Dry Cask Shielding Package

In 2019, Davis-Besse Nuclear Power Station, purchased an array of shielding from NPO for their spent fuel storage campaign. This package included a transfer cask shield bell, canister shield ring, and annulus gap shielding. All of these items contributed to greatly reduced dose rates for both gamma and neutron radiation.

Shield bells are a single-pick piece of shielding that rests directly on the transfer cask and out of the way of the welder. They are typically comprised of steel, borated polyethylene, and lead. The shield bell built for Davis-Besse has cutouts for trunnions to fit the NUHOMS EOS TC as well as chamfered edges to maximize shielding around the openings. DB saw an average reduction of 87% gamma and 62% neutron on contact.

On the EOS canister, a 20-piece T-Flex composite shield ring surrounded the welder. Each piece consisted of 2” of borated polyethylene and 1” of T-Flex® Tungsten. With the neutron shielding (borated polyethylene) facing down, Davis-Besse was able to optimize the shielding effectiveness. A high temperature insulative mat was also provided to protect the shielding pieces.

In addition to those two shielding packages, Davis-Besse utilized annulus gap shielding between the canister and transfer cask. These T-Flex® Tungsten shields are covered in high temperature weld blanket material and ensured that there were no gaps or streaming that would expose workers to unnecessary radiation.

With all the NPO shielding in place, Davis-Besse was able to have a very successful spent fuel storage campaign.

Mobile Shielded B25 Container

This mobile shielded container has all the same internal dimensions as a standard B25 container, but with a few added features. All six sides of the container are 0.5″ thick steel, which provides dose reduction for the rad waste it is intended to store. There are two sliding doors on top and four access doors on the front. All of these doors can be locked and secured with a single hasp. The container is equipped with lifting lugs, has forklift access, and rest on heavy duty lockable casters, which makes this container highly mobile. The entire container weighs approximately 2800 lb, but can easily be moved by a single person.

 

T-Flex Bottle and Vial Shields

NPO uses its custom part and mold making capabilities to produce form fitting shielding for bottles and vials. Bottles and vials filled with radioactive samples require safe storage and transportation. These shields provide just that and can be made to specific attenuation goals for a variety of different isotopes and bottle sizes. Handles and locking lids can also be incorporated into the shielding for added security.

 

60 mL, 250 mL, 500 mL, and 1000 mL

 

4-Pack Vial Shield

 

4-Pack Vial Shield with Handle

RadVision3D – LaSalle Case Study

In the scope of one job, LaSalle Generating Station saved 12 Rem through the use of 3D gamma radiation source mapping and intervention analysis, RadVision3D.

The Reactor Water Cleanup valve is an Anchor Darling valve that needed to be breached and have the internals replaced during a refuel outage. It is located in a locked high rad area. LaSalle needed an optimized shielding package that was not only quick and easy to install, but could also significantly cut down dose. Most work processes were to take place in the area circle in red below in the pre-shielding dose survey.

Pre-shielding dose survey provided by LaSalle

NPO, in conjunction with Transco, performed a RadVision3D scan in the room at four locations. The laser scan 3D point cloud, panoramic camera image, and CZT spectroscopy were combined to create a virtual environment that quantified and located the dose contribution from all sources. Specific locations around the valve were analyzed to determine the impact of various shielding configurations. Several of these configurations were simulated to determine their effectiveness.

 

Data gathering accuracy analyzed at points A and B

Survey Location Survey Data RadVision3D Data
A 1300 mRem/hr 1304 mRem/hr
B 600 mRem/hr 635 mRem/hr
RadVision3D Data is the average of 5 pick points in the approximate area of the dose survey

 

No Shields Hot Spot Shielding
mRem/hr mRem/hr Reduction
Location 1 769 756 2%
2 918 901 2%
3 1283 1242 3%
4 290 286 1%

First, the condition of the room without any shielding was compared to what the dose rates would be like if the hot spots were shielded. This correlated to the shielding plan prior to RadVision3D analysis. Though conventional thinking would point to this being the most effective solution, due to the complex nature and high dose rates in the room, shielding only the hot spots had very little impact on the dose rates.

