Japan Nuclear: NRC threat assessment of Fukushima risks

The 15 page document provides recommendations on how to mitigate them

The New York Times reports today that a March 26 threat assessment by the Reactor Safety Team of the Nuclear Regulatory Commission (NRC) warns that the crippled nuclear plant faces fresh threats that could persist indefinitely.

The newspaper reports that in some cases risks are expected to increase as a result of the very measures being taken to keep the plant stable, according to a confidential assessment prepared by the Nuclear Regulatory Commission.

This blog has obtained the NRC assessment from a source outside the government Click here for a copy in PDF format.

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Note that because of the way the document was scanned it appears to be upside down. Just print out the 15 page report to read it right side up. If you are using Adobe Reader, you can rotate the image to right side up.

New threats for old

The New York Times reports that among the new threats that were cited in the assessment are new levels of stress on the containment structures as they fill with radioactive cooling water. The NRC assessment says that in a worst case scenario, the reactor primary containment structures could break open from earthquake aftershocks.

While the assessment notes that it impossible to inspect the reactor pressure vessels directly, it does not speculate that there is a breach of any of them at the time the assessment was completed (March 26).

U.S. Rep. Ed Markey is claiming in a press release April 6 that the reactor pressure vessel at Unit 1 has been breached and that melted fuel assemblies are on the floor of the primary containment building. He cites the NRC as the source of his information.

Markey said in his press statement, ?The cores of at least two of the Japanese reactors are severely damaged. I have been informed by the Nuclear Regulatory Commission that the core of Unit Two has gotten so hot that part of it has probably melted through the reactor pressure vessel.?

The Wall Street Journal reported April 6 that the Nuclear Regulatory Commission disputed Markey's claim.

Martin Virgilio, deputy executive director for reactor and preparedness programs at NRC, said the regulator believes there was significant fuel damage in all three reactors and the four spent fuel pools.

"But we don't believe at this point in time that that core has left the vessel," he said. "That's not in the situation report that we have from the team in Japan, and that's as of this morning."

Update: On April 7 the New York Times in new coverage of the issue indicated there were several "speculative" scenarios of core breach for unit 2.

Control of Hydrogen

The NRC document reviewed by the New York Times also cites the possibility of new explosions inside the containment structures due to the release of hydrogen and oxygen from seawater pumped into the reactors. According to the newspaper, the document offers new details on how semi-molten fuel rods and salt buildup are impeding the flow of fresh water meant to cool the nuclear cores.

Water cooling to continue

According to the newspaper, experts have said the Japanese need to continue to keep the fuel cool for months.

The steps recommended by the NRC include injecting nitrogen, an inert gas, into the containment structures in an attempt to purge them of hydrogen and oxygen, which could combine to produce new explosions.

On April 6, the Tokyo Electric Power Company, which owns the plant, said it was preparing to inject nitrogen into one of the reactor containment vessels.

No new criticality incident likely

The document also recommends that engineers continue adding boron to cooling water to help prevent the cores from restarting the nuclear reaction, a process known as criticality.

According to the New York Times, the engineers who prepared the document do not believe that a resumption of criticality is an immediate likelihood, Neil Wilmshurst, vice president of the nuclear sector at the Electric Power Research Institute, when contacted about the document, told the newspaper. ?I have seen no data to suggest that there is criticality ongoing.?

The newspaper reports that the document was prepared for the commission?s Reactor Safety Team, which is assisting the Japanese government and the Tokyo Electric Power Company. It says it is based on the ?most recent available data? from numerous Japanese and American organizations, including the electric power company, the Japan Atomic Industrial Forum, the United States Department of Energy, General Electric and the Electric Power Research Institute, an independent, nonprofit group.

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