Qualifying Static and Dynamic Analyses of SSCs

Design of a SSC is often required demonstrate satisfaction of Design Basis Loads (DBLs) to complete its safety evaluation.  HGS excels in qualifying a SSC’s design under the full assortment of loadings and hazards that may arise in its service life. Some examples of loadings and hazards described below illustrate the range of the Company’s proven capabilities:

Earthquake

Seismic Analysis of HI-STORM/HI-TRAC Stack Configuration Using LS-DYNA

Proven capability to prognosticate the structural response of:

  • Anchored and free-standing structures and components with full recognition of soil/structural interaction effects, where applicable
  • Under-water (submerged) structures and components (including sloshing effect of the water mass, hydraulic coupling from proximate bodies, etc.)
  • SSCs with geometric or material non-linearity Structures vulnerable to instability (such as buckling)

 

Tornadic Wind and Hurricane

A large steel frame structure for an Air Cooled Condenser simulated on STATDPRO

High winds and wind-borne missiles are often significant loadings for large exposed structures and buildings. The object of the analysis is to guide the compliance of the subject structure with the governing codes through suitable modifications to the design. The local damage of the structure from a wind-borne projectile is typically performed on LS-DYNA.

Simulation of a Transport Cask dropped on an upright mild steel bar using LS-DYNA

Severe Flood

Usually germane to structures located on a river basin, a flooding event may be dynamic (moving water) or static (hydraulic pressure). In heat generating SSCs, such as ventilated casks that hold fissile material, flood has the additional consequence of blocking rejection of heat. The Company has performed scores of dynamic and static flood analyses on nuclear installations.

Devices for Heavy Load Handling

Handling of heavy loads is a critically important operation because the consequence of an uncontrolled load lowering event can be so catastrophic. The regulatory literature in this area (such as NOG-1, NUREG-0554, NUREG-0612, ANSI 14.6) is fittingly extensive and highly prescriptive. The Company has developed a series of analysis methodologies for qualifying load handling systems and appurtenances such as cranes, crawlers, lift yokes and the like. The expertise includes:

  • Upgrading of cranes to ASME- NOG-1 single-failure-proof (SFP) pedigree
  • Qualification of lifting devices with consideration of non-conformal yoke-to-hook contact interface
  • Seismic qualification of suspended heavy load from a crane hook (to check against possible impact with proximate structures as well as stress compliance)

Analysis of Fluid Systems

Surface Temperature of Dry Storage Casks in Vertical Orientation inside a Building

The analysis mission for fluid systems may be classified into four groups:

  • Simple one-dimensional systems typified by in-pipe flows (single phase and two phase). The main objective is computation of pressure loss and analysis of water hammer scenarios.
  • Forced flows in heat exchangers. The object is to compute pressure loss, heat transfer coefficient and to assess the risk of flow induced vibrations.
  • Systems containing complex 3-D flows with laminar, transitional, and turbulent regimes (such flow conditions are encountered in a variety of equipment such as heat exchangers, reactor cores and fuel baskets of casks) that required a refined articulation of the flow field.
  • Systems vulnerable to fluid-elastic whirling, the Strouhal effect, acoustic resonance, turbulent buffeting, and other forms of instability from fluid flows such as transmission lines, large surface condensers, large evaporators and the like.

• Analysis of coupled thermal and hydraulic systems using RELAP

The Company utilizes mature private domain (company developed) and public domain computer codes (QA validated) for the above class of problems. Not a single system qualified by the Company out of hundreds analyzed has failed in service.

Nuclear Physics


The company performs criticality safety analyses and evaluations of fissile materials such as processes and fuel assemblies used in nuclear reactors, radiation shielding analyses for spent fuel and other radioactive materials, and core design calculations, using state-of-the-art codes such as MCNP, SCALE and CASMO/SIMULATE.

Criticality safety analyses cover a large range of condition. They are performed for various fuel designs (PWR, BWR, VVER), for wet storage conditions (spent fuel pool) and dry storage/transportation casks, for fresh fuel and spent fuel (utilizing NRC approved burnup credit methodologies), and include both UO2 and MOX fuel. Calculations are performed as part of safety analyses reports, but also to optimize existing and new designs of storage and transportation system from a criticality safety perspective. As an example, the figure below shows the neutron flux distribution from fission processes a cross section of a spent fuel transport cask loaded with content of different reactivity in different cells.

Radiological

Radiological evaluations and dose calculations range from simple shielding geometries for radioactive waste as well as complex geometries such as spent fuel storage and transport casks or reactor cores. The company has performed extremely complex evaluations of entire loaded arrays of nuclear fuel storage systems. Calculations cover both neutron and gamma radiation, at any distance from the source, from the cask surface to site boundaries miles away. Calculations also include occupational dose evaluations that analyze dose to personnel based on the operational steps around spent fuel casks. As an example, the figure below shows the combined neutron and gamma dose field in and around an interim storage facility with a large number of storage casks.

Finally, the Company has experience in core design for nuclear reactors, optimizing the fuel design and performing design calculations for normal and accident conditions.

Insidious hazards – Fatigue, creep and brittle fracture

ANSYS simulation of a fatigue analysis of a high pressure heat exchanger subject to thermal transients

Cyclic fatigue is a hazard to any component subject to cyclic thermal and/or pressure loadings. Qualification for cyclic loading is performed using the guidelines of ASME Section III Subsection NB-3222.4 which provides the methodology (Miner’s rule) to compute the cumulative damage factor for components subject to a large number of dissimilar cyclic loadings.

Creep is a concern in pressure vessels operating at elevated temperatures such as liquid sodium reactors. The service life of creep-constrained designs is, of necessity, limited by the amount of permissible creep. The ASME code provides a methodology for computing cumulative creep that the Company has used in certain applications.

Long term settlement of a soil foundations under load is an example of ambient temperature creep. Our methodology for subgrade settlement has been approved by NRC.

Brittle fracture is a concern in thick walled pressure vessels and weldments subjected to thermal or mechanical shock loadings. The ASME Code contains restrictions and required testing to qualify materials for applications vulnerable to brittle fracture.

Material selection and protection from environment-induced degradation

  • Environmental attack on exposed surfaces is an important consideration in the design of a SSC. The selection of the material of construction is a critical decision to protect the SSC from adverse environmental effects. Typical hazards are: Stress corrosion cracking (in stainless steels), surface corrosion (carbon steels), Galvanic corrosion, pitting corrosion, crack propagation from thermal shock, etc.
  • Attack on the internals of a SSC from generation of aggressive species during operation is another form of degradation which is routinely considered as a part of the Company’s design practice. A classic example of severe equipment damage from concentration of solutes produced from operation is the Inconel tubed steam generators in the light water reactor plants.

Holtec maintains a comprehensive information base of potential environmental damage mechanisms that is utilized in the selection of materials.

The operating mantra for our specialists is: “Let us devise the technical solution to an operating problem or deficiency in a SSC without recourse to a physical ‘mod.’”  In their quest for an effective and economical solution, our specialists are not reluctant to escalate the sophistication of the analysis model. Seismic analysis of SSCs provides a typical example: Upon finding that a static seismic analysis is unable to provide a satisfactory answer, a Holtec specialist’s team can invoke the response spectrum method, and if that does not work, then perform a direct time history analysis of the SSC. HGS views many physical modifications made to SSCs in operating plants can be averted (with significant associated savings) by utilizing more advanced and refined analysis techniques. In other words, pragmatic use of advanced analysis methods in the service of safe and economical design is the essence of HGS’ technical approach. Developing optimal solutions to technical problems that require insightful optimization is another forte’ of the Company’s engineering organization.