The PRISME program ("Propagation d’un Incendie pour des Scénarios Multilocaux Elémentaires" or Spread of a Fire for Multi room elementary Scenarios) is an international research program which experimentally studies the propagation of smoke and heat released by a fire located in a nuclear installation equipped with an industrial ventilation network. It is conducted by the IRSN sponsored by the OECD, and groups together 20 partners from 12 different countries. The first part of the PRISME program took place from 2006 to June 2011; the second part, PRISME 2, began in July 2011 for 5 years.
The data collected is used to validate the physical models built into the calculation software that simulates fires, such as Sylvia and Isis.
PRISME 1: objective of the study, experimental approach and results
During the first part of the program, the objective was to study the different mechanisms involved in the propagation of hot gases and smoke from a burning room to adjacent rooms, via all sorts of communicating elements (open doors, leakages around firebreak doors, air vents, piping, etc.) and under the action of ventilation.
Separate effect and global tests
To achieve this, two types of fire tests were carried out: tests specific to the study of a particular physical phenomenon (called "separate effects"), such as the flow through a doorway, and so-called global tests which consist in fires in a configuration that is representative of nuclear scenarios. The three first campaigns were dedicated to the study of separate effects: PRISME Source, for the characterisation of fire sources in open atmosphere and in vitiated environment, PRISME Door, to study the spread of heat and smoke through open doors, and PRISME Leak, to study the propagation of heat and smoke through passages (openings, leakage through a firebreak door, ducting crossing the fire compartment). Finally, the last campaign, PRISME Integral, consisted in carrying out tests according to different accident scenarios and real fire sources (cables and electrical cabinet).
The fire source characterisation tests were carried out in an open environment under the SATURN, a calorimeter used for the study of fire sources in an open atmosphere which is located on the IRSN Cadarache site (Bouches du Rhône). They consisted in burning pans of hydrocarbons, electrical cabinets or cables while especially determining the power of the fire.
Photo of the SATURN hood © IRSN
The other tests were carried on the large-scale facility, DIVA, also located on the Cadarache site, which is composed of three rooms and a hallway which are confined and ventilated like nuclear rooms. The fires are studied there during their complete development until their extinction due to lack of combustible material or depletion of oxidant.
Five experimental campaigns were carried out between 2006 and mid-2011 including over 35 large-scale experiments. The PRISME Source campaign used a single ventilated room. It reproduced the effect of under-oxygenation, caused by the vitiation of atmosphere, on the way in which the power of a hydrocarbon fire evolves depending on the pan size, the type of fuel and the flow rate of the ventilation. The following experimental campaigns covered the flow of the smoke and hot gases through open doors between two or three rooms, through air vents or leaks in firebreak doors. Finally the so-called "global" tests were carried out, in which all the rooms in the experimental installation were used, with complex solid sources and the activation of fire extinguishing systems (especially sprinklers) or fire dampers on the ventilation.
- Effect of ventilation on the power of the fire
The Prisme program made it possible to better understand the effects of ventilation on the power of a fire which breaks out in a confined and ventilated room, and especially on the duration of the fire. Depending on the rate of renewal of the ventilation, the fire can extinguish quickly because of the decrease of the concentration of oxygen in the burning room. But the PRISME tests have shown that a balance can establish itself between the air coming from the ventilation ducts and the power of the source: in this case the combustible material burns more slowly until fire extinction. For example, the same fire source can last 2.5 times longer than in the open air in a room ventilated using an hourly renewal rate of 4.7. The major contribution of the PRISME tests is a better understanding of the effect of under-oxygenation on the evolution of a fire's power.
Correlative and analytical pyrolysis models were developed and validated. These models make it possible to improve the estimations of changes over time of a fire in a confined and ventilated environment using input data obtained from an open atmosphere.
The PRISME program also made it possible to quantify the effect on the spread of smoke of "mixed" convection, which combines the forced convection created by ventilation, and natural convection caused by the high temperature of the smokes. The mechanical ventilation of the burning room can significantly modify the flow of gases that develops naturally through an open door between two rooms. Depending on the ventilation network, mechanical ventilation contributes to unbalancing the flows entering and leaving the burning room, and to changing the position of the neutral zone (height at which the speed of flows is null) at the door.