 

Hot Spots Configuration 1
mRem/hr mRem/hr Reduction
Location 1 756 477 38%
2 901 378 59%
3 1242 534 58%
4 286 192 34%

With the hot spot shielding determined to be ineffective, different configurations of NPO lead wool blanket racks were tried. Each rack is approximately 3 feet wide and had 1″ lead equivalent shielding.

 

Hot Spots Configuration 2
mRem/hr mRem/hr Reduction
Location 1 756 362 53%
2 901 502 45%
3 1242 740 42%
4 286 155 47%

Without the need to go in the room and validate a shielding configuration by actually installing it, LaSalle was free to see the the dose rates of many different options.

 

Hot Spots Configuration 3
mRem/hr mRem/hr Reduction
Location 1 756 522 32%
2 901 531 42%
3 1242 740 42%
4 286 89 69%

 

Hot Spots Optimized Shielding
mRem/hr mRem/hr Reduction
Location 1 756 422 45%
2 901 824 10%
3 1242 414 68%
4 286 169 42%

This is the optimal shielding configuration – three lead blanket racks closing off the valve from the rest of the room. The amount of racks are kept to a minimum to cut on installation time and complexity and their specific positioning significantly drops the dose rates in the crucial work areas. Below is the LaSalle post-shielding survey results compared to the RadVision3D predicted results.

 

Data gathering accuracy analyzed at points A and B – Post-shielding

Survey Location Survey Data RadVision3D Data
A 500 mRem/hr 483 mRem/hr
B 290 mRem/hr 270 mRem/hr
RadVision3D Data is the average of 5 pick points in the approximate area of the dose survey data

The RadVision3D results compare very well with the actual conditions LaSalle experienced. All in all, the scans took 30 mRem, the shielding was installed in 20 minutes and took 163 mRem. With the approximate 50% dose reduction overall, LaSalle was able to save 12 Rem over the course of all work done on this valve.

Bellows Shielding

For BWR plants, shielding the inner bellows is particularly challenging. During refueling operation the concrete shield plugs above the reactor well are removed and replaced by water. This water filled gap becomes a source for both radiation and contamination.

NPO designed a specialized system to address the concerns with shielding this area. Some of the requirements were:

  • Wedge tightly into the inner bellows gap
  • Overlap shielding pieces to prevent streaming
  • Create an FME barrier
  • Withstand being stepped on
  • Have handles for quick and efficient installation
  • Be evaluated for leachables and resistance to corrosion in water

For the last requirement, NPO developed a new T-Flex® material, stainless steel. Stainless steel’s natural resistance to corrosion in water was a perfect fit was this project. The material was tested to MIL-STD-2041E for 1) leachable halides (bromide, chloride, fluoride) and nitrate, 2) leachable sulfur 3) heavy metals per ICP-AES and was determined to be acceptable for use.

Smear Frisker Cave

This small compact solution replaces larger heavier lead brick caves to reduce background when surveying surface smears and other samples.  The standard size (shown below) has 1” of lead on the top and bottom and 3” of lead on the sides and back.  Caves are built to order and can be designed to suit your background reduction requirements.  Finish is stainless steel. Handles are included.

Lead Panels and Water Shields

Sometimes the right shield for the job involves placing a shield as close to the source as possible as with pipe and valves shields, but other times the best solution is to put as much shielding as possible into a wall and create general area shielding for the workers. Lead panel shield walls and water shields have been manufactured by NPO for decades. These shields excel in reducing dose to a large area by blocking a general direction the radiation is coming from.

NPO lead panel shield walls are sheets of solid lead sandwiched between carbon or stainless steel sheets. These panels are modular and designed to link together to form walls of nearly any height and width. The shielding can be configured into a lead cave and create a low dose waiting area in the plant or used to shield exit portal monitors from shine.

