At his christening on December 24, 1904, he was named Volmer HoeghNielsen, and Thora Hoegh was listed as his mother. Walter was convincedhe was an illegitimate child, so as an adult he searched for who hismother was. By 1956, he had identified Justa Tang Beldring as his mother,and he and his family met with her. He continued his search fordocumentation that Justa was his mother. A letter from the Esbjerg cityarchivist dated July 23, 1991, states, "According to census data there isno link between Justa Tang and the Nielsens." Even without documentation,Walter was convinced Justa was his mother.

Walter earned a bachelor's degree and a doctorate in engineering from theUniversity of Michigan ‎(he later earned a second doctorate from theSorbonne in Paris, France)‎. Early in 1942, he was invited to interview inChicago for an important government project. At the Palmer House Hotel hewas interviewed by Enrico Fermi and Ernest O. Lawrence, both of whomoffered him jobs. Feeling his background and interests fit better withDr. Lawrence's work, he accepted the position with Dr. Laerence inCalifornia. Walter's work was instrumental in development of the atombomb and held several patents on key parts of it.

In the early 1990's, Walter expressed a heavy guilt about his involvementwith the bomb. He felt the government had promised the bomb would bedropped in an uninhabited area as a demonstration of its power. He felthis work led to the unnecessary death of over 150,000 people. On beingtold that Jimmie Doolittle's fire-bombing of Tokyo killed over halfmillion people and that the estimate of casualties for the invasion ofJapan was over a half million just for the allied side, Walter decided toreview his feelings. Eventually, he began to think his work may havesaved lives, so he spoke cautiously at a church group of his work on theatom bomb. After the meeting a man came to him and said, "Thank you! Iwas on a troup ship headed for the invasion when your bomb was dropped.You saved my life!" Similar experiences every time Walter spoke, freeinghim of a life-long feeling of guilt.

Walter was a flying enthusiast and held a commercial pilot's license intothe 21st Century.