However after 1 full year with the facility, the condition is still optimal  

 , so I am thinking it may be impossible to set negative value?  
 
 
EDIT:
Name                := Deep Radioactives Refinerie I
Description         := Large fusion reactors which process a planet's deep ore to generate refined high-energy materials.
Facility Group      := Resource Extraction
Facility Family     := 5
Roman Numeral       := 1
Restrictions        := None
Pic Num             := 61
Cost Minerals       := 2500
Cost Organics       := 0
Cost Radioactives   := 0
Number of Tech Req  := 1
Tech Area Req 1     := Radioactives Extraction
Tech Level Req 1    := 1
Number of Abilities := 2
Ability 1 Type      := Resource Generation - Radioactives
Ability 1 Descr     := Extracts 150 radioactives each turn.
Ability 1 Val 1     := 150
Ability 1 Val 2     := 0
Ability 2 Type      := Planet - Change Conditions
Ability 2 Descr     := The condition of the planet is getting worse with time (100%).
Ability 2 Val 1     := -100
Ability 2 Val 2     := 0
EDIT2: add smiley   
 [ June 16, 2004, 22:19: Message edited by: StarBaseSweeper ]
 
 [ June 16, 2004, 22:19: Message edited by: StarBaseSweeper ]