Environmental Engineering Reference
In-Depth Information
lead to the loss of required minimum redundancy.
Regulations NP, 2000 and NP, 2008,b define
redundancy requirements that emergency reactor
protection system should consist at least of two
independent sets, each of them should be designed
in a way that emergency reactor protection was
provided by not less than 3 independent chan-
nels on neutron flux density level, neutron flux
density changing period, and on each technologi-
cal parameter. It is provided that ERP command
should be formed upon coincidence of signals
in, at least, two of three channels. The use of
such a redundancy principle providing fulfilling
the specified requirements to system reliability
(concerning emergency protection failure), at
the same time facilitates reducing of false actua-
tion probabilities, which can lead to the reactor
shutdown in nonhazardous situations.
IAEA, 2002 and IAEA, 2012 international
standards recommend to specify measures which
minimize the probability of operator's actions that
can damage the effective operation of protection
system under normal operation and expected op-
erational event, but that do not prevent operator's
appropriate actions in case of design accident. NP,
2008,b regulations require to provide monitoring
and operability diagnostics of channels and sets
with displaying of fault information and forming
emergency protection or alarm signals in case of
sets or channels failure. There is also stated that
in case or the failure or the removal of one chan-
nel in the operating emergency protection set, the
emergency protection signal should be formed
automatically at the channel output.
In accordance with IAEA, 2012 during op-
eration protection system should permit periodic
checking of each set operability with the operating
reactor. Aim of the testing is to check compliance
with design characteristics and to detect faults,
which could happen after the previous testing
and lead to partial or full loss of redundancy (NP
2008,b). Periodical tests should cover all the sys-
tem components engaged in protective function
performance, from sensors to respective actuating
systems or equipment inputs. The requirements
to removal of one set or one set channel (allow-
able reactor power level limit, condition of other
units, potential duration, etc.) should be defined
and reasoned in a way to minimize the possibility
of any negative impact on operation and power
unit safety condition. The possibility of set or
emergency protection channel removal without
power unit personnel warning should be excluded
with the help of the proper technical equipment.
The main component of E&PRP system is
software-hardware complex (SHC), which is
engaged in performance of all its main and ad-
ditional functions.
The requirements to software-hardware
complexes emergency and preventative reactor
protection (SHC E&PRP) result from:
• General requirements to operation, reli-
ability, durability, quality and indepen-
dence of performed functions, applied to
safety class 2(А) components and their op-
erationally-autonomous component parts;
•
Speciic requirements to redundancy meth-
ods (structure), independence, diagnostics
and controllability of emergency protec-
tion and preventative protection systems.
The diversity requirement directly and ex-
clusively relates to SHC E&PRP. Complexity
and uniqueness of each SHC as a single product,
which is developed (or updated) for a specific
system, increase the risk of hidden faults, being
not detected during validation or acceptance
testing. Hidden faults made during the develop-
ment, configuration, and / or production of SHC
E&PRP, can appear in the process of performance
as a common cause of redundant channel fault
in both sets. Taking into account these features,
IAEA, 2012 standard recommends to use func-
tional diversity maximally, as well as a diversity
of technical and program means or operation
principles to prevent the loss of protective func-
tion (characteristics of possible diversity kinds
are described in Chapter 3).
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