A first year undergraduate engineering student would be able to build a reliable temperature monitor.
Right. Because there are so many combinations of materials that can withstand temperatures in the thousands of degrees F and the intense neutron flux in a commercial reactor core for any prolonged period. Core status is measured by the temperature of the water entering and leaving the core - the core power can be calculated by how much the water heats up. Safety limits are usually given in terms of power, because the behavior has to be calculated.