Thermal Oxide Reprocessing Plant (THORP) Cooling Ponds – CFD Simulation



Reduced costs

Multiple scenarios applied quickly

Improved site safety

SUMMARY

Our client needed to understand the answers to many ‘what if’ scenarios of a cooling system failure while under high load. We decided upon a multi-phase transient computational fluid dynamics (CFD) modelling approach. The amount of available data ensured that the model was highly accurate, allowing multiple scenarios to be demonstrated quickly, presenting our client with sufficient underpinning knowledge to undertake the safety case update with confidence.

SITUATION

Sellafield Ltd has been operating the Thermal Oxide Reprocessing Plant (THORP) in Cumbria, England since the late nineties.  The function of the THORP plant is to reprocess spent fuel from UK and International nuclear reactors; reprocessing separates the uranium (96%) and plutonium (1%) for re-use in new fuel products.

Before reprocessing, all spent fuel is stored in the Receipt and Storage ponds to allow cooling to take place. You can visualise these ponds by imagining three Olympic sized swimming pools joined end to end, then increase the pool depth to 10m and build the pools above ground with a reinforced concrete boundary. The whole pool complex sits inside a portal framed building, so the warm, humid air and multiple layers of security adding to the unique atmosphere.

Under typical operating conditions, there may be 1 to 1.5MW of fuel present. However, the system is designed for up to 8MW of fuel load. The pond temperature is managed via a cooling circuit where pond water is extracted and returned via an arrangement of heat exchangers and cooling towers.

CHALLENGE

All nuclear sites have a safety case that will be frequently reviewed and updated as the regulatory framework, site conditions and phase of operations change over time. As part of the review process, it is often the case that the implications of failure in various pieces of equipment or infrastructure, some of which may be nearing the limit of their original design life, will be investigated and a check as to whether appropriate mitigations are in place is undertaken.

As part of one of these reviews, the original calculations underpinning the cooling system design were interrogated, and it was identified that while there were no inaccuracies, they determined that the assessment process used lacked the detail afforded by modern analytical approaches.

In particular, it was identified that there was a need to understand the answers to a number of ‘what if’ scenarios, one being the outcome of a cooling system failure whilst under high load (8MW of stored fuel) and how it would manifest with respect to temperature over elapsed time from the point of failure. We evaluated many options and decided, with the Sellafield team, that a multi-phase transient computational fluid dynamics (CFD) modelling approach would be appropriate. 

SOLUTION

Firstly, we arranged to meet with the Sellafield team for a kick-off and to visit the THORP facility to gain a feel for the important elements of the facility, the information we would be able to access and the outcomes that our client needed. As an operator of an accredited ‘List- N’ compliant site, we were then able to receive a series of drawings of the facility and various data sets relating to performance metrics under a known fuel load and distribution.

As with all of our analytical work, the first step was to capture the problem statement, the quality plan, all of the inputs/post-processing requirements in the analysis plan. Wherever possible we will try to benchmark any analysis against practical testing or an existing data set; the latter, in the form of a series of temperature measurements of pond water, surrounding air and also relative humidity data was available. Equally, the fuel load and distribution at the time of these measurements was understood, so we could build a CFD model of the facility, including accurate representation of the heat inputs and then compare the analytical results with the data set. Any variances then allowed us to fine tune the model until sufficient accuracy was obtained.

Now that we had a validated model it was possible to undertake a series of ‘what-if’ scenarios and provide Sellafield with sufficient underpinning knowledge to conduct the safety case update with confidence.

BENEFITS
  • Sellafield Ltd gained a significant improvement in their understanding of how the THORP facility responds to the loss of cooling.
  • As a result, Sellafield Ltd was able to improve the site safety case and mitigate future risks.
  • The CFD analysis, once benchmarked, became a tool that could be applied to a multitude of scenarios at a fraction of the time and cost associated with physical testing.


