This particular reactor passed it’s 30 year mark in 2007, it’s a generation II PWR reactor. Ideally we would spend more time building generation III and generation III+’s which have passive safety built into them, but those tend to be blocked by nuclear activists. Any nuclear energy is inherently unsafe to some degree, so the government waits to react to power shortages and ends up using old reactors. This time it was a heat wave in South Korea that has spiked power demands.
Hiromitsu Ino, an emeritus professor of materials science at the University of Tokyo, says that Kori-1 is not safe to operate because the weld material in the pressure vessel has degraded. “Any 50 nuclear power plants in Japan are much better than Kori-1,” he says.
Il Soon Hwang, a nuclear scientist at Seoul National University, says that a thorough government investigation found that the pressure vessel is safe. However, he adds that a more democratic process is needed to get a consensus on the reactor from local residents.
“The most serious issue is that staff in the control room decided not to report the more than ten minutes of blackout and tried to hide this accident,” says Hwang, adding that Korea’s nuclear-safety authority hasn’t explained enough to residents about what is being done to ensure that such a dangerous situation will not be repeated.
South Korea currently has 23 nuclear reactors in operation and three more under construction; about 35% of the country’s electricity comes from nuclear power, according to the IAEA. When it was first opened, the Kori-1 reactor was given an operational lifespan of 30 years, which ran out in 2007. But in 2008, following an IAEA inspection, the reactor was declared safe for another 10 years. Link
The first safety culture issue was the failure of the maintenance worker to follow procedures or the direction of his managers while testing part of the diesel generator system. It was a subsequent mistake by him that caused the initial loss of grid connection. The automatic start-up of the diesel had been prevented by a problem with its air supply valve.
The major safety culture issue then came with a decision by the manager of Kori 1 not to report the loss of power and instead to actually delete records of the incident, despite this being classified as an emergency that must be reported to regulators no matter how quickly the situation is remedied. Yonhap News reported that the manager has been dismissed after admitting to this, and plant owner Korea Hydro and Nuclear Power (KHNP) is facing prosecution for violation of its legal responsibility to report a loss of power.
The incident itself was of low consequence thanks to the redundancy of power supplies and the overall good training and prompt action by staff; what troubles the NSSC is the apparent safety culture issues it may have brought to light. The NSSC’s report said a factor in the manager’s decision not to report was pressure to have a perfectly clean operational record because the reactor had recently been given a licence extension – the first time this had happened in South Korea.
Kori 1 is a 567 MWe pressurized water reactor that came into service in April 1978 and is licensed to operate until 2017. Link
- PWR reactors are very stable due to their tendency to produce less power as temperatures increase; this makes the reactor easier to operate from a stability standpoint.
- PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.
- PWRs can passively scram the reactor in the event that offsite power is lost to immediately stop the primary nuclear reaction. The control rods are held by electromagnets and fall by gravity when current is lost; full insertion safely shuts down the primary nuclear reaction.
- PWR technology is favoured by nations seeking to develop a nuclear navy, the compact reactors fit well in nuclear submarines and other nuclear ships.
- The coolant water must be highly pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure vessel and hence increases construction costs. The higher pressure can increase the consequences of a loss-of-coolant accident. The reactor pressure vessel is manufactured from ductile steel but, as the plant is operated, neutron flux from the reactor causes this steel to become less ductile. Eventually the ductility of the steel will reach limits determined by the applicable boiler and pressure vessel standards, and the pressure vessel must be repaired or replaced. This might not be practical or economic, and so determines the life of the plant.
- Following shutdown of the primary nuclear reaction, the fission products continue to generate decay heat at initially roughly 7% of full power level, which requires 1 to 3 years of water pumped cooling. If cooling fails during this post-shutdown period, the reactor can still overheat and meltdown. Upon loss of coolant the decay heat can raise the rods above 2200 degrees Celsius, whereupon the hot Zirconium alloy metal used for casing the nuclear fuel rods spontaneously explodes in contact with the cooling water or steam, which leads to the separation of water into its constituent elements (hydrogen and oxygen). In this event there is a high danger of hydrogen explosions, threatening structural damage and/or the exposure of highly radioactive stored fuel rods in the vicinity outside the plant in pools (approximately 15 tons of fuel is replenished each year to maintain normal PWR operation).
- Additional high pressure components such as reactor coolant pumps, pressurizer, steam generators, etc. are also needed. This also increases the capital cost and complexity of a PWR power plant.
- The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel); this can cause radioactive corrosion products to circulate in the primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products and adjust the boric acid concentration add significantly to the overall cost of the reactor and to radiation exposure. In one instance, this has resulted in severe corrosion to control rod drive mechanisms when the boric acid solution leaked through the seal between the mechanism itself and the primary system.
- Natural uranium is only 0.7% uranium-235, the isotope necessary for thermal reactors. This makes it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used, it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.