Water shields offer a similar function, but with a different method. When empty, these shields are relatively light and easily moved into place. Once set, they’re filled with water which provides gamma as well as neutron shielding. NPO water shields can be built in a number of configurations that include interlocking rectangular shields and crescent shaped shields that nest into one another and curve around an area. All NPO water shields come with extendable legs to ensure a 2:1 height to base ratio.

Conoseals Shielding

This past spring outage season, NPO collaborated with a plant to provide them with custom shielding for their Conoseals during Core Exit Thermocouple replacement activities. Unit 1 and Unit 2 had different Conoseal structures, and the challenge was to model a shielding package that could be used for both elements interchangeably. The shielding package was designed, engineered and manufactured within 9 weeks of the very first customer communication.  The outcome was a ~90% dose reduction on contact, (Co-60).

Additionally, NPO supplied a modified storage container that the shielding was stored in, crane lifted into the reactor cavity during the activities, and will ease transporting it from Unit 1 to Unit 2 as needed.

Please contact NPO for additional details regarding this very interesting, effective application!

Core Barrel Shielding

In 2013 NPO designed and built a custom Core Barrel Shield for Palisades Nuclear Plant in Covert, MI.  The shielding was implemented in Palisades’ 23rd refueling outage and was a great success.  The story of the project’s success spread in May 2015 when the Palisades plant was recognized during the Nuclear Energy Institute’s best industry practice awards for the innovative shielding package to reduce radiation exposure through a more cost effective, safer method for core support barrel inspections.

For inspection purposes, every 10 years the core support barrel (CSB) must be removed from the reactor vessel and placed on its storage stand on the reactor cavity floor. Once removed from the vessel, the CSB extends approximately 42 inches out of the water.  Shielding must be placed over the 42 inches that extend from the water due to the very high radiation levels emanating from it.

Previous methods of shielding the CSB were difficult to install and not very successful in shielding the top of the CSB. These methods resulted in a lot of exposure during installation as well as elevated dose for workers in other areas of the containment building.

NPO was called into action in April of 2013 to start conceptualizing a design for an effective shielding package that could meet the stringent and challenging requirements of such a large project inside the nuclear power plant containment building.  By May of 2013 we received the purchase order for our design and the engineering and project management teams began turning the wheels for design and production.

The complete package included a 3 piece circular walkway with integrated shielding, removable hand rails that doubled as a track for the inspection robot, an assembly stand, (336) lead blankets with special grommets and (5) custom storage containers.  The 34,000 lb unit had to be small enough to fit through an equipment hatch and, once inside, easily assembled and placed atop the core barrel with a crane.  NPO also provided a structural analysis and assisted in the load testing on-site.

The project yielded a 10% reduction in general area dose rates and an 80% reduction in the proximity of the core barrel.  In addition to reducing radiation exposure, the new package resulted in a time savings for assembly and installation and minimized usage of laydown space inside containment. With the shielding package assembled and installed as one structure, productivity and efficiency also increased.

NPO employed every facet of the company to complete this project in a short timeframe.  Multilevel collaboration between the customer, sales, engineering, quality, production and our vendors was a key component to the overall success.

Thanks to everyone whose contributions made this project such a success!

MCNP Case Study – Dry Cask Shielding

One year ago we introduced our new ability to simulate complex radiation profiles and thereby design the most effective shielding possible (‘NPO uses MCNP to simulate Dry Cask’, March 2016).  Since then we’ve designed and built more than a few shielding systems using this new ability.  Now the day has finally come where we can show how the simulation stacks up against the real world.

We were contracted to build a complete Dry Cask shielding package for a BWR plant that had never done a Dry Cask campaign before and who was using a storage system that had never been used at a BWR before.  There were a lot of firsts but no one was accepting that as an excuse.  This campaign was to come in under dose and ahead of schedule.

