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The new cold source

(Last modified June 2011)


A new liquid hydrogen cold source has been built for placement in the beam port BT-9 to provide an intense beam of long-wavelength neutrons for the Multi-Analyzer Crystal Spectrometer (MACS). MACS must be relocated to allow the installation of additional neutron guides. Optimization of the new source balanced the requirements of MACS and the engineering constraints arising from the small size of BT-9. MACS features a doubly focusing monochromator and it is designed to exploit a large-area beam. The source was designed to provide the largest possible number of neutrons from the maximum diameter geometry, so the LH2 vessel is 110 mm ID by 45 mm thick (see Figure 1). It will have about twice the brightness of the existing cold source.
cold source cutaway view
FIGURE 1. New cryostat assembly installed in BT-9.
cold source cutaway view
FIGURE 2. Photograph of the BT-9 cryostat assembly (left) attached to the BT-9 shield plug.
Monte Carlo transport code calculations (MCNP) showed that the heat load on the new source, 160 W, is sufficiently small that both the existing source and the new source can be cooled using the existing refrigerator. Liquid hydrogen will be supplied by a naturally circulating thermosiphon from a condenser mounted 2 m above BT-9. Full-scale mockups were used to demonstrate that the thermosiphon can remove the heat generated in the source and that the void fraction in the boiling liquid will be about 13%. (These thermal-hydraulic tests used the refrigerant R-134a to simulate the flow of LH2 with the expected vapor flow rate and liquid-to-vapor density ratio.) A separate, 0.5 m3 ballast tank will be installed so that the new hydrogen system will be independent of the existing cold source. It is expected that the new source will operate at 1 bar (cold) and be loaded with 180 g of hydrogen to a pressure of 4 bar (warm).

Hydrogen safety is assured by protecting components from physical hazards, minimizing gas handling, and having at least two monitored barriers preventing the mixing of air and hydrogen. The system is passively safe; the LH2 simply expands into its ballast tank if the refrigerator fails.

The major components of the BT-9 cold source are fabricated and ready for installation. The cryostat assembly (see Figure 2) is scheduled for installation in June 2011. The system should be ready for tests in December 2011.

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Last modified 19-October-2011 by website owner: NCNR (attn: John Copley)