NEEP 541 Design of Irradiated Structures Fall 2002 Jake Blanchard

Outline Design of Irradiated Structures ASME Boiler and Pressure Vessel Code Loads Limits Examples

ASME Boiler and Pressure Vessel Code Designed to enhance safety of pressure vessels 1600 explosions of boilers from 1898 to 1903 (killing 1200 people) Code was adopted in 1915 It has been continuously revised and enhanced ever since It does not cover corrosion, erosion, instabilities, etc.

Organization of Code I.Power Boilers II.Material Specifications III.General requirements (Nuclear Components) Division 1 (Class 1, 2, 3 and Supports) Division 2 (Concrete Reactor Vessels and Containments) IV.Heating boilers V.Nondestructive examination VI.Care and Operation of Heating Boilers VII.Care of Power Boilers VIII.Pressure Vessels IX.Welding and Brazing X.Fiberglass-Reinforced Plastic Pressure Vessels XI.Inservice Inspection

Component Classification Purpose: recognizes different levels of importance in relation to safety Owner is responsible for classification 10-CFR-50 requires that components of reactor coolant pressure bounday be class 1 Others defined with respect to consequences of failure

Design Basis Design conditions Pressure, temperature, mechanical loads Service limits Level A: normal Level B: highly probable, unplanned, component must withstand damage and continue to operate without service Level C: low probability, unplanned, must be recoverable, but may require repair Level D: component may suffer gross deformation

Loading Bulk heating: Level A Coolant pressure: level A Surface flux on cladding or first wall: level A Seismic loads: level B or C Transients: level B, C, or D

Types of Stresses

Definitions Primary Stress=any stress developed by an imposed load which is necessary to satisfy equilibrium of external forces and moments Not self-limiting Examples include pressure and dead- weight

Definitions Secondary Stress=any stress developed by constraint of adjacent material or self-constraint Self-Limiting Examples include thermal stresses

Definitions Peak Stress=an increment of stress over and above the primary and secondary stresses, caused by discontinuities or local thermal stress No gross deformation Can be a concern with respect to cracking Examples include stresses near discontinuities (holes, for instance)

Definitions Membrane Stress=any stress which is uniform over the thickness of a thin component Bending Stress=any stress which varies linearly over the thickness of a thin component

Allowable Stresses The stress limits in the code are based on the yield, ultimate, and creep strengths, with appropriate safety factors The fundamental limit is that the stress should be less than the minimum of S m and S t

Definitions

Design Stress varies with Service Level LevelAllowable Primary Stress AP m <S m P L +P B <1.5 S m BSame as A CP m <1.2 S m P L +P B <1.8 S m

Design Stress varies with Service Level LevelAllowable Primary+Secondary Stress A3S m BSame as A CNo Limit

Why are Primary Stresses Worse? Consider a perfectly plastic material Compare failure due to both a constant force loading and a constant strain loading strain stress YS

Comparison For constant load, a force that causes a stress just beyond the yield stress will cause failure For constant strain, a strain that causes a strain just beyond the yield strain will still be far from the failure strain Pressure stresses are analogous to constant load, while thermal strains are analogous to constant strain

Why is Bending More Forgiving in Terms of Allowable Stress? Consider a beam with an applied moment MM y stress

Bending (continued) Peak stresses are at edge When the beam begins to yield, only the edges will yield and the central portion of the beam will still be elastic (and able to support load) Hence, a beam under pure bending can safely go further beyond the yield point that something experiencing a membrane load This is only true for ductile materials!

Biaxial Stresses Everything discussed so far assumes that stresses are uniaxial Stress is actually a tensor, so it has three normal components and three shear components

Yield Theories There are two theories for yielding under multiaxial stress states Maximum Shear (Tresca): yielding occurs when the maximum shear stress reaches a critical value The maximum shear can be found by taking the difference of the largest and smallest principle stresses (yielding when =YS)

Yield Theories Von Mises: yielding occurs when equivalent stress reaches the yield stress

ASME Code Approach The Code uses stress intensity Stress intensity= All previous allowable stress limits are valid if stress is replaced by stress intensity

Example