By Robin Grimes, Mamdouh El-Shanawany & William Lee
SINCE 1973, Japan has implemented a national strategy to produce energy from nuclear power, in order to reduce its dependence on imported fuels. In 2008 Japan became the third largest nuclear power user in the world, with 53 reactors. Today these provide 34.5 per cent of Japan's electricity.
On March 11 an earthquake of 9.0 magnitude occurred off the north-east coast of Japan, the most powerful recorded earthquake to strike the country. As a result, a tsunami of over 10m height swamped the coast. The area surrounding the Fukushima Daiichi nuclear power plant was badly affected.
The Fukushima Daiichi nuclear power plant was first commissioned in 1971. It consists of six boiling water reactors (BWR). The BWR design is a light water nuclear reactor and is the second most common type of electricity-generating nuclear reactor after the pressurised water reactor (PWR).
As in all nuclear reactors, heat is produced by nuclear fission in the reactor core. In a BWR this causes the large volume of cooling water in the core to boil, producing steam. The steam is used directly to drive a turbine, after which it is cooled in a condenser and converted back to liquid water. This water is then pumped back to the reactor core, completing the loop.
Unit 1 at Fukushima Daiichi is a 460 MW BWR reactor constructed in July 1967, and is an example of an early generation of reactor design. It commenced commercial electrical production in 1971, and was initially scheduled for shutdown in early 2011. Last month, Japanese regulators granted an extension of 10 years for the continued operation of the reactor. Units 2 to 6 have larger electricity-generating capacities than Unit 1.
Prior to the March 11 earthquake, reactors 4, 5 and 6 were shut down for planned maintenance. Reactors 1, 2 and 3 shut down within seconds of the earthquake, as they are designed to do, using the automated Reactor Protection System (RPS). Thus, the main nuclear fission chain reaction ceased in all reactors.
However, considerable residual heat from nuclear decay processes remains in a reactor for some time after it shuts down. This produces the so-called 'decay heat'. While the amount of heat produced naturally ebbs away over time, it is sufficiently great to require active cooling for weeks.
Removal of decay heat is helped by the Emergency Core Cooling System (ECCS). This requires power to be available to the reactors. However, the tsunami that occurred soon after the earthquake was beyond the 5m maximum expected height, which the plant was designed to withstand. This caused the power sources for units 1, 2 and 3 (those that were in operation) to be lost. Also, the back-up emergency diesel electricity generators were damaged and their fuel tanks washed away. Although emergency batteries kept providing electricity, they gradually weakened. This meant the pumps injecting cool water into the reactor cores eventually stopped operating.
Crucially, the flooding and earthquake damage prevented assistance being brought from elsewhere. Over the following days there was evidence of partial meltdown of the nuclear core in reactors 1, 2 and 3; explosions destroyed the upper cladding of the building housing reactors 1 and 3, and an explosion damaged reactor 2's containment.
After nuclear fuel has been in a reactor core for many months, it gives up its useful energy and must be swopped for new fuel - in other words, it becomes 'spent'. It is removed from the reactor core and initially stored within water ponds. The circulating water both removes the 'decay heat', preventing the fuel from overheating, and acts as an effective barrier to harmful radiation. Each of the reactors at Fukushima Daiichi has a spent fuel pond. Unusually, these are situated high up in the reactor building, right next to the reactor core.
Unfortunately, the spent fuel pools also suffered from the earthquake and tsunami. Loss of power meant that water could no longer be added and circulated. Consequently, the spent fuel began to warm up. In the case of pools 3 and 4, it appears the fuel became uncovered. It may be that these ponds were damaged and leaked.
This in turn led to the spent fuel becoming damaged. And in the case of pond4, at least, it resulted in a fire and explosion that damaged the reactor building and caused the release of a small volume of radioactive particles.
The Japanese government and the energy company Tepco have been working tirelessly to control the situation. In anticipation of the loss of function of the emergency batteries, Tepco started diesel fire pumps and got ready to inject sea water when the reactor pressure had lowered. It has also been supplying sea water to the spent fuel pool. Still, the unprecedented combination of Japan's largest earthquake ever recorded and a tsunami above the design criteria has resulted in a very serious situation.
At the time of writing, Unit 1 reactor poses no immediate danger but its core has been permanently damaged, as has Unit 3's. In addition, Unit 3's reactor vessel may have been damaged, according to officials, which raises the possibility of radiation from the fuel in the reactor being released.
Unit 2 is still a concern but external power is being reconnected to all six units. The ponds for units 1 and 2 are being re-filled and their temperatures are stable though a little high. Pond 3 is still a concern but sea water is being introduced. The good news is that units 5 and 6, the newest of the six, seem to have suffered negligible damage.
Tepco can concentrate its efforts on units 1 to 4. Also, the measurements of radiation levels in Tokyo are just above the natural radiation level and do not represent a hazard to people. Local radiation levels will, however, be a concern for some weeks and months but should eventually return almost to normal.
Nuclear power's future
THE nuclear power industry worldwide continued to develop and improve reactor technology in the four decades following the commission of the Fukushima Daiichi nuclear power plant. The industry has recently begun to build the next generation of nuclear power reactors - known as third-generation reactors or Gen III.
Third-generation reactors have:
- A standardised design for each type to expedite licensing, reduce capital costs and reduce construction time;
- A simpler and more rugged design, making them easier to operate and less vulnerable to operational upsets;
- A longer operating life - typically 60 years;
- Additional safety measures to further reduce the impact of core melt accidents;
- Resistance to serious damage that would allow radiological release from an aircraft impact; and
- Nuclear fuel that can safely spend longer in the reactor, the increased efficiency thus reducing the volume of waste produced.
Traditional reactor safety systems are 'active' in the sense that they involve electrical or mechanical operation on command. Some engineered systems, however, operate passively. They function without operator control and despite any loss of auxiliary power. Full passive safety depends only on physical processes such as convection, gravity or resistance to high temperatures.
A second difference to the Fukushima Daiichi plants is that modern designs would not incorporate spent fuel ponds at a great height and adjacent to the reactor core. If the ponds are placed away from the reactor core, the spent fuel will not become compromised in the event of a reactor incident. Also, placing the ponds at a lower elevation makes it easier to refill them if water is lost.
Finally, during the 1990s the International Atomic Energy Agency (IAEA) established the principle of Defence in Depth (DiD). To achieve optimum safety, nuclear plants use the DiD principle, which requires multiple safety systems supplementing the natural features of the reactor core. This ensures the success of the three fundamental safety functions in a nuclear reactor, namely: to control reactivity, to cool the fuel and to contain all radioactive substances.
The safety provisions include a series of physical barriers between the radioactive reactor core and the environment, and the provision of multiple safety systems, each with a back-up and designed to accommodate human error. These barriers are monitored continually.
While it seems that modern reactor designs would not be subject to the same consequences experienced by Fukushima Daiichi units 1 to 4, the international nuclear energy community must not be complacent. This is a difficult time and the community must establish the facts, which are emerging and not yet fully known or understood. It is essential that we understand the implications, both for existing nuclear reactors and any new programme, especially in the selection of sites for new nuclear power plants.
The events unfolding in Japan make it even more important to have a well-educated and trained workforce, which can underpin and ensure the safety of nuclear power.
The writers are with the Imperial College Centre for Nuclear Engineering.