Long-term stability of Sm2Co17-type magnets for control rod drive mechanism (CRDM) in a nuclear reactor

For an application of Sm₂Co₁₇-type magnets for a driving motor of control rod drive mechanism (CRDM) placed inside a nuclear reactor vessel, long-term stabilities of Sm(Co_0.61Fe_0.28Cu_0.08Ni_0.01 Zr_0.02)_7.3 magnets were evaluated under the severe conditions. Initial magnetic properties of the specimens at room temperature were: B_r=1.03 T, H_cJ=1400 kA/m and (BH)_max=207 kJ/m³. Irreversible losses of open-circuit remanent flux of the specimens exposed for 19000 hours in 1 atm Ar atmosphere were 5~10% at the temperature (573~603 K) and the operating point (permeance coefficient of 1.7~2.4) of the actual driving motor application. Large fraction of the irreversible loss is attributed to permanent flux loss due to oxidation of the specimen. Losses due to thermal fluctuation aftereffect of these specimens are estimated to be less than 5%. Multi-layer coating of Ni, Cu, Ni and Au was found to be effective to protect the magnets from the oxidation. The coated specimens exhibited a small permanent loss value of 0.5% after the exposure to 120 atm water for 2000 hours at 613 K