«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
In the course of severe accident sequences there are a number of complicated phenomena that are taken into account. In the early phase of nuclear power development the processes involved were not exactly known, so that rather pessimistic assumptions used to be taken on their probabilities and consequences. US NRC identified them as “Unresolved Safety Issues” (USI) and a large programme of experimental and analytical work was undertaken to clarify and resolve them. Presently most of the USIs connected with severe accidents have been resolved, but some of them are still being studied on international level. The phenomenological issues of most importance for Temelín NPP safety are briefly presented below.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 99
4.3.1 Vessel Failure
The consequences of Reactor Pressure Vessel failure depend very much on the pressure inside the RCS at the moment of vessel melt-through. If the pressure inside the RCS could not have been decreased below 1-2 MPa before the vessel break, the sequence of events is called “high pressure scenario”, otherwise “low pressure scenario” occurs. The scenarios of severe accidents are divided accordingly into the two following classes
- High pressure scenarios, including total blackout, total loss of feed water, small break LOCA with failure to depressurize the RCS in time before RPV break
- Low pressure scenarios, including LB LOCA, large Primary to Secondary leakage, and those severe accident scenarios where a successful RCS depressurization is achieved.
Each class of scenarios has its own specific features and the results are plant specific.
184.108.40.206 High Pressure The results of German Risk Study Phase B [GRS 89] indicated that without accident management measures 98% of severe accidents would lead to high-pressure (HP) core melt. In high-pressure core melt scenarios the delivery of cooling water to the core is more difficult or impossible, and the reactor pressure vessel (RPV) rupture results in high pressure melt ejection (HPME). The ejected materials are likely to be dispersed out of reactor cavity into surrounding containment volumes as small particles, quickly transferring thermal energy to the containment atmosphere. In addition, metallic components of the sprayed core debris, mostly zirconium and steel, can react with oxygen and steam in the atmosphere, raising a large quantity of chemical energy that can further heat up and pressurize the containment. The term “direct containment heating (DCH)” is used as a summary description of the involved physical and chemical processes.
The magnitude of the containment loading that could be caused by HPME/DCH depends on various features of the plant design, especially the design of the reactor cavity, and also the availability of flow paths to the upper regions of the containment. Recent containment analyses have shown that, for a PWR with a typical large dry containment, DCH is only a threat if a large quantity of entrained debris (typically 50 % of the core inventory) is involved. The implicit assumptions made in these analyses are that the entrained mass is finely fragmented and that flows are unimpeded.
In reality, there are several mitigating factors that would considerably limit this energy transfer. The key factors are fragmentation of ejected corium, its de-entrainment by structures and efficiency of chemical and thermal interactions. Recent analyses [Henry 91], [Werner 94], [Ang 95], [Morozov 03] have all concluded that large dry containments are less vulnerable to the loadings from DCH than previously suggested by scoping containment analysis and the containment failure probabilities are small.
Another process can that was considered a hazard to containment integrity is the possible steam explosion due to molten corium interaction with water. The steam explosion loads on the containment were first considered in WASH-1400 and, because of the assumptions made about the nature of this event at that time, the failure of containment (due to in-vessel steam explosion generated missile) contributed a substantial fraction of the conditional probability for early containment failure. The work on steam explosions since that time led to more realistic estimates of the probability of containment failure due to in vessel steam explosions. The current estimation is that this conditional probability (i.e., given a core melt) is less than 0,001. [Seghal 96].
The hazard of ex-vessel steam explosion depends on the amount of water that is present in the reactor pit and the spreading compartment at the time of the RPV melt-through. There are many facets to the determination of the containment failure probability due to interaction 100 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues of a corium jet with water in deep water pool, e.g. jet characteristics, the corium composition, the extent of fragmentation, the strength of the trigger required, the pressure pulse generated in the steam explosion and the fragility of the containment.
In addition to the pressure increases due to DCH and water evaporation the pressure increase due to simultaneous hydrogen burning should be considered. This issue is discussed in the Sections 4.3.2 and 5.5.
The ejection of molten corium under high pressure involves the hazards of molten corium attack against the containment liner and possibly against other surfaces involved in providing containment leak tightness. Such hazards should be evaluated and prevented as needed.
220.127.116.11 Low Pressure The best protection against various threats listed above consists in bringing RCS pressure down before RPV failure. It is generally agreed, that below 1 MPa no HPME phenomena are of importance.
Moreover, depressurization of the RCS brings important advantages much before the RPV break, because it facilitates using various water sources to inject into the RCS and cool the core. For example, in phase A of German Risk Study where no AM measures to depressurize RCS were considered the core melt frequency was estimated as 10-4 [1/a], while in Phase B with depressurization strategies implemented the CDF was reduced to 2,6 ×10-6 [1/a], of which HP scenarios constituted only 4,5 ×10-6 [1/a] [GRS 89, p. 67]. Therefore, the intensive depressurization of RCS in case of severe accidents is generally accepted as one of the main SAM strategies.
