ORNL-4191 - the Molten Salt Energy Technologies Web Site

N,, CH,, and CO are separated on the molecular
sieves column, and the H,O and CO, are separated
on the Porapak column. The chromatograph will
require separate compartments controlled at dif-
ferent constant temperatures. The sample valve,
columns, and detectors may require different tem-
peratures for optimum operation.
The requirement that determination of impurities
be made at sub-, low-, and also at higher-ppm
levels necessitates the use of two different types
of systems or detectors. A helium breakdown
detector can be operated in either the high- and
low-sensitivity modes or in the high-sensitivity
mode in conjunction with a thermal conductivity
detector.
The chromatograph which is used for analyzing
the reactor off-gas will be subjected to radiation
from samples at the I-curie/cc level. This re-
quires a system free of organic construction
maferials. An all-metal sampling valve and a
detectoi unaffected by this radiation are necessary.
The organic Porapak column cannot be used at
this level of radiation. This eliminated the pos-
sibility of H,O analysis on these samples. A
silica gel column will be used to resolve CO,.
The problem of radioactive samples and the
transmittal of gas samples containing ppm levels
a€ H,O from the source to the detector require the
use of a heated all-metal sampling valve. Such
a valve has been designed and constructed. The
valve is similar to the conventional Phillips six-
way pneumatically actuated diaphragm valve (Fig.
14.3). The Teflon diaphragm has been replaced
by a 1-mil-thick Inconel diaphragm which contacts
and seals the redesigned entry orifices (Fig. 14.4).
With this metal diaphragm a spacer is required to
allow gas flow without excessive backpressure.
Spacers made from 2-mil gold were necessary to
give a leak-tight seal between the diaphragm and
the valve faces. The gold was annealed at 1500°F
and, after installation in the valve, subjected to
32,000 psi pressure. Pressure maintained by the
assembly screws was sufficient for retaining this
seal.
The choice of detectors which are sensitive to
sub-ppm levels of permanent gases is limited to
the helium ionization types. A helium breakdown
173
voltage detector was used on the MrR Capsule
Test Facility (test 47-63' and was not aEEected
by the radiation present in the gas samples. 'This
may be explained by the reldively high current
levels (microamperes) used with this type of
detector. A constant-current power supply is used
with this detector, and a decrease in breakdown
voltage, rather than an increase in ionization current,
indicates an increase in the electrical conductivity
of the gas. The breakdown voltage of
pure helium is about 500 v and is lowered by about
SO v by 1 ppm of impurity. The minimum detectable
limit is controlled primarily by the noise level
present, but under optimum conditions is below 1
PPb .
In previous attempts to improve the stability of
the detector discharge, various electrode radii
were used with the anode and cathode in a concentric
arrangement. In this early model of the
detector, which was contained inside a Swagelok
tube fitting, it. was impossible to observe the
helium discharge. A test detector was therefore
constructed which has a glass body with Kovar
seal tube connections through which the electrodes
are mounted (Fig. 14.5). These electrodes have
temovable tips which allow the testing of various
electrode shapes and spacings. The effects on the
helium discharge can be observed through the
glass. Tests with this detector indicate that a
minimum noise level is obtained with a smooth flow
discharge on the anode probe. Maximum sensitivity
dictates the use of a very pure helium carrier gas,
but this purity level also causes a sparking or
arcing in the helium discharge. The addition of
mercury vapor by the presence of a small source
of the metal in the tip of the anode stabilized the
discharge. The more practical solution of adding
a contamincant by a controllable gas flow is being
tested. This approach may also permit addition of
larger amounts of contaminant to allow the detector
to operate in the less-sensjtive mode necessary
for the determination of h.i@ levels of impurities
in the blanket gas samples.
'MSR Program Semiann. Pro&. Rept. JIJIY 31, 1964,
ORNL-3708, p. 328.