Nuclear Production of Hydrogen, Fourth Information Exchange ...

TRANSIENT MODELLING OF S-I CYCLE THERMOCHEMICAL HYDROGEN GENERATION COUPLED TO PEBBLE BED MODULAR REACTOR
At steady state several significant results of this coupling are noted. The energy deposited via the
IHX into Section 2 is 68.01 MWth, and the energy deposited via the IHX into Section 3 is 199.9 MWth.
Thus, 25.3% of the PBMR-268 power is deposited into H 2 SO 4 decomposition whereas 74.7% of the
PBMR-268 power is deposited into HI decomposition. These percentages are dependent on the model
assumptions, namely single constant volume reaction chambers which are sized according to simple
chemical kinetics expressions. Nevertheless, they give important insight into the relative importance
of the two chemical plant sections as portions of the nuclear reactor heat sink.
Test matrix and results
In this paper two transient scenarios are considered that are initiated via the nuclear reactor side of
the test loop. The scenarios consist of reactivity insertions of USD 0.25 and USD -0.25 respectively.
These reactivities were chosen to represent minor reactivity adjustments via control rods. The response
due to these reactivity insertions are observed in a hypothetical S-I cycle plant with a design hydrogen
generation rate of 142.3 mol/sec. These transient scenarios are simple, in that they involve a very
explicit driving force. In these scenarios, the nuclear reactor is the real driving force behind the
transient behaviour, with the chemical plant providing feedback in a responsive role. Eventually, this
model will be used to investigate benchmark problem type accident scenarios, such as loss of flow
accidents or load following transients. However, these simple control rod adjustments were chosen
for this paper due to relative simplicity and the preliminary nature of this work.
Results for the insertion of USD -0.25 of reactivity are shown in Figures 5-10. This reactivity
removal causes a precipitous drop in reactor power, followed by a new steady-state power condition.
In Figures 8 and 9, it is evident that a new accumulation of reactants occurs in both reaction chambers.
This accumulation of reactants is due to slower reaction rates, because less energy is available to
drive the reactions. Figures 8 and 9 also illustrate the difference in response times between the two
reactions, with Section 2 having a response time of approximately ~50 seconds and Section 3 having a
response time on the order of ~500 seconds. This is a very important conclusion, as it suggests that
Section 3 is the rate-limiting step of the entire S-I cycle chemical plant. Finally, in Figure 10 we note the
response of the hydrogen generation rate, which adjusts to a rate of ~80 mol/sec in about ~300 seconds.
Similar results are indicated for the insertion of USD +0.25 of reactivity. These results are shown
in Figures 11-16. The insertion of this level of reactivity causes a spike in reactor power followed by a
new steady-state condition. In contrast to the reactivity removal, we note in Figures 14 and 15 that
there are fewer reactants “available” for reaction in each reaction chamber. This is due to the increase
in reaction rates caused be the greater availability of energy. Finally, in Figure 16 we note the response
of the hydrogen generation rate, which adjusts to a rate of ~200 mol/sec. However, due to feedback
from the chemical plant a new steady-state condition has not quite been reached, and the hydrogen
production rate continues to adjust.
Conclusions
A transient control volume model of the S-I and HyS cycle is presented. An important conclusion based
on the results of this model is that the rate-limiting step of the entire S-I cycle is the HI decomposition
section. In the HyS cycle, the rate-limiting step is the H 2 SO 4 decomposition. A generalised methodology
for coupling these thermochemical cycle models to a nuclear reactor model is overviewed. The models
were coupled to a THERMIX-DIREKT thermal model of a PBMR-268 and a point kinetics model. Key
assumptions in the PBMR-268 model include flattening of the core and parallelisation of the flow
channels.
Two reactivity insertions of USD -0.25 and USD +0.25 were investigated. Interesting results
included the relative accumulation or dissipation of reactants due to a respective decrease or increase
of available energy. It is noted that these two transient scenarios, while interesting, are quite simple
with the nuclear reactor providing a distinct driving force and the chemical plant responding.
Chemical plant driven transient scenarios provide an interesting opportunity for further study. Events
including piping system failures, tank failure and heat exchanger failure, could prove to be important
as each scenario represents a partial or total loss of heat sink for the nuclear reactor.
370 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010