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PROPOSED CHEMICAL PLANT INITIATED ACCIDENT SCENARIOS IN A S-I CYCLE PLANT COUPLED TO A PEBBLE BED MODULAR REACTOR
Introduction
The S-I cycle chemical plant is split into three main reactions, which are described here, as “sections”
of the chemical plant:
• Section 1 (Bunsen reaction): I 2 + SO 2 + 2H 2 O → 2HI + H 2 SO 4
• Section 2: H 2 SO 4 → H 2 O + SO 2 + 1/2O 2
• Section 3: 2HI → H 2 + I 2
Section 1 operates at 393 K, Section 2 operates at 1 123 K, and Section 3 operates at 773 K. The
section of the plant dedicated to the Bunsen reaction is dubbed Section 1, H 2 SO 4 decomposition is
dubbed Section 2 and HI decomposition is dubbed Section 3.
Any heat source which provides both high amounts of heat energy and the high temperatures
required for the endothermic Section 2 and Section 3 reactions is acceptable for the S-I cycle. The heat
transfer to each section consists of a series of heat exchangers manufactured out of a ceramic material
or a special metal, such as silicon carbide or Hastelloy. The selection of these special materials is due
to the high relative degree of corrosion and temperature resistance. A high power density is also
required for the S-I cycle, making nuclear energy a particularly attractive option. General Atomics,
Sandia National Laboratories and the University of Kentucky produced a comprehensive steady-state
flow sheet for an S-I cycle system (Brown, 2003).
Several reactors are candidates for use as a high temperature heat source for the S-I cycle.
Candidates include the modular helium reactor (MHR) and pebble bed modular reactor (PBMR). One of
the most thoroughly investigated candidates is the PBMR. Recent work has been performed in
benchmarking the THERMIX code to the PBMR-268 design (Reitsma, 2004; Seker, 2005).
In a coupled nuclear hydrogen generation system, the reactor loop will be coupled to the
chemical loop via an intermediate heat exchanger (IHX). This coupling is illustrated in Figure 1.
Figure 1: S-I cycle coupling through IHX
Safely implementing a thermochemical nuclear hydrogen generation scheme requires a robust
understanding of the interaction between the nuclear plant and the chemical plant. In turn, this requires
robust models of the chemical plant, reactor thermal-hydraulics and reactor physics. Efforts have
been conducted in both the transient modelling of the sulphur-iodine (S-I) and hybrid sulphur (HyS)
thermochemical cycles, as well as coupling to models of the pebble bed modular reactor (PBMR-268)
(Brown, 2009).
Importance of chemical plant initiated scenarios
Accident scenarios initiated in the PBMR plant have been described and thoroughly modelled as
benchmark problems (Reitsma, 2004). While modelling these scenarios in a coupled nuclear reactor/
chemical plant scheme is interesting, it should be noted that in most of these scenarios the nuclear
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