16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 103 / 295Document ID.No.UKEPR-0002-162 Issue 04• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with partial cooldown (p.c.) and delayeddepressurisation (SB(LOCA)/D),• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with fast secondary cooldown (f.s.c.) andre-flood (SB(LOCA)/R).Sub-section 16.2.2.3 - Figure 2 shows the histories of the potential AICC pressure (solid curves)for all scenarios on a shifted time scale (hydrogen release starts at time zero) together with thecalculated combustion pressure histories (dashed curves) for three scenarios: SB(LOCA) coldleg f.s.c., SB(LOCA)/D and SB(LOCA) cold leg p.c. for the ex-vessel phase. The pressurecalculated with GASFLOW does not exceed 5.5 bar. The maximum AICC pressure of 6.3 barwas calculated for the bounding scenario SB(LOCA)/D. The highest pressure increase of 4.2 barwas calculated for the SB(LOCA)/R scenario with a peak pressure of 5.6 bar. Note that theAICC pressure curves show the pressure value that would result from combustion at any giventime. They do not describe combustion histories, in contrast to the curves resulting fromGASFLOW calculations.2.3.3. Assessment of the Combustion ModeFlame acceleration up to sound velocity has been found in many hydrogen combustionexperiments, when the hydrogen concentration was above about 10% by volume. Flameacceleration originates from turbulence generated by structures in the flow path (e.g. orifices intube experiments). Fast hydrogen combustion leads to dynamic pressure loads, which meansnon-isotropic loads with strong variation in time.In extreme cases, a fast flame can even evolve into a detonation (DDT). Direct initiation of adetonation is not possible within a containment due to the high energy required. Peak pressuresare comparable to those generated by fast deflagration, but the dynamic variation of thepressure with time (and thus the slope of pressure variation) is much higher.Potential hydrogen combustion in a nuclear reactor containment is quite well understood today.This is the result of many experiments performed under different conditions in previous years.Hence, procedures are now available for the direct prediction of the possible mode ofcombustion, either based solely on the distribution of gases and temperature, or additionallyusing information about the containment geometry.Flame acceleration can only occur if the expansion ratio, which is the ratio of the atmosphericdensity before the combustion to the density after the combustion, exceeds a threshold value(sigma criterion). This threshold value is derived directly from experiments and depends only onthe gas concentration and the temperature.In order to reach sound velocity, the flame must be sufficiently accelerated, and thus the path ofthe flame must be long enough. This so-called run-up distance is less well established and wasnot applied in the analyses of this section. The minimum distance for reaching sound velocityalso depends on the gas concentration. Finally, DDT can only happen if the characteristic spatialextent of the compartment(s) under consideration is more than seven times the detonation cellsize (lambda), which depends on the mixture composition (7 lambda criterion). This factor ofseven has been derived from experiments.