Trip Report from SNO with Details of How to Build an

Acrylic Sphere

L. BartoszekBARTOSZEK ENGINEERING

6/26/05

What’s to be covered here:

• Things I learned about the construction of SNO, particularly the acrylic sphere– Many thanks to Art McDonald and others from

SNO

• Comparing cost information for SNO and Braidwood– This needs to be rescaled to Krasnoyarsk size

• This document assumes Krasnoyarsk will use an acrylic sphere—it may not

Building the Acrylic Sphere

• SNO’s acrylic plate was cast by Polycast, then thermo-slumped into female molds by Reynolds Polymer Technology to produce the spherical sections

• After slumping, the sheets were machined to shape on a 5 axis milling machine– Only pure water was used as a cutting fluid

• AV was pre-assembled above ground to check fits

Dry fit of panels prior to shipping to the mine—photo courtesy of SNO

Bonding the acrylic• The secret to the success of the

construction of SNO was:– A specially formulated polymerizable cement

made by Renolds: LUC6751-RT• Room temperature curing• Pot life of ~8 hours to allow pouring of very long

joints

– Development of repair techniques for imperfectly bonded joints

– Much of following comes from the “Handbook of Acrylics” by Jerry Stachiw (who worked on SNO)—very useful book (thanks, Art)

Surface Preparation of panels to be joined

1. Edges of acrylic parts wetted by cement must be planed or routed smooth

2. Next is wet sanding with 60 and 120 grit paper on block sander, followed by 240 and 300 on orbital sander

3. Surfaces adjacent to to the bond gap are wet sanded with 60 and 120 grit paper on block sander

4. After sanding, parts must be annealed at no less than 185°F for time determined by thickness

Surface Preparation 2

5. Wash surfaces of the joint with a water based detergent solution (4% Alconox detergent)

6. Rinse the surfaces with distilled water

7. Wipe rinsed surfaces with 20% isopropyl alcohol and dry with paper towels (Santara #2500)

8. Mask bond gap with masking tape (3M #2090 or equivalent)

Preparation of dams

• The cement shrinks by 20% at curing, so the dams must allow a reservoir of cement, and a means of adding more.

• The dams are silicon rubber (see next figure) and are cleaned same as acrylic, except no alcohol is used.

• Aluminum tape is placed on dams where they will be wetted by cement (3M 591)

Picture from “Handbook of Acrylics” by Stachiw

These pictures make it clear that we must have access to both sides of the joint, even for the last joint.

The chimney must be designed to accommodate passage of a person.

Scale drawing of a man inside a Krasnoyarsk acrylic sphere

The chimney as shown is large enough for access.

Size shows need for internal scaffolding to polish bond joints.

Surface prep 3• The acrylic and the dams must be primed

with adhesive primer to ensure the dams do not fall off from the hydrostatic pressure of the cement.

• The aluminum tape on the dams is covered with masking tape

• Dow Corning 1200 primer is applied to all the surfaces where the dams are glued to the acrylic– Allow to dry for 60 minutes

Bond Preparation• Coat the primed areas of the acrylic and

the dams with Dow Corning 790 building adhesive– Allow to cure to tacky to the touch for 30

minutes

• Remove the masking tape from the acrylic bond gap surfaces– Do not let any 790 get onto the bonded

surfaces because it will prevent polymerization

Bond Prep cont’d• The acrylic panels are put into position

using steel scaffolding and turnbuckle jacks to create the desired bond gap of .180 inches– .120” < Bond gap < .250” (more later)– Significant cost of fixturing

• Once the glue is tacky, center the dams over the acrylic plates and press them down– Use roller to drive out bubbles, cure for 12

hours

Picture from “Handbook of Acrylics” by Stachiw

Preparation of the cement

1. Cement held at 65-75°F for 72 hours2. Mix the components of the cement3. De-gas the mixture in vacuum bell jars4. Pour the cement into the bond gap

without entrapment of air5. Provide opportunity for entrapped air

bubbles to rise to the surface of the bond before it turns viscous

• A stainless steel needle can be used to puncture large bubbles

Curing and final surface prep• After 24 hour cure remove the dams

• Route off the excess cement

• Perform visual inspection of the joint

• Sand surfaces in steps up to 600 grit

• Post-cure the joint at 175°F for 12 hours– Do not post cure any joint shown by visual

inspection to have voids or flaws

• If all went well, the joint should be practically invisible

QA checks• Joints are visually inspected, and checked

with crossed polarizers and circularly polarized light

• Counting fringes quantifies the residual stress in the joint

• Working stress on joints must be kept below 1000 psi, and bonded joint tensile strength should be above 9000 psi– Repaired joints rarely get above 6000 psi