Our portion of the project included a Shield Bell with an integrated floor shield (another first), a lid shield, and annulus gap shielding.  The design challenge was to work within a given weight limit and provide the maximum dose reduction possible without adding any weight to the work platform.  It was a perfect opportunity to try out our new tool and the first project to do so.

The simulation works by inputting a gamma/neutron spectrum along with the geometry and chemical composition of every material present.  The subsequent calculation allows you to see the predicted dose rate at any point in space.  A number of different shielding configurations are analyzed until the best combination of material and geometry is revealed.

We are very pleased to say that the shielding was very well received, successfully implemented, and performed better than expected.  We are also very pleased to say the accuracy of our simulation tool has been validated and predictions are conservative (Actual reduction greater than estimated).  For locations we were able to obtain measured dose rates before and after shielding the actual reductions were 50% and 60%, predicted values at those locations were 34% and 44% respectively.

T-Flex Pipe and Valve Shields

T-Flex is the most versatile shielding material that NPO has to offer. Its ability to be poured as a liquid and cured as a flexible solid makes it ideal for fitting the maximum amount of shielding around a component while keeping space and weight to a minimum. The most common way this is put into practice it through pipe and valve shielding. NPO is able to conform the shielding snugly around a pipe and valve which brings the most shielding possible closest to the source while staying clear of any interferences in the immediate area. Below is a showcase of different pipe and valve shields NPO has made over the last few years.

 

Custom Valves, Elbows, Straight Pipe Sections

NPO was able to create custom fit valve shields based on pictures and reference dimensions. The first group of pictures are from a plant that was wanted to remove the locker high rad status from the area. They were able to do so using T-Flex Tungsten shielding around two valves and straight sections of pipe.

This second group of pictures shows off pipe and valve shields for a vertical application. This shielding is intended to be permanent and features lockable steel banding to secure it all to the pipe. In addition, NPO made and elbow shield as well as a base for weight to be supported on and keep shielding from sliding through the gap in the grating. The base is also made of T-Flex Tungsten.


 

Pipe and End Caps for Pipe Removal

This T-Flex Bismuth shielding is 3″ thick and provides and estimated 82% dose reduction at Co-60 energy. This shielding was secured around a high dose pipe that was cut out and removed. The end cap pieces shield the cut out section on all sides, keeping the pipe shielded as it is moved for disposal.

Neutron Radiation: The Concern, Reason, and Solution

FULL ARTICLE

Executive Summary

Nuclear Power Outfitters (NPO) offers a wide range of materials that can be used to attenuate neutron radiation. In contrast to the high density/high atomic weight element materials used for gamma attenuation (lead, bismuth, tungsten), neutron radiation is most effectively shielded using relatively low density materials with high hydrogen content, such as water or high density polyethylene.

Hydrogen attenuates neutron radiation primarily through scattering interactions which reduce the energy/speed of neutrons (thermalization). Neutrons are effectively thermalized by hydrogen because there is a high probability of interaction between neutrons and hydrogen (high scattering cross section) and because scattering reactions with hydrogen reduce the energy of neutrons more per scattering interaction than other elements.

As the neutrons are thermalized, the probability of neutron interactions with matter increase, including absorption reactions. Additionally, the thermalization and capture of neutrons produces secondary radiation in the form of photons (gamma), secondary neutrons, and other charged particles (alpha, beta, etc.). Neutron shielding material often includes elements with high neutron absorption cross sections, such as boron. Inclusion of boron reduces dose from neutron radiation in two ways: 1) by absorbing thermalized neutrons and 2) by reducing secondary photon production during neutron capture through the 10B(n,α)7Li reaction.

Determination of the most appropriate material(s) for the attenuation of neutron or combined neutron and gamma radiation dose involves the consideration of attenuation properties, chemical and physical compatibility, weight and budget limitations, and potential neutron activation of shielding materials. Nuclear Power Outfitters utilizes over 40 years of experience, experimental attenuation data, and Monte Carlo (MCNP) calculations to help customers choose the most appropriate shielding for their needs. Please contact NPO for examples of NPO neutron and neutron/gamma attenuation projects.

