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ImprovingperformanceInside WANO peer reviews (below) page 23

Rajasthan 5 tops 2013 full-year load factors league tables page 26

Dealing with primary-circuit cracks and leaks page 20

NewneutrontransportsimulationsUsing Monte Carlo to estimate extent of reactor activation page 13

Going pin-by-pin in AP1000 first core page 33

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Decontamination & decommissioning

NUCLEAR ENGINEERING INTERNATIONAL | www.neimagazine.com May 201416

Aqueous, in situ primary circuit decontaminationA technology that removes activity and corrosion products from inner pipe and vessel surfaces yielded overall decontamination factors of 80-90 at Unterweser and Neckarwestheim 1, and factors as high as 150 for steam generator tubing. By Christian Topf, Luis Sempere-Belda, Kai Tscheschlok and Klaus Reuschle

Germany was confronted with

the decontamination and

decommissioning (D&D) issue when

the 13th amendment to the German Atomic

Energy Act was passed in the backlash

following the Fukushima accident. With it,

Germany’s nuclear programme reversed

almost overnight from life extension to

immediate and permanent shut down. This

caused a major overhaul in the utilities’

operational and financial strategies, forcing

them to re-examine the cost-effectiveness

of their D&D programmes.

All existing decommissioning strategies

were considered, but immediate

decommissioning after the operational

phase soon emerged as the favoured

option for all the country’s utilities. The

experience accumulated in D&D in Europe

over the last few decades, especially in

Germany, has established full system

decontamination (FSD) – simultaneous

decontamination of the complete primary

circuit and auxiliary systems – as the key

element of immediate decommissioning.

It is considered to be state-of-the-art

best practice. It has been used in recent

decommissioning projects and is planned

for projects in preparation.

Over the last few decades AREVA has

performed FSD for return-to-service and

decommissioning at 18 reactors worldwide

– some immediately or several years after

final shutdown. The complexity of such

FSD projects always requires a flexible

approach regarding process chemistry and

control to achieve the best possible result.

The multi-cycle CORD concept (chemical-

oxidation-reduction-decontamination), with

its dynamic process control and flexible

application times, makes best use of the

chemicals employed, helping minimise the

waste generated during decontamination.

FSDs at Unterweser in 2012 and

Neckarwestheim 1 in 2013 were the first

such applications in Germany in the

aftermath of the Fukushima event and the

German decision to phase out nuclear.

Engineering and chemistry conceptThe key to a successful FSD application is

symbiosis between system operation and

the chemical concept.

The first and vital step is to clearly

define the main systems in the

decontamination area. To achieve the best

possible decontamination the area should

include all the systems that are in contact

with primary coolant during normal plant

operation. A variety of auxiliary systems

(for example reactor coolant storage

system, waste water treatment system and

the nuclear exhaust system) support the

application.

For PWRs the typical decontamination

area is the primary circuit, including the

RPV with internals, pressurising system,

residual heat removal system (RHR),

chemical volume control system (CVCS),

and reactor water clean-up system. For

BWRs, it includes the reactor vessel,

recirculation system (depending on the

design of the plant), RHR system, and

reactor water clean-up (RWCU) system.

To make sure the decontamination

solution reaches all parts of the system,

the number of dead legs (runs of piping

ending in a termination) are minimised by

making temporary connections to them

with hoses and pipes. A flushing/rinsing

programme at the end of decontamination

helps to flush the remaining dead legs.

Figure 2 shows the schematic drawings

of the decontamination area of the FSDs

per-formed recently at Unterweser (2012,

left side) and Neckarwestheim 1 (2013,

right side).

The detailed engineering approach

for these plants was designed in close

collaboration between AREVA and the

utilities. The basic technical data of the

plants are listed in Table 1.

The system was adapted and the

technical modes of operation were

substantially developed and configured by

experienced plant operational personnel

in cooperation with AREVA engineering

and chemical personnel. Plant operators

controlled the plant systems during the

FSD.

Top: Inspection of a post-FSD pump (Neckarwestheim). Bottom: Pressurizer spray valves (left) and DN 50 valve after FSD, revealing clean surfaces (Unterweser)

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Decontamination & decommissioning

www.neimagazine.com | NUCLEAR ENGINEERING INTERNATIONALMay 2014 17

The main reactor coolant pumps

and the RHR system pumps were

used to recirculate the solution in the

decontamination area, and the thermal

energy liberated by operating them

supplied the necessary process heat.

The negative pressure suction head to

run the coolant pumps during FSD mode

was achieved by using one of the RHR

accumulators as a pressurizer. Based on

the different main coolant pump designs

(see Table 3) the operational pressure

during FSD differed from around 21

bar (~300 psi) to 26 bar (~375 psi). The

plant pressurizer was operated in solid

mode to obtain the highest possible

decontamination on the complete

surface. To ensure exchange of the

decontamination solution the main and

auxiliary spray lines were operated either

in parallel or alternately during the FSD.

Different approaches used the plants’

RHR systems for process temperature

control. At Unterweser temperature was

regulated using all four RHR redundancies.

