4th International Technical Meeting on Small Reactors - 2016 Nov. 02-04

Presented at:
4th International Technical Meeting on Small Reactors
2016 Nov. 02-04
Ottawa, ON Canada
Session Title:
Parallel Session F: Updates on Research Reactors

A. Röhrmoser (TU Munich)


The German multi-purpose neutron source FRM II is a 20 MW steady state reactor based on a very compact fuel element centered in a huge heavy water reflector tank. For reactor restart FRM II features two sources, the primary and fadingboth sources are located in the same small tube, which penetrates in the HW tank vertically at a radius of 42.5 cm, so somewhat aside from the fuel element. At restart the sources feed the fuel element and the three wide range detectors, which are about 2m away from the fuel element and rather Cf-252 and the secondary SbBe-source, which is rechargeable during operation; asymmetrically from the tube with the restart neutron sources. And there exist clear minimum rate expectations for all three signals before allowing any move¬ment of the low lying control rod at restart. Our detailed calculations had shown, that after a longer shut down of nearly one year in 2011, there was still a huge contribution from the Cf-252 primary source, the secondary one alone wouldn’t have had the necessary n-source strength after about five times the half-value period of Sb-124. The author presents the idea of inserting a very small ring of Sb in the central hole of the existing Be cylinder as part of the control and moderator follower rod. This measure would ensue huge benefits; the new Sb-source would be recharged automatically during operation and with more than an order of magnitude more efficiency, allowing very long reactor stops in future without an expensive primary one; and the cylindrical symmetry of this measure ensures an equable signal answer at the three detectors, what is also welcome.

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