Maybe I'm reaching way out on a limb, but here goes, and remeber I'm not a Chemist or Biologist...Just a Fire Fighter / Investigator. If there is Urine / Feces in a diaper it would begin to decompose, as the moisture evaporates and the decomposition continues an amount of Uric Acid and Ammonia would be produced. Now, according to the North American Emergency Response Guidebook, Guide #113 the following is said for Urea nitrate, wetted with not less than 10% water
Urea nitrate, wetted with not less than 20% water. Yes, I realize that the following is for bulk Haz-Mat situations, but could you conceivably reach the percentages stated, and could this occurr on a smaller scale?


GUIDE
113 FLAMMABLE SOLIDS - TOXIC (Wet/Desensitized Explosive)

POTENTIAL HAZARDS
FIRE OR EXPLOSION
• Flammable/combustible material.
• May be ignited by heat, sparks or flames.
• DRIED OUT material may explode if exposed to heat, flame, friction or shock; Treat as an explosive (GUIDE 112).
• Keep material wet with water or treat as an explosive (GUIDE 112).
• Runoff to sewer may create fire or explosion hazard.
HEALTH
• Some are toxic and may be fatal if inhaled, swallowed or absorbed through skin.
• Contact may cause burns to skin and eyes.
• Fire may produce irritating, corrosive and/or toxic gases.
• Runoff from fire control or dilution water may cause pollution.
PUBLIC SAFETY
• CALL Emergency Response Telephone Number on Shipping Paper first. If Shipping Paper not available or no answer, refer to appropriate telephone number listed on the inside back cover.
• Isolate spill or leak area immediately for at least 100 meters (330 feet) in all directions.
• Keep unauthorized personnel away.
• Stay upwind.
• Ventilate closed spaces before entering.
PROTECTIVE CLOTHING
• Wear positive pressure self-contained breathing apparatus (SCBA).
• Structural firefighters' protective clothing will only provide limited protection.
EVACUATION
Large Spill
• Consider initial evacuation for 500 meters (1/3 mile) in all directions.
Fire
• If tank, rail car or tank truck is involved in a fire, ISOLATE for 800 meters (1/2 mile) in all directions; also, consider initial evacuation for 800 meters (1/2 mile) in all directions.
EMERGENCY RESPONSE
FIRE
CARGO Fires
• DO NOT fight fire when fire reaches cargo! Cargo may EXPLODE!
• Stop all traffic and clear the area for at least 800 meters (1/2 mile) in all directions and let burn.
• Do not move cargo or vehicle if cargo has been exposed to heat.
TIRE or VEHICLE Fires
• Use plenty of water - FLOOD it! If water is not available, use CO2, dry chemical or dirt.
• If possible, and WITHOUT RISK, use unmanned hose holders or monitor nozzles from maximum distance to prevent fire from spreading to cargo area.
• Pay special attention to tire fires as re-ignition may occur. Stand by with extinguisher ready.
SPILL OR LEAK
• ELIMINATE all ignition sources (no smoking, flares, sparks or flames in immediate area).
• All equipment used when handling the product must be grounded.
• Do not touch or walk through spilled material.
Small Spills
• Flush area with flooding quantities of water.
Large Spills
• Wet down with water and dike for later disposal.
• KEEP "WETTED" PRODUCT WET BY SLOWLY ADDING FLOODING QUANTITIES OF WATER.
FIRST AID
• Move victim to fresh air.
• Call 911 or emergency medical service.
• Give artificial respiration if victim is not breathing.
• Administer oxygen if breathing is difficult.
• Remove and isolate contaminated clothing and shoes.
• In case of contact with substance, immediately flush skin or eyes with running water for at least 20 minutes.
• Ensure that medical personnel are aware of the material(s) involved and take precautions to protect themselves.