Prior to 1969 the design and construction code for piping systems in nuclear power plants was ASME B31.1 Power Piping, with supplementary requirements specific to nuclear power. To this day, ASME B31.1 remains the applicable code for pre-1969 plants, and for non-safety related piping in all plants.
In 1962 a nuclear code case was published (ASA B31.1 Code Case N-1 “General Requirements for Nuclear Power-Plant Piping”) to address the design requirements specific to nuclear piping, what today is referred to as safety class 1, 2 and 3 piping systems.
With time, Code Case N-1 evolved in a new code in 1969, the ASME B31.7 Code for Nuclear Power Piping. This code remained applicable until 1971, when the design and construction requirements for nuclear power plant piping were folded into the ASME Boiler and Pressure Vessel Code, Section III Division 1, which until then was a nuclear pressure vessel code.
The Allowable Stress Multiplier
Throughout this evolution, the design of piping systems in nuclear power plants relied, in part, in limiting the calculated stresses in the piping system to a multiple k of the allowable stress S, so that in its simplest form
Calculated stress < k ´ S
The first k multiplier was 1.2 in ASME B31.1-1967 for occasional loads, a term introduced to differentiate these loads from sustained loads. This factor k of 1.2 remains applicable to this day in ASME B31.1.
In 1970, Code Case 70 introduced four classes of events: normal, upset, emergency, and faulted. Later, in 1977, this nomenclature was changed to Service Levels A, B, C and D. Based on these four classes of events, in 1972 (with piping design now part of ASME III) an allowable stress multiplier k of 2.4 was assigned to faulted events. The k factor was set at 1.2 for upset conditions, and 1.8 for emergency conditions with, in each case, additional limits against yield stress.
In 1981, together with the introduction of stress indices for Class 2 and 3 piping, the k factor was increased to 1.5 for design (sustained loads), 1.8 for Level B, 2.25 for Level C, and 3.0 for Level D with, as before, additional limits against yield stress.
The Allowable Stress
Unlike the k multiplier, the allowable stress S has had few changes. In fact it remained unchanged since its introduction in 1955 in B31.1 until 1999.
For steel piping operating below the creep regime, the allowable stress S is the lesser of a fraction of the minimum yield stress or the minimum ultimate stress, in other words
S = smaller of (yield / DMy) and (ultimate / DMu)
For Class 2 and 3 piping, the design margin against ultimate DMu has always been 4, until 1999, when it was reduced to 3.5. The same reduction occurred in ASME B31.1, first as a Code Case in 2000, then in the Code itself in the 2005 addendum.
This change in design margin from 4 down to 3.5 has practical repercussions. Taking for example the common SA 106 Grade B carbon steel pipe, the allowable stress at 100oF which used to be 15 ksi (1/4 of the minimum ultimate strength of 60 ksi) is now 17.1 ksi (1/3.5 of the minimum ultimate strength of 60 ksi).