Carrying Lead Blankets for Fun

How many times in your career have you seen someone excited to carry lead blankets?  At “The Combine” hosted by Frontline OCR and the local USMC recruiting office, there were more than you could count.  Frontline OCR’s mission is “To create a race that replicates the demands that our first responders and military personnel experience every day”.  Race formats vary but always include some form of carrying, lifting, dragging, throwing weight around.  Enter the lead blanket…  Heavy and floppy like a body, not so rough on the skin, grommets for towing, you name it.  We were proud to sponsor and participate.  The event was a total blast for everyone.

Next time you’re thinking of ordering some lead blankets you might want to think about adding a few more for fun…

Neutron Shielding – Borated Polyethylene

Borated Polyethylene (BPE) is one of the primary materials NPO uses for shielding Neutron Radiation. Our standard BPE is 5% Boron by weight. The benefit of adding Boron is that capture by B-10 reduces the dose from secondary gamma production. It primarily captures through (n, alpha), and the alphas are easily re-absorbed in the material. Spent fuel handling and storage is the area with the biggest application for BPE.  It is commonly used in canister lid shields, panels around the work platform, and bell shielding for transfer casks. BPE can be used in conjunction with T-Flex®, lead, and/or steel to greatly reduce for neutron and gamma radiation. NPO keeps a large stock of BPE sheets and we’re ready to manufacture any shielding your site may need.

 

Transfer Cask Shield Bell (left) and T-Flex® Tungsten and BPE Lid Shield (right)

 

General area shielding at VC Summer (left) and Cyclotron Shielding at PETNET (right)

 

Steam Dryer Flood Up Water Shielding

Tungsten T-Flex® was selected as the best material to shield divers during a Steam Dryer repair.   Approval for use in flood up water required chemical testing for leachables and heavy metals.  The end result was an 8 ft X 5 ft shield wall constructed by attaching T-Flex panels to a metal frame.  Each side of the frame had 8 ft X 1.5 ft stationary panels.  The middle had several removable T-Flex ‘windows’ hanging on hooks to allow work access in the center. The shield panels are 0.56 inches thick and provide and approximate 42% dose reduction at C0-60 energy.

 

Saint-Alban Steam Generator Shielding

EDF Saint-Alban plant, located in Saint-Maurice-l’Exil, France, collaborated with NPO Europe to address a few opportunities for shielding the steam generators during sludge lancing operations. Saint-Alban wanted to reduce shielding installation and removal time from the benchmark of 5.5 hours, reduce total project dose from 476 mRem, and remove the physical clutter of shielding/scaffolding as much as possible to provide a better work environment. With NPO’s custom fit magnetic lead wool blankets, Saint-Alban was able to improve in every area of focus. They experienced an overall 60% estimated dose savings and eliminated the need for scaffolding installation all together. Below is a table detailing the dose reductions as well as pictures of the before and after with NPO blankets.

Braidwood Dry Cask Annulus Gap Snakes

Exelon Braidwood Station recently saw very good dose reduction with their dry cask campaigns by utilizing NPO T-Flex® Annulus Gap Shielding. This was designed to fit in the annulus gap between the canister and transfer cask and had a fire blanket covering specified by Braidwood. The annulus gap shielding, in addition to more NPO dry cask shielding, made for a near record low dose campaign. Below is the Braidwood shielding survey that compares the annulus gap dose rates and the two types of NPO supplied shielding.

 

“I cannot express my happiness at the dramatic improvement (from 800mrem/hr-to-500-150 see attached survey from Braidwood) we have made with the annulus gap shielding recently. The shielding for CPS will have a few tweaks to make it easier to use by the welders, and maybe nudge the dose reduction down a little more as well. Much thanks to all who have helped with this effort.”

