Safety first in decommissioning work / Speed no longer top priority at N-plant


With the government’s approval of a revised road map for the decommissioning of nuclear reactors at the Fukushima No. 1 nuclear power plant, the government and Tokyo Electric Power Co. are shifting to a policy focused on “reducing risks” rather than “speedy operations.”
On Friday, the government decided on a revised road map for decommissioning the nuclear reactors that reflects the current circumstances surrounding the nuclear plant four years after the outbreak of the crisis, following the Great East Japan Earthquake in 2011 that forced the government to take urgent measures.
The schedule includes some practical content such as delays to the start of removing spent nuclear fuel rods that are stored in fuel pools at the Nos. 1 to 3 reactors.

Risk assessment
“We’ll continue facing this unprecedented challenge and proceed with decommissioning work by giving utmost consideration to safety,” TEPCO President Naomi Hirose said Friday during a meeting of concerned Cabinet ministers at which the revised schedule was endorsed.
Eschewing an emphasis on speed, the government has shifted to a policy that stresses the reduction of risks that could negatively impact people and the environment.
The shift stems from a review of the government’s previous commitment to follow a schedule that put excessive pressure on workers at the site, leading to increased cases of problems and accidents that eventually resulted in delayed operations.
There were initially about 3,000 plant workers after the outbreak of the crisis. Now, there are around 7,000 involved in such projects as the construction of additional tanks to store radioactive contaminated water and installing subterranean ice walls around reactor buildings to block groundwater from flowing in.
Work-related accidents are on the rise. In January, operations were suspended for two weeks following fatal accidents at both the Fukushima No. 1 and No. 2 nuclear power plants.
Based on the policy shift, the road map has sorted operations into several categories ordered by priority depending on their risk assessments. For example, contaminated water disposal is deemed a high priority because of leakage risks, meaning measures should be taken immediately.
In terms of the most difficult task — removing melted fuel debris — the road map stipulates that a cautious stance be taken out of concern that “the risk of failure would actually increase if [operations] are hastily conducted.”
“It’s important to classify the risks since decommissioning work involves a range of procedures,” said Hiroaki Yoshii, a professor emeritus at Tokyo Keizai University.
Identifying effective methods
Preparation work such as debris removal is expected to be a lengthy process, prompting the road map to indicate that spent fuel extraction from the pools at the three reactors will be delayed by from four to 40 months.
But the extent to which the delays would affect the overall timetable of completing decommissioning work, projected to take 30 to 40 years, remains unclear. The outline of the overall timeline remained unchanged.
The extraction of melted fuel from the containment vessels is expected to start in 2021. The operation faces an unprecedented challenge involving the use of a robot arm, however, meaning deciding on the best extraction method for each reactor will take about two years.
One option is a “submersion method” in which the vessel is submerged in water to extract fuel debris. Other ways include a dry approach that doesn’t involve water.
The submersion method has the advantage of using water as a radiation shield, but potential leak points need to be repaired. Containment vessels would also need to be tested for their ability to withstand earthquakes when filled with water.
A dry method would not require the leaks to be stopped, but measures would be needed to control emissions from radioactive substances and shield workers from radiation.
The government and TEPCO plan to deploy robots to investigate the position and state of melted fuel in the Nos. 1 and 2 reactors after summer.
“If we can learn about the conditions of the fuel, we can develop an efficient retrieval method. Operations in the next few years will be important,” said Hajimu Yamana, vice president of the Nuclear Damage Compensation and Decommissioning Facilitation Corporation.
Tainted water still flowing in
Radioactive contaminated water generated from groundwater flowing into the plant continues to stand at 300 tons a day. The flow needs to be blocked before melted fuel can be extracted.
The road map also outlines a new goal of reducing groundwater flow to less than 100 tons a day by fiscal 2016 as part of efforts to complete contaminated water disposal.
To achieve the target, contaminated groundwater pumped up from areas enclosed by ice walls and wells called “subdrain pits” must be purified and directed to the ocean — but the effectiveness of the unprecedented scale of the ice walls remain unknown.
The government and TEPCO have also failed to obtain consent over the subdrain pit plan from local governments and residents after rainwater contaminated with radioactive material was found to have escaped into the ocean through a trench at the power plant in February.
Source: Yomiuri
http://the-japan-news.com/news/article/0002218963

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