Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
65

Completion of “Joyo” Experimental Fast Reactor
Upper Core Structure Replacement- Development of Fast Reactor Core Internal Equipment
Replacement method MASAHIDE HARA*1

JUNYA TANAKA*2

HIROYOSHI OKAZAKI*3

KENICHIRO ODANI*3

KAZUYA TAMURA*3

TAKUMA TERAMAE*4

In the “Joyo” experimental fast reactor of Japan Atomic Energy Agency, the upper core
structure was damaged due to an accident with the experimental equipment in the reactor vessel.
Under high radioactive atmosphere (with fuel loaded) and with the inert gas boundary of aboutmaximum of 160°C, the old upper core structure was extracted through a gap of millimeters by
remote control and was replaced with a new upper core structure. Such unprecedentedly difficult
operation was successfully completed after a prior study of the risks, design validation using afull-scale mock-up and operational training. This replacement contributed to the development of
the fast reactor core internal equipment replacement method.

|1. Introduction
In the “Joyo” experimental fast reactor (sodium-cooled fast reactor: hereinafter called“SFR,” thermal output: 140 MWt) of Japan Atomic Energy Agency (hereinafter, “JAEA”), a
problem occurred in the extraction of the Material Testing Rig with Temperature Control(hereinafter “MARICO-2”) that was conducted in May 2007, and part of the testing rig (hereinafter“test subassembly”) was damaged and deformed onto the in-vessel storage rack, disabling access to
the reactor core fuel in some areas. It was also confirmed that the contact with the MARICO-2 test
subassembly caused damage to the bottom of the upper core structure (hereinafter “UCS”) (SeeFigure 1)(1).

Figure 1

State of damage of core internal equipment(2)By observing the inside of the core, damage of MARICO-2 was found.
Through the observation of the underside of the UCS, damage of the UCS was also confirmed.
*1
*2
*3
*4

Chief Staff Manager, Reactor Core & Safety Engineering Department, Nuclear Energy Systems Division, Energy & Environment
Nuclear Plant Construction & Maintenance Engineering Department, Nuclear Energy Systems Division, Energy & Environment
Nuclear Plant Component Designing Department, Nuclear Energy Systems Division, Energy & Environment
Nuclear Plant Manufacturing Department, Nuclear Energy Systems Division, Energy & Environment

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
66

JAEA decided that to recover from this problem, the MARICO-2 test subassembly should be
retrieved and the UCS should be replaced with a new one. Under a commission from JAEA,
Mitsubishi Heavy Industries, Ltd. (MHI) cooperated in the development of techniques and aconstruction method utilizing those techniques, both are required for this UCS replacement work,and designed and manufactured the UCS for replacement as well as the equipment used to carry out
the work, which was conducted over May to December 2014. This report describes theachievements in the development of techniques and the methods for the replacement of UCS.

|2. Outline of UCS replacement work
2.1

Overview of UCS

An outline structure of the reactor vessel, the rotating plug and UCS of “Joyo” are shown in
Figure 2. The reactor vessel has a cylindrical container structure with inside diameter of about
3.6m and height of about 10m, and contains the core internals, sub-assemblies, etc. In addition, it
has sodium filled inside it to cool sub-assemblies, and its temperature is kept at about 200°C during
reactor shutdown. The sodium in the reactor vessel has a free liquid level, and inert gas (argon gas)
is filled in the space above the liquid level as cover gas. The rotating plug (including the UCS)
installed on top of the reactor vessel defines the boundary of this cover gas.

Figure 2

Outline structure of the reactor vessel, rotating plug and UCS of “Joyo”(2)The UCS, which forms a part of the rotating plug, has a cylindrical structure, as shown in
Figure 2, with the maximum diameter of about 1345 mm and a height of about 6330 mm. Its
bottom is provided with a flow-regulating grid and thermocouples for measuring the coolant
temperature at each sub-assembly outlets. The UCS is also structured to have a large diameter at its
lower part, specifically, with the outside diameters of the body being φ1130 mm, φ1010 mm
andφ1060 mm from the top.

2.2

Purpose of UCS replacement work

For the purpose of recovering the fuel exchange function and restoring the reactor to anormal state, the UCS replacement work was conducted using the following procedure: (1) the
damaged old UCS was extracted; (2) the MARICO-2 test subassembly interfering with the UCS
was retrieved (MARICO-2 retrieval work, under control of JAEA), and (3) a newly manufactured
UCS was loaded (new UCS installation work).

