H. Quack, 27.10.2015
Discussions on Ortho-Hydrogen at J-PARC on Oct. 19, 2015
1. General
The moderation of neutrons in a moderator bath is more efficient in p-H2 than in o-H2. Therefore the
o-H2 content should be kept to a minimum. ESS has specified a maximum o-H2 content of 0,5 % at
the entry to the moderators.
Fig. 1 o-H2 content in equilibrium hydrogen
When one inspects the equilibrium curve of the hydrogen allotropes in function of the temperature
(fig. 1), one finds that an o-H2 content of 0,5 % corresponds to a temperature of about 23 K. Since
the actual temperature in the loop varies between 17 and 21 K there should be in principle no
problem to fulfil the < 0,5 % requirement, if somewhere in the loop there is some catalyst, which
secures the establishment of equilibrium – and if no new o-H2 is generated.
But there exist theoretical predictions that high energy neutrons can cause the production of o-H2,
even if the temperature of the bulk fluid is very low. The theory is that the neutrons cause a
dissociation of the H2 molecule into two hydrogen atoms. But if two atoms later re-combine to form
again a molecule, quite some energy would be released locally, which causes the resulting hydrogen
to have the composition of normal hydrogen, i.e. 75 % o-H2 and 25 % p-H2.
To keep the o-H2 level in the loop below 0,5 % there has to be a converter with catalyst in the loop,
which converts the such-produced o-H2 back to e-H2, which means practically to p-H2. The practical
question is, whether this converter has to be in the main loop or whether it can be placed in a bypass
loop.
A placement of the converter in the bypass has the advantage, that the converter does not cause any
pressure drop in the main loop, which is nice for the duty of the circulating pumps. Compared to this
an additional flow rate through the bypass is of smaller concern.
Concerning the cryostat design it makes quite a difference whether one has to install a converter for
1000 g/s with an allowed pressure drop of 0,1 bar or one for 50 g/s with an allowed pressure drop of
1 bar.
2. Definition of “small” and “large” production rate of o-H2
Since at the moment we have no information, whether the o-H2 production in the moderator is small
or large, we turn the definition around and define the o-H2 production in the moderator as “small”, if
it is possible to compensate it by a converter in a bypass stream. And we define it as “large”, if it is
necessary to install the converter in the main loop.
To come to numbers, we assume the following conditions:
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The o-H2 content at the entry to the moderator shall be less than 0,5 %.
The maximum flow rate through the bypass should be less than 5 % of the total flow.
The conversion in the bypass converter is from 0,5 to 0,2 %. Equilibrium hydrogen at 17 K has
a 0,05 % content of o-H2. So there is sufficient “driving force” for the conversion.
This leads to a maximum allowed increase of o-H2 of the full flow in the moderator,which is
20 times smaller than 0,3 %, i.e. 0,015 %, i.e. from 0,5 to 0,515 %.
If the hydrogen flow through the moderators is 1000 g/s, an increase of o-H2 content by
0,015 % means a production rate of o-H2 of 0,15 g/s.
The conversion energy from p-H2 to o-H2 is about 700 J/g. A production rate of 0,15 g/s
requires an energy input of 0,15 g/s * 700 J/g = 105 W.
If we assume that at ESS the total absorbed energy in the moderators is 20 kW, half of which occurs
in the aluminum and the other half in the hydrogen, the just mentioned conversion energy of 105 W
means that about 1 % of the neutron energy absorbed in the hydrogen is used for the production of
o-H2.
If the production rate is smaller than 0,15 g/s, a converter in a bypass will be sufficient. If the
production rate is larger, the converter has to be placed in the main stream.
3. Measurements at J-PARC
At J-PARC measurements of the o-H2 content were performed in a period, when the accelerator was
operated at 300 kW.
Fig. 1 Hydrogen loop at J-PARC
The converter is located in the main loop. Probes were taken upstream of the converter.
Gaseous probes were taken at ambient temperature and atmospheric pressure. The gas probes were
brought to an outside company to measure the o-H2 and p-H2 content by Raman spectroscopy. The
spectroscopy was done about six hours after taking the probe.
The analysis showed no difference between pure p-H2 and the taken probe within the accuracy of
the measurement, which was stated as about plus/minus 0,4 %.
As a conclusion one can state:
The method to take probes and the analysis by Raman spectroscopy at atmospheric pressure and
ambient temperature seems to be safe against re-conversion into normal hydrogen by the handling
method. Otherwise one should have seen a certain amount of o-H2. This means that this
measurement method is in principle suited to determine o-H2 production in the moderator.
The stated absolute accuracy of 0,4 % is not sufficient to detect phenomena like the difference
between “small” and “ large”, which lies in the order of a change of 0,015 %.
The “Dresden” method to measure the o-H2 content is based on a small bypass flow through an
oversized converter and the measurement of the temperature increase of the flow in this converter.
The theoretical sensitivity is a 0,8 K temperature increase for an o-H2 content of 1 %. The expected
accuracy of the measurement of the temperature increase by the pressure difference in two vapor
pressure thermometers is about 0,01 K. So the expected accuracy of the determination of the o-H2
content is about
(0,01/0,8)* 1% = 0,0125 %
This seems to be about 30 times more accurate than the Raman spectroscopy. But it is still on the
limit of the determination of the difference between “small” and “large” production rate.
This means that a really convincing measurement of the production rate of o-H2 in the moderator
can be obtained only from an accumulation method, i.e. first establishment of pure p-H2 with the
help of a converter and then operation for a while without converter, so that the produced o-H2 can
accumulate to a level, which can be measured with high accuracy.
J-PARC is considering such a more accurate measurement:
The hydrogen loop at J-PARC is equipped with a bypass around the converter. The system shall be
operated for a while with the converter to establish a nearly pure p-H2 flow. Then the converter will
be taken out of the loop and the system will be operated using the bypass. Periodically probes will be
taken to investigate whether one can observe an increase in the o-H2 content.
This kind of measurement looks rather simple. But it requires that operating changes in the hydrogen
loop are being made while the spallation neutron source is in full operation and producing neutrons
for experiments. It may not be easy to get permission for such kind of actions from the laboratory
management.