation in the eight prefectures adjoining Fukushima as well as at various monitoring points (MPs) around the NPP complex in a moving car. The data show that, generally, the levels in the adjoining prefectures are not significantly higher than the normal level for that region, except for one or two spikes. Indeed, the levels also seem to be falling or plateauing. Absolute reduction will occur only when cold shutdown of all the reactors is achieved.

The most intriguing thing about this accident is the very similar nature of the cooling failure in all the reactors. This is indicative of two things: one, lack of adequate water inventory at each of these units; two, the failure of backup EDGs to be able to pump seawater even though the inventory is infinite. The latter could be the result of the tsunami damaging the cabling, the switchgear and the pipings of the entire complex.

Even if there is external backup power and a working pump, functional switchgear is necessary to get the cooling going. Only detailed technical information on the sequence of events, which the Director General of the IAEA has asked for, will tell us what actually brought the NPP so perilously close to catastrophe.