westinghouse pwr design comparison: gen ii versus gen iii

Running head: GENERATION II PWR VERSUS AP1000 1

Generation II versus Generation III+: A Comparison of Westinghouse PWR Designs

NUC-350 Plant Systems Overview

Jonathan Varesko

Excelsior College

GENERATION II PWR VERSUS AP1000 2

Abstract

One of the benefits of living in a modern and industrialized society is that massive advances in

technology are a common occurrence. But technology takes time to develop, test, and become

implemented into everyday life. In the nuclear power industry time is money, and a lot of it. But

that’s not all, there also exists the process of proving the technology safe and reliable, ensuring it

is not prohibitively expensive to implement, and finally obtaining the licenses and permits to

build and operate the new reactor. This paper serves to compare and contrast one such

technological advancement by the nuclear division of Westinghouse Electric - the general plant

design and the advantages and/or disadvantages that two separate generations of Westinghouse

Pressurized Water Reactors (PWRs) provide.

In modern electronics a ‘generation’ of new technology is often superseded within a 12-18

month period after release; in nuclear, a ‘generation’ of reactor technology can and has spanned

up to 30 years. By comparing two designs stemming from Westinghouse - a two-loop Generation

II PWR (R.E. Ginna Nuclear Power Plant) and a two-loop Generation III+ PWR better known as

the AP1000 - the answers to questions such as “What has and has not changed about design since

the early 1960s” and “What are the advantages and/or disadvantages by the advancing

technology” can be obtained.

Key Words: Westinghouse, Generation III+, AP1000, R.E. Ginna, Passive Safety System,

Reactor Design, Pressurized Water Reactor,


Prior to analysis of the general plant design and systems within, it is pertinent to get a brief

history of the two plants. Generation II reactors such as R.E. Ginna were built as the successors

to the original atomic power reactors (Shippingport, Fermi 1, Dresden 1) that resulted from the

efforts of commercialization by the U.S. Atomic Energy Commission of the Navy’s Nuclear

Propulsion Program (Beaver, 1998). These successors to the Generation I reactors started

construction and operations anywhere between 1965 and 1995 and several Generation II plants

still operate to this day, R.E. Ginna being the 3rd oldest with regards to length of time operating.

Generation III+ reactors are those which the Argonne National Laboratory (2006) considers in

near-term deployment and contain “evolutionary designs offering improved economics”

compared to the Generation III reactors such as the AP600 and EPR – which would see their

deployment last from 1995 thru approximately 2010. The AP1000 is one such Generation III+

design and is the newest NRC approved reactor design (approved in 2004) and has since gone

through several revisions, the latest of which is Rev. 19 which was approved in 2011 (“Issued

Design Certification – Advanced Passive 1000 (AP-1000)”, 2013).

Inside the containment of any nuclear reactor it can be expected to find a reactor vessel, a couple

steam generators, two or more reactor coolant pumps, a pressurizer, and a lot of piping, valves,

and cabling runs. R.E. Ginna is no exception and a top down view of the general containment

layout can be seen in Figure 1 (R. E. Ginna NPP UFSAR, 2013). It is a fairly spaced out

arrangement with the vessel in the middle with hot leg piping runs on either side of the vessel to

a steam generator. Behind the missile shielding and sitting vertically next to each steam

generator is a single centrifugal reactor coolant pump attached to the cold leg piping runs. –

cutaway image found in Figure 2.


Despite the maintenance friendly layout in regards to keeping adequate space between large

pieces of equipment, more floor space leads to higher cost for the concrete pour and construction

of containment because the entire containment structure is required to be a safety related seismic

structure. Combined with the reality that lengthier RCS hot and cold leg piping runs require

larger diameter piping and thus more material (Belles, 2006), the initial capital investment

continues to compound. Additionally, as the RCS loops require maintenance or replacement,

more material must be procured and more man hours spent dismantling and reconstructing the

loops which ultimately leads to higher O&M costs.

Looking at the primary system layout of the AP1000 reactor, shown in Figure 3, a few things

instantly stand out as different from R.E. Ginna. Perhaps the least striking difference is the four

cold leg design, two legs per steam generator. This is to be expected as the AP1000 has a sizably

larger power output and the extra loops are required for adequate mass flow cooling. The

differences start with the reactor coolant pumps, continue on with the somewhat familiar

integrated reactor head package, and end with pressurizer.