2.3

Constraint conditions and special conditions for UCS replacement work

In the study of the UCS replacement work method, the constraint conditions and special
conditions described below were considered, the risks in the work were extracted and the work
procedure for avoiding the risks and the specifications of jigs and devices used in the work were
examined.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
67(1)

Constraint conditions(a) Sub-assemblies in the reactor: Since the Sub-assemblies are loaded inside the reactor, theconsideration must be given to radiation.
(b) Liquid sodium in the reactor: Chemically active liquid sodium is present in the reactor, and
it is necessary to keep the inert gas atmosphere and take measures against high temperature.
(2) Special conditions(a) The structure of UCS: Since the UCS was designed without assuming it would be replaced,
it has never been extracted in more than 30 years since the reactor was constructed.
Therefore, it is necessary to study the avoidance of interference and extraction force at thetime of extraction.

|3. Efforts by MHI toward the completion of the replacement work
UCS replacement work is broadly divided into two tasks: the “old UCS removal work” inwhich the existing damaged UCS is extracted, put in a storage cask and transferred to a storage
place; and the “new UCS installation work” in which a newly manufactured UCS is loaded into theremoval hole (guide sleeve). Taking into consideration the previously described constraint
conditions and special conditions, MHI studied the work procedure and the specifications of jigs
and devices used in the work and manufactured them. Then, using these jigs and devices, afull-scale mock-up test was conducted for the validation of the work method, and training wasprovided before the field work.

Figure 3

Outline of UCS replacement work procedure

The damaged old UCS was extracted using screw jack equipment and wire jack equipment,
and the new UCS was inserted using the wire jack equipment.

3.1

Work procedure and problems and countermeasures

An outline of the UCS replacement work procedure is shown in Figure 3. In addition, themajor problems and countermeasures in the replacement work are described below.
(1) Problem such as interference in the old UCS extraction work
The UCS of “Joyo” was designed as an eternal structure, as were the reactor vessel androtating plug, and it was not assumed that the UCS might be replaced during the lifetime of theplant. In fact, it had never been replaced. Therefore, before the old UCS extraction work was
conducted, the risks described below were considered. The concept of the risks is shown in
Figure 4.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
68

Figure 4 Concept of risks during extraction of UCS
At the time of the extraction of the UCS, there is a risk of damage to the UCS caused bysodium shearing resistance or interference.

(a) When the gap between the old UCS and the guide sleeve is obstructed by sodium deposited
during over 30 years of use, resistance is produced by the sodium being sheared during
extraction of the old UCS. The guide sleeve in which the old UCS was inserted is a thin
structure with a wall thickness of 5 mm. If the sodium shearing resistance becomes
excessively large, the guide sleeve may be deformed or damaged.
(b) The design value of the outside diameter of the old UCS large-diameter part is φ1060 mm,
while the design value of the inside diameter of the guide sleeve small-diameter part is
φ1070 mm. During extraction of the old UCS, the gap becomes 5 mm at one side. The
UCS is positioned with reference to the central axis of the core internals (core support
plate), and the UCS has been set up being displaced from the central axis of the guidesleeve. As such, the gap may become lopsided. Therefore, if the large-diameter part of theold UCS is largely inclined during extraction, the old UCS and the guide sleeve interfere orcome into contact with each other, producing resistance (hereinafter, “interference
resistance”). If the interference resistance becomes excessively large, the old UCS may be
deformed or damaged.
To prevent the above (a) and (b), it is effective to monitor the inclination (levelness of the
top face) and load of the old UCS during the extraction work and to stop the work byinterlocking if the inclination or load becomes large.
(2) Establishment of method using vinyl bag
The O-ring replacement and its associated O-ring groove cleaning work before theinstallation of the new UCS had to be conducted manually, in the state of the cover gas
boundary being secured, while being visually checked. Therefore, a gas boundary made from atransparent vinyl bag with work gloves attached thereto was adopted. This work must be
conducted via the vinyl bag in a very confined space above the rotating plug (See Figure 5.)
The O-ring replacement and O-ring groove cleaning method were established after theverification of work through the full-scale mock-up test and changes in the design, such as the
positioning of the gloves, were based on the verification results.