4.3.2 Hydrogen Production, Distribution and Combustion
Controlling the release of large quantities of hydrogen during the course of a severe accident poses one of the greatest challenges for the design of NPPs. The main potential hydrogen source during a core heat up accident is due to the oxidation of zirconium fuel cladding and zirconium containing structural core materials with steam. The reaction of zirconium /steam is highly exothermic (587 kJ/mol) so that the reaction is self-accelerating once the fuel temperature has reached approximately 1100 oC. The sole limitation is given by the amount of available steam. This fact explains that very large quantities of hydrogen can be generated in short time spans, being eventually limited by steam starvation, although the core is still heating up. At the time of core slumping into the water of the RPV lower head there will be another short period of intensive hydrogen generation.
The hydrogen produced during in-vessel phase can be released from the RCS through pressuriser PORV or safety valves in case of such scenarios as blackout of LOFW, and flow to the relief tank and then to the containment dome, or through the break in the RCS piping in case of SB LOCA. In the latter case the concentration of the hydrogen near the release point depends on the room geometry. In the case of WWERs with horizontal SGs and flat SG boxes, the hydrogen will initially stay in the boxes before flowing into the containment dome, so that its concentrations will be locally higher than in PWR containments with vertical SGs and high rooms containing steam generators.
Additional hydrogen is generated during Molten Corium-Concrete Interaction (MCCI) after RPV failure. The quantities of generated hydrogen and carbon monoxide have been studied in many experiments and the calculation results obtained by means of CORCON code used in MELCOR code package to describe MCCI are well comparable with the experimental data.
The rates of ex-vessel hydrogen generation are generally lower than those typical for invessel processes.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 101 In terms of the threat posed by hydrogen combustion to containment integrity, several different combustion regimes should be recognized. Hydrogen deflagration - simple burning, producing no shock loading - is not likely to pose a threat to containment integrity. Generally the pressure rise due to deflagration is smaller than the containment design pressure, and no dynamic (shock) loads are involved.
There are three more energetic hydrogen combustion modes to be considered: (a) flame acceleration, (b) deflagration-to-detonation transition (DDT), and (c) detonation. Flame acceleration represents an intermediate and transitional behaviour between deflagration and detonation. The resulting pressure loads are higher, but still not typically threatening to containment integrity for a large dry containment. Transition to detonation (DDT) is more of a threat to containment integrity because detonation involves shock loading on structures. DDT requires specific conditions in order to take place. Global detonation is also possible, but very unlikely since conditions conducive to its occurrence are not normally encountered in large dry PWR containment plants.
It was recognized by the late 1990s that the WWER 1000 design might be more at risk from energetic hydrogen combustion modes due to structural layout considerations, especially the potential for DDT in the lower part of the containment [Kujal 97]. Since such hydrogen combustion modes are not modelled in the MELCOR code (or other similar lumped parameter codes), in PN7 this issue was addressed by the use of a three-dimensional CFD code (GASFLOW II) to identify the extent to which conditions conducive to DDT might be present in the WWER 1000 design.
Hydrogen hazards in large dry containments are primarily mitigated by the large free volume and high structural strength (compared with other designs) of the containment. Hydrogen combustion is also prevented under situations in which the containment atmosphere contains sufficient steam to suppress combustion. (Ignition is not possible with a steam concentration of 55% or higher, a condition which is often encountered in severe accidents not involving containment bypass.). Moreover, the hazard of deflagration decreases with pressure and according to Czech estimates at high pressures the probability of deflagration with significant steam contents is negligible.
In addition, the hydrogen source in the containment atmosphere in severe accidents is depleted in the long term by twenty-two passive autocatalytic recombiners (PARs) installed in the containment. The specific model installed at Temelín is rather small (Siemens FR 90/1-150), and is intended for design basis accident conditions to maintain the hydrogen concentration in the containment at less than 4 vol.%. Under severe accident conditions these PAR units do over the long term deplete both the hydrogen and oxygen concentrations in the containment by catalytic recombination of hydrogen and oxygen to form water vapour, but in view of their small size it is a slow process. Higher capacity PARs would be more appropriate for severe accident conditions.
Finally, venting the containment atmosphere to the environment can reduce the concentration of hydrogen in the containment. The Temelín SAMGs provide for a filtered venting path, which is nominally intended to reduce containment pressure. However, actuation of this filtered venting pathway necessarily removes hydrogen from the containment (and also reduces the pressure in the containment which could be present if hydrogen combustion takes place, thus reducing the threat of overpressure failure of the containment). The effectiveness of venting for reducing hydrogen combustion hazards has been widely recognized within the EU [EUR 14037] and the US [NRC 03].
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4.3.3 Core-Concrete Interactions and Base mat Penetration
Molten corium-concrete interaction (MCCI) results in concrete erosion, which decreases base mat thickness and can lead to complete base mat penetration by molten corium. The temperature of corium at the moment of RPV failure is about 2700 K, while the solidification temperature is 2173 K. Since at the initial moment of corium-concrete interaction the corium contains significant amount of metal and the temperature of concrete dissolution is higher than the temperature of metal-steam reaction, the release of combustible gases (H2 and CO) occurs during all the period of corium-concrete interaction.
Thus, the process of MCCI gives rise to two hazards:
1. Containment pressure increase due to production of non-condensable gases, including hydrogen and carbon monoxide, which may eventually burn, thus increasing the containment atmosphere temperature and pressure.
2. Base mat penetration, with loss of leak tightness of containment envelope, mixing of hydrogen with air, which may result in deflagration, and fission product leakage outside containment.