Residual fuel in Fukushima reactors 'is draining away'

7 December 2011


A technical analysis by plant operator Tepco has concluded that most of the residual fuel in Fukushima Daiichi unit 1 has melted out of the reactor pressure vessel (RPV) and into the primary containment vessel (PCV). It also concluded that fuel has melted in units 2 & 3, but has mostly remained within the RPVs.

In the case of unit 1, Tepco now estimates that most of the fuel has drained, via the bottom-mounted control rod tubes or instrumentation penetrations, into the concrete primary containment vessel. It reasons that the unit's decay heat before seawater injection 'significantly exceeded' the RPV water and materials' heat absorption capacity; this reasoning also explains why the RPV temperature has been low from an early stage. Based on what Tepco calls 'realistic assumptions', and PCV gas analysis results, the company estimates that the corium has eroded the 2m-thick PCV floor by about 70cm, but is now assumed to have stopped.

In the case of units 2&3, lesser amounts of fuel have dropped out of the RPV and into the PCV, because decay heat was less than the total heat absorption capacity of water in the core, and because of RPV temperature trends after water injection restarted.

• Tepco has explained how it calculates the total about of radiation currently being emitted by the damaged reactors: 60 million Bq/hr.

At unit 1, the amount released is combined from two numbers: the product of radioactive dust concentration around the top of the reactor building time the assumed amount of steam generation, plus the product of the dust concentration near the equipment hatch, times the air flow rate (total: 10 million Bq/hr). Deduced from that amount is the effectiveness of the exhaust filtration system. At unit 2, the amount is the dust concentration at the blowout panel times the air flow rate. Air flow in unit 2 is caused by a air from leaks in from the truck bay floor and by warm vapour from decay heat rising (total: 10 million Bq/hr). The calculation method for unit 3 is the same as at unit 1 (unit 3 total: 40 million Bq/hr). From these figures, Tepco concludes that this discharge equates to a personal exposure of 0.1 mSv/yr at the site boundary, assuming average climactic conditions, and including external atmospheric exposure, external ground-based exposure and internal exposure through inhalation.

• Tepco has changed the method of nitrogen gas injection in units 1-3 so that it can reach the RPV as well as the PCV. All three systems will inject nitrogen into the RPV via the head spray line, in addition to their current nitrogen injection arrangements into the PCV by various means. The unit 1 nitrogen supply will be via a hose attachment point in series to PCV injection via the AC line; the unit 2&3 RPV line will be attached through jigs to test valves on the instrument rack.

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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