NEEP 541Design of Irradiated Structures

Fall 2002Jake 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 CodeI. Power BoilersII. Material SpecificationsIII. General requirements (Nuclear Components)

Division 1 (Class 1, 2, 3 and Supports) Division 2 (Concrete Reactor Vessels and Containments)

IV. Heating boilersV. Nondestructive examinationVI. Care and Operation of Heating BoilersVII. Care of Power BoilersVIII. Pressure VesselsIX. Welding and BrazingX. Fiberglass-Reinforced Plastic Pressure VesselsXI. 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

m em b ra ne b en d ing

P rim a ryS tre ss

m em b ra ne b en d ing

S e con d a ryS tre ss

P e akS tre ss

S tre sses

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 Sm and St

Definitions

creeptertiarytostress

straintostress

rupturetostress

S

eTemperaturOperatingyield

eTemperaturRoomyield

eTemperaturOperatingultimate

eTemperaturRoomultimate

S

t

m

min%80

%1min%100

min3/2

min

@2/1

@2/1

@3/1

@3/1

min

Design Stress varies with Service Level

Level Allowable Primary Stress

A Pm<Sm

PL+PB<1.5 Sm

B Same as A

C Pm<1.2 Sm

PL+PB<1.8 Sm

Design Stress varies with Service Level

Level Allowable Primary+Secondary Stress

A 3Sm

B Same as A

C No 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

stressYS

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 1-3=YS)

Yield Theories Von Mises: yielding occurs when

equivalent stress reaches the yield stress

222222 62

1zyxzxyzzxxyyzzyyxxeq

ASME Code Approach The Code uses stress intensity Stress intensity= 1-3

All previous allowable stress limits are valid if stress is replaced by stress intensity

Example