Figure 5 Work via vinyl bag
Through the full-scale mock-up test in the closely simulated field work environment, the problems
in the work were identified and improvements were made, resulting in success in the work.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
69(3) Ascending of high-temperature cover gas
In the period from the old UCS extraction work to the completion of new UCS
installation, the high-temperature cover gas in the reactor vessel will ascend while top of the
UCS guide sleeve is opened, and the ability of the boundary formed by the vinyl bag which is
set up on the rotating plug cannot be ensured. To prevent this, the cover gas circulating system
was constructed, so that low-temperature argon gas was blown down from upper side of theargon gas boundary to prevent the high-temperature cover gas from ascending from the reactor
vessel. This system was planned and provided by JAEA (See Figure 6)(3).

Figure 6 Concept of the measures for high-temperature cover gas(3)An inert gas circulating system was constructed, so that the down blow of cooling gas
prevents the high-temperature cover gas from ascending. (By JAEA)

3.2

Full-scale mock-up test

Prior to the UCS replacement work, a full-scale mock-up test was conducted using theequipment used in the actual work and mock-ups with the same sizes as those of the actual devices,
the work procedure and operational methods of jigs and devices were checked and the obtained
results were reflected in the work procedural manuals. In the full-scale mock-up test, mock-ups
with the actual dimensions of the small rotating plug, guide sleeve and UCS were set up on a test
stand with dimensions of about 12m × 12m × 8m, and the working conditions of the actual work
were simulated. The lower body of the guide sleeve mock-up was designed so that part of the gap
between the guide sleeve mock-up and the UCS mock-up would be smaller than the design value,in consideration of the manufacturing tolerance in the actual guide sleeve and any distortion createdduring its installation. The UCS mock-up was designed to simulate the weight and the position of
the center of gravity of the actual equipment.
The outlines and results of the “jacking work procedure test” and the “extraction andinsertion performance check test” in the full-scale mock-up test, which were implemented to obtaindata on the UCS removal and insertion work, are described below.
(1) Jacking work procedure test
All the operations from the installation to the removal of the screw jack equipment were
simulated and the setting data for parameters (extraction height, extraction speed, allowableloads thereto, etc.) required for extraction by the screw jack equipment were obtained. Throughthe simulation, the prospect was obtained that the work for jacking from the state of being
placed on the bottom to the position at 1000 mm could be conducted without any problems,
while the load and levelness were maintained within the allowable values. Furthermore, tocheck the procedure of O-ring removal work that will be conducted after the jacking work of
the old UCS in the actual equipment and the handling ability of jigs, training for removing
O-rings via the vinyl bag in the same way as in the actual work was conducted.
In the old UCS jacking work and the new UCS insertion work, there is an especially highrisk that the UCS and the guide sleeve may interfere or come into contact with each other,resulting in difficulty in continuing the work. To reduce these risks, the interference simulation

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
70

test was conducted using a mock-up for the purpose of estimating the position whereinterference occurs and establishing a procedure for avoiding interference. The results of theinterference simulation test are shown in Figure 7. In this test, an interference plate with awidth of 50 mm was set up on the guide sleeve mock-up, and the data on the loading behavior
in the directions from 0 to 360 degrees when it interferes with the UCS mock-up was obtained.
If the UCS interferes or comes into contact with the guide sleeve during the extraction of the
UCS, the loads applied to the three jacks become unbalanced due to the resistance at theinterfering position. In the old UCS jacking work, etc., the position where interference occurred
was calculated or estimated based on the obtained offset load data. As shown in Figure 7, theoffset load data obtained when the mock-ups were directly interfered with generally agrees withthe load value obtained when the interference plate was set up, which shows that this procedure
is effective(2).

Figure 7 Results of the interference simulation test using the mock-up
In the mock-up test, interference was intentionally produced, so that a method of
identifying the interfering direction could be established.

Next, the procedure for avoiding interference is described. The screw jack equipment has
a high rigidity and is integrally structured with the old UCS so that precise level control isrealized. Therefore, as shown in Figure 8, the screw jack equipment was set up on the top faceof the guide tube, the guide tube was slid in horizontal and right-angled two directions by thepush bolt for the adjustment of the horizontal position provided on the guide tube, thereby
correcting the eccentricity of the old UCS to avoid interference between the old UCS and theguide sleeve, and the UCS mock-up was extracted up to the position of 1000 mm. Thus, it was
confirmed that by using the same method, the old UCS could be extracted in the actual fieldwork(2).