At Neckarwestheim 1 RHR system 30 was

excluded from the decontamination area.

Within the regulatory framework RHR

system 30 had to stand by under normal

conditions at any time because it provides

redundancy for the spent fuel pool cooling

system, so temperature was regulated using

only the two other RHR 10/20 systems.

For both applications proprietary

decontamination equipment Automated

Modular Decontamination Appliance

(AMDA®) was used with the plant´s own

systems during FSD. This allowed AREVA’s

radwaste volume-reducing technologies

to be applied, such as UV decomposition

of the decontamination chemicals. AMDA

was also used for sampling during

the application, for detailed process

control and mechanical filtration of

the decontamination solution. AMDA

equipment, which can include pumps,

heaters, ion exchange columns, filters, UV

reactors, sampling modules, monitoring

systems, automation and remote controls

and chemical injection equipment, was

connected in each case to two different

RHR systems (see Figure 3).

The CVCS and RWCU systems in

Neckarwestheim were part of the

decontamination area and were operated

under normal operating conditions and

temperature restrictions. In Unterweser

the sensitive centrifugal charging pumps

in the CVCS were replaced by an external

pump to ensure undisturbed operation

during FSD application but remaining in

standby mode for redundancy. In contrast

to Neckarwestheim, the Unterweser RWCU

system supported the clean-up of the

decontamination solution during certain

phases of the application. The increase in

the cleaning rate reduced treatment time

and supported process control during the

FSD operation.

To have maximum decontamination

effect at minimum material impact, a

five-cycle chemical oxidation reduction

decontamination (CORD) UV chemical

decontamination concept using

permanganic acid (HP) was developed

by AREVA and the plants. Generally

speaking, the formulation is adjusted to

the characteristics of the oxide present.

Table 1: Basic technical data

Unterweser Neckarwestheim 1

Reactor type PWR, 4 loop, 1410 MWe PWR, 3 loop, 840 MWe

Decontamination areaPrimary circuit / 4 RHR systems

/ CVC systemPrimary circuit / 2 RHR systems / CVC system / RWCU system

Auxiliary systems RCS / WWTS / NES / RWCU RCS / WWTS / NES / RWCU

Total surface area 22,500 m2 (~ 278,000 ft²) 18,000 m2 (~ 194,000 ft²)

Total system volume 540 m3 (~ 142,000 gal) 360 m3 (~ 95,100 gal)

Reactor coolant pump supplier KSB Andritz

Operational pressure ~ 21 bar (~ 300 psi) ~ 26 bar (~ 375 psi)

Operational temperature 60 - 95 °C (140-200 °F) 60 - 95 °C (140-200 °F)

AMDA connection points RHR 30 / 40 RHR 10 / 20

Application time 35 days 25 days

Collective radiation exposure for on-site activities

70 mSv 61 mSv

Loop20

Loop10

Loop30 Loop

30

Loop10

Loop20

Loop40

AMDA AMDA

RHR 20

RHR 20

RHR 30

RHR 10

RWCU

RHR 10 RHR 40

RPV

CVCS

RWCU

CVCS

RHR 24

RHR 30

Spent fuelpool

RPV

Primary circuit Residual heat removal system Chemical volume control system

Pressurizing system Reactor water clean up system AREVAs decontamination equipment AMDA

Figure 2: Schematic Drawings of the decontamination area at Unterweser (left) and Neckarwestheim 1 (right)

NPP-Systemlow dose-rate

Demineralizedwater

ActivityCorrosion productsMn**Ion exchange resins

Demineralizedwater

Metallicallyclean surface

ActivityCorrosion productsMn**Ion exchange resins

NPP-Systemlow dose-rate

Demineralizedwater

Demineralizedwater

Metallicallyclean surface

CORD UV Cydes OxidationReductionDecontaminationDecomposition

CORD UV Cydes OxidationReductionDecontaminationDecomposition

Carbondioxide

CORDchemicals

Carbondioxide

CORDchemicals

ActivityCorrosion products

ActivityCorrosion products

NPP-Systemhigh dose-rateNPP-System

high dose-rate

Figure 3: Logistics of the HP CORD UV process

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Decontamination & decommissioning

NUCLEAR ENGINEERING INTERNATIONAL | www.neimagazine.com May 201418

The main process variables are volume,

surface, construction materials, oxide

characteristics, operating history, process

temperature, ion exchange rate and

chemical concentration.

The entire process takes just one fill of

water. Each cycle is divided into the following

steps:

■ Oxidation with HP

■ Reduction of HP with decontamination

chemical

■ Decontamination

■ UV decomposition of decontamination

chemicals and clean-up.

Oxidation prepares the oxide layers for

their removal, but does not mobilize large

amounts of activity and corrosion products

itself. The mobilization of activity and

oxide dissolution take place during the

decontamination step. The water is not

flushed. The dissolved corrosion products

and activity are removed via ion exchangers

and the water is reused in the next cycle.

Bypass purification is performed during

the decontamination step to fix the

dissolved activity and corrosion products

on ion exchange resins (Co-60 is the

dominant nuclide in recently shut-down

plants, and the main cause of dose).

