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We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.

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Polarized 3 He produced by spin-exchange optical pumping ͑SEOP͒ has potential as a neutron spin filter for polarization and polarization analysis in many neutron-scattering and neutron particle physics applications. The advantage of the SEOP method is its suitability for providing continuous stable polarization over the course of long experiments. However, we have discovered that exposure to high neutron flux leads to additional strong relaxation mechanisms in the optically polarized alkali-metal vapor used to polarize the 3 He. At a neutron flux density of 4.7ϫ 10 9 cm −2 s −1 , the alkali-metal relaxation rate increased from 100 to 1000 s −1 leading to reduced alkali-metal polarization. Other effects such as time dependence and gas composition dependence were explored to help understand the processes. In this paper we discuss our observations and present possible solutions for practical use of SEOP as a neutron spin filter for high-flux density applications.

The incident, which happened in the Swedish nuclear power plant Barsebäck 2 on 28th of July, 1992 brought about a re-evaluation of the design requirements for the control of loss-of-coolant accidents (LOCA) involving the release of insulation material that were valid in Germany up to that time. The current paper provides an overview on the aspects that need to be taken into consideration for analyses as well as a description of the important boundary conditions. In giving details of the analyses, the actual state is also described. It is also illustrated that sufficient examinations and tests have been performed for the German PWR plants and corresponding measures have been taken providing a sufficient basis for a final positive evaluation of the issue “LOCA involving release of insulation material”.