The RCPs, while still being vertically mounted centrifugal type motor driven pumps, are now

inverted in orientation (Baumgarten, Brecht, & Bruhns, 2010) compared to R.E. Ginna and

attached to the bottom of the steam generators instead of sitting upright and off to the side behind

the steam generator missile shielding. By welding the RCPs to the steam generators a small

footprint for the primary system is achieved; a smaller footprint which reduces the size

requirement for the containment building as well as the amount of piping material required for

the RCS loop runs. As an added benefit of the shorter piping runs, the diameter of the piping can

also be reduced by a significant amount while still maintaining the necessary RCS flow capacity

for cooling during operation. In addition to this, the RCPs no longer have seals (Baumgarten et


al., 2010) which allows for the elimination of a small break LOCA path during accident analysis

(Belles, 2006).

Having a reactor vessel head package is nothing new from a design standpoint, as all PWRs have

at minimum the cabling for various reactor instrumentation as well as the control rod drive

mechanism and its associated cabling entering through the top of the vessel. Westinghouse in

their AP1000 design takes this one step further and integrates all cabling and instrumentation

that enters and exits the reactor vessel and integrates it into a single removable assembly

(Schene, 2007). This centralizes the location for all electrical and I&C work involving

disconnecting and reconnecting the reactor head as well as keeping all penetrations of the vessel

above the RCS hot and cold leg penetrations.

Westinghouse has also made one change to the pressurizer in the AP1000 design, it is

approximately 50% larger than the pressurizer from a unit with comparable power output. This

allows for the elimination of the PORV (Schene, 2007) as well as the addition of a dedicated

quench tank because the In-Containment Refueling Water Storage Tank (IRWST) is connected

to the remaining pressurizer depressurization valves.

Arguably the most important part of a nuclear reactor is the ability to safely shut down the plant

and maintain core cooling even during the most challenging of accident conditions, the most

challenging of which is a station blackout – where all off-site power is lost and the diesel

generators fail to start. It is paramount to keep in mind the difference in terminology regarding

the ECCS systems as they are discussed below. “Active” systems are those which require AC

power to perform their intended function while “passive” systems are those which do not rely

upon AC power to perform their intended function. With respect to the safety related systems


required to provide emergency core cooling and safe shutdown to the R.E. Ginna plant, those

systems are considered active in nature while the AP1000 emergency core cooling and safe

shutdown systems are passive in nature.

R.E. Ginna’s safety related ECCS systems include the RHR/LPCI, SI accumulators, HPCI,

containment and core spray, the CVCS and charging systems, containment cooling HVAC,

CCW, and the emergency diesel generators (R.E. Ginna NPP UFSAR, 2013). A generic P&ID

for the R.E. Ginna ECCS system interactions can be seen in Figure 4. As previously mentioned,

all of these systems are considered active and rely on AC power provided by either the diesel

generators or off-site power from the grid. Due to the safety related nature of these systems,

redundancy is required which means extra pumps, valves, and train separation – all of which lead

to increased costs, maintenance activities, and more surveillances that can put the plant into a

violation of a tech spec if the surveillance fails. It is unnecessary to go into a detailed analysis of

each system as the general design of ECCS systems has not changed in well over 40 years since

R.E. Ginna’s start of operation. However, in the event of an accident the system response is

governed by primary system pressure – CVCS makeup and charging systems at high pressure, SI

at medium to high pressure, and the LPCI/RHR at low pressure – as well as the presence of AC

power – if off-site power is available the diesels will not start, if a loss of off-site power occurs

the diesels will start and pick up the vital ECCS loads that are required to cool the core (R.E.

Ginna UFSAR, 2013).

AP1000 reactors still have systems such as the RHR, SI accumulators, CVCS and charging

systems, CCW, and emergency diesel generators; however, these systems are no longer deemed

safety related (Belles, 2006) due to the passive ECCS systems’ ability to safely shut down and

cool the reactor core. Due to the reduced classification to non-safety related, redundant pumps,


valves, cross-ties, piping, and breakers can all be eliminated from the system design which

results in substantial space and cost savings in regards to plant footprint and equipment capital

costs as well as O&M. There are two passive systems, one for containment cooling (Passive

Containment Cooling System – PCCS) and one for core cooling, both of which rely on clever

designs and the principles of natural circulation and condensation to achieve safe shutdown

(Schene, 2007). These can be seen in Figures 5.

An analysis of the AP1000’s ECCS is required to better grasp how the reactor can stay cool and

covered using only the principles of natural circulation and condensation, without the need for

AC powered equipment such as pumps and motor-operated valves, particularly in the event of a

station blackout. During the initial onset of an accident scenario, the water level in the steam

generators will decrease substantially indicating a rising RCS temperature and a loss of cooling

to the core (Westinghouse Nuclear, 2011). Automatic actuation of a couple of valves align the

RCS to the Passive Residual Heat Removal Heat Exchanger (PRHRHX) which resides within

the In-Containment Refueling Water Storage Tank (IRWST). This prompts the start of the

natural circulation and heat transfer process as hot reactor coolant is run through the PRHRHX

and dumping some of the waste heat to the IRWST (Westinghouse Nuclear, 2011). As RCS

inventory decreases further over the next hour and water in the IRWST starts to boil, an

automatic alignment of the Core Makeup Tank (CMT) occurs based on a low pressurizer level

signal. The inventory inside the CMT is enough to continue natural circulation out to hour 36

after the initial accident onset (International Atomic Energy Association, 2011) without any

operator interaction due to the condensation of steam and return of the condensate to the IRWST.