After the decontamination step the

ultraviolet decomposition of the remaining

decontamination chemicals takes place

in-situ. The decontamination chemicals are

decomposed to water and carbon dioxide

while the remaining activity and corrosion

products are being purified. Through this

procedure, the system water reaches a

purity that is close to demineralised water

at the end of each cycle, so the water can

be reused in the next cycle.

The HP CORD UV process does not

require a predetermined number of cycles

to be performed. The number of cycles is

varied depending on the decontamination

tasks and targets.

The cycle duration depends on the speed

of dissolution of deposits, the total amount

of deposits present, and the speed of

removal of dissolved species from solution.

During application the decontamination

solution is analysed in the hot lab and

the progress of the process is determined

through analyses of corrosion products and

radionuclides in solution.

ResultsThe decontamination targets for

Unterweser and Neckarwestheim

assumed immediate decommissioning, but

maintained the possibility of a return to

service. They were:

■ Minimizing the total activity inventory

■ Reducing dose rates at plant systems,

especially the primary circuit with its

heavy components, to facilitate further

handling

■ Minimizing ambient dose rates to lower

future personnel doses (ALARA)

■ Avoiding a significant shift of the

gamma-to-alpha ratio (which can

have consequences in later stages

of decommissioning operations,

particularly concerning health physics

and radiation protection monitoring).

Based on these targets, five HP CORD UV

cycles were applied in both PWRs. During

on-site application AREVA’s chemical

experts provided 24-hour supervision.

Table 2 summarises the results. The

decontamination factors are the ratio of the

contact dose rates before and after FSD at

specific measuring points.

Based on these results and the total

surface of the decontamination area, the

average oxide layer removed is 4.3 µm.

Considering the amount of corrosion

products removed and the total surface

of the decontamination area, the average

oxide layer in Unterweser is 4.9 µm.

During the commissioning phase and

in the course of the FSD applications at

Unterweser and Neckarwestheim the

primary coolant was permanently filtered.

In the Unterweser plant 9x1012 Bq of

activity (Co-60) was additionally removed

by filtration.

A number of measuring points were

identified in the decontamination area to

determine the average decontamination

factor. In Unterweser (a four-loop PWR)

83 measuring points were used, and in

Neckarwestheim (a three-loop PWR) 66

were used. Before FSD, at Unterweser

100% of the points in the decontamination

area had dose rates above 0.5 mSv/h (0.05

Rem), while at in Neckarwestheim the

distribution was about 90%. The initial

dose rates in the primary systems differed

significantly, from an average of 5 mSv/h

(0.5 Rem) in Unterweser to 630 µSv/h

(0.063 Rem) in GKN 1.

The contact dose rates in the primary

systems after FSD were significantly lower,

even in the area of plugged tubes. They

averaged 30 µSv/h (0.003 Rem) in both

plants. The distribution of dose rates in

the decontamination area of both plants

shifted significantly, with 90% of the

measuring points lower than 0.1 mSv/h

(0.01 Rem).

AREVA’s HP CORD UV concept, adjusted

to the needs of these plants, resulted

in a very high decontamination while

keeping all systems operational. Extensive

inspection programmes after treatment

revealed no detrimental effects on the

material. Technically, the plants could

return to operation.

Performing the decontamination right

after the operational phase proved to be a

great advantage. The key to success was

involving the plant’s own personnel during

all phases of the decontamination. Their

detailed system knowledge, expertise, and

extended operational experience, together

with the close and open cooperation with

AREVA’s decontamination personnel,

produced the best result. ■

References

Dr. Christian Topf (christian.topf@areva.com), Luis Sempere-Belda (luis.sempere-belda@areva.com), AREVA GmbH, Abteilung Chemistry Services, Paul-Gossen-Str. 100, 91054 Erlangen; Kai Tscheschlok (kai.tscheschlok@eon.com), E.ON Kernkraft GmbH, Kernkraftwerk Unterweser, Abteilung TMT, Dedesdorfer Str. 2, 26935 Stadland. Klaus Reuschle (k.reuschle@kk.enbw.com), EnBW Kernkraft GmbH, Kernkraftwerk Neckarwestheim, Im Steinbruch, 74382 Neckarwestheim, Germany

Table 2: Full-scope decontamination applications

Unterweser (KKU)

Neckarwestheim-1 (GKN1)

Activity & corrosion products removed

Corrosion products (Fe, Cr, Ni) 459 kg 260 kg

Total activity removed (> 99% Co-60) 9.1 x1013 Bq (2,460 Ci) 1.1x1013 Bq (297 Ci)

Decontamination factors (DF) achieved

DF overall 94.5 (83 MP) 81 (66 MP)

DF of primary circuit (loop & pressurizer) 158 (26 MP) 31 (20 MP)

DF of steam generator tubing area 147 (16 MP) 224 (15 MP)

DF of auxiliary systems (RHR / CVCS) 35 (41 MP) 44 (31 MP)

Waste

Ion exchange resins 21 m³ (planned 23 m³) 11 m³ (planned 15 m³)

MP = measuring point

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