Nuclear Production of Hydrogen, Fourth Information Exchange ...

TRANSIENT MODELLING OF S-I CYCLE THERMOCHEMICAL HYDROGEN GENERATION COUPLED TO PEBBLE BED MODULAR REACTOR
Sulphuric acid (H 2 SO 4 ) and hydriodic acid (HI) are produced via the Bunsen reaction. The Bunsen
reaction is exothermic. Utilising a nuclear heat source, these two acids are then decomposed into
their relevant constituents. The H 2 SO 4 decomposition reaction produces oxygen, sulphur dioxide and
water. The HI decomposition reaction produces hydrogen and iodine. With the exception of hydrogen
and oxygen, the other products are reused in the Bunsen reaction. As the name suggests, the S-I cycle
is a cyclic process. The intent of the S-I cycle is to act as a veritable “black box” where water is split
into hydrogen and oxygen. In reality, a variety of reactants are involved in the process, but they are
recycled continuously, so that the only inputs to the system are heat and water and the only outputs
are hydrogen and oxygen. The heat input is supplied via a high temperature nuclear reactor.
A simplified flow sheet for the S-I cycle is shown in Figure 2.
H 2
SO 4
, SO 2
,
O 2
, H 2
O
Figure 2: Simplified S-I cycle schematic
O 2
product
H 2
SO 4
Preheater
Evaporator
Decomposer
H 2
O
Bunsen
reactor
H 2
SO 4
,
HI, I 2
,
O 2
, H 2
O
3-phase
separator
H 2
SO 4
,
H 2
O
To IHX
From
IHX
HI, I 2
,
H 2
O
HI, I 2
,
H 2
O
HI
Preheater
Evaporator
Decomposer
H 2
product
The Westinghouse sulphur cycle, also known as the HyS cycle, was developed in the United States
by Westinghouse during the oil crises of the 1970s (Brecher, 1976). In the HyS process, unlike the
electrolysis of water, hydrogen is actually produced at the electrolyser cathode (Brecher, 1976). Similarly,
sulphuric acid is produced at the electrolyser anode (Brecher, 1976). The HyS cycle consists of a
decomposition step and an electrolysis step expressed as the following equations.
• Decomposition (sulphuric acid, 1 123 K): H 2 SO 4 → H 2 O + SO 2 + ½O 2
• Electrolysis: 2H 2 O + SO 2 → H 2 SO 4 + H 2
In the HyS cycle, the decomposition of sulphuric acid is the same as Section 2 of the S-I cycle.
Thus, the difference between the S-I cycle and HyS cycle is the removal of the Bunsen reaction and
the replacement of Section 3 with an electrolysis process. A simplified flow sheet for the HyS cycle is
shown in Figure 3.
Both a nuclear reactor and a thermochemical hydrogen generation plant can be subject to a
variety of transient scenarios. Each plant has its own start-up and shutdown scenarios, in addition to
a wide variety of other transients. For the nuclear plant, this includes minor reactivity adjustments,
off-normal operation and accident scenarios. In addition, the chemical plant is also subject to a wide
variety of potential transient events, including start-up, shutdown, piping system failures, tank failure,
heat exchanger failure and a wide variety of other accidents (Marsh, 1987). The point of interaction
between the chemical plant and the nuclear reactor is the intermediate heat exchanger. The function
of the intermediate heat exchanger is to transfer energy from the nuclear reactor to the chemical
plant, thereby driving the chemical reactions.
Because the nuclear reactor and hydrogen generation plant are so strongly coupled via the IHX,
any event that occurs on one side of the loop will by definition effect the other side of the loop.
By coupling two independently validated models of a high temperature nuclear reactor and a S-I/HyS
NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010 365