Delivery Guide (DG) for the Inspection of Ammonium Nitrate (AN) Strategic Topics


1. To ensure that inspectors and other relevant staff are aware of the priority topics for inspection associated with the storage and handling of ammonium nitrate.

2. Inspectors should be familiar with the guidance and documents listed in the section on supporting information.  Internet links are provided.


3. In this document AN is used as shorthand for ammonium nitrate.

4. This DG covers ammonium nitrate storage and blending sites subject to regulation by HID SI2. These will include COMAH TT and LT sites, sub COMAH consent sites and sub consent sites.

5. The DG is not intended to cover farms or AN manufacturing sites regulated by FOD and HID CI respectively, although advice has been given to HID CI on the standards required at AN storage facilities within AN manufacturing sites.

6. There is an inconsistency in the definition of AN products in the various regulations and associated guidance. Unless specified otherwise, the products within the scope of this document are those fertiliser grade AN products which fall within Schedule 1, Part 2, Note 2 of the COMAH Regs 1999 (as amended);

Ammonium nitrate (1,250/5,000): fertiliser grade.

This applies to straight ammonium nitrate-based fertilisers and to ammonium nitrate-based compound/composite fertilisers in which the nitrogen content as a result of ammonium nitrate is—

  1. more than 24.5% by weight, except for mixtures of ammonium nitrate with dolomite, limestone and/or calcium carbonate with a purity of at least 90%;
  2. more than 15.75% by weight for mixtures of ammonium nitrate and ammonium sulphate; or
  3. more than 28% by weight for mixtures of ammonium nitrate with dolomite, limestone and/or calcium carbonate with a purity of at least 90%,

and which satisfy the detonation resistance test (DRT).

These products will be classified for transport as UN 2067.  For ease of reference the following table gives the nitrogen content in fertilisers derived from AN

AN % 100 90 80 70 60 45 30
N % 35 31.5 28 24.5 21 15.75 10.5

Whilst HSE's guidance document INDG230 focuses on >28% nitrogen from AN, it does refer to AN fertilisers that contain 28% nitrogen or less at page 9 of the guidance. Essentially the guidance for >28% nitrogen from AN applies except that relating to stack limits (as these grades do not normally present an explosion hazard) and separation from incompatible materials.

7.Technical Grade AN (TGAN) is not included in the scope of this document because its storage in significant quantities is seldom encountered.


8.The worst major accident in Europe this century was an explosion involving AN at Toulouse in September 2001 which killed 30 people including 8 off-site. There have also been other incidents (in Spain and Romania) were lorries carrying AN have been involved in traffic collisions, which has led to vehicle fire and subsequent explosion of the AN. There were 19 fatalities in the Romanian incident in 2004. These recent incidents highlight the potential hazard from AN and emphasize the potential for incidents and consequences in the AN supply chain.

9. Evidence from recent inspections has highlighted several instances of systemic failures to adequately manage the major hazards aspects of AN storage. Patterns of non-compliance have been seen both within larger and smaller duty-holders.

What are the issues with AN sites?

10. The hazards from AN are twofold. In a fire, AN will melt and decompose with the release of toxic fumes. Whilst AN will not explode due to the friction and impact from normal handling and blending operations, detonation can occur under heat and confinement or by severe shock.  The likelihood of an explosion is significantly increased if the AN becomes contaminated with incompatible materials, particularly organic matter. The likelihood of a detonation is also greater where the product has failed the DRT test which is required for the supply and storage of fertiliser grade AN.

As pointed out above HSE's guidance focuses on greater than 28% nitrogen from AN as fertilisers that contain 28% or less nitrogen from AN normally present a reduced explosion hazard.

11. HSE guidance on the safe storage of AN has been published and, although this is due for review and revision, the principles set out in the current edition provide the standard to be achieved, unless this has been specifically dealt with elsewhere (eg the currently accepted standards of tarmac flooring in stores).

12. There are a number of current issues associated with the storage and handling of AN;

  1. As highlighted in the Toulouse incident, if non-conforming products are not managed effectively they present a significant hazard. The aftermath of Toulouse led directly to the amendment to SEVESO and subsequently to COMAH in 2005. The effective management of non-conforming products remains an on-going issue to ensure appropriate standards are in use.
  2. Recent evidence from inspections has identified a number of facilities where combustible or incompatible materials have been co-stored in the same building with AN without adequate separation eg wood pulp, grain. Such occurrences increase the likelihood of a fire or detonation involving the AN. The AN handling and storage facility should be designed and operated to (a) prevent any fire associated with the combustible material from effecting the AN and (b) prevent any possible mixing with incompatible materials. Such arrangements will minimise the possibility of the ammonium nitrate becoming involved in an incident.
  3. A topic that has caused much discussion and debate is the use of asphalt or tarmac flooring in ammonium nitrate stores. An action plan has been agreed and implemented with the industry regarding this issue which is reflected in this document. The focus on this issue will primarily be on non AIC (Agricultural Industries Confederation) members.
  4. When ammonium nitrate is found to have failed the DRT, the keeper should take measures to reduce the risk of detonation of that material to an insignificant level which, either by: dissolving the material in water to produce a liquid fertiliser, or blending it with other suitable material.  Both of these activities will take time to deal with the failed material and will require appropriate equipment. The site operator should have a suitable contingency plan in place to be able to store the failed material safely until it has been dealt with and be able to access the necessary equipment for such eventualities if it is not available on site. It should be noted that the COMAH TT threshold for DRT failed material is 50 tonnes and that some sites (particularly sites that have no Hazardous Substances Consent) may be unsuitable from an off-site risk perspective.

