Consequences of a severe reactor accident
Severe accidents have the capability to cause large damage to the reactor, up to and including total destruction of the reactor and the surrounding civil structures. If also the containment is damaged (which is highly likely), then a radiological release and contamination may occur, both on-site and off-site, and thereby creating risk for public health and safety, and environmental damage. Releases may also cause societal disruption and large economical damage.
On-site consequences
If the containment is still intact, the radiation consequences on-site will be more limited, although the plant will likely be a total loss. It is relevant to monitor the radiation levels in working areas, such as the control room, the room(s) for the Emergency Response Organisation, local areas where manual actions must be performed, locations where equipment must be restored or temporary equipment hooked on, etc.
Special precautions should be taken in case the containment needs to be vented. A containment filter will trap many aerosols, but noble gases escape for 100%, and capture of gaseous iodine may be limited. Additionally, the vent lines themselves will become highly contaminated following venting operations and pose a serious personnel dose threat in the case that the vent lines pass near critical areas such as the reactor control room.
In case of containment failure access to the site may be limited, thereby aggravating the possibility for intervention and support. It may be needed to evacuate parts or all of the site and the surrounding areas.
Accident management is then only possible from protected rooms. Protective equipment (e.g., breathing apparatus) may be needed for operating personnel and workers. Changing shifts is hampered by the radioactive contamination in the environment.
Staff may be exposed to radioactivity and may worry about the radiation risk to their families and community. This will result in elevated stress for staff at work and possibly reduced effectiveness.
Off-site consequence analysis
A loss of containment will result in a potentially large energetic and highly radioactive plume, which will disperse from the site through the environment to the surrounding population, by expanding and downwind movement.
Measures in the environment are taken to protect people by sheltering (staying indoors), by distribution of iodine pills (to protect the thyroid from the absorption of radioactive iodine) or by evacuation, in dependence of the severity of the actual and/or the anticipated releases.
Analysis of off-site consequences
For the analysis of the consequences of a severe accident, knowledge is needed about the 'source term'. This is the amount, timing and isotopic composition of material released (or postulated to be released) from the reactor or spent fuel pool.
The assessment of the source term can be used:
   • To assess the robustness of the containment features to retain fission products and gases;
   • To develop or improve SAM systems for the mitigation of releases;
   • To provide adequate protection for direct radiation on operating staff in the different reactor areas and control rooms    (on-site consequences).
The characterisation of the source term and its calculation as originating from a damaged containment is the input to off-site consequence analysis and environmental impact and protection to the public. Such estimates of the atmospheric releases can then be used for:
   • Transport and dispersion modelling;
   • Emergency response modelling;
   • Estimation of health impact on the public.
The off-site consequences will be evaluated in two sequences
   • The emergency phase during and shortly after the accident:
   • The long-term phase evaluating mainly radiological consequences.
Influencing Factors
The release of radioactive substances depends on the following factors:
   • Fraction of fission products released from the fuel;
   • Progression of fuel damage;
   • Retention of fission products in the RCS, retention and deposition in the containment and on the containment walls, chemical interactions, resuspension/revaporisation of fission products;
   • Effect of containment spray;
   • Filtered venting at containment, or containment leakage or break.
Experimental research worldwide is a crucial input for the understanding of these factors →. See more →.
Further guidance for the understanding of the source term is in IAEA SSG-4, sec. 6. Read more →
Severe Accident Analysis Codes and Tools
Two different types of computer codes are used for the analysis of severe accidents:
   • Integral codes. These are capable of simulating the whole event, from the start of core damage until the release of fission products. The codes use simplified models for the various physical phenomena, in order to being able to capture the whole event. Where lack of detail exists, sometimes user-specified values need to be provided. It should be noted that these days, the generation of integral or system level codes has become increasingly mechanistic, so this distinction between mechanistic and integral codes has become largely indistinct.
   • Mechanistic codes, using a mechanistic approach, i.e. trying to approach the physical phenomena from their physical basis. They usually focus on a single phenomenon, within known boundary conditions. An example is the distribution of hydrogen, upon the hydrogen source been provided by another code (e.g., an integral code).
Worldwide, the progression of severe accidents is modelled by integral system level code systems. Today, these integral codes are state-of-the-art tools for source term calculations and serve as reservoir of knowledge of severe accident phenomenology.