Figure 8

Screw jack equipment

Three screw jacks are placed in a circle, with each of them equipped with a load meter anda stroke meter. They are set up on the guide tube so that horizontal adjustment can be made.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
71(2)

Extraction and insertion performance test
In the extraction and insertion performance test, the UCS test body was visually checkedfrom above and from the lateral side, and all the operations of the old UCS extraction andhousing work and the new UCS insertion work using the wire jack equipment were simulated
while checking the run-out state of the UCS test body occurring during extraction and insertion,and the setting data for the necessary parameters (extraction/insertion height,
extraction/insertion speed, allowable loads thereto, etc.) were obtained. Thus, the prospect was
obtained that the housing work (1000 mm to about 11400 mm) and the insertion work (about
11400 mm to 0 mm) could be conducted without any problems, while the load and levelness
were maintained within the allowable values(2).
Next, concerning the means of avoiding interference, since the wire jack equipment has aflexible structure，the eccentricity of the new UCS was corrected by the guide system provided
at three points in the circumferential direction on the top face of the rotating plug, so thatinterference between the new UCS and the guide sleeve could be avoided.
Furthermore, in the preparatory work for the extraction and insertion performance test,
the procedure for setting the large, heavy devices, such as the door valve and cask, in alignment
with the central axis of the UCS was verified, and through the measurement of the deformation,etc., of the shield floor associated with the setting-up of the large, heavy devices, the datarequired for securing the verticality during the installation of the cask, etc., in the actual fieldwork and for preventing eccentricity, etc., of the UCS and the guide sleeve lifted up by the wirejack equipment was also obtained.
In addition to the above operations, in the full-scale mock-up test, a series of tasks
including the removal and attachment of bolts were implemented. Through this work, theinstallation positions and connection of devices, installation procedures, handling methods,,
etc., were clarified, the data contributing to the development of work procedural manuals was
obtained, and the skills of workers were enhanced, toward the actual field work.

|4. UCS replacement work in the field
The UCS replacement work implemented in the field based on the previously describedfull-scale mock-up test results is described below.

4.1

Old UCS removal work

In the old UCS removal work, the monitoring of levelness with high precision and the load
control of the jacking equipment and the wire jack equipment effectively functioned and the work
was completed as planned.

Figure 9 Changes of load during jacking of UCS

During the extraction of the UCS, no sodium shearing resistance was observed.
Interference during the extraction was avoided by horizontal movement.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
72(1)

Old UCS jacking work
At the position of 0 to 2 mm, to check for the presence or absence of sodium shearing
resistance and its behavior, the old UCS was jacked up by manual handle operation (with ajacking speed of about 2 mm/h). The load measurement results at the position are shown in
Figure 9(a). The extraction load increased with the movement of the jack, but it became almost
constant at about 16.8 tons, and resistance associated with sodium shearing was not observed.
However, in the observation after 1000 mm jacking, adherence of porous sodium with fineparticles that had aggregated or deposited on the side face of the upper body of the old UCS
was observed(2).
At the position of 2 to 1000 mm, the old UCS was jacked up by automatic operation withinterlocking triggered by levelness and load restriction. The load measurement results at thisposition are shown in Figure 9(b). Here, the load value was set by canceling the weight of theold UCS, etc. to make it easy to determine the load produced by interference. During jacking,load equalization measures (level adjustment only by screw jack operation) were conducted as
appropriate, so that the unbalanced load produced by the offset between the center of gravity ofthe old UCS and the center of jacking was corrected. As shown in Figure 9(b), at the position of
about 750 mm, it was detected that interference was produced between the old UCS and theguide sleeve. According to the prepared procedure for avoiding interference, the extraction
position was moved by about 0.7 mm using the push bolt for the adjustment of the horizontal
position on the guide tube, resulting in the avoidance of interference. It was confirmed that theprocedure for avoiding interference was appropriate(2). Thus, the prospect was obtained that in
the old UCS jacking work, after the old UCS was jacked up to the position of 1000 mm, thesubsequent old UCS removal and containing work could be conducted without any problems as
initially planned.
(2) Old UCS removal and containing work
In the old UCS removal and containing work using the wire jack equipment, the total
initial load was about 18 tons as shown in Figure 10, but while the removal work was being
conducted, the resistance which may be caused by the contact between the old UCS and theguide sleeve gradually decreased, and it became almost stable at about 17.3 tons in the position
between about 4000 mm and the upper limit, where the removal and containing work could becompleted. The storage cask in which the old UCS was contained was transferred from thereactor containment vessel to the old waste treatment building, an additional shield body forstorage was mounted, and the storage cask was put into storage.