Our client needed to understand the answers to many ‘what if’ scenarios of a cooling system failure while under high load. We decided upon a multi-phase transient computational fluid dynamics (CFD) modelling approach. The amount of available data ensured that the model was highly accurate, allowing multiple scenarios to be demonstrated quickly, presenting our client with sufficient underpinning knowledge to undertake the safety case update with confidence.



Sellafield Ltd has been operating the Thermal Oxide Reprocessing Plant (THORP) in Cumbria, England since the late nineties.  The function of the THORP plant is to reprocess spent fuel from UK and International nuclear reactors; reprocessing separates the uranium (96%) and plutonium (1%) for re-use in new fuel products.

Before reprocessing, all spent fuel is stored in the Receipt and Storage ponds to allow cooling to take place. You can visualise these ponds by imagining three Olympic sized swimming pools joined end to end, then increase the pool depth to 10m and build the pools above ground with a reinforced concrete boundary. The whole pool complex sits inside a portal framed building, so the warm, humid air and multiple layers of security adding to the unique atmosphere.

Under typical operating conditions, there may be 1 to 1.5MW of fuel present. However, the system is designed for up to 8MW of fuel load. The pond temperature is managed via a cooling circuit where pond water is extracted and returned via an arrangement of heat exchangers and cooling towers.



All nuclear sites have a safety case that will be frequently reviewed and updated as the regulatory framework, site conditions and phase of operations change over time. As part of the review process, it is often the case that the implications of failure in various pieces of equipment or infrastructure, some of which may be nearing the limit of their original design life, will be investigated and a check as to whether appropriate mitigations are in place is undertaken.

As part of one of these reviews, the original calculations underpinning the cooling system design were interrogated, and it was identified that while there were no inaccuracies, they determined that the assessment process used lacked the detail afforded by modern analytical approaches.

In particular, it was identified that there was a need to understand the answers to a number of ‘what if’ scenarios, one being the outcome of a cooling system failure whilst under high load (8MW of stored fuel) and how it would manifest with respect to temperature over elapsed time from the point of failure. We evaluated many options and decided, with the Sellafield team, that a multi-phase transient computational fluid dynamics (CFD) modelling approach would be appropriate. 



Firstly, we arranged to meet with the Sellafield team for a kick-off and to visit the THORP facility to gain a feel for the important elements of the facility, the information we would be able to access and the outcomes that our client needed. As an operator of an accredited ‘List- N’ compliant site, we were then able to receive a series of drawings of the facility and various data sets relating to performance metrics under a known fuel load and distribution.

As with all of our analytical work, the first step was to capture the problem statement, the quality plan, all of the inputs/post-processing requirements in the analysis plan. Wherever possible we will try to benchmark any analysis against practical testing or an existing data set; the latter, in the form of a series of temperature measurements of pond water, surrounding air and also relative humidity data was available. Equally, the fuel load and distribution at the time of these measurements was understood, so we could build a CFD model of the facility, including accurate representation of the heat inputs and then compare the analytical results with the data set. Any variances then allowed us to fine tune the model until sufficient accuracy was obtained.

Now that we had a validated model it was possible to undertake a series of ‘what-if’ scenarios and provide Sellafield with sufficient underpinning knowledge to conduct the safety case update with confidence.



  • Sellafield Ltd gained a significant improvement in their understanding of how the THORP facility responds to the loss of cooling.
  • As a result, Sellafield Ltd was able to improve the site safety case and mitigate future risks.
  • The CFD analysis, once benchmarked, became a tool that could be applied to a multitude of scenarios at a fraction of the time and cost associated with physical testing.
Thorp Pond
Thorp air velocity streamlines
Thorp CFD Streamlines
Thorp water velocity streamlines

SECTOR

PROJECT ATTRIBUTES

PTC Creo

PTC Mathcad

ANSYS DesignModeler

ANSYS CFD

MathCAD Calculations


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The CFD analysis, once benchmarked, became a tool that could be applied to a multitude of scenarios at a fraction of the time and cost associated with physical testing

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