This condensation of steam within containment and containment cooling is achieved by the

Passive Containment Cooling System (PCCS) (“Engineering Energy”, 2012). Due to the


containment vessel being made entirely of steel and having direct contact with the atmosphere,

the steam present in containment (due to either lost RCS inventory or boiling within the IRWST)

is able to naturally condense on the colder metal surface. This was not possible in previous

generations of reactors because there was four feet of concrete between the half-inch steel liner

on the inside containment and the atmosphere. Surrounding the containment vessel is the

concrete shield building which has several airflow channels allowing for both natural (density

and temperature differences) and induced draft (via circulation fans) to circulate the air between

the shield building and containment vessel, expelling the hot air out of the top of the shield

building (Westinghouse Nuclear, 2011). Because of this ‘always on’ cooling of the containment

vessel, steam can readily condense on the inside of containment and return via gutters to the

IRWST. Additional containment vessel cooling is achieved by the automatic alignment of the

Passive Containment Cooling Water Storage Tank (PCCWST) drain valves once a high

containment pressure signal is received (“Engineering Energy”, 2012). The volume of the

PCCWST, which sits on top of the shield building, is enough to keep the outside of the

containment vessel cool enough for effective natural circulation and condensation to continue for

up to 72 hours (Westinghouse Nuclear, 2011).

After hour 72 of the initial onset of the accident and station blackout, operators are finally

required to take actions to maintain the safe shutdown of the reactor. This involves starting the

ancillary diesel generators (“Engineering Energy”, 2012) which provide up to four days of

emergency power to recirculation pumps that replenish the PCCWST from either the ancillary

storage tank (which is large enough to maintain PCCWST inventory for four days) or from

various other on-site and off-site sources. There are two important takeaways from the operation

of these passive containment and core cooling systems. The first is that safe shutdown can be

achieved within 36 hours (and maintained for up to 36 additional hours) following the onset of


an accident event coupled with a station blackout; a safe shutdown that requires zero operator

interaction and no reliance on AC power during the entirety of those 72 hours (International

Atomic Energy Association, 2011). The second takeaway is that with minimal operator

interaction, the alignment of valves to the PCCWST and starting of the ancillary diesel

generators, containment – and thus core – cooling can be maintained for up to an entire week

after the initial accident coupled with a continuous seven day station blackout (“Engineering

Energy”, 2012).

Perhaps the greatest advancement of design and technology is the control and instrumentation

advancements made in the AP1000 design compared to that of R.E. Ginna. The AP1000 design

advances traditional process control and relay logic from electromechanical to digital in addition

to overhauling the control room to become more modernized in appearance and functionality.

These changes accomplish several things such as increasing plant reliability, reducing operator

stress by streamlining information and plant system status, and reducing the amount of cabling

and thus the cost of the process control and trip logic instrumentation.

Digital controls increase plant reliability in several ways, the first of which is accomplished by

the elimination of multiple metal-to-metal terminations which can lead to high resistance

connections in the analog 4-20 mA signal processing that all I&C instrumentation use. By

converting process controls to digital signaling and using fiber optics, plant transients or

malfunctioning equipment due to any one of several thousand terminations becoming high

resistance is virtually eliminated. Moving to digital logic also allows for the more expensive 2 of

4 trip logic to be implemented in every reactor safety function without significantly increasing

the costs to the system. This allows channels to be placed in bypass during testing and calibration

to maintain a 2 of 3 logic (Belles, 2006) instead of having to trip out a channel in a 2 of 3 system


which would lead to increased susceptibility of transients tripping the unit offline.

Figure 6 shows a computer generated model of the AP1000’s control room. The largest change is

the array of monitors that replaces most of the standard small digital readouts, gauges, dials,

indicating lights, and alarm panels. This array of monitors simplifies the operator’s job by

providing indication of multiple plant systems at once and even sorting and prioritizing alarms

(Belles, 2006) as they are received by system the alarms are attached to and importance of the

alarm. This not only increases the amount of information immediately visible to the operator, but

also helps reduce mental stress on the operator (Belles, 2006) by eliminating the need to find the

information across a dozen or more panels and PPCX tabs. Though an operator should not be

one to make continual mistakes, previous events in history have shown that the operations

personnel are human just as we all are, and that excessive stress and a large mental loading

combined with time pressure can lead to bad decision making. The AP1000 design takes this into

consideration to make the operator’s job as easy as possible in order to maintain reactor

reliability and nuclear safety (Schene, 2007).

