A method for operating a turbine engine, wherein the turbine engine comprises: a compressor; a combustor; a turbine; a gaspath passing downstream from the compressor through the combustor and then through the turbine; a fuel source; a fuel flowpath from the fuel source to the combustor; and a heat exchanger (102; 400) for transferring heat from the gaspath to the fuel flowpath, wherein the heat exchanger (102; 400) comprises: an inner wall in heat transfer relation with the gaspath; an outer wall; tubes (180, 182) between the inner wall and the outer wall bounding respective segments of the fuel flowpath; and a heat transfer fluid between the inner wall and the outer wall and in heat transfer relation with the tubes (180, 182) and the inner wall, wherein the method comprises: passing fuel along the fuel flowpath from the fuel source to the combustor; combusting the passed fuel in the combustor to generate combustion gas; and transferring heat from the combustion gas to the fuel via the heat transfer fluid prior to the passing of the fuel to the combustor to heat the fuel, wherein the method further comprises: measuring a pressure of the heat transfer fluid; and responsive to an increase in pressure of the heat transfer fluid, venting heat transfer fluid to an accumulator (210A; 210B); and/or responsive to a decrease in the measured pressure of the heat transfer fluid, shutting down the turbine engine and/or shutting off at least portion of the passing fuel. a compressor; a combustor; a turbine; a gaspath passing downstream from the compressor through the combustor and then through the turbine; a fuel source; a fuel flowpath from the fuel source to the combustor; and a heat exchanger (102; 400) for transferring heat from the gaspath to the fuel flowpath, wherein the heat exchanger (102; 400) comprises: an inner wall in heat transfer relation with the gaspath; an outer wall; tubes (180, 182) between the inner wall and the outer wall bounding respective segments of the fuel flowpath; and a heat transfer fluid between the inner wall and the outer wall and in heat transfer relation with the tubes (180, 182) and the inner wall, an inner wall in heat transfer relation with the gaspath; an outer wall; tubes (180, 182) between the inner wall and the outer wall bounding respective segments of the fuel flowpath; and a heat transfer fluid between the inner wall and the outer wall and in heat transfer relation with the tubes (180, 182) and the inner wall, wherein the method comprises: passing fuel along the fuel flowpath from the fuel source to the combustor; combusting the passed fuel in the combustor to generate combustion gas; and transferring heat from the combustion gas to the fuel via the heat transfer fluid prior to the passing of the fuel to the combustor to heat the fuel, wherein the method further comprises: measuring a pressure of the heat transfer fluid; and responsive to an increase in pressure of the heat transfer fluid, venting heat transfer fluid to an accumulator (210A; 210B); and/or responsive to a decrease in the measured pressure of the heat transfer fluid, shutting down the turbine engine and/or shutting off at least portion of the passing fuel. passing fuel along the fuel flowpath from the fuel source to the combustor; combusting the passed fuel in the combustor to generate combustion gas; and transferring heat from the combustion gas to the fuel via the heat transfer fluid prior to the passing of the fuel to the combustor to heat the fuel, wherein the method further comprises: measuring a pressure of the heat transfer fluid; and responsive to an increase in pressure of the heat transfer fluid, venting heat transfer fluid to an accumulator (210A; 210B); and/or responsive to a decrease in the measured pressure of the heat transfer fluid, shutting down the turbine engine and/or shutting off at least portion of the passing fuel.