Special considerations for divertors

DOE-STD-6003-96

Due to the challenging nature of its loading conditions and developmental status, it is desirable

that the divertor system not be classified as a safety-class system. However, since the divertor

structure will become radioactive and its plasma-facing surfaces will become contaminated with

absorbed tritium, the impact of the divertor system on other safety-class systems must be

considered.

The goal of the divertor design should be that a single failure of the divertor system does

not threaten any in-vessel or ex-vessel safety-class system. The divertor system design should

prevent damage to safety-class components, which might include the vacuum vessel, fueling

and vacuum pumping system piping, and in-vessel coolant system pipes. If this goal cannot be

met and individual components within the divertor system are designated as safety-class, the

design guidance listed in Section 6.1 should apply to these components. Divertor components

designated as safety-class should be designed, fabricated, inspected, and tested in accordance

with an approved structural acceptance criteria. Because the divertor environment and material

candidates differ significantly from more conventional applications with regard to (1) handling

the high, steady-state heat and energetic particle fluxes on the surface of the divertor target,

(2) withstanding the intense thermal and electromagnetic loads during plasma disruptions, and

(3) experiencing fluences of high-energy neutrons leading to property changes, embrittlement,

and irradiation-induced creep, existing safety-related codes are largely inapplicable. The design

of the divertor should meet the safety design criteria of this Standard and should employ a

design and analysis methodology that is consistent with a recognized safety-related code.

Design standards and practices for non-safety-class divertor components are not addressed in

this document.

b. Special considerations for divertors

In addition to conventional materials and effects, special consideration of the following

items, which are unique to the fusion divertor environment, must be included in the design and

analysis of the divertor and in defining appropriate design practices and criteria:

1. Armor Tile Materials--Many of the armor tile material candidates considered for

use in protecting the divertor target structure are brittle metals or nonmetals. The

behavior of these materials and their influence on the structures to which they are

attached must be considered in evaluating the integrity of the coolant confinement

structure.

2. Erosion and Redeposition--Reduced or increased armor tile thickness due to

erosion or redeposition of previously eroded material could have a significant

effect on the thermal stresses in the coolant containment structure to which the

armor tiles are attached.

3. Plasma Disruptions/VDEs--The transient dynamic mechanical and thermal effects

during plasma disruptions/VDEs, which are extremely intense but of very short

duration, must be considered in evaluating the integrity of the divertor structure

and in defining appropriate structural design criteria.

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