Nuclear Production of Hydrogen, Fourth Information Exchange ...

USE OF PSA FOR DESIGN OF EMERGENCY MITIGATION SYSTEMS IN A HYDROGEN PRODUCTION PLANT
Introduction
General Atomics’ sulphur-iodine process (SI) for hydrogen production is currently one of the best
candidates for the production of the energy carrier on a large scale, chiefly due to its high efficiency
(57% upper bound) (Kasahara, 2007) and because it is a zero emissions technology. However, due to
the nature of chemicals involved in the cycle, appropriate mitigation systems should be taken into
account to avoid unwanted situations and contingencies.
The cycle consists essentially of three process sections (Norman, 1978), which are: synthesis of
sulphuric and iodhidric acid (Bunsen section), concentration and decomposition of sulphuric acid
(Section II) and concentration/decomposition of iodhidric acid (Section III).
The chemical reactions involved in each section are:
I 2 + SO 2 + 2H 2 O • 2HI + H 2 SO 4 (Bunsen) (1)
H 2 SO 4 • H 2 O + SO 2 + ½O 2 (2)
2HI • H 2 + I 2 (3)
The first reaction is carried out exothermically at about 120°C (Brown, 2003) and is succeeded by a
three-phase separation system and a second reactor whose function is to increase the conversion of
reactants. For this part of the process, temperature and pressure conditions do not represent a
dangerous situation, even so, the chemical activity makes it necessary to take security measures
oriented to minimise the effects of a possible leak.
The third reaction takes place in a reactive distillation tower where the iodhiric acid is
concentrated and decomposed simultaneously, to produce hydrogen at a temperature range from
200 to 310°C and pressure up to 22 bar (Brown, 2003). This section requires analysis of iodhidric acid
leak and hydrogen explosions, though these sections are not covered in this paper as they will be the
subject of future developments.
To carry out the second reaction (Section II, decomposition of sulphuric acid), which is the section
of interest in this paper, it is required to concentrate the sulphuric acid from its initial 20% by weight
concentration to a final 90% mass fraction. This is achieved through the use of three concentrating
systems, which are: isobaric flash, isoentalpic flash and vacuum distillation. Once concentrated, its
pressure and temperature are raised up to 7 bar and 800°C in order to achieve the necessary conditions
for progress of reaction (Brown, 2003). The high operating temperatures required in this section,
coupled with the need for primary energy sources free of carbon, make high temperature gas-cooled
reactors unbeatable as a first option to supply energy to the process.
In the case that any of the mechanical elements of this section presented a failure, the formation
of a concentrated acid puddle would occur with the simultaneous release of sulphur oxides. Subsequent
to this release, and in the absence of mitigation systems, an acid cloud could form (Greenberg, 1991),
extremely dangerous due to its high toxicity (LC50 = 320 mg/m 3 /2hr) (McAdams, 2006).
It is precisely in this second section, where the dangers due to the leakage of sulphuric acid and
associated oxides require a special mitigation system, which is intended to avoid toxic cloud formation
and its consequent effect on the surrounding civilian population. For example, in October 2008, a leak
of concentrated sulphuric acid, at the Indspec Chemical Corporation, caused the evacuation of more
than 2 500 people in Pennsylvania, United States (Associated Press, 2008).
For this type of mitigation system, probabilistic safety assessment (PSA) can be used as a tool for
evaluation of and feedback to designs. Originally developed on the basis of rules and standards of
engineering, resulting in a design enriched by statistical aspects and which meets acceptable risk
levels for a plant in normal operation.
When this kind of PSA study is developed, wherein top event quantification, sensitivity analysis
and dynamic management of the proposed systems are required, the complexity of data processing
and calculations requires the use of appropriate software, such as the processing PSA package
Saphire® 6.0, developed by Idaho National Laboratory for the Nuclear Regulatory Commission of the
United States and which is used in this work (Smith, 2005).
Using Saphire® 6.0 the system can be defined and data entered through a clear and graphic
interface for further processing and analysis.
398 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010