Main actions

13. Inspectors should carry out pro-active inspections in accordance with Unit risk ranking, the lead inspector programme (for large duty holders) and the COMAH Competent Authority's expectation. In no case should the inspection interval for COMAH LT sites be greater than 3 years, whilst for consent sites, the maximum interval should be no greater than 5 years and for sub-consent sites: 8 years.

14. The intervention priorities for 2012/13 are derived from the 'HID SI2 Ammonium Nitrate Strategy 2011–2013' and align with – as appropriate – COMAH priorities and SI2 strategic topics, including a number of 'AN Strategic Topics'. This document details the AN Strategic Topics.  'Success Criteria' are included with a read across to the level of performance encountered and the initial enforcement expectation in the circumstances.

15. The HSE, SI2 or AN strategic topics do not preclude inspectors following up other company or site-specific issues where these are matters of evident or significant concern.

Management of non conforming material

16. Industry guidance defines non-conforming materials as "those materials which do not meet the characteristics of the intended products at the time of production or when marketing".   This includes both off-spec and reject materials and ranges from relatively minor non-conformances eg pH out of specification to DRT failures.

17. The non con-forming materials that are of primary interest here are those that may have a lower resistance to detonation, although this reduction will be somewhat dependant on the specific reason for the non conformance.  This will include fines produced as a result of the processing activities carried out, caked materials, materials which have deteriorated in storage (particularly long duration storage), low bulk density materials and AN that has been contaminated.  It should be noted that where there has been contamination with organic matter, the mixture is a potential class 1 explosive. For transport purposes, if the organic content is greater than 0.2%, the material is class 1.

18. The storage of large quantities of non con-forming AN was one of the principal factors in the Toulouse explosion in 2001. This directly led to the amendment in SEVESO and subsequently COMAH in 2005, when a new category of AN (category 4: "off-specs" material and fertilisers not satisfying the detonation resistance test) was inserted with qualifying inventories of 10tes and 50tes for LT and TT respectively. These qualifying inventories are equivalent to those specified for the more hazardous category of explosives.

19. Toulouse also led to the introduction of the Ammonium Nitrate (High Nitrogen) Safety Regulations 2003, which are Defra regulations enforced by Trading Standards. It should be noted however, that the scope of these regulations is different to COMAH in that these regulations only apply to materials with >28% nitrogen from AN.

20. There are guidance documents which are relevant here;

  1. Guidance note for Manufacturers, Importers, Blenders, Transporters, Store Keepers and Suppliers of AN based fertilisers produced jointly by AIC, Defra and HSE and published on the AIC website
  2. Guidance for the disposal or utilisation of AN based fertiliser which fails to meet the requirements of the ammonium nitrate safety regulations 2003 for material that has failed the DRT

21.  The documents at referenced in paras 20(i) and (ii) are UK produced and where the focus is on non-conforming materials relating to DRT failure.  These are appropriate for material that has either failed the DRT or is contaminated or in such poor condition that it might reasonably be expected to fail a DRT. The guidance referenced at para 20(i) recommends that a contingency plan be prepared to deal with the eventuality of DRT failure.

22. If material is found that is 'caked' ie lumpy, sticking together (typically water contaminated) or there are signs of contamination with other materials eg oil, combustibles (including 'dirty' looking product) it is possible that it could fail a DRT.  In these circumstances, samples of suspect material should be taken in accordance with sampling protocol and tested – Can be arranged via HSL. Note – Consideration on the extent of any 'caking' or contamination is required and may require separation and segregation of the suspect material from good material.

23. The EFMA documents at referenced at paras 20(iii) and (iv) are produced for the European industry and are wider in scope in that they identify sources of non-conforming material, bring out relevant aspects of their properties and outline various methods available for their safe utilisation and disposal. They also detail the general principles of a safety management system associated with non-conforming material. These principles are considered appropriate for managing non-conforming material generally and include written procedures for handling and treatment, records of non-conforming materials, minimisation of quantities accumulated, segregation from other products etc.

