Just how dangerous are the Hydrazine Fuels? The hydrazine fuels (anhydrous hydrazine and mono-methylhydrazine MMH) are currently going through close scrutiny in Europe because of the new ‘REACH’ legislation. Consequently, the question of their toxicity is a hot subject.
Having worked with those propellants for about 30 years the one thing I can say with certainty is that I could find some published toxicity studies which say the dangers (to humans) are very real and other published studies suggesting the dangers have been overestimated. Although this may seem extraordinary, there are two key issues which could explain the apparent contradiction;
- Low concentrations of hydrazine vapours are notoriously difficult to detect and quantify reliably
- Relatively small numbers of people get exposed to hydrazine, so the database is limited.
Whereas the vapour of NTO/MON oxidiser can be detected by inexpensive but highly sensitive equipment, the detection of hydrazine vapour at suitably low levels has always been difficult to do. The complications include such issues as the way that hydrazine adsorbs on surfaces and this may affect the responsiveness of sensors. Once adsorbed, hydrazine may then decompose in to products including ammonia, and as human sweat contains some ammonia, false positive indications can arise.
Tests to assess the toxicity of hydrazine on animals do give worrying results. The exposure limits (as prescribed in all countries) for hydrazine have come down significantly over the years. So the bottom line is that anyone working with hydrazine needs to adopt very disciplined and professional approach.
The UK occupational exposure limits for all chemicals can be found in HSE document “EH40”
The hydrazine fuels have been around a long time, have served the space programmes well, and until a few years ago you could have been forgiven for thinking that they were irreplaceable. However, there has now been great progress made with non-toxic fuels based on aqueous solutions of either ammonium dinitramide (ADN) or hydroxyl ammonium nitrate (HAN), as developed in Sweden and the USA, respectively. These propellants look as though they could replace hydrazine as a mono-propellant sometime in the future (10 years?). However, the bi-prop combination of MMH and NTO looks like it will be around for some time to come (:-)).
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