—John Burns, DCS Project Coordinator

 

“Thanks very much got the shielding yesterday just in time to use!! It also fit perfectly. We got a large gamma reduction, with the old shielding in place it was 500/120 mr/hr gamma and with the new shielding it went to 100/55 mr/hr reduction … That fire blanket makes for a smooth in set in the annulus area.”

—Donna Fay, RP ALARA

Bio-Shield Door Shielding

Limerick Generating Station was in need of custom shielding for their bio-shield doors. Even though shielding was already installed, there was still a gap that continued to be a source of dose. NPO worked closely with ALARA Specialist, Joe Bruno, at Limerick to design a 4-piece magnetic set of custom lead blankets. Each piece had magnets placed on the outside edge in order to stick to the steel doors with the inner portion consisting of only lead wool. An additional inch of material at the ends allowed for the blankets to overlap and eliminated the possibility of streaming. Finally, each blanket had printing on one side showing the total weight and magnet location. Over all, Limerick station saw about a 30-35% dose rate reduction after implementing NPO custom lead blanket shielding.

Lower Dry Cask Barrel Shield

Some NPP’s process Dry Casks in a such a way that the entire cask produces a general area dose concern. To reduce these dose rates NPO developed a shield package that sits on the floor and shields the cask to a height of 9 feet. NPO is currently under contract to provide shield package with a HVL (half value layer) neutron and HVL gamma shielding factor for this application. It must allow the Dry Cask to enter on one side and exit the opposite. To keep costs down and win the project award NPO utilized its comprehensive knowledge of attenuation properties to select 4 inches of Borated Polyethylene as the shielding material and its engineering prowess to design a simple barn door design that allows the cask to pass through. The shielding package bolts to the floor and to overhead I – beams. In addition to the shielding package, NPO is providing a 2 over 1 seismic analysis.

 

New Cono Seals Shielding

NPO’s new Cono Seals shield package exemplifies another unique and ideal application for T – Flex®. The Cono Seals are a significant dose contributor during reactor head disassembly and, most recently, reactor head cavitation peening in PWR’s. NPO used Tungsten T – Flex to achieve a HVL (half value layer) of shielding from Co – 60. Simple Velcro straps are used for quick securement. The shielding was designed with flexibility to accommodate some unknown dimensions in the Cono Seals themselves. For example, the middle piece acts as a ‘slip joint’ to cover a 4 inch uncertainty/variation in height.

NPO uses MCNP to simulate Dry Cask

Dry Cask processing operations present a number of complex radiological challenges. The presence of both neutron and gamma radiation coupled with the varying amounts of shielding already provided by the cask and canister make it difficult to know where to put shielding and what type shielding to use. Additionally, shielding limitations often exist in the form of max weight and temperature. To help solve these challenges NPO employs the expertise of our affiliate, PG Research Foundation, and MCNP code. MCNP (Monte Carlo N – Particle) was developed by Los Alamos National Laboratory to simulate nuclear processes for applications such as criticality safety, accelerator target design, reactor design, etc. Within the software 3D shapes and radiation spectrums can be created to produce numerous outputs including color plots of attenuation and dose (mRem/hr). In the case of Dry Cask shielding, the canister, transfer cask, and work platform are modeled in a 3D environment and a combination of the Watt Fission Spectrum and a gamma profile is used to simulate aged spent fuel. From here, various combinations of shielding are applied until the optimal solution is revealed. This capability is a tremendous asset for comprehensive shielding design. Attenuation estimates, particularly with complex sources and geometries, are much more accurate as they account for all important attenuation interactions including scattering and secondary radiation production. Additionally, shielding can be increased where it is needed most and decreased, or eliminated, where it is not as beneficial. Dry Cask is not the only application for MCNP. Currently NPO is also running a simulation on a filter shielding cabinet, any complex shielding problem can be analyzed.