The volume of data collected during this program made it possible to assess the capacity of software to simulate different fire scenarios. The new models, especially for pyrolysis, were built into the software from the different partners (the Sylvia and Isis software for the IRSN) and validated thanks to the experimental data obtained during this program.
Further, inter-software comparisons were organised by the IRSN in the context of a group attached to the PRISME program: the different partners compared the results of fire simulations with the experimental data. This group especially tested and analysed several "metrics" (i.e. mathematical norm) indicators used during the validation process to objectively assess the differences between the experimental data and the simulation results. Amongst those recommended by the standards, two metrics were revealed to be complementary to assess the capacity of software to simulate a fire. The first metric assesses the relative differences between the numerical and experimental results for local values, as extrema. The second metric is the standardised Euclidean distance which is used to assess the difference between the numerical and experimental results on the duration of the fire. This metric behaves as a global error measurement.
A sensitivity test was also carried out using six different calculation software programs, including the IRSN's Sylvia. The influence of six input parameters (power of the fire, radiative fraction of the flame, thermal properties of the walls, etc.) was tested to calculate nine scales of interest (the temperature of the gases and walls, the concentration of oxygen, the heat fluxes, etc.). For all the software, the results show that the most influential input parameter is the power of the fire, which demonstrates the need to continue the efforts to improve its modelling.
Simplified diagram of the DIVA installation © IRSN
Objectives and procedure
Under the auspices of the OECD, a second international PRISME program was launched in July 2011 to study fire-related topics that could not be treated during the first experiment program, such as the propagation of smoke between stacked rooms, propagation of a fire between cableways and the performance of spray systems.
The PRISME 2 program will include four test campaigns to be conducted at the DIVA facility.
Test campaign 1: Vertical smoke propagation (VSP)
This campaign will study vertical smoke propagation using a horizontal opening between two stacked, mechanically ventilated rooms. It involves analysing and understanding the role and relative importance of natural convection caused by temperature differences in the gaseous environment and forced convection caused by ventilation.
It includes four tests:
- the first, or reference, test (VSP_1) will be performed only in the lower room, with a centred combustion chamber and set ventilation rate;
- VSP_2 will be performed with the same combustion chamber and ventilation rate as the reference test, but with a connection to the upper room open just above the combustion chamber and smoke exhaust taking place through the upper room;
- VSP_3 differs from the previous by the off-centring positioning of the combustion chamber and smoke exhaust and new gas intake taking place in each of the two rooms;
- VSP_4 is identical to the previous test except there will no longer be smoke exhaust in the lower room.
Test campaign 2: Propagation of a fire in cableways, or cable fire spreading (CFS)
This campaign studies propagation of a fire between several stacked cableways and an electrical cabinet open to cableways passing over it, and the propagation of smoke from the room with the fire to an adjacent room.
It includes two test series (see figure below):
- tests CFS_1-CFS_4 concern propagation of a fire between five stacked cableways with the goal of testing three types of cables and two ventilation levels;
- tests CFS_5-CFS_7 study propagation of the fire from an electrical cabinet open to three cableways located above it; for these tests, a third room will be used to study the effects of pressure on a fire door (connecting a third room to the room adjacent to the room with the fire).
These tests are representative of fire scenarios in nuclear facilities that are initiated by an electrical fault in a control panel or electrical cabinet, for example.
Test campaign 3: fire extinction system (FES)
The objective of this test campaign is to assess the performance of sprinkler-type fire extinction systems. In four to six tests (FES_1-FES_6), the effectiveness (ambient environment cooling and fire extinction) of two or three types of industrial sprinkler nozzles will be evaluated for two water flow rates under conditions that represent a fire in a confined and ventilated environment.
Test campaign 4 and additional tests
The last test campaign will involve three to five fire tests to be performed in conditions that will be defined in view of the results of the first three test campaigns. It may involve all five rooms at the DIVA facility.
Sixteen experiments are planned as part of these four campaigns in order to characterize the combustible materials (physicochemical properties and behaviour when exposed to fire) and provide more extensive analysis for the PRISME 2 tests. Initial results and analyses from these experiments are expected in late 2013.