Meanwhile in Japan..

Paper; Gamma Radiation Damage To Fukushima Units 1- 4 Spent Fuel Pools
[link to www.simplyinfo.org]

[snip]

CONCLUSION

We have learned many things about our research on gamma heating or radiation damage that can be caused by the fuel racks being so close to the spent fuel pool (SFP 4) walls. This review has raised a host of questions which, at the time of this writing are unanswered as follows:

Analysis to support moving the racks containing the fuel assemblies so close to the wall and adjacent to the concrete (1/2 inch) has not been able to be found. Not even in the documents to support a more dense SFP fuel loading.
Operation in an unanalyzed condition at NPPs (nuclear power plants) must be evaluated to support the continued operation which must be done at Japan’s NPPs.

SFP 4 has had the total fuel loading from the last period of operation centrally placed in the pool. To our knowledge this has not been done at any other reactors and introduces a critical element of trying to complete assessments on the severity of heating and intense gamma radiation induced on the wall in that location
At this point our assumption is that concrete degradation is occurring in the SFP, concrete overheating, spalling, chipping and chemical changes are ongoing.

The annular space between the concrete and the liner is being impacted by potential hydrogen, Co2, hydrogen peroxide or other gas concentrations which produce new challenges.
In addition to the gamma fields there are also spontaneous neutrons which are generated and may escape the racks and interact with the steel liner or concrete structure.

We have established our choice of the boundary conditions within the concrete around which more detailed analysis can be done. The limit is 2 x10e20 rads of dose for gamma radiation. The estimated radiation level of a fuel assembly which has been discharged from the reactor core is on the order of 1 x 10e6Rem/Hr.
We must take into consideration all of the accumulated radiation exposure that has been received since the beginning of operation for unit 4.