Why things are the way they are

• The bond gap must not be larger than .250”– The cement cures in an exothermic process– If the gap is larger, the heat runs away and

boils the syrup causing voids and residual stress

• The cement and acrylic must be held in a narrow temperature range before bonding– Not doing so results in a weaker cold joint, or

a weaker boiled joint

More reasons for things• Bond gap must be not be less than .120”

– Small gaps prevent adequate flow of cement which leads to voids and cracks

– Strength of joints with smaller bond gaps is less than for optimal bond gaps

• Rigid spacers must not be used to create bond gap– Residual stress goes way up during curing– SNO used adjustable turnbuckles and

manually shrunk the gap during curing

One way to go wrong

• If the joint is overfilled, the flexure strength of the joint is reduced by 50%– Routing and sanding brings it back up to

parent material strength

• Underfilling of the joint reduces the flexural strength by 70%– Routing and finishing makes it worse

• That’s why the dams make sure the joint is overfilled

When it’s wrong, cut it out• 5 meters of the 1 km of joints in SNO had

to be cut out and repaired• That 5 meters took as long to fix as all the

rest of the joints took to make• Repair is a big deal with an elaborate,

must be done properly, process (or you do it again. And again.)

• Sometimes custom plugs of acrylic must be fabricated for large holes– Big time penalty here

Comments specific to Krasnoyarsk

• SNO accomplished what they did from a development effort of several years and millions of dollars– Can we learn enough from them and

Reynolds to substantially lower this cost?– We need to make joints at Krasnoyarsk

similar to SNO’s because of the part size limitation from the corridors

– Can we train Russian technicians here in the techniques?

More Krasnoyarsk comments• SNO’s acrylic vessel was two inches thick

in places because of its size and the need to protect the heavy water from any chance of leakage– Simple calcs on Krasnoyarsk-size vessels

would give a much thinner shell thickness– People still need to walk inside the vessel to

do the polishing– I don’t know how the bonding technique would

change for much thinner acrylic plates

The next slides• The next slides are on costs for SNO and

Braidwood

• I have no idea how to cost this for Krasnoyarsk

• Can the acrylic shells be made, cut and shipped entirely inside Russia? – Can we transport acrylic parts made in the US

to Krasnoyarsk if the answer is no to above?– What are shipping costs in Russia?

Costing the Braidwood spheres• Art McDonald estimated that SNO’s acrylic

sphere cost ~$5M, dominated by labor– There were 120 panels at $25K each—

totaling $3M– There was about $2M in development costs

• Labor was 10 people for 2 years for assembly– I get an extra $2M from this labor ($50/hr total

includes bennies)– I don’t know whether some of these numbers

are mixed in with each other or is total $7M?

Ratios

• SNO AV surface area = 452.4 m2

• Braidwood AV surface area = 85 m2

• Ratio of surface areas (Br/SNO) = .188

• If SNO cost $5M, one Braidwood sphere could be $.94M– I put $500K/sphere in the estimate– Four spheres would cost $3.76M

Alternate calculation• SNO had 120 pieces at $25K each

• Vic gets 11 pieces per sphere

• 11 pieces at $25K each give $275K per sphere for parts, $1.1M for four

• Assume assembly labor scales with ratio of the number of parts– $2M X .092 X 4 = $.73M

• Total = $1.83M for four spheres– less than $500K ($458K) per sphere

Some conclusions• SNO AV numbers are >7 years old now

• This analysis does not include inflation

• Reynolds probably lost money on SNO, even though they revolutionized acrylic construction technology

• Our estimate ranges from ~$.5M to ~$1M per sphere, a 100% uncertainty range– We need to beat on this number– We may not have to pay (as much) for

development

More conclusions• I don’t know whether Art’s numbers

included all the custom fixturing, and supplies for cleaning and bonding

• SNO hired an engineering firm to design all the steel fixturing

• There are lots of tricky bits here that are not well documented even in the good documentation– No details on exactly how to anneal– Ports in dams for pouring are a mystery to me

Final conclusions

• I would use the higher cost estimate

• Cleanliness is going to be a big, costly deal– How low do backgrounds need to be?