Update radiation.lua

I think this will work?

technic/radiation.lua

@@ -185,22 +185,30 @@ technic.register_group_resistance("tree", 3.4)
 technic.register_group_resistance("uranium_block", 500)
 technic.register_group_resistance("wood", 1.7)
 
--- Function to calculate radiation resistance
-local function node_radiation_resistance(node_name)
-	local def = minetest.registered_nodes[node_name]
-	if not def then
-		return 0
-	end
+technic.resistance_cache = {}
 
-	local resistance = 0
-	-- Add rad_resistance group value if it exists
-	if def.groups.rad_resistance then
-		resistance = resistance + def.groups.rad_resistance
+function technic.cache_resistances()
+	for node_name, node_def in pairs(minetest.registered_nodes) do
+		local resistance = 0
+		if node_def.groups and node_def.groups.rad_resistance then
+			resistance = node_def.groups.rad_resistance
+		end
+		technic.resistance_cache[node_name] = resistance
 	end
+end
 
-	return math.sqrt(resistance)
+local function node_radiation_resistance(node_name)
+	local cached_resistance = technic.resistance_cache[node_name]
+	if cached_resistance then
+		return math.sqrt(cached_resistance)
+	else
+		return 0
+	end
 end
 
+-- Initialize cache
+technic.cache_resistances()
+
 --[[
 Radioactive nodes cause damage to nearby players.  The damage
 effect depends on the intrinsic strength of the radiation source,