Gunpowder safety is one of the activities with which TNO-PML acquired a great name and fame worldwide. The nitrated cellulose-based gunpowder is in principle unstable and can ignite spontaneously when stored for a long time at an elevated temperature. This can lead to significant explosions (such as the major explosion in Lapua Finland in 1976, in White Oak Washington in 1992, and in Brazil in 1995; in addition to many deaths and injuries, there was 1 billion in damage). By adding a stabiliser, this decomposition process (due to ageing) is slowed down. The gunpowder, however, must be continuously monitored during its lifetime to ensure that it is still sufficiently stable for a subsequent storage period. This is done through the gunpowder guard method. an important activity to prevent loss of life and material damage.
Since the end of 1800, the stability of gunpowder has been checked in the Netherlands, in the beginning by the Artillerie Inrichtingen on the Hemveld near Zaandam.
On January 1, 1949, the name of the T.N.O. Laboratory Poortlandlaan was changed to “Technological Laboratory of TNO-RVO”, headed by ir der Weduwen. When the Technological Laboratory was founded, gunpowder research was the most important core of its research. During the rearmament of the Armed Forces after WWII, a large part of the research was performed on this topic for many years. All gunpowder entering the Netherlands is examined because gunpowder is inherently unstable and can therefore cause large explosions. This concerns hundreds of powders from abroad, including the powders obtained under the Marshall aid for the 155 mm howitzers.
After the transfer of sovereignty from Indonesia on 27 December 1949, it was ensured that the gunpowder stored at tropical temperatures in Indonesia by the Dutch army was examined to see whether the gunpowder could still be used in the Netherlands.
In the first half of the 1970s, research was directed jointly by the three Dutch armed forces. In the early 1990s, this resulted in an effective and well-systematised method of gunpowder monitoring using the heat development test in a microcalorimeter (HFC-Heat Flow Calorimeter) that fits well with the Defence needs and that is also internationally standardised and recognized (NATO STANAG 4582). Such a development process takes many years to complete. The effectiveness of this gunpowder monitoring method can be seen from the fact that since TNO carries out this monitoring, no explosions have ever occurred in ammunition storage complexes in the Netherlands.
Technology development gunpowder protection
In the 1950s and 1960s gunpowder research mainly used laborious (wet) chemical methods (Abel Heat Test, Bergman JunkTest, etc.), in which the effectiveness of the stabilizer and the possible decomposition at elevated temperatures were determined. In the second half of the 1960s, there was the first “functioning test”, namely the Closed Vessel test, in which the pressure build-up during combustion of the gunpowder is measured in a static set-up. Ultimately, this results in a total gunpowder investigation, in which all relevant gunpowder data is recorded in a ‘propellant description sheet’, such as appearance, composition (with nitrogen content of the guncotton), results of heating tests (stability tests), calorific value, specific weight, combustion rate. These data together with the data from the closed vessel test are important to calculate the internal ballistics of gun systems and thus the muzzle velocity of a shell. In the 1970s, much more efficient chemical methods were developed such as high-pressure liquid chromatography (HPLC), e.g. for the determination of the stabiliser content. However, with this method, one can only approve or reject a certain gunpowder.
Heat development test (WOP or later ‘heat flow calorimeter’)
The more unstable the gunpowder, the more heat is released. At some point, the gunpowder can heat itself and it will ignite. When stored, this means that a burning grain ignites other powder grains, which can lead to a chain reaction that causes an explosion. At normal ambient temperature, however, the amount of heat released is very small and measurements must be carried out over a longer period (days to weeks at different temperatures). Dr.ir. J.L.C. van Geel from TNO introduced the heat development test (WOP) based on a very sensitive measurement of heat development (better than 10 microwatts per gram) with heat flow meters developed by the TPD-TNO-TU Delft that allows better predictions about the gun powder lifespan.
It appears possible to find a relationship (safe diameter) between the composition/geometry of the powder grain and the amount of heat released over time. It can be shown that the powder grain will heat itself if this safe diameter is exceeded.
Also, using the Arrhenius equation, accelerated ageing at high temperatures can be translated into a service life under storage conditions. In this way, a prediction can be made about the life span during which the gunpowder can still be stored safely.
To ultimately investigate the large quantities of gunpowder that must be measured regularly, a multi-WOP has also been developed, in which a maximum of eight gunpowder samples are measured simultaneously in one device during the duration of the experiment. This means that one has a very powerful instrument, because the powder stock does not have to be checked every year, but only at the end of the predicted service life.
For the Dutch Ministry of Defence, TNO subsequently developed a method to regularly measure the total gunpowder stock – with an age up to about a hundred years – for the risk of spontaneous combustion. This method looks at the current stability, as well as a prediction of the stability development for the subsequent eight years. This method was then introduced into NATO AC/310 (later AC/326) “On Safety and Suitability for Service of Munitions”. The method is now a NATO standard for the quality control of nitrocellulose-based powders. In addition to research into normal gunpowder, all kinds of newly developed gunpowder types are also examined in the WOP for possible spontaneous combustion.
Brazil contacted TNO in the second half of the 1990s because they had an explosion at one of their gunpowder stores at their Navy depot in Rio de Janeiro. They were charmed by the TNO method with the microcalorimeter and would like to introduce it in the Brazilian Army. Wim de Klerk developed a modern version of the WOP for this purpose, the Heat Flow Calorimeter. The HFC is relatively cheap and robust and can be manufactured in limited numbers in collaboration with the industry. A completely new layout has been made, with a new control and data acquisition system. TNO-PML also switched to this modern Heat Flow Calorimeter in 2004. After extensive negotiations and test phases in 2005, the TNO concept was adopted by Brazil in its entirety and Brazilians were trained to work with the equipment. Later, the systems were installed at more Defence and defence-industry locations in Brazil.
PML Vaarwel (2019) [Dutch]