steam gen design

8/12/2019 Steam Gen Design

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Design of Monju Steam Generator- with regard to Maintenancean.fiRepair -

T. Takahashi, Y. Yamagishi, R. Mukaibo

AbstractThe steam generator design of Monju started in 1963 .and

since then, extensive research and design work has been done.Monju steam generator consists of one evaporator and

one superheater to each of the three independent coolingsystems. Ultrasonic and eddy current tubing inspection de-vices have been developed for maintenan ce. And for failedtubes,welding or explosive plugging is applied.

Following the completed safety review and the comingdesign and construction licensing, Monju is expected to reachcriticality in fiscal year 1990.

IntroductionMonju is the first power generating fast breeder

reactor in Japan. The design, which started in.1968,la nowin the final stage and the construction is targeted to reach

initial criticality in fiscal year 1990.In the steam generating system, the helical coiled tube

type steam generator has been chosen and. at the same tiiie,the straight tube type has been under consideration as asecond choice.

Research work on the steam generator, using mock-up nodels,has been extensively done at Oarai Engineering Center. In1971,a l.MV steam generat or st arted testing and since 19742 different pOMlv steam generators have been installed andtasted. The first ran about 3500 hour s and the second about14,000 hours , now under endurance testing. In parallel tothese mock-up tests, material t ests, tube to tube-sheetwelding tests, sodium-water reaction tests, development ofleak detectors have been under way. These significant basetechnologies have been a great support to the development ofthe Monju steam generator.

Monju Power Station, which is rated at 7l4MVth, 28OMWin electric power generation, has 3 separated once throughcooling systems and each has one evaporator and one super-heater. In 1984, the evaporator of Monju was awarded toMitsubishi and the superheater to Hitachi.

For Presentation at IAEA IWGFR Specialist's Meeting onMaintenance and Repair of LMFBR Steam Generators (Oarai,Japan, June 1984)

Design of Steam GeneratorThe steam generator is composed of a separated evaporator

and superheater, both helically coiled tube type with free

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surface iodi un insid e. The water stsani cycle is a once throughtvoe arvi the reheater whic h was installed in the originalddiiin was eliminated.

The steam generat or system is shown in Fig. 1 and thestructure in Fig- 2. The input heat condition from the ec-ondary sodium coolant system is 5Q5C sodium at the entranceof the superheater and 32?'C at the exit of the evaporator.Steam condition at the outlet of the superheater is kS7 C

2and 133 =c?/c:n sa nd the feed water during normal operationis 2'iO C. The ma teri al used for the evaporator is 2- -Cr-lMofar both the heat transfer tubes and the shell of the evapo-rator. For the superheater, considering the high temperature,SUST21 material has been selected. The outer diameter of theheat transfer tube is 31.8mra and the numb er, 1^7. Sodiumlevel control is done by cover gas pressure for the super-heater and overflow line for the evaporator.

Specific numbers for the steam generator are shown intable 1.

In a rare case when water/steam leaks inside the steamgenerator and sodium-water reaction occurs, it will be detectedby a hydrogen detector system which is installed both insidesodium and in cover gas. In order to accommodate thepres-sure rise, a pressure relief and reaction product separatorsystem is prepared. They are normally isolated from the so-dium system by a rupture disc at the cover gas. This system

is shown in Fig. 3-Vhen the plant is shut down, for instance, by reactor

scram, during power operation, the steam generator is isolatedfrom the plant cooling system anrf the decay heat is removedby an air blast cooler which is installe d in the secondary'main cooling system, parallel to the steam generator.

Desizn Base yents of MonjuAs a design base' accident for the Monju steam gener-

ator, a double ended guillotine failure of a single heattransfer tube is the basic condition and in order to accommo-date the successive rupture propagation, a maximum of 1 + 3guillotine failures of neighboring tubes is considered.Mien the accident occurs, it is recognized by a rupturesignal from the rupture disc and the steam gen erato r is iso--lated from the cooling system. The water/steam will blowdown and when the pressure reaches down to a certain degree,nitrogen gas is supplied.

In a case where the leak rate of water/steam is small,and the leak detection is done by the hydrogen detector,rather than by the burst of the rupture disc, manual scramof the reactor by the decision of the plant operator isachieved and,consequently, the steam generator is isolatedfrom the cooling system.

In both cases mentioned, the sodium inside the steam


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jeneratar will be drained after a total inspection of the5y tem.

Valves and pro cess i nstrume ntatio n are shown in Fig. t.

Design Co nsider ations for >fainxenanc e and RepairDurini_N'or^ial Operation

The three sxaam generators belong to three independentcooling systems and are all located in the reactor auxiliarybuilding where access is free. The evaporator and superheaterare each located in separate rooms and the reaction producxstank is installed on the roof of the building. This enableseasy access for maintenance and repair. The steam generatoris installed in each room in such a way that access is pos-sible not only from the side s, but also from the top andfrom the bottom.

The hydrogen detectors are designed so that absol utecorrection can be done, as installed, independently from thesystem,Small_Leaks Occurrences

Inspection technology of heat transfer tubes, aftersmall leaks have occurred, has bee n developed and establishedusing either ultrasonic testing devices or eddy current test-ing devices for defect indication detection. Howeve r, inorder to facilitate accessibility and have direct access tothe water/steam inlet and outlet tube to tube-sheet area ,

a cao cover type structure has been introduced. Furthermore ,to help -nake the insertion of the detector into the tubesmoothly, certain limits co [he curvaturesof the tubes havebeen applied.

Tube plugging technology by explosives and by weldinghas been developed. The height of stand pipes are decidedto enable easier access for the plugging.Lar-re Leak Accidents

When a large leak inside the steam generator occurs,inspection of heat transfer tube bundlesar ri repair work isrequired. In order Co do this,a flangs structure is adoptedat the top of the steam generator and this enables the tubebundle to be pulled out together without cutting off any partof the shell. This method of inspection and repair has al-ready b een applied and ex perienced at the 50>flv* mock-up steamgenerator facilit y. The space above the steam generator atthe plant is left open and vacant in spite of the ratherhigh elevation where it is installed, and this is to accommo-date the withdrawal.

The layout of the steam generator is shown in Figs.5and 6. Fig. 7 shows the features that have been mentioned.

ConclusionFollowing the conceptual design of the steam generator

of Monju, the related safety review, including the evalua-

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