Fukushima Daiichi: US Nuclear Regulatory Commission assessment indicates serious challenges remain to stabilize plant

The New York Times has released details of a confidential assessment dated March 26 prepared by the US Nuclear Regulatory Commission's (NRC) Reactor Safety Team of the situation at the Fukushima Nuclear Power Plant.  NRC and Department of Energy experts have been in Japan since March 16.  At that time both the NRC and Department of Energy suggested that their assessment was that the situation was more serious than was thought.

The NRC team was assisted by the Japanese government and TEPCO. The report is based on the “most recent available data” from TEPCO, JAIF, the US Department of Energy, General Electric and EPRI, an independent electric power research foundation.

Issues

According to the New York Times, the NRC assessment and interviews with nuclear experts indicate that  Fukushima Daiichi is clearly not out of the woods yet. Very serious issues remain that need to be surmounted to stabilize the reactors.

• Increasing stresses placed on the containment structures as they fill with radioactive cooling water, make them more vulnerable to rupture in one of the aftershocks after the earthquake
• Possibility of explosions inside the containment structures due to the release of hydrogen and oxygen from seawater pumped into the reactors
• Semimolten fuel rods and salt buildup are impeding the flow of fresh water meant to cool the nuclear cores.
• Whether pouring water on nuclear fuel in the absence of functioning cooling systems can be sustained indefinitely. 
• Suggests that fragments or particles of nuclear fuel from spent fuel pools above the reactors were blown “up to one mile from the units,” and that pieces of highly radioactive material fell between two units and had to be “bulldozed over,” presumably to protect workers at the site.
• The ejection of nuclear material, which may have occurred during one of the earlier hydrogen explosions, may indicate more extensive damage to the extremely radioactive spent fuel pools than previously disclosed.

Recommendations

The NRC document makes some recommendations.

• Injecting nitrogen into the containment structures to purge them of hydrogen and oxygen, which could combine to produce explosions. [TEPCO has already begun injecting N₂ into the Unit 1 reactor vessel.]
• Continue adding boron to the cooling water to help prevent recriticality.

On the positive side, the document does not suggest that recriticality has happened since the reactors were shutdown immediately after the earthquake.  According to the New York Times article, there is also nothing in the data that suggests that recriticality is "an immediate likelihood".

Comments

[Dr.M.R.Iyer]

In the analysis report of March 15, TEPCO reported substantial amount Chlorine 38, a shortlived activation product of Chlorine in sea water, indicating the presence of neutrons in the reactors which in turn is possible only if there was a localized criticality. Otherwise how do you explain the presence of Chlorine 38?

[Geoff]

April 20 TEPCO has reevaluated its nucleotide analysis and reported that the concentration of Cl 38 is below detectable levels.
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110420e11.pdf