24. An aspect to be aware off is the possibility of the duty holder mixing AN with urea if both are handled at the same time in the operation or where there may be residual non-conforming urea followed by AN or vice-versa.  Mixtures of AN and urea are more hazardous as they form a low melting point eutectic mixture (~45oC vs melting point of AN ~167oC) and the potential explosive yield (energy available for release) is enhanced.  INDG 230 currently states "When it is absolutely necessary to store urea and ammonium nitrate products in the same building, keep them in such a manner that they cannot mix in any likely accident." This is equally relevant to waste or "off-spec" materials.

25. Things to Verify

  • Check that the duty holder is aware of the implications of contamination and it possible effect that the product could fail a DRT
  • Does the duty holder have a scheme in place supported by written procedure for dealing with non conforming materials? Does this recognise potentially hazardous materials? eg those likely to fail a DRT, is a risk assessment carried out prior to different non conforming materials being mixed?
  • Has the duty holder maintained a record of the types, sources and quantities of non conforming materials and how they were utilised?
  • Do the conditions in the storage area reflect the documented procedure?
  • Check the condition of the AN in any 'loose heaps'. The bagged product should be checked if there is any reason to suspect that the product may have been contaminated on route – especially sea water.
  • Is the quantity of non conforming materials in the storage area managed? [Typical maximum quantities of untreated material should be circa 1te and for treated materials, circa 20-25te (ie equivalent to a lorry load) respectively.]
  • Are the non conforming materials segregated from other raw materials and conforming materials?

The relevant extracts from the guidance are detailed in the 'fully compliant' column in Table 1. Criteria for Judging Success – AN Strategic Topics.

Incompatible Storage

26. Evidence from inspections during 2010/11 identified a number of instances were ammonium nitrate had been co-stored with flammable, combustible or incompatible materials eg wood pulp, sawdust, grain. HID SI2 approached AIC regarding these findings and AIC agreed to issue a 'Member Briefing' to all their members (attached).

27. There are two relevant guidance documents; HSE's 'Storing and Handling AN' - INDG230 and EFMA's 'Guidance for the Storage, Handling and Transportation of solid mineral fertilisers'.  HSE's guidance note in several places refers to a suitable firebreak or partition.  The expectation for the firebreak or partition being that it will be sufficient to prevent any damaging effect on the ammonium nitrate stored. EFMA's guidance states 'Do not store fertilisers where they may be affected by any source of heat or stored combustible materials' and 'Do not store fertilisers where they can become contaminated with incompatible or combustible materials'.

28. Things to Verify

  • Is the duty holder aware of the relevant guidance documents?
  • For duty holders who are AIC members, are they aware of the AIC Member Briefing?  If not, the reasons for this should be explored.  For non AIC members, if they are not aware of the AIC Member Briefing, the contents of it should be shared with them and discussed.
  • Has the duty holder a policy / risk assessment for AN storage and are staff aware of it?
  • Are the storage area(s) compliant with the guidance note?

The relevant extracts from the guidance are detailed in the 'fully compliant' column in Table 1. Criteria for Judging Success – AN Strategic Topics.

Detonation Resistance Testing

29. This section does not include the management of DRT failed material – that is covered in the section on the 'Management of non-conforming material'.

30. The focus here is on those fertilisers which fall within the scope of Schedule 1, Part 2 Note 2 of the COMAH Regulations 1999 (as amended).  Products which fall within this scope should pass the DRT and the duty holder should have a copy of the DRT certificate. If the DRT certificates are not available, their COMAH category cannot be confirmed and examination of such materials should be undertaken.

31. The DRT test is also relevant to the Ammonium Nitrate (High Nitrogen) Safety Regs 2003, but these are DEFRA regulations enforced by Trading Standards.  It should be noted that the definition of AN within the scope of these regulations is different than COMAH as it only applies to those AN products with > 28% nitrogen from AN.

32. The DEFRA regulations require that the keeper should have a contingency plan in place for dealing with DRT failed material.  This plan should ensure that they have or can get access to the necessary equipment (solution plant, blending equipment) to deal with the failed material.  It should also ensure appropriate arrangements are in place such that the failed material can be stored safely in the interim period.  This requirement is referred to in the joint AIC/DEFRA/HSE 'Guidance Note for Manufacturers, Importers, Blenders, Transporters, Store Keepers and Suppliers of Ammonium Nitrate based Fertilisers'.  The appropriate considerations for the contingency plan have now been further described in the document entitled 'Guidance Note for Manufacturers, Importers, Blenders, Transporters, Store Keepers and Suppliers of Ammonium Nitrate based Fertilisers'.

33. Such a contingency plan is also a reasonable expectation for all material included within the scope of the COMAH Regs.

