Water has been frequently proposed as the coolant for fusion reactor in-vessel components including the shielding blanket and vacuum vessel being designed for the International Thermonuclear Experimental Reactor (ITER). DT burning plasmas generate high fluxes of energetic (14-MeV) that react with the blanket and vacuum vessel structural and shielding materials producing large concentrations of activated products that introduce serious problems for reactor operation, safety, maintenance, and, ultimately, disposal. Water is no exception.

Cooling the plasma facing components, particularly the first wall (FW) and sections of the blanket immediately behind it, expose the coolant water to intense fluxes of high energy neutron that trigger reactions with hydrogen and oxygen producing large concentrations of ³H, ¹⁶N, ¹⁷N, ¹⁸N, ¹⁹O and other radio nuclides. Of these, ¹⁶N and ¹⁷N are the most serious in terms of reactor design and shielding. ¹⁶N is produced from fast neutron (E ³ 10-MeV) reactions with ¹⁶O(99.76%) via the ¹⁶O(n,p)¹⁶N reaction and decays with a half-life of 7.13s by the emission of 6.13 and 7.12 MeV gamma-rays. ¹⁷N results from fast neutron (E ³ 10-MeV) reactions with ¹⁷O(0.038%) by way of the ¹⁷O(n,p)¹⁷N reaction and decays with a half-life of 4.12 s by the emission of neutrons having an average energy of 0.9 MeV. Although the natural abundance of ¹⁷O in water is small compared to ¹⁶O, large amounts of water circulate through the blanket so the total concentration of ¹⁷N produced during reactor operation is non-negligible. For reactors operating at high power and long pulse duration, the ¹⁶N and ¹⁷N that are produced affects system design particularly after the water exits the in-vessel components and passes through the cryostat and biological shield in transit to heat exchange equipment located outside the biological shield. ¹⁶N decay gamma-rays are an added source of heating in components inside the cryostat while the ¹⁷N introduces a neutron source outside the shielded region that activates equipment. Activation of equipment outside the biological shield impacts maintenance and personnel access in otherwise well shielded areas.

The production of ¹⁶N and ¹⁷N radio nuclides in a blanket cooling loop having water circulation and residence times similar to those anticipated in ITER are discussed. Effects of activated water on cryogenic equipment located inside the cryostat and in piping and equipment outside the biological shield are presented. Amelioration of the effects of activated water are also reviewed.