Examples of codes calculating severe accident sequences (based on the older thermo-hydraulics codes SCDAP/RELAP5 (US) and AC2 and ICARE/Cathare (EU)) are the integral parameter codes MELCOR (US) and ASTEC (EU) with large detailed analysis of source term release and chemistry. Japan has developed the mechanistic SAMPSON code, Russia develops its SOCRAT code system. (See also IAEA TECDOC-1872)→.
Industry uses fast operating commercial codes in MAAP (US and EU improved in version 5). These integral codes are also used in sensitivity studies and risk evaluation for PSA Level 2 studies, estimating the risks of PSA scenarios.
The codes can also be used to investigate potential strategies (CHLAs), both their beneficial and possibly negative effects. In developing the guidelines, they can be used to find set points.
They also serve to define scenarios for exercises, in which the SAMG are trained with plant staff. More information is given in the chapter 2.4 of this module.
Today also very finely meshed mechanistic CFD (Computational Fluid Dynamics, i.e. finite elements in 3D) codes can be used for specific and more detailed phenomena within the spectra of a severe accident, e.g. particular 3D-phenomena inside containment. It should be understood however that fidelity or accuracy of highly detailed codes is generally limited by the comparatively larger uncertainty in the boundary conditions of the accident under study. Indeed, uncertainty characterization of accident sequence analyses is of principal importance in understanding the significance of code results. Often what is more useful for safety and consequence assessments are a large number of calculations that produce a distribution of key results by means such as Monte Carlo sampling of sequence boundary conditions and their variability.
Processes relevant for the source term and considered in computer codes:
- Inventory of fission products;
- Physics of aerosol formation and behaviour, e.g. retention in the RCS;
- Phenomena in the containment;
- Leakages of buildings;
- Thermophoresis: motion of suspended particles following a temperature gradient near a surface;
- Diffusiophoresis: spontaneous motion of dispersed particles in a fluid induced by a diffusion gradient (also called 'concentration gradient') of molecular substances that are dissolved in the fluid;
- Electrophoresis: motion of dispersed particles (having an electric charge) in an electric field;
- Reaction kinetics: speed of chemical reactions between substances, which depend on their reactive surfaces, temperatures, etc;
- Revaporisation/resuspension: volatile fission products that have been deposited before becoming volatile again;
- Sedimentation;
- Chemical combination.
Severe accident consequences codes 
Some computer software available for the assessment of severe accident consequences include:
RASCAL makes dose projection after accidental release of nuclides; it is distributed by the USNRC through the RAMP program.
MELCOR Accident Consequence Code System (MACCS) has also been developed by the USNRC to evaluate the impacts of severe accidents at nuclear power plants and surrounding public. MACCS2 is the latest package enhanced for more flexibility, extended library of nuclides and a semi-dynamic food-chain model. This code determines health consequences of a severe accident both in terms of Early Fatality Risk and Latent Cancer Fatality Risk, as well as providing estimated of land contamination and damage to the economy.
RODOS: In case of a nuclear accident in Europe, the Real-time On-line Decision Support system for off-site emergency management in Europe (RODOS) provides consistent and comprehensive information on the present and future radiological situation, the extent and the benefits and drawbacks of emergency actions and countermeasures, and methodological support for taking decisions on emergency response strategies. Main users of the system are those responsible at local, regional, national and supra-national levels for off-site emergency management. The application of the system for training and exercises was a further important consideration in its development.
Emergency Preparedness
To reduce the consequences of a radiological event, it is required to provide adequate protective measures against this radiological emergency, e.g. evacuation, sheltering, respiratory protection, relocation, potassium iodide (KI) blockage, decontamination of people, decontamination of land and buildings, food chain protection, medical treatments.
The International Commission on Radiological Protection gives recommendations on radiological protection (1-2 mSv/yr livable, 20 mSv/yr limit for attack, etc.). See ICRP work.
An important feature in emergency preparedness is periodic exercises of emergency response capabilities, providing and maintaining adequate facilities and equipment, established procedures to notify the local response organisations and emergency personnel. Notice on the amount and description of the radiological signature has to be given nationally and internationally.
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