Issue 41

G. G. Cunto et alii, Frattura ed Integrità Strutturale, 41 (2017) 332-338; DOI: 10.3221/IGF-ESIS.41.44 333 I NTRODUCTION he LABGENE is a 48 MW thermal PWR prototype reactor under development in Brazil, that aims to develop and test the capability to design small and medium power reactors for electricity production and for nuclear propulsion. Leak-Before-Break (LBB) is a method used in design of nuclear power reactor coolant loop piping to eliminate consideration of the dynamic effects of pipe rupture. The exclusion of dynamic effects associated with pipe rupture from the design basis is allowed when analyses demonstrate that the probability of pipe rupture is extremely low for the applied loading resulting from normal conditions and a postulated safe shutdown earthquake (SSE). A deterministic LBB evaluation performed using guidance from NUREG-1061 Volume 3 [1] and SRP 3.6.3 [2] is usually used to demonstrate this low probability of pipe rupture. This examination is based on fracture mechanics analysis that is used to demonstrate that a crack leak, present in a pipe, can be detected by the plant leak detection systems, with a factor of 10 between the predicted leakage and the plant leakage detection capability. If the leakage crack size is smaller than the critical crack size by a factor of two, then LBB requirements are satisfied and the dynamic effects of pipe rupture need not be considered in the plant design basis [1]. For the critical flaw sizes determination, it can be used either limit load with net section collapse criterion or J-Integral and Tearing Modulus (J/T) analysis for tearing instability criterion. For the leak rate determination, a well know and extensively validated computer program called PICEP [3] is usually used for determination of leakage. This program contains a methodology for computing crack opening area based on elastic plastic fracture mechanics analysis and its flow rate equations are based on a modification of Henry’s homogeneous non-equilibrium critical flow model. Nonetheless, the small size of the reactor coolant piping, considered in LABGENE project, can makes it difficult to meet the same LBB standards that were developed for large commercial reactors, when NUREG-1061 Volume 3 [1] and NUREG-0800 Standard Review Plan 3.6.3 [2] were initially developed. Furthermore, the small pipe diameter makes the usual ferritic coolant pipe with inside austenitic cladding, impossible to be performed, then a fully austenitic pipe shall be used. Austenitic stainless steels pipes made by material type AISI 316 and its variants have been chosen for applications in nuclear power plants owing of their good high temperature mechanical properties and creep and corrosion resistance. A low carbon choice alloyed with nitrogen of this steel, designated as AISI 316LN, is a possible chose for the LABGENE reactor coolant piping. Moreover, due to the need of connecting the pipes spools by use of weld, a shielded metal arc welds (SMAW) using AISI 316L coated electrode was select as the most appropriate commercial weld join material. This proposal welded pipe is tested and analyzed as three-component composed of weld metal, base material and heat affected zone (HAZ). Inadequate mechanical properties in any of these three zones is a threat to the LBB analysis. M ETHODOLOGY ollowing is provided a summary of the methodology used for the LBB evaluation in the LABGENE’s reactor coolant loop piping. Determination of Loads and Stress The loads to be used in LBB evaluations is considered as normal operating loads for leakage determination and normal plus seismic SSE loads for critical crack determination. The normal operating loads consist of pressure, dead weight, and thermal expansion loads. For calculation of critical flaw size, it was considered the maximum of SSE loads added to the normal operating loads. The piping dimension is considered with an outside diameter of 273 mm and a thickness of 28.57 mm, the operating conditions for the LBB feasibility evaluation are considered as the usual one for a PWR plant, with operation temperature of 288 ˚C and internal pressure of 25.17 MPa in the reactor coolant line. Determination of Material Properties Material properties to be used in the LBB evaluation shall be evaluated by stress-strain curve determination using Ramberg- Osgood (RO) analysis, and material toughness is obtaining by resistance curve test method with the determination of material J-R curve. T F