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.
FIGURE 1. New cryostat assembly installed in BT-9.
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FIGURE 2. Photograph of the BT-9 cryostat assembly (left) attached to
the BT-9 shield plug.
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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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