Figure 10

Changes of load during removal and insertion of UCS

During the removal of the UCS, the load fluctuated until the UCS completely passed
through the small-diameter part of the guide sleeve. During the insertion of the new
UCS, the load was stable except when the fore-end came into contact.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
73

4.2

New UCS installation work

The new UCS installation work could also be completed as planned, because a detailed work
plan was prepared and the procedure was mastered through training in advance.
(1) O-ring attachment work for new UCS
Since the O-ring attachment work for the new UCS must be manually done by workers, it
was conducted in the state of a temporary shield plug being inserted in the guide sleeve. The
cleaning of the O-ring groove and the attachment of the O-ring must be conducted using gloves
via the vinyl bag in a confined place. Using the jigs and the procedure developed based on thework training results in the full-scale mock-up test, the O-ring for the new UCS could be
successfully attached to the O-ring groove. The state of the O-ring installation work for the new
UCS is shown in Figure 5.
(2) New UCS insertion work
In the new UCS insertion work using the wire jack equipment, the total load was
generally stable at about 16.6 tons as shown in Figure 11. Interference between the new UCS
and the guide sleeve was detected at the position of about 4500 mm, but according to theinterference avoidance procedure established through the full-scale mock-up test, theinterference could be avoided by moving the insertion position by about 5 mm in the horizontal
direction using the guide equipment. Thus, it was confirmed that the avoidance procedure wasappropriate. With the levelness control and load control during the insertion work and thefunction of the guide equipment, no interference that prevented the insertion of the new UCS
was produced after that(2).
Furthermore, the new UCS was required to be installed within ±1.02 mm from theplanned position based on the old UCS installation position and the new UCS manufacturing
dimensions. In the placement/installation of the new UCS on the bottom, as shown in Figure
11, after it was confirmed that the tip of the guide bolts were projected out through the new
UCS flange bolt holes, each guide ring adjusted to the given dimensions was inserted into thegap between the bolt hole and the guide bolt. The installation accuracy was measured after the
new UCS was placed/installed on the bottom, and the results showed that the error was within
the maximum of 0.35 mm ± 0.1 mm from the planned position(2).

Figure 11 Method of positioning in installation of the new UCS

Using the guide bolt and the guide ring, the new UCS was installed at the given position.

|5. Conclusion
The maintenance and repair works for reactor core internal equipment in sodium-cooled fast
reactor must be conducted in a high-radiation dose, high-temperature environment and inert-gas
sealed environment. Therefore, development of maintenance and repair work techniques which are
different from those for light water reactor power plant is required. For this replacement work of
the upper core structure of “Joyo,” the special equipment and construction method under the newconcept were developed, the full-scale mock-up device of the actual equipment was manufactured,Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)74

and the technical verification, feasibility check and training of operators were carefully repeated
before the actual replacement works, and finally, the replacement of the upper core structure was
successfully completed as initially planned. The techniques, experiences and findings obtained
through this replacement will greatly contribute to the progress of maintenance and repair
techniques for the fast reactor in the future.
Lastly, we would like to express our gratitude to JAEA and the concerned parties for theirguidance and cooperation in the study of the methods, the manufacturing of the equipment and thefield work for the upper core structure replacement operation, and would like to ask for further
understanding and support for the development of fast reactors in the future.
[Explanation of terms]yyyUpper Core Structure: One of the fast reactor core internal structures. It has the functions of
guiding and holding the control rod driving mechanism and measuring the temperature of
the coolant at the core fuel assembly outlet.
Gas boundary: The space in which the target gas is enclosed or the material which forms thespace.
Full-scale mock-up: A facility composed of full-scale mock-ups of all objects for target
actions.

References
1.

T. Kobayashi, “Development of Inspection and Repair Techniques in Reactor Vessel of Fast Reactor Endeavour to retrieve damaged components in the experimental fast reactor Joyo -,” Journal of theAtomic Energy Society of Japan, 54 (2012), pp. 664–666. [in Japanese].
2. H. Ito, et al., “Inspection and repair techniques in the reactor vessel of the experimental fast reactor Joyo;
Replacement of upper core structure,” JAEA-Technology, 2016-008, Japan Atomic Energy Agency,(2016),[in Japanese].
3. H. Ushiki, et al., “Inspection and repair techniques in the reactor vessel of the experimental fast reactor
Joyo; Development of cover gas recycling system with precise pressure control,”JAEA-Technology,
2015-042, Japan Atomic Energy Agency, (2016),[in Japanese].