		2018			2017
	US$m			US$m
Net book value
At 1 January		1,037			951
Adjustment on currency translation		(125	)		86
At 31 December		912			1,037
– cost		15,861			17,942
– accumulated impairment		(14,949	)		(16,905	)

At 1 January
– cost		17,942			17,144
– accumulated impairment		(16,905	)		(16,193	)

At 31 December, goodwill has been allocated as follows:
		2018		2017
	US$m		US$m
Net book value
Richards Bay Minerals		474		552
Pilbara		351		389
Dampier Salt		87		96
		912		1,037

Richards Bay Minerals’ annual impairment review resulted in no impairment charge for 2018 (2017: no impairment charge). The recoverable amount has been assessed by reference to FVLCD, in line with the policy set out in note 1(i) and classified as level 3 under the fair value hierarchy. FVLCD was determined by estimating cash flows until the end of the life-of-mine plan including anticipated expansions. In arriving at FVLCD, a post-tax discount rate of 8.8% (2017: 8.7%) has been applied to the post-tax cash flows expressed in real terms.

The key assumptions to which the calculation of FVLCD for Richards Bay Minerals is most sensitive and the corresponding decrease in FVLCD are set out below:

Other assumptions include the long-term pig iron and zircon prices and operating costs. Future selling prices and operating costs have been estimated in line with the policy set out in note 1(i). The recoverable amount of the CGU exceeds the carrying value when each of these sensitivities are applied whilst keeping all other assumptions constant.

The annual impairment review of the Pilbara CGU has been assessed by reference to FVLCD using discounted cash flows, which is in line with the policy set out in note 1(i) and is classified as level 3 under the fair value hierarchy. In arriving at FVLCD, a post-tax discount rate of 6.8% (2017: 6.7%) has been applied to the post-tax cash flows expressed in real terms. The recoverable amount was determined to be significantly in excess of carrying value, and there are no reasonably possible changes in key assumptions that would cause the remaining goodwill to be impaired.