A Tepco remote-controlled robot has reached the fifth floor level of the Fukushima Daiichi unit 2 reactor, has taken photos and video, and has surveyed radiation levels there. And plant operator Tepco has released more information about its decontamination plans.
The Quince 2 robot found that average atmospheric radioactivity levels (caesium-134/-137) are only slightly (1.3-1.5 times) above the maximum legal airborne radiation limits for workers. Those measurements—3x10^-3 for caesium-134, 3.9x10^-3 for caesium-137—are expressed in Bq/cm3. The robot also measured the atmospheric dose rate in mSv/hr, and found, not surprisingly, the highest rates (220 mSv/hr) near the reactor well itself. Away from the the well, the rate declined to about 60 mSv/hr.
The robot was able to climb the stairs to the top floor, where it encountered radiation levels of about 20 mSv/hr. That the robot reached the fifth floor at all was a technical success, since the first Quince unit snagged up on the third floor of unit 2 during an attempt several months ago, and was abandoned. There were barriers in place that prevented the robot from reaching the side of the spent fuel pool.
• In other developments – Tepco has published more information about its plan to remove fuel debris, a process considered to be the most complex aspects of decommissioning Fukushima Daiichi units 1-4. The information comes from a 24 February government workshop.
The first step, which is forecast to take perhaps six or seven years, is decommissiong of the reactor building. In order to easily access primary containment vessels (PCVs), work areas will be decontaminated by high-pressure washing and coating, to be carried out by user-operated and remote-control devices.
The second step, forecast to take about two years and to begin in 2013, will be inspections of PCVs to discover where the PCVs are leaking. Inspections will be both visual and using dose measurement.
The third step, forecast to take less than a year and to start in about 2016, will be to repair the PCV leakage points and stop the leaks. This work will be complicated by high radioactive dose, and by flowing water. The lowest areas will be repaired first.
The fourth step, forecast to take a little less than a year, will be to flood the lower part of the PCV. Once the leaks are repaired, the sources of water will be switched from the accumulated water in the turbine buildings to the reactor building torus. The priority is to establish a robust boundary at the bottom of the PCV.
The fifth step, forecast to take about 18 months, will be to perform PCV inspection and sampling, to ascertain the distribution of fuel debris. The work will be challenged by highly radioactive conditions and uncertainty about working conditions, particularly water clarity and the existence of debris.
The sixth step, expected to take about a year, will be to repair the upper parts of the PCV with user-controlled and remote-controlled devices.
The seventh step, also expected to take about a year, will be to flood the entire PCV and RPV with enough water to ensure shielding, and then to remove the RPV upper cover.
The eighth step (about 18 months) will be to sample and inspect the RPV to ascertain the state of the fuel debris and the RPV internals. A telescoping arm mounted on a gantry travelling on rails above the RPV will lower tools down to the fuel debris, including cameras, cutters, drills, grippers, and suction devices.
The ninth step in the process, to remove the fuel debrist is expected to last 10-15 years. The same telescoping gantry will pick up fuel debris and load it into canisters. Tepco notes that R&D may be a critical issue here, since the other major occasion on which fuel debris had to removed – at Three Mile Island I – it was not removed from inside the PCV. Tepco believes it is in a position to develop more sophisticated technologies and methods than were employed at TMI.
Tepco also notes that the working environment inside the reactors is extremely adverse. Scattered debris blocks access; some stairwells are impassable. There are differences in the levels of different floors. It is hot and humid, and many areas are highly radioactive. There are many different types of areas to decontaminate, and in addition both water and air need decontaminating. Investigation work will need to take place in contaminated water, and in narrow places, and with difficult access. Despite the difficulty, some delicate work is best carried out by manual labour in short shifts. Remote operation requires relays because of the distance between the machine and the operator.
Regular updates may be found on the website of our sister journal, Nuclear Engineering International, reachable on <www.neimagazine.com> or by clicking on the link in the navigation bar to the left of this page.
The following sites are also posting continuing updates:
<http://www.tepco.co.jp/en/index-e.html>
<http://www.iaea.org/>
<http://www.jaif.or.jp/english/>
<http://bravenewclimate.com/2011/03/15/tepco-reactor-status/>
<http://www.world-nuclear-news.org/>
<http://www.kantei.go.jp/foreign/index-e.html>
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