The terrifying meltdowns and hydrogen explosions at the Fukushima Daiichi nuclear power station in the days following 11 March 2011 made the importance of backup electricity generators painfully clear.
But a year later, the best way to provide backup power is still being debated. Newer reactor designs have yet to prove that they can provide survivable backup. A US plan to throw lots of backup generators and other emergency equipment at the problem 鈥� without assessing the risks that each plant faces 鈥� has come in for severe criticism.
Power needs to be provided for what nuclear engineers call 鈥溾��. In this state, reactors require constant power to keep them cooler than 100掳C and prevent them producing turbine-driving steam. This is the case because even when fission is stopped by driving neutron-absorbing control rods into a reactor core, radioactivity still warms the fuel rods. This residual heat must be removed by a cooling system. For safety reasons, cooling power is supplied from an offsite source, but when that goes down, backup generators and then batteries kick in.
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Inadequate sea walls
Backup power was needed at Fukushima after the magnitude-9.0聽quake struck and the six power lines bringing in offsite electrical power to Fukushima Daiichi were severed, says Michael Weightman, Britain鈥檚 chief nuclear installations inspector and a member of the International Atomic Energy Agency鈥檚 .
Sensors detected the quake and switched the plant鈥檚 three working reactors into shutdown mode. The power for cooling was then supplied by 12 onsite diesel generators in non-waterproofed buildings. These generators were inundated with water 45 minutes later when the 15-metre tsunami swept over the sea wall, which had been built to withstand a maximum wave height of only 5.7 metres.
The plant was left relying on reserve battery power. Then came what the nuclear industry dreads: 鈥� no power at all for cooling, sensing or control. That led to heating, cracking fuel rods, hydrogen generation and meltdowns 鈥� until the ad hoc measures of the eventually brought a measure of control.
Higher ground
In light of the Fukushima disaster, an obvious move might now be to recommend placing the backup generators for nuclear power stations on higher ground to guard against tsunamis or floods. But that might put the backup generators for power stations in some parts of the world vulnerable to tornados or hurricanes. If that happened, and a flood took out lower-lying generators too, it would still lead to station blackout.
To ensure that some backups will endure extreme events, says Weightman, a strict process of 鈥渄iversity and segregation鈥� should be enforced. This is one of many safety issues currently being investigated by his team at the UK in Bootle, as it considers licensing new, supposedly safer, reactor designs from companies like Japanese-owned Westinghouse Electric and French-owned Areva.
鈥淒iversity鈥�, says Weightman, involves using different fuels to power backup generators. In a long outage, one type of fuel could run out or become contaminated if, say, a quake ruptured a diesel tank, or flood waters got into it.
A mixture of gas turbine and diesel generators should be used, he says, and water and air cooling systems should both be considered for diesel generators in case a shortage of air or water might be an issue at the generator location.
By taking into account the most likely threats that a plant faces, the backup generators should be carefully distributed around the site, Weightman says. This segregation must also be matched in terms of the architecture of a plant鈥檚 control systems, he says, for instance by ensuring that the emergency control system is separated (and separately powered) from the plant鈥檚 regular control system. This would stop problems with the control system preventing safe shutdowns.
Inflexible approach?
A different idea is being adopted in the US. The US Nuclear Regulatory Commission (NRC) is proposing a scheme called 鈥淔LEX鈥�, which will order nuclear power companies to around nuclear sites (and off site too). In theory, this means that there will always be enough generators, portable pumps, batteries, compressors and debris clearing equipment to 鈥渕itigate the effects of any extreme event鈥�, Stephen Kuczynski, CEO of , a nuclear power company headquartered in Birmingham, Alabama, told a meeting of the American Physical Society in Boston on 1 March.
But the NRC plan has not gone down well everywhere. 鈥淭he US industry is proposing to purchase lots of commercial-grade emergency equipment and distribute it in many different places, with the hope that there will be something left no matter what happens,鈥� says , a nuclear industry analyst with the Union of Concerned 杏吧原创s in Cambridge, Massachusetts.
His worry is that the emergency generators and pumps may end up in harm鈥檚 way, because every nuclear plant has its own topography and mix of likely threats, be they hurricanes, earthquakes, tornados, floods, tsunamis or a calamitous cocktail of these. What is needed, Lyman says, is for nuclear energy firms to undertake a detailed threat analysis for each plant with 鈥渁 set of severe accident scenarios鈥� computer-simulated exhaustively for each. That way, the safest sites for each cache of emergency equipment can be established.
It is unlikely that the US nuclear industry will be keen on that idea, says Lyman. The costs of computer simulations, emergency exercises to test the response of staff and new, secure buildings for emergency pumps and generators are high compared to the FLEX approach. As Charles Pardee, chief operating officer of power company Exelon in Chicago, Illinois, : 鈥淚t鈥檚 cheaper to buy three pumps than one and a heckuva big building to put it in.鈥�
