Leak before break

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

Hardness controls rednce the risk of crack initiation. Crack propagation risks are reduced by either increasing the fractnre tonghness of the material (usually by heat treatment) or by reducing stresses. The objective is to eliminate crack propagation or to ensure that it is slow and stable in order to obtain leak-before-break (i.e., failure is not catastrophic). 

Design Considerations (irwin s Leak-Before-Break Criterion)... 

Figure 5.1. Schematic diagram showing Irwin s leak-before-break criterion.

Table 5.1. Minimum fracture toughness requirement as a function of yield strength and thickness based on Irwin s leak-before-break criterion...

In addition there will be greater acceptance of the leak-before-break approach to protect against major failure and large leaks. This requires close attention to the reliable detection of small leaks at an early stage, before they can give rise to significant fire. There... 

Oil systems have been avoided wherever possible to minimise the risk of oil fires.. The quick actuating valves on the water steam side and sodium side are pneumatic driven. Cooling is by nitrogen for primary cold trap and by air for secondary cold trap. For austenitic stainless steel piping, leak before break concept is used where leak monitoring provision exists. To minimise sodium fires, all the sodium pipes within the reactor containment building are double walled. The dump valves are duplicated in sodium circuits to enhance reliability of dumping, in case of a sodium leak. 

L. Martin et al., "Leak before Break operating experience from European fast reactors". Proceedings of International conference on Fast reactor and related fuel cycles, FR-91,, Kyoto, Japan, Oct 28-Novl,1991. 

The Leak-Before-Break (LBB) concept and the associated leak detection requirements... 

No break is postulated in the crossduct in the primary system between the reactor vessel and the steam generator. Fracture mechanics techniques as part of the Leak-Before-Break approach (Ref. 2), as is currently being done for some light water reactor (LWR) primary system piping, will be used to demonstrate a failure probability sufficiently low to be of no concern. 

Given the difficulty of determining leak sizes and the frequencies of their occurrence these are usually fixed in safety reports (deterministic approach). An important role is played by the leak before break criterion, which implies that a leak of stable size formed before large area leaks in vessels or full cross section ruptures of pipes occur. However, the applicability of this criterion is subject to numerous prerequisites being fulfilled. Details can be found in [7]. 

In this scenario, it is assumed that one of the outlet lines of a steam generator suddenly breaks. The concept of leak before break , which excludes the guillotine break of the large primary lines, cannot be applied to the steam lines because it is difficult to demonstrate that a steam leakage from a quasi-critical crack can be detected in time with certainty. This accident, therefore, has to be considered less unlikely than a primary pipe break. 

The residual stresses are important factors of fatigue crack growth and of evaluations of leak before break . They are less important for the evaluation of failure danger. 

Problems of strength of materials (irradiation effect on the pressure vessel, behaviour of the pipes in various loading conditions, leak before break and detection of pipe leaks, steam generator problems). 

EUR allows for considerations of Leak before Break and for Break Preclusion ... 

Examples of comprehensive procedures to qualify piping systems are the European safety practices on the application of the leak before break concept (EUR 18549), the United States Nuclear Regulatory Commission s leak before break appUcation (SRP 3.6.3.), the break preclusion procedure used in Germany (RSK guidelines) or the leak before break guideline used in Japan (JEAG 4613). 

A pipe break need not be assumed if a successful qualification for leak before break, for break preclusion or for low probability of failure has been performed for the piping under consideration, resulting in a sufficiently low frequency of the occurrence of a spontaneous break In general, a fracture mechanics analysis should be performed to calculate the leak size. In lieu of such an analysis, a subcritical crack corresponding to a leak size of 10% of the flow cross-section should be postulated The leak detection system should be shown to have a sensitivity that is adequate to detect the minimum leakage from a crack that is just subcritical. 

For primary or secondary piping without qualification for leak before break or for break preclusion, the probability of a pipe break can be reduced significantly if additional safety orientated measures are applied, such as surveillance measures (increased in-service inspections or monitoring for leakage, vibration and fatigue, water chemistry, loose parts, displacements, and erosion and corrosion). 

In some States a limiting probability of 10 per year is recommended for high quality piping qualified for leak before break or break preclusion. 

B. The flywheels will be designed to withstand normal operating conditions, anticipated transients, and the largest mechanistic pipe break size remaining after application of leak before break as described in Section 3.6, combined with the Safe Shutdown Earthquake. 

Each motor is provided with an anti-reverse rotation device. The device is designed to prevent impeller rotation in the reverse direction due to each of the following conditions motor starting torque, if the motor was incorrectly wired for reverse rotation and, reactor coolant flow through the pump in the reverse direction due to the largest remaining pipe break after application of leak before break as described in Section 3.6, which could result in reverse flow through the pump. 

In the event of a break which is not eliminated by leak before break which would result in reverse flow through the pump, the anti-reverse rotation device prevents impeller rotation in the reverse direction. In the event of a break which is not eliminated by leak before break and which could result in increased flow through the pump tending to accelerate the pump impeller,the highest predicted pipe break induced overspeed is less than the lowest critical speed of the flywheel. 

The criteria applied in the design of the Reactor Coolant System supports are that the specific function of the supported equipment be achieved during all normal, earthquake, safety valve actuation and Branch Line Pipe Break (BLPB) conditions. (BLPB includes feedwater line breaks and all loss-of-coolant-accident conditions resulting from breaks not eliminated by leak-before-break analysis in piping to branch nozzles of the reactor coolant system.) Specifically, the supports are designed to support and restrain the Reactor Coolant System components under the combined Safe Shutdown Earthquake and Branch Line Pipe Break loadings in accordance with the stress and deflection limits of Section III, ASME Code. 

Reduced vessel fluence reduce initiator frequency Pnmary circuit integration limits small LOCA consequences Canned pumps suppress initiator (seal caused small I,OCA) Adaptation of the leak before break limits accident consequences,... 

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