Continuous
Mobile
Boration
By Johan Hallén and Ryan Vanston,
Westinghouse Electric Company.
Johan Hallén
Johan Hallén brings 25 years of
experience in the nuclear industry
to his current role as vice president
of Operating
Plant Automation
Engineering and
Products for
Westinghouse
Electric Company.
In this position, he
is responsible for
leading the delivery
of instrumentation
and control products
and pressurized
water reactor
engineering services
globally.
Prior to this position,
Mr. Hallén was
vice president and
managing director for Westinghouse
in Northern Europe, where he was
responsible for project delivery for
operating plants business in Finland,
Sweden and Ukraine. Mr. Hallén
progressed through increasingly
responsible positions beginning through
roles with ABB and Westinghouse in
project management, design engineering
and management to his current executive
role.
Mr. Hallén attended the Royal Institute
of Technology, KTH, in Stockholm,
Sweden, where he earned a Master
of Science Degree in Mechanical
Engineering.
In a pressurized water reactor,
regularly replenishing borated water
during normal plant operations is
required, as borated fluid is necessary for
the safe operation of the reactor coolant
system and other systems throughout
the plant. As a result of the Fukushima
Daiichi event, utilities identified the need
to quickly and flexibly generate borated
water sources as part of new beyond-
design-basis accident response strategies.
Westinghouse responded to this need
by developing a more efficient, higher
flow rate, skid-style mobile boration sys-
tem, called the Mo-
bius™ system, as part
of a mitigation strat-
egy for core cooling
that would address
international regula-
tory
requirements.
The Mobius system
uses water, boric acid
powder and energy to
create a continuous
output of borated flu-
id at a specified con-
centration, flow rate
and pressure and will
be used for the first
time in the field in the
spring of 2017.
In
parallel
with Westinghouse’s development of
the Mobius system, the U.S. Nuclear
Regulatory Commission (NRC) issued
Order EA-12-049, “Order Modifying
Licenses With Regard To Requirements
For Mitigation Strategies For Beyond-
Design-Basis External Events.” This was
closely followed by the NRC-endorsed
“Diverse and Flexible Coping Strategies
(FLEX) Implementation Guide” (NEI
12-06) published by the Nuclear Energy
Institute. In the emergency situations
addressed by the NRC Order and FLEX
guide, it may be difficult to have ready
access to borated fluid if current systems
or storage tanks are not designed to
withstand the postulated severe external
event scenarios.
Requirements for core cooling and
heat removal, as outlined in the FLEX
guide, stipulate that all plants have ameans
to provide borated water to replenish
the reactor coolant system indefinitely.
During cold shutdown and refueling,
the normal methods for core cooling are
assumed to be inoperable. The reactor
coolant system is itself designed to vent
heat through steam into containment as
part of the core cooling process, but the
boric acid from the borated fluid within
the system will remain as precipitate. A
flow of borated water must be available to
keep the boric acid concentration below
precipitation limits and remove decay
heat.
The FLEX guide also stipulates
that there are diverse connections to
the reactor coolant system capable of
handling flow rates that are sufficient
for simultaneous heat removal and
boron flushing, as the mode of operation
demands. Flushing with a continuous
flow of borated water allows the boric
acid concentration that remains in the
reactor coolant system to be maintained
below the precipitation limit while still
maintaining sub-criticality. The current
industry response plans do not include
a continuous system as the requirement
can be alternately satisfied with an onsite
tank or an offsite resource depending on
analysis or timing results.
For
normal,
emergency
or
augmented boric acid supply systems,
plants have relied on single-batch tank
systems due to the significant amount of
energy required for heating to maintain
the ability to continually dissolve boric
acid crystals in water. This high-energy
heating and dissolution process limits
the overall capacity for a single-batch
system. System capacity can be directly
translated to an overall discharge flow
rate of borated fluid, which may be too
low to meet the high flows necessary to
alleviate early post-accident decay heat
rates.
The Westinghouse Mobius system,
by contrast, is a specialized, continuous
flow, contained mobile boration system
that resolves the challenges presented
by single-batch tank systems and can be
used for normal and accident operations.
The continuous flow capability addresses
the indefinite nature of the FLEX
requirements, and the Mobius system
reduces the days required by other
Mobius™ System for Operational Flexibility
40
NuclearPlantJournal.com Nuclear Plant Journal, September-October 2016