34. Things to Verify

  • Does the duty holder have DRT certificates for the material stored? Note. More than one DRT certificate may be required to cover all the batches of product.
  • Any AN that is not covered by a DRT certificate or where the DRT certificate is more than 12 months old, check the condition of the AN, 'loose heaps' and bagged stacks. If material is found to be 'caked' ie lumpy, sticking together (typically water contaminated) or there are signs of contamination with other materials eg oil, combustibles (including 'dirty' looking product) it is possible that it could fail a DRT.  In these circumstances, samples of suspect material should be taken in accordance with sampling protocol and tested – Can be arranged via HSL. Note – Consideration on the extent of any 'caking' or contamination is required and may require separation and segregation of the suspect material from good material.
  • Is the duty holder aware of the need for a 'contingency plan' and the guidance that is available?
  • The key elements of the contingency plan are that the duty holder has identified the key facilities which will be required and that they are aware of the storage arrangements required for failed material.
  • Were the duty holder doesn't have a contingency plan or if the key elements are missing or inadequate, the requirement for an appropriate plan should be discussed and a timescale agreed for completion.

Building Construction (inc. flooring)

35. Buildings used for the storage of ammonium nitrate should be constructed from materials that will not burn.  A significant query over the past few years has been the suitability of tarmac or asphalt flooring.  An action plan has been agreed with AIC and implemented by them on the use of such flooring.

36. The action plan required that all AIC members had established the bitumen content of their floors either by sampling and testing or via documentation.  Where the bitumen levels were found to be > 9%, the duty holder was required to develop a programme of remedial action within the agreed timescales.

37. Things to Verify

  • Establish whether the duty holder is a member of AIC.
  • If a member of AIC, check that the duty holder has appropriate information (via sampling analysis or documentation) which confirms the bitumen level in any stores which are used for >28% nitrogen from ammonium nitrate.
  • Where bitumen levels greater than 9% have been established, check that the duty holder has an appropriate programme for remedial action which is within the agreed timescales.
  • If the duty holder is not a member of AIC, check if the duty holder is aware of the issue and the AIC action plan and whether they know what the bitumen levels are in the tarmac in their stores.
  • Excluding the floor, check that the building is constructed from materials that will not burn. If there is wood in the structure of the building, specialist advice should be sought to assess its significance.


38. As arson is a significant cause of warehouse fires, it is important that the site has considered security (particularly out of working hours).  Obviously HSE inspectors are not security experts but it is possible to establish some confidence that the duty holder is considering the full range of hazards.  It is likely that the site will have been visited by the local CTSA (counter terrorist security advisor) but it may have been more than 5 years since the last visit.

39. Things to Verify

  • Has the duty holder sought and acted upon competent security advice?  Insurance companies and police crime reduction advisors (including CTSA's) may provide these services. Contact information for the CTSA's is available from the local police force web-site.
  • Has the duty holder developed policies and procedures for site security?  Are they up to date and implemented?
  • Is there an established system for checking perimeter security and any other precautions? Are fences and systems such as CCTV well maintained?
  • Is a record of potential or actual breaches of security kept and used to inform improvements?

40. Positive responses to such questions should be recorded. Concerns should be referred to the duty holder for action.  Advice should be taken if enforcement action is considered – this will also likely involve discussion with the CTSA for the site.

Help and support

41. Where more specialist support is required, HID SI2E should be contacted in the first instance.

Supporting information

42. The guidance and documents listed below provides inspectors with additional information to that given in this DG.

  1. INDG 230 – Storing and Handling Ammonium Nitrate; HSE
  2. Guidance for the Disposal or Utilisation of AN based fertiliser which fails to meet the requirements of the Ammonium Nitrate Safety Regulations 2003; AIC
  3. The Ammonium Nitrate Materials (High Nitrogen Content) Safety Regulations 2003
  4. The Ammonium Nitrate Materials (High Nitrogen Content) Safety Regulations 2003 – FMA Guidance Note July 2003; AIC

Judging success and moving on

43. Table 1 is linked to the performance 'rating' to be used under either instructions from the COMAH CA or HID SI2 (for non-COMAH sites).  Its purpose is to tie in inspection findings, performance rating scores and initial enforcement expectation (under EMM). The table only provides guidance; the EMM1 and local or strategic factors will ultimately determine the appropriate action and close-out of an issue.

Recording the intervention/outcome

44. Inspectors should assign the performance score for each AN strategic topic according to the criteria in Table 1 above and input this data onto the Performance Rating field which is located in the middle of the Inspection Rating Form (IRF) page of the COIN Service Order along with any other SI or SI2 strategic topics which were inspected.

45. The AN strategic topics have been added to all the COMAH Intervention Plan Service Orders (IPSO's) for AN sites for 2012/3.  For non-COMAH interventions the AN strategic topics will need to be added onto the bottom of the SO.

46. There is no expectation that inspectors will carry out a retrospective rating exercise; the rating will be based on intelligence gathered during the site visits.

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Updated 2022-07-12