A Study of the Neutron Noise Transmission Characteristics of Nonmultiplying Media in Liquid-Metal Fast Breeder Reactors by Neutron Wave Propagation Technique

Abstract

In this paper, an attempt has been made to investigate the noise transmission characteristics of nonmultiplying media of liquid-metal fast breeder reactors (LMFBRs) and study its implications on the detection of malfunctions in LMFBR cores by using out-of-core detectors and noise analysis methods. Neutron wave propagation technique has been used to study the problem by employing different approximations such as infinite and finite medium, one- and two-group diffusion theory, and multiregion and multigroup diffusion theory approximations. It has been found that reactor core noise will be transmitted to the out-of-core detectors with equal attenuation for all frequencies, ω < (ωΣt)min where Υ is the speed of neutrons and Σt is the total macroscopic removal cross section of the medium. For normal in-reactor vessel nonmultiplying media, (ΥΣt)min is of the order of 1 kHz. However, for materials like graphite if used as a moderator surrounding the out-of-core detectors, the limit (ΥΣt)min can be as low as 10 Hz. Reactor noise of malfunctions due to thermal events inside the reactor core such as sodium boiling lies in the frequency range of 2 to 15 Hz for integral boiling and goes up to 1 kHz for local boiling. Noise due to mechanical events is also a high frequency phenomenon. Therefore for detecting the malfunctions due to thermal and mechanical events in LMFBR cores by out-of-core detectors and noise analysis methods, one has to keep in mind that for moderating materials like graphite used in the surroundings of detectors, a band limited noise in reactor may be transmitted to detector locations in a distorted way and since high frequency noise is likely to be attenuated more, it will pose a problem in detecting the malfunction in its incipient stage.