Lessons from Japan

The earthquake and tsunami of March 11, 2011, in northern Japan killed over 20,000 people and a few thousand may suffer from slow radiation poisoning over the next few decades. Global worries about radioactive material entering the human food chain are real as winds and ocean currents spread the radioactive waste.
On April 3, newspapers reported the following about Fukushima reactors: An eight-inch crack was discovered in the maintenance pit with radioactive seawater flowing into the sea.

The air above this pit showed a radiation level of 1,000 milliseverts per hour (the maximum permissible radiation dose is 20 milliseverts per year). Highly radioactive water was discovered in a steam turbine generator compartment.
The basic reason for Japan’s Fukushima nuclear disaster was the delay in utilising seawater cooling in four operational reactors after the main and standby cooling systems failed.
Once the Fukushima reactors were shut down due to the earthquake, it required a few weeks’ cooling (using standby generators) to remove the “decay heat” of reactor fuel “fission byproducts”. Unfortunately, 15 minutes later the tsunami came and shut down the standby diesel generators and cooling was done for the next eight hours using batteries and boric acid to absorb neutrons. Once the batteries were exhausted there was a very small time window in which seawater cooling should have been used even though seawater would have corroded and permanently “written off” the reactors. Apparently it was decided to try and save the reactors for future use by delaying seawater cooling, resulting in the reactor fuel meltdown.
Since there are numerous lessons for India, which hopes to import a couple of dozen light water reactors (LWRs), let me first explain that LWRs are of two types: the pressurised water reactor (PWR), which is used in both civil nuclear power plants and nuclear submarines, and the boiling water reactor (BWR) of the type that is in the news in Fukushima.
Both the PWR and BWR use uranium oxide (UO2) as fuel and this fuel heats up water in a “closed loop” to drive a steam generator to produce electricity. To ensure a higher boiling point of water (about 250ºC), the reactor pressure vessel (RPV) which houses the fuel and the pure demineralised water (know as the “first loop”) is kept at high pressure to ensure a higher boiling point of water for greater heat transfer. The RPV is similar to a pressure cooker where excess pressure can be vented out by a safety valve.
Unlike the PWR (where the “first loop” is kept isolated from the steam turbine generators), the simpler BWR allows the radioactive steam from the reactor core to go directly to the steam turbine generators. But this requires special precautions to be taken in the steam turbine generator compartments of the nuclear plant due to the radioactive steam. This explains the April 2 discovery of radioactive water in the turbine compartment of a reactor.
The UO2 fuel has a melting point of about 3,000ºC. These UO2 pellets are put in sealed Zircaloy tubes (which have a melting point of 2,200ºC, and these tubes form the “fuel core”. This core is stored inside a RPV, which can withstand high temperatures and the very high pressures of the boiling water, which is converted to steam, which then drives the steam turbines to produce electricity. This “closed loop” steam is subsequently “externally cooled” by seawater in a seawater cooled “condenser” that converts the steam into water and the pure water is then pumped back into the RPV to cool the core and become steam again in the closed cycle. If the fuel in the reactor core is not constantly cooled by water the Zircaloy tubes can melt in about 45 minutes, followed soon thereafter by the UO2 fuel, resulting in a “core meltdown”. This nightmare scenario is known as LOCA (loss of cooling accident).
Should the reactor core “melt down” due to LOCA, then the fuel is prevented from coming into contact with the atmosphere by a “containment vessel”. The explosions one saw on TV at the Japanese nuclear plants were basically hydrogen explosions outside the containment vessel, of steam released from the RPV (at high temperatures steam breaks up into hydrogen and oxygen). With the earthquake of March 11, the reactors were immediately shut. However, each reactor core had “fission byproducts”, which require water cooling for a few weeks to reduce decay heat. As explained earlier, this cooling failed and the seawater cooling was delayed, leading to LOCA and reactor fuel meltdown.
The radiation level spikes, followed by the discovery of plutonium (plutonium 239 has a half life of 24,400 years) in the soil on March 29, 2011, indicate a “containment vessel leak” in at least one reactor. The decision of March 30 to decommission four of the six Fukushima reactors was inevitable. Also, the reactors, apart from UO2 fuel, have a few fission byproducts with long “half lives”, eg. Cesium 137 is 30 years, Strontiun 90 is 29 years. Hence, apart from entombing the four reactors in sand, lead and concrete, the Fukushima region will need careful monitoring for a long time.
Nuclear power is important for India but a transparent safety audit needs to be done of our existing (and future) nuclear plants, site locations, the tsunami warning system, the national disaster management system and the nuclear emergency response teams. Also, our Nuclear Liabilities Bill must not be diluted, indeed, it needs to be made even more stringent. Safe renewable energy sources like wind, solar and hydropower too require a fresh relook.

Vice-Admiral Arun Kumar Singh retired as Flag Officer Commanding-in-Chief of the Eastern Naval Command, Visakhapatnam