9 Mar (NucNet): Japan’s nuclear regulator has outlined measures that need to be taken for the reduction of mid-term risks at the Fukushima-Daiichi nuclear station, which was destroyed by an earthquake and tsunami four years ago.
In a document published online, the Nuclear Regulatory Authority (NRA) says that one of the key issues remaining at the facility is dealing with contaminated water that has resulted from water being used to cool the reactors.
The document also gives target dates for removing the remaining spent nuclear fuel and understanding the situation inside damaged facilities.
According to the NRA, the effective radiation dose at the Fukushima-Daiichi nuclear station site boundary is now in line with the natural background radiation level in Japan of around two to five millisievert a year (mSv/yr), with the country’s nuclear regulator aiming to maintain these levels, or lower them, throughout the decommissioning process.
The aim is to manage the effective offsite dose during the decommissioning process by keeping it to an average of 0.2 microsievert per hour (µSv/hr) or less in 2015. This will be done by continuous radiation monitoring and by treating high-radioactive contaminated water. The target for 2016 onwards is an average of 0.1 µSv/hr or less, the document says.
Within 24 hours of the March 2011 accident, radiation dose-rate levels at the site boundary showed more than 1,000 times the normal value of 0.07 µSv/hr or less.
There was a measured peak value of 1,015 µSv/hr at the most exposed point during the first venting of Unit 1 on 13 March. In the days after the accident, the average dose rate reached a range of five to 50 µSv/hr.
For comparison, in most countries, the natural background radiation dose-rate level is in the range of 0.2 to 0.6 µSv/hr (including the natural radon background radiation in buildings), or about two to five mSv/yr.
The NRA document outlines other measures for the reduction of mid-term risks at the site, including dealing with contaminated water, beginning the operation of incineration plants for “miscellaneous radioactive waste” and, in the event of another tsunami, preventing the outflow into the sea of stagnant, contaminated water.
On the issue of contaminated water, which has been a major problem for station operator Tokyo Electric Power Company (Tepco) since shortly after the accident, the NRA document says targets in 2015 include the continued treatment of highly radioactive water being stored on tanks onsite. The radioactive water mainly stems from water that is being pumped into the reactors to keep them cool.
A new high-performance system for treatment of the contaminated water came online last year. The system, which cleans radionuclides from the water being used to cool the reactors, is a third version of the Alps (advanced liquid processing system) water treatment system, two versions of which are already in place at Fukushima-Daiichi.
The new system will reduce radioactive strontium to non-detectable levels, which will reduce any risk in the event of a leak, reduce offsite radiation doses, and reduce the amount of radiation exposure of workers on patrol, Tepco said.
Another target outlined in the document is to prevent the outflow of contaminated groundwater into the sea by completing an underground impermeable wall on the ocean side of the site. This will be completed in 2015.
A longer-term target, by 2017, is to reduce the volume of contaminated water stored in onsite tanks by treating it and discharging it into to the sea.
Spent fuel has already been removed from Unit 4 at the station, but still needs to be removed from the spent fuel pools at Unit 3 and Unit 1. According to the NRA, the target is to complete spent fuel removal from the Unit-3 in 2017 and Unit-1 pools in 2019.
Experts also need to understand the situation inside damaged facilities at the station. By 2017 Tepco is planning to analyse contamination levels inside the reactor buildings and during 2019 to see inside primary containment vessels and reactor pressure vessels using remote-controlled robots, which Tepco is developing with specialised manufacturers. The aim is to assess the situation in detail in order to prepare for removal of the highly radioactive destroyed fuel elements and melted fuel debris, the so-called corium.
Background
The accident at the Fukushima-Daiichi nuclear plant in northeast Japan happened after the facility was struck by an earthquake and tsunami on 11 March 2011. This resulted in an extended loss of onsite electrical power supplies followed by a series of equipment failures, fuel meltdowns, and releases of radioactive materials.
The station comprised six separate boiling water reactors operated by Tepco. At the time of the earthquake, Unit 4 had been de-fuelled while Units 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, and emergency generators came online to supply control electronics and coolant systems.
The tsunami broke the reactors' connection to the power grid and resulted in flooding of the rooms containing the emergency generators. Those generators stopped working and the pumps that circulated coolant water in the reactors ceased to work, causing the reactors to begin to overheat. The flooding and earthquake damage hindered external assistance.
Radioactive material was released from the containment vessels of Units 1-3 as the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and associated uncontrolled events.
Further reading: NucNet Fukushima-Daiichi Fact File (for NucNet subscribers) http://bit.ly/1EdwHyl
The NRA document is online: http://bit.ly/1B4QZYA
