Application of gamma densitometer for measurement of void fraction in liquid hydrogen moderator of HANARO cold neutron source Myong-Seop Kim à , Jungwoon Choi, Gwang-Min Sun, Kye-Hong Lee Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong, Daejeon 305353, Republic of Korea article info Article history: Received 11 September 2008 Received in revised form 14 January 2009 Accepted 3 February 2009 PACS: 28.50.Dr 29.30.Kv 92.60.hv 07.20.Mc Keywords: Gamma densitometer Void fraction HANARO Cold neutron source Liquid hydrogen abstract The void fraction in the liquid hydrogen used for the moderator of the HANARO cold neutron source (CNS) was measured by using a gamma densitometer technique. A mock-up of the HANARO CNS facility with an electric heating system as the heat source instead of radiations was constructed. The photon transmissions through the hydrogen moderator were simulated to search for an optimum experimental condition. From the simulation, it was confirmed that Am-241 was suitable for the measurement of the void fraction in the liquid hydrogen medium. A gamma densitometer using the Am-241 gamma-ray source was designed and installed at the mock-up of the CNS. The attenuation of 59.5 keV gamma-rays from the Am-241 through the hydrogen medium was measured by using an HPGe detector. The void fraction was determined using the amount of the gamma-ray attenuation. The void fractions in the hydrogen moderator were measured for stable thermo-siphon loops with several electric heat loads applied to the moderator cell of the CNS mock-up. The longitudinal distribution of the void fraction inside the moderator cell was also determined. The void fraction measured at a heat load of 720 W had values of 8–41% depending on the height from the bottom of the moderator cell. The overall void fraction was obtained by volume-weighted averaging of its longitudinal distribution. The void fraction at the nuclear heating power expected at the normal operation condition of the HANARO CNS facility was determined to be about 20%. The large uncertainty was expected in the void fraction determination by a gamma densitometer for the liquid hydrogen medium with the void fraction less than 10%. When the void fraction of the liquid hydrogen was near 20%, the uncertainty in the void fraction determination by using a gamma densitometer became relatively small, and it was regarded as an acceptable level. The measurements for the void fraction will be very useful for the design and operation of the HANARO CNS. & 2009 Elsevier B.V. All rights reserved. 1. Introduction The design and installation of a cold neutron source (CNS) facility for HANARO, a 30 MW research reactor, are in progress. The in-pool assembly (IPA) of HANARO CNS facility consists of a two- phase thermo-siphon loop and a vacuum chamber. Liquid hydrogen has been selected as the moderator to slow down neutrons in the moderator cell of the IPA. The evaporated hydrogen from the moderator cell by the nuclear heating flows up along the transfer tube. It is liquefied in the heat exchanger, and the liquid hydrogen flows back down to the moderator cell. The moderator cell, the heat exchanger, and the piping are put into the vacuum chamber for the thermal insulation which will be loaded in the CN hole of the HANARO reflector tank in the reactor pool. In order to validate the design of the IPA, the thermo-siphon mock-up test was conducted. A full scale mock-up of the HANARO CNS facility was constructed. It contained an electric heating system as the heat source instead of radiations to simulate the nuclear heating applied to the moderator cell in the reactor. The design validation tests using the mock-up included a vacuum test, a heat penetration estimation, a thermo-siphon confirmation, and a void fraction measurement. Also, the dynamic properties of the thermo-siphon and the effects of the malfunction of the refrigerator and a vacuum loss in the IPA were investigated by using the mock-up to simulate the abnormal events of the facility. Since the HANARO CNS adopted a two-phase thermo-siphon loop driven by the natural circulation, boiling of the liquid hydrogen by radiation heating in the moderator cell was allowed. Thus, the void fraction in the hydrogen moderator inside the moderator cell is one of the important parameters for the opera- tion of the cold neutron facility since it affects the moderation capability and the stability of a CNS [1]. Therefore, one of the major purposes of the mock-up test was to provide the informa- tion related to the void fraction in the two-phase hydrogen moderator to confirm the performance of the HANARO cold neutron facility. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.02.008 à Corresponding author. Tel.: +82 42 868 8666; fax: +82 42 8610209. E-mail address: mskim@kaeri.re.kr (M.-S. Kim). Physica B 404 (2009) 1695–1700