Information from Oeko-Institut on “cold shutdown” in Fukushima

16.12.2011

From the viewpoint of Oeko-Institut, use of the term “cold shutdown” in the context of the destroyed reactor in Fukushima is misleading. The term “cold shutdown” is usually used to describe a reactor having been cooled down and depressurized as part of a regular shutdown (i.e. the termination of the chain reaction). But in this state the fuel rods still need to be cooled and an (active) residual heat removal chain still needs to be running – now, however, with a longer time lag (i.e. with longer periods remaining in the event of loss of cooling, before overheating and radioactive releases into the atmosphere are to be expected).

“Cold shutdown” is a term used in the context of reactor operation; it presupposes that the cooling system is intact, a closed cooling circuit – reactor pressure vessel, pipelines, etc. – is intact and the reactor shutdown is carried out as normal. This is not the case in Fukushima because of the extent of destruction there. It was only with great difficulty that cooling water could be fed in to the reactor and removed again as part of provisional measures.

The statement made by Japan could be politically motivated. From a technical viewpoint, it can be interpreted to mean that conditions have now been reached whereby the heat from the reactor can be dissipated steadily and at comparatively low temperatures. This is an important partial step to restorung a safe status, but certainly does not mean that the cooling circuit is of the usual quality, “normal” operation has been achieved or there is no longer any risk deriving from the plant.

Condition of reactor core still not known

The condition of the reactor core and inside the containment is generally still largely unknown, with the result that the associated risks cannot yet be fully determined. It is also questionable whether current operational status is reliable in the longer term. This applies particularly in the event of new impacts – for example, as a result of an earthquake – which could cause additional damage to initially damaged structures and systems.

The reactor pressure vessels, containment and building structures – including the foundations, as the case may be – are still largely destroyed and are thus not watertight. There could be further leakages or radioactive releases; however, the release due to vapour generated from the cooling water will probably decrease under the current lower temperatures.

In addition to radioactive inventory in the reactor core, there are large quantities of contaminated cooling water within the reactor site; structures and systems are highly contaminated and involve a risk potential. In view of this continuing risk potential, the extent of the destruction, and the above-mentioned uncertainties, it is not appropriate to talk of a “controlled” condition in this context.

Contact

Dr. Christoph Pistner
Researcher
Nuclear Engineering & Facility Safety
Oeko-Institut, Darmstadt office
E-Mail Contact

Stephan Kurth
Researcher
Nuclear Engineering & Facility Safety
Oeko-Institut, Darmstadt office
E-Mail Contact

Beate Kallenbach-Herbert
Head of Division
Nuclear Engineering & Facility Safety
Oeko-Institut, Darmstadt office
E-Mail Contact