Despite the impressive advantages of the modernized digital design of the AP1000, the most

impressive part stems from the ability to dramatically cut equipment and cabling costs. Though

fiber wire is not cheap, extensive amounts of copper are also not budget friendly and require

more consideration in terms of maintenance and environment in which it is placed. Fiber optics,

being completely digital, also presents the advantage of being able to send multiple signals down

one wire simultaneously. It also allows the mixing signals together by use of a multiplexer

(Belles, 2006), logic flow diagram as seen in Figure 7, to change multiple signals from multiple

wires into one signal that travels on one wire, a single signal which can later be de-multiplexed

or decoded into the original individual signals prior to multiplexing.


The real advantage however lies in the size of these IC (integrated circuit) chips which are

arranged together to form microcontrollers or microprocessors. The microcontroller array

equivalent of R.E. Ginna’s control room and relay room would most likely not be any bigger

than three or four of the dozen already existing control room panels, barring of course that

everything those panels were tied into was also capable of being digitally fed instructions. A

microcontroller array is also very easy to maintain and test because testing is a matter of

checking all of the “if this, then that” programming code and ensuring that the correct signal (a 1

or a 0) is being output on a specific pin. It does not stop at physical size though, ICs are also low

voltage DC (between 5 and 15 volts for operating power) and low power consumption

components. This means the elimination of 125 VDC control power for most applications and

allows for the use of smaller power systems and battery backups (Belles, 2006).

Analyzing the trends in the comparison thus far, one of the major advantages of the AP1000 in

every case is the capital and O&M costs of the plant. But how much cost cutting is really taking

place and will it allow for a nuclear plant to compete with the natural gas industry which is

thriving in the market where electricity is bought from providers for $32-$35 per MWe? Figure 8

provides a clear image of just what advanced digital design and controls coupled with passive

safety systems can do in terms of dropping costs - 50% fewer valves, 35% fewer pumps with no

safety related pumps, 80% less safety related piping, 80% fewer HVAC units, 45% reduction in

seismic building volume, and 70% less cable (Belles, 2006). While the numbers touted are

impressive, what truly matters is the monetary value those reductions provide, and as it would

turn out that monetary value happens to be quite significant. Whereas older generation plants like

R.E. Ginna (which require a $40-$45 per MWe pricing to be profitable) are struggling in the

current market where electricity is hovering around $32-$35 per MWe, AP1000 reactors can


generate profits with pricing as low as $30-$35 per MWe. This is a staggering 20-30% reduction

in capital (Belles, 2006) and O&M costs which coupled with the reduction in safety related

equipment, surveillances, and number of components and connections means that operations and

maintenance staffing can be reduced by approximately 1/3 (Belles, 2006).

In summary, the Generation III+ AP1000 plant has many advantages over the Generation II R.E.

Ginna plant while showing no real disadvantages in any category of comparison. The AP1000 is

cheaper to build, has less safety related buildings, piping, and equipment while still maintaining

excellent nuclear safety standards, is easier to operate by providing operators with more readily

available information that is prioritized by system function, and makes maintenance easier with

the reduced plant footprint meaning smaller components in smaller quantities (International

Atomic Energy Association, 2011). The only disadvantage seems to be getting the money and

consensus of the surrounding public to build one in the current economy despite the AP1000

being able to provide a necessary grid base-load power at prices competitive with natural gas.

Despite the challenges of getting reactors built and operating, the AP1000 reactor is indeed a step

in the right direction for nuclear power by touting increased safety (including immunity to the

station blackout – which is still fresh in the public’s mind from the Fukushima accident in 2011),

reduced capital building and O&M costs, and showcasing that the industry is still progressing

into the future and not stuck in the primitive age of reactor technology from 45 years ago – a

showcasing which is vital to gaining the trust of the general public because they live under the

perception that old is bad, old is unreliable, and old is unsafe.


Figure 1 – R.E. Ginna Containment, Top-down View


Figure 2 – R.E. Ginna Reactor Coolant Pump


Figure 3 – AP1000 Primary System Layout


Figure 4 – Simplified R.E. Ginna ECCS P&ID (not all systems shown)


Figure 5 – AP1000 Passive ECCS, Core Cooling (top) and Containment Cooling (bottom)


Figure 6 – CGI Model of AP1000 Control Room


Figure 7 – Multiplexed/Digital Control Reactor Trip Flow Diagram


Figure 8 – Breakdown of Component Reduction Due to AP1000 Design Changes


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